Oct 6, 1986 - found to be inhibited 2,4-dichlorophenoxyacetic acid and 5,7-dichloroin- ... 4, 15). In retarding senescence of soybean leaves on pod-bearing.
Plant Physiol. (1987) 83, 334-340 0032-0889/87/83/0334/07/$01.00/0
Cytokinin Biochemistry in Relation to Leaf Senescence' II. THE METABOLISM OF 6-BENZYLAMINOPURINE IN SOYBEAN LEAVES AND THE INHIBITION OF ITS CONJUGATION Received for publication June 9, 1986 and in revised form October 6, 1986
REN ZHANG, DAVID S. LETHAM*, 0. CHOON WONG, LARRY D. NOODEN, AND CHARLES W. PARKER
Research School of Biological Sciences, Australian National University, Canberra, A.C.T. 2601, Australia (R.Z. D.S.L., O.C.W., C.W.P.), and Botany Department, University of Michigan, Ann Arbor, Michigan 48109 (L.D.N.) peared to be an inactivated form of BA, the inhibition of its formation has been studied.
ABSTRACI` The metabolism of 13Hj6-benzylamino purine was studied in presenescent and early senescent soybean (Glycine max [L.] Meff.) leaves. In both types of leaves, the metabolism was essentially the same. The principal metabolite was identified as j-(6-benzylaminopurin-9-yl)alanine by mass spectral studies, which included discharge ionization-secondary ion mass spectrometry and pulsed positive ion-negative ion-chemical ionization mass spectrometry. Conversion to this alanine conjugate was found to be inhibited 2,4-dichlorophenoxyacetic acid and 5,7-dichloroindoleacetic acid.
Repeated application of 6-benzylaminopurine and a-naphthalene acetic acid to intact soybean plants is known to delay leaf senescence markedly, and even when pods are dry, the leaves of such treated plants may still be green (16, 19). However, repeated applications of BA2 or NAA alone were much less effective and a single application of BA evoked little response (3, 4, 15). In retarding senescence of soybean leaves on pod-bearing stem explants, BA, dihydrozeatin, and dihydrozeatin riboside were the three most effective compounds in a group of tested cytokinins which included zeatin (9). BA was also more effective than zeatin in delaying the senescence of discs excised from primary leaves of soybean (27). Retardation or prevention of soybean leaf senescence is of basic physiological interest, and could provide insight into the correlative controls which govern the terminal phase of plant development. It is also of potential agronomic significance. Because of this, it may be important to find cytokinins which are more effective than BA and which retard soybean leaf senescence markedly with only one or two applications. The design of such compounds would be facilitated by a study of the metabolism of BA in soybean leaves. Such a study is the subject of this paper. The principal metabolite has been identified, and since it ap'For part I, see Tao GQ et al. (24).
2Abbreviations: BA, 6-benzylaminopurine; NAA, a-naphthalene acetic acid; 9R-BA, 9-fl-D-ribofuranosyl-BA; 9G-BA, 9-t-D-glucopyranosylBA; 9R-5'P-BA, 5'-monophosphate of 9R-BA; 9Ala-BA, ,-46-benzylam-
inopurin-9-yl)alanine; DCCC, droplet counter current chromatography; DCI, desorption chemical ionization; DISI, discharge ionization-secondary ion; pfBz, pentafluorobenzyl; EI, electron impact; PI, positive ion; PPINI, pulsed positive ion-negative ion; MH+, protonated molecular ion; ZR, zeatin riboside.
