Journal of Plant Physiology Metabolomic and

Journal of Plant Physiology 232 (2019) 200–208

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Journal of Plant Physiology journal homepage: www.elsevier.com/locate/jplph

Metabolomic and transcriptional analyses reveal the mechanism of C, N allocation from source leaf to flower in tea plant (Camellia sinensis. L) Kai Fan, Qunfeng Zhang, Meiya Liu, Lifeng Ma, Yuanzhi Shi, Jianyun Ruan

T

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Tea Research Institute, Chinese Academy of Agricultural Sciences, Hangzhou, 31008, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Camellia sinensis Flowering Nitrogen remobilization Metabolome Gene expression

Tea flowering in late autumn competes for a large amount of nitrogen and carbohydrates, potentially undermines the storage of these resources in vegetative organs, and negatively influences the subsequent spring tea yield and quality. The mechanism underlying the re-allocation N and carbohydrate from source leaf to flower in tea plant has not been clearly understood. In this study, 15N allocation, changes in metabolomics, and gene expression in flower buds, flowers, and adjacent leaves were characterized. Total N content of the adjacent leaves significantly decreased during flowering while such a decrease could be reversed by flower bud removal. Foliar-applied 15N in the adjacent leaves markedly decreased and was readily allocated to flowers. Metabolomic analysis revealed that most sugars and benzoic acid increased by more than two-fold whereas theanine, Gln, Arg, Asp, and Asn decreased when flower buds fully opened to become flowers. In this process, Gly, Pro, and cellobiose in the adjacent leaves increased considerably whereas sucrose, galactose, benzoic acid, and many fatty acids decreased. Removal of flower buds reversed or alleviated the above decreases and led to an increase of Asn in the leaves. The expression of genes associated with autophagy (ATG5, ATG9, ATG12, ATG18), sucrose transporters (SUT1, SUT2, SUT4), amino acids permease (AAP6, AAP7, AAP8), glutamine synthetase (GS1;1, GS1;2, GS1;3), and asparagine synthetase (ASN1, ASN2) was significantly up-regulated in leaves during the flowering process and was strongly modulated by the removal of flower buds. The overall results demonstrated that leaves are the ready source providing N and carbohydrates in flowering and a series of genes related to autophagy, protein degradation, turn-over of amino acids, and phloem loading for transport are involved.

1. Introduction Tea, one of the most popular non-alcoholic beverages in the world, is made from fresh young shoots of tea plants (Camellia sinensis). Flowering, the transition from the vegetative to reproductive phase often occurs from mid-September to early December and peaks in October to November in most Chinese tea planting areas (Yang, 2005). Tea flower contains versatile components, rendering it a potentially important resource for functional by-products (Chen et al., 2016). Tea flower also contains abundant contents of nutrients such as nitrogen (N), phosphorus (P), and potassium (K). It is estimated that the dry mass of tea flowers in a typical tea plantation can range from 3,000–12,000 kg/ha (Chen et al., 2018a,b). Consequently approximately 5.3–21.1 kg P/ha and 31–124 kg K/ha are consumed in the flower development and flowering process assuming concentrations of 1.76 g/kg and 10.3 g/kg (Jia et al., 2016), respectively. In addition to

mineral nutrients, many studies also demonstrated that a large portion of carbohydrates and nitrogenous compounds are transported from vegetative (e.g. leaves) to reproductive organs during the phase of reproductive growth (Clifford et al., 1995; Gladun and Karpov, 1993; Han et al., 2017; Sklensky and Davies, 2011). For example, N stores were more commonly depleted after reproductive growth in masting tree species (Han et al., 2017). The overall requirement for carbohydrates of a single flower of grapefruit (Citrus paradisi Macf. cv. ‘Marsh seedless’) is estimated as 8·33 × 10–3 mol C over three weeks, and the amount invested each year in bloom at the whole-tree level is 166–400 mol C per tree depending on the number of flowers (Bustan and Goldschmidt, 1998). The mineral nutrients in flowers may be supplied from immediate root uptake and/or storage organs such as mature leaves (Brant and Chen, 2015). Martinez-Alcantara et al. (2011) demonstrated that in citrus more than 70% N in flowers is re-translocated from mature leaves during fully flowering stage. Consumption of such a large amount of

Abbreviations: ATG, autophagy-related gene; AAP, amino acid permease; GS, glutamine synthetase; ASN, asparagine synthetase; SUT, sucrose transporter; SAG12, senescence-associated gene 12; SCPL, serine carboxypeptidase ⁎ Corresponding author. E-mail address: [email protected] (J. Ruan). https://doi.org/10.1016/j.jplph.2018.11.007 Received 29 June 2018; Received in revised form 5 November 2018; Accepted 5 November 2018 Available online 10 November 2018 0176-1617/ © 2018 Elsevier GmbH. All rights reserved.


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and ASN in leaves was analyzed simultaneously. The objective was to uncover the allocation of N and carbohydrates from source leaves to flower and the underlying mechanism in tea plant.

nutrients competes strongly with vegetative growth (Han et al., 2017; Martinez-Alcantara et al., 2015). Tea flowering occurs in late autumn when storage N and carbohydrates are accumulating in vegetative organs (leaves, roots, and branches). Such storage is an important source for the growth of young shoots in the following spring, and its pool size of remobilization greatly influences the spring tea quality (Liu et al., 2016). Therefore, it is important to clearly understand the mechanism of N allocation to flowers in tea plants. The development of reproductive organs often causes physiological and morphological changes in vegetative organs. For example, leaf N, chlorophyll, and net photosynthesis decrease in mango and Chinese chestnut trees after flowering (Urban et al., 2004; Xie and Guo, 2015). The starch and total soluble sugar are lowest in leaves of Phalaenopsis during flower opening (Yuan et al., 2016). On the other hand, removing flowers showed obvious changes in leaf metabolites in many experiments (Brown and Hudson, 2017; Chen et al., 2018a,b; Turner et al., 2012). Furthermore, the re-allocation of nutrients to reproductive tissues often triggers senescence of other tissues (Sklensky and Davies, 2011). Senescence, a type of programmed cell death, is a complex and highly regulated process that begins with the degradation of macromolecules. Autophagy is a universal mechanism which facilitates the vacuole-dependent degradation of unwanted cell constituents in plant (Masclaux-Daubresse et al., 2017). With the involvement of autophagyrelated genes ATGs, the autophagosome can be formed in a stepwise process, resulting in the engulfment of cytoplasmic material and its translocation to the vacuole (Avin-Wittenberg et al., 2018). Studies with ATG mutants provide strong evidence that ATGs play an important role in autophagy, which is required for N remobilization (Guiboileau et al., 2013, 2012). When autophagosomes reach the lytic vacuoles, protease begins to degrade protein, mainly Rubisco, to remobilize N. (Kato et al., 2004, 2005). Following vacuolar proteolysis, amino acids are released for other interconversions. It is widely accepted that Gln and Asn are the preferential exported forms in plants (Guak et al., 2003; Malaguti et al., 2001). The GS/GOGAT and asparagine synthetase (ASN) genes play important roles in the metabolism of these amino acids (Cooke and Weih, 2005; Gaufichon et al., 2013, 2010). Finally, these amino acids are loaded into phloem and transported to sink tissues (Tegeder and Hammes, 2018). Many studies suggest that the proton-coupled amino acid permeases (AAPs) are involved in the transport of a broad spectrum of amino acids (Tegeder, 2012; Tegeder and Ward, 2012). In the present study, we investigated the levels of metabolites in flower buds and flowers and their changes in the adjacent leaves during the process of flowering by metabolomic analysis. The expression of genes related to ATGs, protease, sucrose transporters (SUTs), AAPs, GS,