MATERIALS AND METHODS Chemicals. GA3, NAA, 2,4-D, BA, BA riboside (9R-BA), adenine, and adenosine were purchased from Sigma. The following compounds were synthesized by procedures detailed in the cited references: [G-3H]BA 20 mCi mmol-' (26), 9-fl-D-glucopyranoside of BA (9G-BA) (2), BA riboside 5'-monophosphate (9R-5 'P-BA) (24), and j-(6-benzylaminopurin-9-yl)alanine (9Ala-BA) (13). N-Benzyl-N'-phenylurea was the gift of Dr. J. A. Zwar (CSIRO, Canberra) while 5,7-dichloroindoleacetic acid (5,7-dichloro-IAA) was provided by Dr. G. F. Katekar (CSIRO, Canberra). BA Metabolism during Leaf Maturation: Plant Material. Soybean (Glycine max [L.] Merr., cv Bragg) seedlings were inoculated and grown in pots containing potting soil mixture in a glasshouse during winter with controlled temperature (26°C). Plants at different stages of development, namely, middle podfill, late podfill, and early leaf yellowing, were moved into a growth cabinet (10 h daylight, 300 ME m-2s-', 28°C day, 22°C night) 24 h before treatment. BA Application and Tissue Extraction. One each of the selected uniform leaflets at the middle of the plants, a lanolin ring (diameter 27 mm) was made avoiding the midrib. [3H]BA solution (50 ,l, 50 Mm in 0.05% [v/v] Tween 80) was spread within the ring and allowed to dry. The area was then rewetted with 25 Ml of distilled H20 to facilitate [3H]BA uptake. After 24 and 48 h, the treated areas were excised, weighed, and dropped into methanol-water-formic acid (15:4:1, v/v/v; 30 ml g-' of tissue) chilled to -20°C. After 2 to 3 d at -20°C with occasional shaking, the samples were homogenized with a mortar and pestle, left overnight or longer at 2°C, stirred, and finally centrifuged. The supernatant was evaporated to dryness in vacuo with a rotary evaporator (bath temperature, 35°C) and the residue was redissolved in 50% ethanol (1 ml g"' of tissue). TLC Methods. Layers (0.3 mm unless stated otherwise) spread in the laboratory were prepared with the following: A, Merck Silica Gel 60 PF254; B, Merck Silica Gel 60 GF254 ( 15 Mm particle size); C, Serva cellulose with added fluorescent indicator (21). Impregnation of layer B with paraffin (17) yielded a layer termed B/P. Solvents were as follows (proportions by volume): A, l-butanol- 14 N NH40H-water (6:1:2, upper phase); B, methanolwater (2:3); C, methyl acetate-water-acetic acid (100:25:8). D, acetonitrile-water (2:3); E, chloroform-methanol (9:1); F, l-butanol-acetic acid-water (12:3:5); G, isopropanol-14 N NH40Hwater (14:3:3).
334
METABOLISM OF 6-BENZYLAMINOPURINE IN SOYBEAN LEAVES To elute for rechromatography the appropriate zones were scrapped, packed into a small column and then eluted with ethanol-water-acetic acid (50:50:3, v/v/v). For determination of radioactivity in zones, the layer was placed in a scintillation vial containing water (usually 0.5 ml), and left for 24 h at 23°C when 10 volumes of scintillation fluid (10) were added for counting in a LKB 1215 Rack Beta liquid scintillation counter. HPLC. Equipment used for HPLC has been described previously (8). Three HPLC systems were used: A, Zorbax C8 semipreparative column (9.4 x 250 mm; du Pont), solvent methanolwater-acetic acid (40:60:1, v/v/v), flow rate 4.5 ml min-'; B, ,MBondapak phenyl column 3.9 x 300 mm, Waters Associates), solvent methanol-water-acetic acid (50:50:1, v/v/v), flow rate 2.0 ml min-'; C, C8 radial compression column (8 x 100 mm, Waters Associates), solvent methanol-water-acetic acid (40:60:1, v/v/v), flow rate 3.5 ml min-'. Evaporated TLC elutes were dissolved in HPLC solvent and clarified by centrifuging through a cellulose filter (0.20 um pore, Bioanalytical Systems, West Lafayette, IN). DCCC. A Buchi 670 chromatograph (294 columns each of diameter 2.2 mm and volume 1.52 ml) was used at a flow rate of 20 ml h-' and fractions of 1.7 ml were collected. The upper and lower phases of the mixture ethyl acetate-chloroform-methanol-water-acetic acid (20:70:70:56:1, v/v/v/v/v) were used as solvents, the lower serving as stationary phase. Purification of Metabolite M. For the isolation of M (the principal metabolite of BA, see "Results") for mass spectral studies, BA was supplied to derooted soybean seedlings (cv Anoka) which had primary