2. Materials and methods 2.1. Plant materials and treatments Tea plants (C. sinensis cv. Longjing 43) from the experimental plantation at the Tea Research Institute in Hangzhou, Zhejiang province (latitude N30.17, longitude E120.09) were used in this study. The bushes were planted in single rows with a 1.5-m row distance and 33-cm space between bushes within a row. The plantation was fertilized with N, P, and K (285, 60, and 90 kg/ha, respectively, annually) and routinely maintained according to local practices. The N fertilizer was urea (LingGu, China, N ≥ 46.4%), and applied four times each year, i.e. one in October (30%) as basal fertilization followed by another three in late February (30%), mid May (20%), and mid June (20%) of the next year. P and K were supplied as single superphosphate (TaiShan, China, P2O5 ≥ 12.0%) and potash sulfate (KaLi, Germany, K2O ≥ 50.0%) in October as basal fertilization. All fertilizers were applied to small temporary furrows (about 10 cm wide and 10 cm deep) excavated in the middle between two lines of bushes, which were filled with soil after fertilizer application. Two experiments were carried out. In the first experiment, samples of flower buds, flowers, and their immediate adjacent leaves were taken at two time points during the flowering period. The first sampling was performed on October 15, 2016, at the bud stage. Each of ten flower buds (FB) and their immediate adjacent leaves (hereafter referred to as Leaves_FB) were randomly collected from the middle canopy of a tea bush. The second sampling was performed two weeks later when most of the buds were in full blossom (on Oct 31, 2016). Again, each of ten flowers (F) and their immediate adjacent leaves (hereafter referred to as Leaves_F) were randomly collected from similar canopy locations. Samples were taken from three bushes and referred to as three independent replications. To verify the assumption that the N in leaves can be allocated to the adjacent flowers, a 15N pulse-chasing was conducted. On the same day, each of another 20 leaves adjacent to flower buds was randomly selected. A volume of 100 μL urea solution (15N enrichment 99%) at a concentration of 1% [weight/volume (w/v)] was applied carefully using a pre-equilibrated paintbrush to both surfaces of each selected leaf. One day later, each of 10 labeled leaves and their adjacent flower buds were sampled. Another 10 leaves receiving 15N labeling and their adjacent flowers were also sampled 15 days after the first sampling (November 1, 2016).

Fig. 1. The morphological characteristics of different flower development stage in this study. 201


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In the second experiment, flower buds on one side of tea bushes were completely removed by hand on October 16, 2016 while those on the other side were reserved (as control) (Fig. 1) (hereafter referred to as flower removal experiment). Each of 10 leaves with their adjacent flower buds removed (Leaves_−F) or reserved (Leaves_+F) was sampled from tea bushes on the same day. Four tea bushes were randomly selected and regarded as four replications. When most of the buds had fully opened to flowers (14 days after the bud removal), each of 10 leaves immediately adjacent to flowers and those with flower bud removal were sampled (on November 1, 2016) from the two sides of the treated bushes, respectively. The samples of flower buds, flowers, and leaves were washed in deionized water, and divided into two parts, one treated shortly with liquid N and stored in a -80℃ ultra refrigerator for gene expression and metabolomic analysis. The other part was immediately dried in an airforced oven at 70℃ to constant weight for C and N content measurements.

detector voltage was 0.9 kV. 2.4. Data processing and analysis The data files from GC × GC-TOF/MS were processed by LECO Chroma TOF software at S/N threshold 500. Metabolite identification from these selected variables was achieved by NIST 05 Standard mass spectral databases (NIST, Gaithersburg, MD, USA). The resulting data containing sample information, peak retention time, and peak intensities were normalized to the area of the internal standard and then mean-centered. Multivariate statistical analyses such as principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) were worked using SIMCA-P 11.0 (Umetrics AB, Umea, Sweden). The notable metabolite differences were screened by the loading plot in PLS-DA. Variables with VIP (Variable Importance in the Projection) > 1 which played significant roles in the classification were selected for further analysis. Subsequently, independent t-test was utilized to exclude the variables that were not significantly (P > 0.05) different among the groups. Pathway analysis was performed using MetaboAnalyst 3.0, open-source bioinformatics website for metabolite data interpretation.

2.2. Measurement of total C, N, and amino acid contents Total C and N contents of the samples were determined by an elemental analyzer (Vario Max CN Analyzer, Elementar Analysensysteme GmbH, Germany). Samples (100 mg) were extracted by 5 mL water in a boiling water bath for 5 min and centrifuged for 10 min. Free amino acids in the supernatant extract were then quantified by an automatic amino acid analyzer (S-433D, Sykam GmbH, Germany) (Yang et al., 2013). The 15N abundance of plant samples was determined by an isotope ratio mass spectrometer (Finigan Corp., Bremen, Germany) coupled to an elemental analyzer (Carlo Erba, Milano, Italy). The parameters representing 15N in plant organs were calculated by the following equations (Ourry et al., 1994; Ruan and Gerendás, 2015).