leaves and partly expanded first trifoliate leaves, and also to expanded leaves excised from plants (cv Bragg) just before flowering. The stems of the former and petioles of the latter were placed in an aqueous solution of 150 Mm BA. After 2 to 3 d under continuous weak fluorescent light (4 ME m-2s-'), the leaf blades were extracted (see above) and 3Hlabeled M (purified by TLC, see below) was added to the extract which was evaporated. The residue was suspended in water (2 ml g-' of tissue) and the resulting solution was clarified by centrifugation and then shaken 3 times with equal volumes of ethyl acetate. The extracts were discarded and the extracted aqueous solution was purified on a column of cellulose phosphate (0.5 g g-' of tissue; NH4' form equilibrated to pH 3.0; see Ref. 20) and the fraction eluted with 0.5 N NH40H was subjected to preparative TLC (layer A, 1 mm; developed twice with solvent A). The radioactive zone was eluted and the evaporated eluate was purified by HPLC (system A). The radioactive fraction which co-eluted with -a UV-absorbing component (retention time 13.3 min) was then subjected to DCCC. The main UV-absorbing fraction (peak in fraction 20) was collected and purification was completed by HPLC with system B which yielded purified M (retention time 3.1 min). This was used for UV spectra, desorption chemical ionization (DCI) mass spectra, and for derivatization. For determination of the discharge ionization-secondary ion (DISI) mass spectra, M obtained by HPLC was further purified by chromatography on a prespread reverse phase high performance TLC plate (RP-8 F2_u,, E. Merck) which had been washed by allowing 80% (v/v) acetonitrile to run to the top of the layer twice. The developing solvent was D and the chromatographed M (RF 0.54) was eluted by stirring with 80% (v/v) acetonitrile (50 MAl). [3H]M to serve as a chromatographic marker (see above) and for some chromatographic comparisons was prepared by the following method. Crude extracts of soybean blades which had been supplied with [3H]BA as above were subjected to TLC (layer A, solvent A). The eluate of the radioactive zone just below 9GBA was rechromatographed (layer A, solvent F). The principal radioactive zone (RF 0.44) was eluted to yield [3H]M. DCCC indicated
only
one minor radioactive
impurity.
335
Derivatization Methods. Metabolite M (10 ,g) was dissolved in 150 Ml of an aqueous solution containing tetrabutylammonium hydrogen sulfate and sodium carbonate (20 and 25 mg ml-', respectively; pH adjusted to 11.5 with NaOH). Chloroform (150 ,l) followed by pentafluorobenzyl bromide (13 Ml) were then added and the biphasic system was stirred vigorously for 4.5 h. The aqueous phase was discarded, while the chloroform solution was shaken with four 150 Ml volumes of water and then evaporated in vacuo. The above method is similar to that of Gylledhaal and Ehrsson (1 1) for derivatization of sulfonamides. Derivatization in single phase systems using several polar solvents proved unsatisfactory. The residue was subjected to TLC using layer B (previously washed with ethyl acetate) and solvent E. The M derivative of RF 0.78, a di-pfBz derivative, was eluted with redistilled ethyl acetate (50 Ml) while that of RF 0.40 (mono-pfBz derivative) was eluted with ethyl acetate-ethanol (1:1, v/v). When the di-pfBz derivative (5 Mg) was heated at 7O°C for 16 h with a mixture of anhydrous ethanol (50 Ml), diethoxypropane (10 Ml) and concentrated HCI (4 Mul), it yielded by transesterification a N-pfBz ethyl ester. Heating at 90°C with anhydrous n-butanol (75 Ml), dimethoxypropane (10 Ml), and concentrated HCI (4 M1) yielded the corresponding n-butyl ester. Both esters were purified by TLC (layer B, solvent E). For permethylation, potassium tert-butoxide (30 mg) was first heated at 70°C with dry DMSO (1 ml) under anhydrous conditions for 30 min. The resulting solution (50 Ml) was added to the dried sample, followed by purified methyl iodide (10 Ml). After 30 min, water (100 Ml) was added and the mixture was then extracted with chloroform (three 100-Mul volumes). The combined extracts were washed with water (100 Ml) and evaporated for MS. The above procedure is based on the methods of Eagles et al.