N derived from fertilizer (Ndff , %) =

2.5. Identification of related genes in the tea genome and relative expression analysis Candidate genes involved in autophagy (ATG5, ATG7, ATG8, ATG9, ATG12, ATG18), protease (SAG12, CND41, SCPL, RD21), H+/sucrose symporters (SUT1, SUT2, SUT3, SUT4, SWEET1, SWEET2, SWEET16, SWEET17), amino acid permease (AAP3, AAP6, AAP7, AAP8), glutamine synthetase (GS1;1, GS1;2, and GS1;3), and asparagine synthetase (ASN1, ASN2) were identified from the tea genome database (http:// www.plantkingdomgdb.com/tea_tree/). Full-length amino acids sequences of related proteins in Arabidopsis thaliana (http://www. arabidopsis.org/) were used as query sequences. A BLASTp search was performed and E-value of e−6 was used as the threshold (Wang et al., 2014). The candidate genes ID in tea genome database and the primer sequences are listed in Table S1. Melting curves of each pair of primers are presented in the supplementary figures (Fig S2, Fig S3, and Fig S4). For gene expression analysis, total RNA was isolated using Trizol reagent (Invitrogen, CA, USA) following the manufacturer's instruction. The total RNA was reverse transcribed using PrimeScript™ 1 st Strand cDNA Synthesis Kit (Takara, Japan). The quantitative real-time PCR (qRT-PCR) was performed on the Applied Biosystems 7300 machines (Carlsbad, CA, USA). qRT-PCR was performed for three independent biological replications. For each gene, triplicate reactions were performed. The GAPDH was used as the reference gene, and ΔCt [ΔCt = (Ct target gene–Ct GAPDH gene)] was calculated. The expression level of SAG12 in Leaves_FB was set as control, and ΔΔCt=[ΔΔCt=(ΔCt target gene -ΔCt sag12 in Leaves_FB)] was calculated. The relative expression of each gene was quantified using the 2−ΔΔCt method.

15N excess in plant × 100 15N excess in fertilizer

where 15Nexcess (%) = 15N abundance (%) − natural 15N abundance (0.3660%); The amount of 15N in flowers or leaves (15N) = Ndff × total N concentration × biomass; Change of 15N amount in flowers or leaves (Δ 15N) = 15Nt2 − 15Nt1, where 15Nt1 and Nt2 are the 15N amount at the first (t1) and second (t2) time, respectively. 2.3. Metabolic analysis based on GC × GC-TOF/MS For metabolomic analysis, plant samples were extracted with methanol/chloroform solvent (3:1, v/v) (Liu et al., 2015). L-2-Chlorophenylalanine (0.3 mg/mL in water, 10 μL) was added in the extraction as internal standard. After centrifugation at −4 °C for 10 min at 12,000 rpm, the supernatant (400 μL) was transferred to a new 2-mL Eppendorf tube, dried in a vacuum concentrator with moderately blowing N2 gas without heating, and finally derivatized according to the previously described method (Lisec et al., 2006). Each 1-μL aliquot of the derivatized product was injected in splitless mode into a two-dimensional gas chromatograph (GC × GC, Agilent 6890 N, Agilent Technologies, Santa Clara, CA, USA) connected to a mass spectrometer (Pegasus HT, Leco Co., St. Joseph, MI, USA). The GC × GC was equipped with a DB-5 ms column (30 m × 250μμm i.d. × 0.25 μm) as the first-dimension column and a DB-17H (2.5 m × 0.1 mm i.d. × 0.1 μm) as the second-dimension column. The instrumental settings referred to the previously described method (Zhang et al., 2018). The carrier helium gas was set at a constant flow rate of 1 mL /min. The injection temperature was 280 °C, and that of the ion source was 220 °C. The GC column temperature was set at 90 °C for 2 min, increased to 180 °C at a ramping rate of 8 °C/min, then to 240 °C at a rate of 3 °C/min, and finally to 290 °C at a rate of 25 °C/min, kept for 18.25 min. Mass spectra were obtained by full scan monitoring mode with the mass scan range of 30–600 m/z at a rate of 50 spectra per second. The ionization mode was electron impact at 70 eV and a

3. Results 3.1. Total C, N, and 15N enrichment in flower buds, flowers, and their adjacent leaves During the process of blossom (flower bud to flower), the contents of total C and N in flower buds, flowers, and adjacent leaves changed significantly (Table 1). Total N and C contents in fully opened flowers were significantly higher than those in flower buds. Total free amino acids, which represents the size of the immediate N pool, was higher in fully opened flowers than in buds. Conversely, total N contents in the adjacent mature leaves decreased significantly in the flowering process. Total C in leaves however changed insignificantly. Removal of flower buds led to significant increases of total N and free amino acids in the 202


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Table 1 Total N, C, amino acids and 15N amount in flower bud (FB), flower (F), leaves with adjacent flower bud (Leaves_FB) and flower (Leaves_F), and leaves with (Leaves_ +F) or without (Leaves_−F) adjacent flower (mg g-1). Labeled urea (15N) was applied to leaves adjacent to flower or bud.

N (mg g−1) C (mg g−1) Amino acids (mg g−1) Ndff (%) 15 N amout (μg/per 10) Δ15N (μg/per 10)

Flower bud

Flower

Leaves_FB

Leaves_F

Leaves_−F

Leaves_+F

31.4 ± 0.4a 454.9 ± 0. 4a 47.9 ± 2.2a 0.0024 ± 0.0001a 0.62 ± 0.006a /

35.1 ± 0. 4b 483.2 ± 5.3b 55.1 ± 3.6b 0.14 ± 0.004b 57.4 ± 2.84b 56.8 ± 2.84

31.7 ± 0.6b 476.8 ± 3.8 16.7 ± 0.06a 0.11 ± 0.001b 102.6 ± 1.84b /

30.7 ± 0.3a 485.5 ± 2.1 22.0 ± 1.2b 0.08 ± 0.002a 71.1 ± 1.80a −31.5 ± 2.15

33.5 ± 0.6b 478.1 ± 2.1 26.0 ± 1.2b / / /

31.3 ± 0.9a 487.9 ± 0.6 17.7 ± 0.08a / / /

Note: Different letters within the same lines means significant difference (p < 0.05) between each comparison group (flower and flower bud, leaves_FB and leaves_F, leaves_+F and leaves_−F).

ATG8 which was down-regulated (Fig. 3C). Removal of flower buds reversed the change in the expression of these genes in the adjacent leaves, where the expression of CND41 was induced whereas those of SCPL, ATG5, ATG8, ATG9, ATG12, and ATG18 were repressed (Fig. 3B, D). The expression of ATG7 was not affected by the removal of flower buds. Flowering significantly up-regulated the expression levels of GS1 (GS1;1, GS1;2; GS1;3) and ASN (ASN1; ASN2), which participate in the assimilation of amino acids (Fig. 4A, C). Among them, the expression levels of GS1 members in Leaves_F were up-regulated by more than five-fold compared with Leaves_FB (Fig. 4A). Expression of amino acid permeases (AAP6, AAP7, and AAP8) in the adjacent leaves was also dramatically up-regulated whereas AAP3 expression was slightly downregulated during the flowering process (Fig. 4C). Again, the removal of flower buds reversed the change in gene expression, repressing the expression of GS1;1 and GS1;2 but up-regulating the expression of GS1;3 and ASN1 (Fig. 4B). Flower bud removal moderately enhanced AAP7 and AAP8 expression, and tremendously increased AAP6 expression by more than eight-fold, which increased by much more than four-fold in comparison to Leaves_F vs Leaves_FB (Fig. 4C). Based on the results of the metabolomic analysis, we found that sucrose significantly changed in the source leaves during the flowering process (Table 2). Sucrose was loaded from source leaves into the phloem and unloaded into sink tissues by transporters (Remi et al., 2013). Two groups of sugar transporters, SUTs and SWEETs, which are assumed to be involved in the process, were analyzed (Fig. 5). The SUTs in the flower adjacent leaves were significantly higher than in flower bud adjacent leaves, showing up-regulation during the flowering process (Fig. 5A). By contrast, the SWEETs expression in the adjacent leaves was repressed after blossom. SWEET2 expression was strongly repressed while the expression of SWEET1 and SWEET17 was moderately and SWEET16 was slightly down-regulated (Fig. 5C). Flower bud removal repressed the expression of SUT1, SUT4, SWEET1, and SWEET17 but greatly induced SUT3 (by more than three-fold) and SWEET16 (by 15-fold) expression (Fig. 5B, D).

adjacent leaves. To better understand the N allocation between flowers and their adjacent leaves, leaves were labeled with 15N urea. The 15Ndff significantly increased in fully opened flowers compared to in flower buds whereas the corresponding values in their adjacent leaves decreased in the meantime (Table 1). The net accumulated amount of 15N in flowers compared to flow buds was about 56.8 μg 15N (per 10 unit). During this period, about 31.5 μg 15N in their adjacent leaves was mobilized and likely transported to flowers. 3.2. Metabolomic analysis of flower buds, flowers, and adjacent leaves Metabolomic analysis was performed on flower buds, flowers, and adjacent leaves by GC × GC-TOF/MS. The Supervised Partial Least Squares Discrimination Analysis (PLS-DA) was used to investigate the metabolites, demonstrating the greatest differences between any of the two pairs, buds (FB) vs flowers (F), leaves with the adjacent flower buds (Leaves_FB) vs those with flowers (Leaves_F), and leaves with (Leaves_ +F) vs without (Leaves_−F) the adjacent flowers. Metabolic differences were enriched in several metabolic pathways including Ala, Asp, and Glu metabolism; Gly, Ser, and Thr metabolism; starch and sucrose metabolism; and pyruvate metabolism (Fig. S1). Twenty-four differential metabolites were screened based on VIP > 1 and p < 0.05 of t-test between each of the two pairs and mainly amino acids, sugars, and organic acids. The differences in amino acids and sugars were more significant in the pair of flower buds vs flowers than in other pairs. Most sugars, Ala, Gly, malate, and benzoic acid increased by more than two-fold whereas Asp, Asn, and Ile decreased when flower buds fully opened. During the process of blossom, Gly, Pro, and cellobiose in the adjacent leaves increased considerably whereas sucrose, galactose, benzoic acid, and many fatty acids decreased. Removal of flower buds reversed or alleviated the decrease of sucrose, benzoic acid, and fatty acids (except butanoic acid). Flower bud removal led to an increase of Asn and a decrease of cellobiose but hardly affected galactose. The contents of free amino acids were further quantitatively analyzed by HPLC (Fig. 2). The amino acids Asn, Arg, Gln, and Theanine, which play a storage role, decreased immensely after the flower bud opened. Asp, Ser, and Gly in the adjacent leaves showed opposite change and increased during this process. Removal of flower buds led to an increase of Asn but a decrease of Gln in leaves (Leaves_−F) compared to those (Leaves_+F) with flowers intact.

4. Discussion 4.1. The sink-source relation of metabolites between flowers and the source leaves Flowering and later the development of seeds, representing reproductive growth of tea plants, cost large amounts of carbohydrates and nutrients. Flowering costs at least 105 kg N and 1440 kg C per hectare each year, according to the present results that flowers contain N 35 mg g−1 and C 480 mg g-1. Such consumption may impose competition of assimilates and nutrients between vegetative and reproductive growth and consequently negatively affect the productivity of tea plantations as young shoots are the harvested subject (Bustan and Goldschmidt, 1998). The mineral nutrients in flowers may be supplied from immediate root uptake and/or storage organs such as mature leaves (Brant and Chen, 2015). In the present study, the contribution to

3.3. Expression of genes involved in C, N metabolism in flower buds, flowers, and adjacent leaves Flowering remarkably influenced gene expression in the adjacent source leaves (Fig. 3A, C). The protein degradation includes two pathways: autophagy and protease pathway (Liu et al., 2008). For genes involved in the protease pathway, RD21 showed moderate repressive expression while SCPL was slightly up-regulated (Fig. 3A). Most autophagy-related gene ATG members in leaves were up-regulated, except 203


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Fig. 2. The amino acid profile in flower bud (FB), flower (F), leaves with adjacent flower bud (Leaves_FB) and flower (Leaves_F), and leaves with (Leaves_+F) or without (Leaves_−F) adjacent flowers. * and ** indicate significant difference at p < 0.05 and p < 0.01, respectively.

flowering of nutrients in mature leaves adjacent to flower buds was assessed. We found that the total N content of the leaves significantly decreased during flowering while such a decrease could be reversed by flower bud removal (Table 1). Moreover, 15N pulse-chasing results showed significant accumulation at the start of flowering in flowers of

15

N with previous foliar application to the adjacent leaves whereas the N enrichment in the leaves decreased markedly in the meantime. These findings suggest that N in the adjacent leaves can be readily transported to and consumed by flowering. The present finding is in line with previous works with other plants (Han et al., 2017; Brant and 15

Fig. 3. Expression of genes involved in protein degradation in leaves with adjacent flower bud (Leaves_FB) and flower (Leaves_F), and leaves with (Leaves_+F) or without (Leaves_−F) adjacent flowers. The relative expression values were calculated by the 2−ΔΔCT method, and GAPDH was used as the reference gene. Expression level of SAG12 in Leaves_FB was set as 1. * and ** indicate significant difference at p < 0.05 and p < 0.01, respectively. 204


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Fig. 4. Expression of genes involved in amino acid metabolism and transport in leaves with adjacent flower bud (Leaves_FB) and flower (Leaves_F), and leaves with (Leaves_+F) or without (Leaves_−F) adjacent flowers. The relative expression values were calculated by the 2−ΔΔCT method, and GAPDH was used as the reference gene. Expression level of SAG12 in Leaves_FB was set as 1. * and ** indicate significant difference at p < 0.05 and p < 0.01, respectively.

Chen, 2015; Chapin et al., 1990; Martinez-Alcantara et al., 2011; Milla et al., 2005). Martinez-Alcantara et al. (2011) demonstrated that in citrus more than 70% N in flowers is re-translocated from mature leaves during the full flowering stage. Interestingly, we found that the increment of 15N amount in flower was more than decrement of 15N amount in the adjacent leaves. This indicated that there were other forms of 15N exported to the flower. We speculated that part of the labeling urea might export to flowers directly. The inconsistent change might also be caused by the experimental method in which the plant samples were sampled one day after labeling (referred to as t1). A significant amount of 15N may have moved out of the leaves but not yet reached the flower buds during the one-day period between labeling and first sampling. A previous study showed that N in leaves from foliar application is immediately transported to active organs such as young shoots (Ruan and Gerendás, 2015). Amino acids are considered the main forms of N allocation. In the present study, we observed significant changes of free amino acids in flower buds, flowers, and adjacent leaves in the process of flowering. The contents of Asp, Asn, Theanine, and Arg in flower buds were significantly higher than those in blooming flowers (Fig. 2 and Table 2). These amino acids might play the N storage role in flower buds and may thereby be consumed after blooming. After flowering, Pro, Gly, and Ser increased in the adjacent leaves. Pro was reported as a candidate signal accelerating flower growth and meristem formation, and its biosynthesis is required during the development of reproductive organs (Funck et al., 2012; Mattioli et al., 2009). In the present study, we found significant changes of Asn and Asp in flowers and the adjacent leaves, in the latter being affected by removal of flower buds. Asn is one of the dominant organic N compounds in phloem and xylem sap for transport between source and sink (Gaufichon et al., 2016; Yabuki et al., 2017). We speculate that Asn is the N form transported between flowers and

Table 2 Fold changes of metabolites in flower bud (FB), flower (F), leaves with adjacent flower bud (Leaves_FB) and flower (Leaves_F), and leaves with (Leaves_+F) or without (Leaves_−F) adjacent flower. Only those metabolites with VIP > 1.0 and significantly changed (t-test, p < 0.05) are presented in the table. The positive and negative values indicate increase and decrease of the metabolites. Metabolite

Amino acids Asp Ala Gly Asn Ile Pro Thr Sugars Fructose Sucrose Maltose Mannose Cellobiose Galactose Organic acids Malate Pyruvate Benzoic acid Butanoic acid Gluconic acid Fatty acids Arachidic acid Nonadecanoic acid Oleic acid Hexadecanoic acid Linolenic acid

Flower/ Flower bud

Leaves_F/ Leaves_FB

−5.02 2.42 2.20 −1.94 −4.62

5.85

Leaves_―F/Leaves_+F

1.23 3.92 −2.17

9.73 5.48 4.63 3.12

3.66 −1.92 2.38

−5.58

−2.04

4.35 −6.69

−2.96 −7.00

−8.69 −2.79 −3.70

7.00 −3.69 4.73

−2.38 −2.74 −4.69 −6.27

5.68

3.94

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Fig. 5. Expression of genes involved in sugar transport in leaves with adjacent flower bud (Leaves_FB) and flower (Leaves_F), and leaves with (Leaves_+F) or without (Leaves_−F) adjacent flowers. The relative expression values were calculated by the 2−ΔΔCT method, and GAPDH was used as the reference gene. Expression level of SAG12 in Leaves_FB was set as 1. * and ** indicate significant difference at p < 0.05 and p < 0.01, respectively.

leaf senescence, showing the CLPP functions in Clp degradation (Gombert et al., 2006; Roberts et al., 2012). However, in the present study, the expression of protease genes in leaves either exhibited no obvious change (SAG12, SCPL) or was even down-regulated (CND41, RD21) during the flowering process. This result suggests that the protease pathway might not be the main pathway of protein degradation for providing N remobilization in leaves during flowering. Autophagy is an important pathway before proteolysis. Recently, phenotype of ATG mutants provided strong evidence that ATG plays an important role in N remobilization in maize (Li et al., 2015). Most atg mutants show low remobilization efficiency at the whole-plant level (Avila-Ospina et al., 2014; Guiboileau et al., 2012, 2013). Our results show that the expression of ATG encoding genes was significantly up-regulated in leaves during the flowering process. Removal of flowers decreased the expression levels of ATGs (Fig. 3). These findings provide explicit evidence that the ATGs were probably involved in facilitating the protein degradation for N re-allocation to flowers during flowering. The amino acids released by protein degradation are further transformed by amino transferase into the form for transport. Plenty of studies indicate that the GS/GOGAT cycle is the primary route for ammonium assimilation in senescing leaves and GS1 plays important role in N remobilization (Lothier et al., 2011; Tabuchi et al., 2005). For example, GS1 gene is up-regulated during autumn senescence in poplar, suggesting its role in N remobilization (Castro-Rodríguez et al., 2011). Our results showed that the GS1 genes (GS1;1, GS1;2, and GS1;3) were all up-regulated in leaves during flowering whereas such activation was repressed by removing flower buds, except for GS1;3. This finding suggests that the expression of GS1 was strongly influenced by flowering. Another important enzyme for N remobilization is asparagine synthetase transferring the glutamine-amide group to the 2-position of aspartate and releasing Asn and Glu (Gaufichon et al., 2010). Recent

source leaves. Due to lack of photosynthesis pigments in flowers, carbon is another important nutrient transported from source leaves. We observed an increase of total C and nonstructural carbohydrates (fructose, sucrose, maltose, and mannose) in flowers compared to flower buds, which might indicate net entry from source organ (Tables 1 and 2). On the other hand, sucrose in the adjacent leaves concomitantly decreased in this process, which is consistent with the previous results (Jia et al., 2016), and such a decline was alleviated after removing the flower buds. Interestingly cellobiose, a representative product of cellulose degradation in leaves increased during the flowering process. However, removal of flowers led to a decrease in cellobiose. Cellobiose can be converted to glucose and is thereby a supportive source of sugars. A very recent study showed that cellobiose plays a signaling role following plant cell wall breakdown during cell wall remodeling (Souza et al., 2017). We found that fatty acids (arachidic acid, nonadecanoic acid, oleic acid, and hexadecanoic acid) in leaves decreased during flower development whereas such a decrease was reversed if the flower buds were removed. Flowers contain abundant aromatic components including fatty acids, which are released at blooming. A decrease in fatty acids in the leaves during flowering might suggest their remobilization and reutilization from source leaves to flower sink. 4.2. The effect of flowering on gene expression in the source leaves Re-allocation of nutrients from source to sink often triggers leaf senescence. This complicated progress often initiates the degradation of macromolecules, including proteins, nucleic acids, and lipids (Havé et al., 2016). The chloroplast proteins (clp), which account for up to 70% of all leaf proteins, are thought to be a major source of N for remobilization. Most of the genes encoding ClpP were identified during 206


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Fig. 6. The possible mechanism of C, N allocation from source leaves to flowers. indicates gene up-regulation. represents the autophagosome.

studies show that the ASN1-encoding asparagine synthetase in floral organs contributes to N filling in Arabidopsis seeds (Gaufichon et al., 2017). In the present study, ASN1 and ASN2 were up-regulated in leaves during flowering. Corresponding to such change, the contents of Asn and Asp were also significantly altered. These results suggest that the ASN genes might be involved in regulating the Asn metabolism to participate in N re-allocation to flowers. During remobilization and re-utilization of N, amino acids in the source leaves are transported to the sinks via membrane transporters. Recent studies have shown that the source to sink transport of amino acids is assisted by amino acid permease (AAP), localizing in the vasculature or phloem of leaves (Mechthild et al., 2007; Tegeder and Ward, 2012). In the present study, most AAPs were up-regulated by more than three-fold, especially AAP7 and AAP8 whose expression was less activated by removing flower buds. These findings indicate that the expression of AAPs was associated with the flowering process. Sucrose is known as the primary carbohydrate for long distance transport in many plant species. SUTs are H+/sucrose symporters which utilize the proton motive force (PMF) to load sucrose against its concentration gradient into the phloem (Reinders et al., 2012). In Arabidopsis, SUTs are highly expressed when sucrose export is high (Durand et al., 2018). Inhibiting the expression of SUT1 and SUT2 affects fruit and seed development in tomato (Hackel et al., 2006). On the other hand, over-expression of SUT enhances sucrose uptake and plant development (Leggewie et al., 2003). Previous studies have shown that most SUT genes are expressed in phloem cells including sieve elements and companion cells, confirming the phloem loading functions of SUTs (Williams et al., 2000; Xu et al., 2018). In the present study, the expression of SUTs was induced during the flowering process whereas their expression was reduced by removing flower buds. Therefore, the present finding suggests that SUTs in tea leaves are up-regulated to export sucrose to flowers during flower development. Interestingly, the present work showed that the expression of CsSWEETs in the adjacent leaves decreased in full blossoming compared to the flower bud stage (Fig. 5), following similar expression tendency in flowers (Wang et al., 2018). This result suggests that the expression of these genes in the adjacent leaves is likely regulated coordinately with flowers as the gene expression are intimately associated with the development of reproductive organs such as flower and seed (Wang et al., 2018). Furthermore, we found that removal of flower buds strongly up-regulated the expression of CsWEET16 in the adjacent leaves, which occurred committantly with the change in carbohydrates (Table 2). It is speculated that, similar to its Arabidopsis homologue AtSWEET16, CsSWEET16 locates at the tonoplast membrane and plays a role in maintaining sugar homeostasis, although its function has not been clearly understood (Guo et al., 2014; Wang et al., 2018). Therefore, it is likely that SUTs contribute to the allocation of sucrose between leaves and flowers.

5. Conclusions In conclusion, leaves are the sources readily providing N and carbohydrates to flowers. The possible mechanism of C and N allocation from source tea leaves to flowers is summarized in Fig. 6. During the process of blossom, ATG genes (ATG5, ATG7, ATG9, ATG12, ATG18) in leaves are significantly up-regulated to form autophagosomes and deliver cell material to the vacuole where proteases reside. Then, the amino acids released from proteolysis are transformed to Gln and Asn for phloem loading. The GS1 genes (GS1;1, GS1;2, GS1;3) and ASN genes (ASN1; ASN2) are induced in this process. Finally, the amino acids are loaded into phloem via amino acid permease (as indicated by the up-regulated expression of AAP genes AAP6, AAP7, AAP8) for transport to the flowers. On the other hand, sucrose is loaded for phloem transport by SUT genes (SUT1, SUT2, SUT4) in the source leaves. Author contributions statement KF and JR designed the research; KF performed most the experiments and wrote the paper; QZ helped to analyze the metabolome data; ML, LM, and YS discussed the data; JR revised the article. All the authors have read and approved the manuscript. Conflict of interest The authors declare that they have no conflicts of interest. Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2016YFD0200901), the Chinese Academy of Agricultural Sciences through an Innovation Project for Agricultural Sciences and Technology (CAAS-ASTIP-2018-TRICAAS), and the National Natural Science Foundation of China (No. 31572199). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jplph.2018.11.007. References Avila-Ospina, L., Moison, M., Yoshimoto, K., et al., 2014. Autophagy, plant senescence, and nutrient recycling. J. Exp. Bot. 65, 3799–3811. Avin-Wittenberg, T., Baluska, F., Bozhkov, P.V., et al., 2018. Autophagy-related approaches for improving nutrient use efficiency and crop yield protection. J. Exp. Bot. 69, 1335–1353. Brant, A.N., Chen, H.Y.H., 2015. Patterns and mechanisms of nutrient resorption in plants. Crit. Rev. Plant Sci. 34, 471–486. Brown, A.V., Hudson, K.A., 2017. Transcriptional profiling of mechanically and

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change of metabolites and quality of green tea during the short duration of a single spring season. J. Agric. Food Chem. 64, 3302–3309. Lothier, J., Gaufichon, L., Sormani, R., et al., 2011. The cytosolic glutamine synthetase GLN1;2 plays a role in the control of plant growth and ammonium homeostasis in Arabidopsis rosettes when nitrate supply is not limiting. J. Exp. Bot. 62, 1375–1390. Malaguti, D., Millard, P., Wendler, R., et al., 2001. Translocation of amino acids in the xylem of apple (Malus domestica Borkh.) trees in spring as a consequence of both N remobilization and root uptake. J. Exp. Bot. 52, 1665–1671. Martinez-Alcantara, B., Quiñones, A., Primo-Millo, E., 2011. Nitrogen remobilization response to current supply in young citrus trees. Plant Soil 342, 433–443. Martinez-Alcantara, B., Iglesias, D.J., Reig, C., et al., 2015. Carbon utilization by fruit limits shoot growth in alternate-bearing citrus trees. J. Plant Physiol. 176, 108–117. Masclaux-Daubresse, C., Chen, Q.W., Havé, M., 2017. Regulation of nutrient recycling via autophagy. Curr. Opin. Plant Biol. 39, 8–17. Mattioli, R., Falasca, G., Sabatini, S., et al., 2009. The proline biosynthetic genes P5CS1 and P5CS2 play overlapping roles in Arabidopsis flower transition but not in embryo development. Physiol. Plant. 137, 72–85. Mechthild, T., Tan, Q., Aleelk, G., et al., 2007. Amino acid transporter expression and localisation studies in pea (Pisum sativum). Funct. Plant Biol. 34, 1019–1028. Milla, R., Castro-Díez, P., Maestro-Martínez, M., et al., 2005. Relationships between phenology and the remobilization of nitrogen, phosphorus and potassium in branches of eight Mediterranean evergreens. New Phytol. 168, 167–178. Ourry, A., Kim, T.H., Boucaud, J., 1994. Nitrogen reserve mobilization during regrowth of Medicago sativa L. (relationships between availability and regrowth yield). Plant Physiol. 105, 831–837. Reinders, A., Sivitz, A.B., Ward, J.M., 2012. Evolution of plant sucrose uptake transporters. Front. Plant Sci. 3, 22. Remi, L., La, C.S., Rossitze, A., et al., 2013. Source-to-sink transport of sugar and regulation by environmental factors. Front. Plant Sci. 4, 272. Roberts, I.N., Caputo, C., Criado, M.V., et al., 2012. Senescence-associated proteases in plants. Physiol. Plant. 145, 130–139. Ruan, J.Y., Gerendás, J., 2015. Absorption of foliar-applied urea-15N and the impact of low nitrogen, potassium, magnesium and sulfur nutritional status in tea (Camellia sinensis L.) plants. Soil Sci. Plant Nutr. 61, 653–663. Sklensky, D.E., Davies, P.J., 2011. Resource partitioning to male and female flowers of Spinacia oleracea L. in relation to whole-plant monocarpic senescence. J. Exp. Bot. 62, 4323–4336. Souza, C.D.A., Li, S., Lin, A.Z., et al., 2017. Cellulose-derived oligomers act as damageassociated molecular patterns and trigger defense-like responses. Plant Physiol. 173, 2383–2398. Tabuchi, M., Sugiyama, K., Ishiyama, K., et al., 2005. Severe reduction in growth rate and grain filling of rice mutants lacking OsGS1;1, a cytosolic glutamine synthetase1;1. Plant J. 42, 641–651. Tegeder, M., 2012. Transporters for amino acids in plant cells: some functions and many unknowns. Curr. Opin. Plant Biol. 15, 315–321. Tegeder, M., Hammes, U.Z., 2018. The way out and in: phloem loading and unloading of amino acids. Curr. Opin. Plant Biol. 43, 16–21. Tegeder, M., Ward, J.M., 2012. Molecular evolution of plant AAP and LHT amino acid transporters. Front. Plant Sci. 3, 21. Turner, G.W., Cuthbertson, D.J., Voo, S.S., et al., 2012. Experimental sink removal induces stress responses, including shifts in amino acid and phenylpropanoid metabolism, in soybean leaves. Planta 235, 939–954. Urban, L., Lu, P., Thibaud, R., 2004. Inhibitory effect of flowering and early fruit growth on leaf photosynthesis in mango. Tree Physiol. 24, 387–399. Wang, L., Zhu, W., Fang, L., et al., 2014. Genome-wide identification of WRKY family genes and their response to cold stress in Vitis vinifera. BMC Plant Biol. 14, 103. Wang, L., Yao, L.N., Hao, X.Y., et al., 2018. Tea plant SWEET transporters: expression profiling, sugar transport, and the involvement of CsSWEET16 in modifying cold tolerance in Arabidopsis. Plant Mol. Biol. Rep. 96, 577–592. Williams, L.E., Lemoine, R., Sauer, N., 2000. Sugar transporters in higher plants- A diversity of roles and complex regulation. Trends Plant Sci. 5, 283–290. Xie, P., Guo, S.J., 2015. Patterns of photoassimilate translocation between shoots in Chinese chetnet trees during flowering and fruit growth. Aust. J. Forensic Sci. 78, 86–91. Xu, Q.Y., Chen, S.Y., Ren, Y.J., et al., 2018. Regulation of sucrose transporters and phloem loading in response to environmental cues. Plant Physiol. 176, 930–945. Yabuki, Y., Ohashi, M., Imagawa, F., et al., 2017. A temporal and spatial contribution of asparaginase to asparagine catabolism during development of rice grains. Rice 10, 1–10. Yang, Y.J., 2005. Tea Plant Cultivation in China (in Chinese). Shanghai Scientific and Technological Press, Shanghai. Yang, Y.Y., Li, X.H., Ratcliffe, R.G., et al., 2013. Characterization of ammonium and nitrate uptake and assimilation in roots of tea plant. Russ. J. Plant Physiol. 60, 91–99. Yuan, X.Y., Xu, S.P., Liang, F., et al., 2016. Comparative proteomic analysis of Phalaenopsis leaves in the vegetative and flowering phase. Acta Physiol. Plant. 38, 1–13. Zhang, Q.F., Tang, D.D., Liu, M.Y., et al., 2018. Integrated analyses of the transcriptome and metabolome of the leaves of albino tea cultivars reveal coordinated regulation of the carbon and nitrogen metabolism. Sci. Hortic. 231, 272–281.

genetically sink-limited soybeans. Plant Cell Environ. 40, 2307–2318. Bustan, A., Goldschmidt, E.E., 1998. Estimating the cost of flowering in a grapefruit tree. Plant Cell Environ. 21, 217–224. Castro-Rodríguez, V., García-Gutiérrez, A., Canales, J., et al., 2011. The glutamine synthetase gene family in Populus. BMC Plant Biol. 11, 119. Chapin III, F.S., Schulze, E.D., Mooney, H.A., 1990. The ecology and economics of storage in plants. Annu. Rev. Ecol. Syst. 21, 423–447. Chen, Y.Y., Fu, X.M., Mei, X., et al., 2016. Characterization of functional proteases from flowers of tea (Camellia sinensis) plants. J. Funct. Foods 25, 149–159. Chen, Y.Y., Zhou, Y., Zeng, L.T., et al., 2018a. Occurrence of functional molecules in the flowers of tea (Camellia sinensis) plants: evidence for a second resource. Molecules 23, 790. Chen, Y.Z., Kong, X.Q., Dong, H.Z., 2018b. Removal of early fruiting branches impacts leaf senescence and yield by altering the sink/source ratio of field-grown cotton. Field Crop. Res. 216, 10–21. Clifford, P.E., Neo, H.H., Hew, C.S., 1995. Regulation of assimilate partitioning in flowering plants of the monopodial orchid Aranda Noorah Alsagoff. New Phytol. 130, 381–389. Cooke, J.E.K., Weih, M., 2005. Nitrogen storage and seasonal nitrogen cycling in Populus: bridging molecular physiology and ecophysiology. New Phytol. 167, 19–30. Durand, M., Mainson, D., Porcheron, B., et al., 2018. Carbon source–sink relationship in Arabidopsis thaliana : the role of sucrose transporters. Planta. 247, 587–611. Funck, D., Winter, G., Baumgarten, L., et al., 2012. Requirement of proline synthesis during Arabidopsis reproductive development. BMC Plant Biol. 12. Gaufichon, L., Reisdorf-Cren, M., Rothstein, S.J., et al., 2010. Biological functions of asparagine synthetase in plants. Plant Sci. 179, 141–153. Gaufichon, L., Masclaux-Daubresse, C., Tcherkez, G., et al., 2013. Arabidopsis thaliana ASN2 encoding asparagine synthetase is involved in the control of nitrogen assimilation and export during vegetative growth. Plant Cell Environ. 36, 328–342. Gaufichon, L., Rothstein, S.J., Suzuki, A., 2016. Asparagine metabolic pathways in Arabidopsis. Plant Cell Physiol. 57, 675–689. Gaufichon, L., Marmagne, A., Belcram, K., et al., 2017. ASN1-encoded asparagine synthetase in floral organs contributes to nitrogen filling in Arabidopsis seeds. Plant J. 91, 371–393. Gladun, I.V., Karpov, E.A., 1993. Production and partitioning of assimilates between the panicle and vegetative organs of rice after flowering. Russ. J. Plant Physiol. 40, 629–633. Gombert, J., Etienne, P., Ourry, A., 2006. The expression patterns of SAG12/Cab genes reveal the spatial and temporal progression of leaf senescence in Brassica napus L. with sensitivity to the environment. J. Exp. Bot. 57, 1949–1956. Guak, S., Neilsen, D., Millard, P., et al., 2003. Determining the role of N remobilization for growth of apple (Malus domestica Borkh.) trees by measuring xylem-sap N flux. J. Exp. Bot. 54, 2121–2131. Guiboileau, A., Yoshimoto, K., Soulay, F., et al., 2012. Autophagy machinery controls nitrogen remobilization at the whole-plant level under both limiting and ample nitrate conditions in Arabidopsis. New Phytol. 194, 732–740. Guiboileau, A., Avila-Ospina, L., Yoshimoto, K., et al., 2013. Physiological and metabolic consequences of autophagy deficiency for the management of nitrogen and protein resources in Arabidopsis leaves depending on nitrate availability. New Phytol. 199, 683–694. Guo, W.J., Nagy, R., Chen, H.Y., et al., 2014. SWEET17, a facilitative transporter, mediates fructose transporter across the tonoplast of Arabidopsis roots and leaves. Plant Physiol. 164, 777–789. Hackel, A., Schauer, N., Carrari, F., et al., 2006. Sucrose transporter LeSUT1 and LeSUT2 inhibition affects tomato fruit development in different ways. Plant J. 45, 180–192. Han, Q.M., Kabeyam, D., Inagaki, Y., 2017. Influence of reproduction on nitrogen uptake and allocation to new organs in Fagus crenata. Tree Physiol. 37, 1436–1443. Havé, M., Marmagne, A., Chardon, F., et al., 2016. Nitrogen remobilisation during leaf senescence: lessons from Arabidopsis to crops. J. Exp. Bot. 68, 2513. Jia, S.S., Wang, Y., Hu, J.H., et al., 2016. Mineral and metabolic profiles in tea leaves and flowers during flower development. Plant Physiol. Biochem. 106, 316–326. Kato, Y., Murakami, S., Yamamoto, Y., et al., 2004. The DNA-binding protease, CND41, and the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase in senescent leaves of tobacco. Planta 220, 97–104. Kato, Y., Yamamoto, Y., Murakami, S., et al., 2005. Post-translational regulation of CND41 protease activity in senescent tobacco leaves. Planta 222, 643–651. Leggewie, G., Kolbe, A., Lemoine, R., et al., 2003. Overexpression of the sucrose transporter so SUT1 in potato results in alterations in leaf carbon partitioning and in tuber metabolism but has little impact on tuber morphology. Planta 217, 158–167. Li, F.Q., Chung, T., Pennington, J.G., et al., 2015. Autophagic Recycling Plays a Central Role in Maize Nitrogen Remobilization. Plant Cell 27, 1389–1408. Lisec, J., Schauer, N., Kopka, J., et al., 2006. Gas chromatography mass spectrometrybased metabolite profiling in plants. Nat. Protoc. 1, 387–396. Liu, J., Wu, Y.H., Yang, J.J., et al., 2008. Protein degradation and nitrogen remobilization during leaf senescence. J. Plant Biol. 51, 11–19. Liu, G.D., Dong, X.C., Liu, L.C., et al., 2015. Metabolic profiling reveals altered pattern of central metabolism in navel orange plants as a result of boron deficiency. Physiol. Plant. 153, 513–524. Liu, J.W., Zhang, Q.F., Liu, M.Y., et al., 2016. Metabolomic analyses reveal distinct

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