(5).
Determination of Mass Spectra. All spectra were determined with a Finnigan 4530 mass spectrometer. El spectra were determined at an ionization energy of 70 eV and a source temperature of 150°C. Samples were introduced via a direct inlet probe fitted with a rhenium wire (see below). DCI spectra were determined at 140 eV with a source temperature of 120°C. The sample was applied to a rhenium wire mounted at the end of the direct inlet probe which was inserted into the plasma of the CI source. The wire was heated at the rate of 50 mamp s-' and spectra, were recorded in the PI or PPINI mode. The reagent gas was either ammonia or methane at 1.0 Torr pressure. For determination of DISI mass spectra the sample was applied to glycerol on a copper tip at the end of a direct insertion probe. The sample was bombarded with argon ions (5 kV) and the source temperature was 120°C. Inhibition of 9Ala-BA Formation. Intact Plant Experiment. [3H]BA solution (10 Ml, 50 M in 0.05% (v/v) Tween 80) containing inhibitor (0.25 mM) was spread within a lanolin ring (diameter 14 mm) on leaf blades of soybean plants (cv Bragg, at middle podfill stage). An adjacent area received [3H]BA solution without inhibitor to serve as control. When the above solutions had dried on the leaf surface, S Ml distilled H20 were spread within the rings. The treated areas were excised after 24 h and were then washed with 15 Mm unlabeled BA solution, blotted with tissue paper, and extracted as above. Aliquots were taken for determination of 3H and for TLC (layer A, solvent A followed by F). Excised Leaf Disc Experiment. Leaf discs (diameter 6.5 mm) were excised from Anoka soybean plants at mid-podfill. They were each placed abaxially on 10 Ml ofa 0.05% Tween 80 solution containing 7.5 AM [3H]BA and inhibitor. Plastic Petri dishes (diameter 9 cm) each containing about 12 discs were kept under weak fluorescent light (4 ME m 2s-') at 22°C for 65 h. The leaf discs were then washed sequentially with 0.05% Tween 80, 40 Mm BA in 0.05% Tween 80 solution, and distilled H20. They
336
ZHANG ET AL.
Plant Physiol. Vol. 83, 1987
were then blotted with tissue paper and extracted as above. Aliquots were taken for determination of 3H and TLC (as above).
adenosine, adenine, and 9R-BA did not make appreciable -contributions to the extracted radioactivity either at 24 or 48 h. A small proportion of the extracted 3H co-chromatographed with 9G-BA, especially in extracts from leaves at mid-podfill (24 and RESULTS 48 h) and early leaf yellowing (48 h). However, elution of this Metabolism of BA in Leaves of Differing Maturity. 3H-labeled radioactivity and further TLC showed that it was largely due to BA was applied to soybean leaves at three stages of plant devel- metabolites other than 9G-BA. An interesting feature of the opment, namely, mid-podfill, late podfill, and early leaf yellow- results shown in Table I is the very small decline in the level of ing. The leaf extracts prepared after 24 and 48 h were subjected free BA between 24 and 48 h. Active metabolism of BA appears to TLC on silica gel (layer A, solvent A) with the following to be confined to the first 24 h after BA application in the present cochromatographed markers: BA, 9R-BA, 9R-5'P-BA, 9G-BA, experiment. adenine, and adenosine. For extracts of all three types of leaves, When [3H]BA was supplied to the blades of first trifoliate and the distributions of radioactivity over the chromatograms were primary leaves of young soybean plants through the transpiration similar and each showed two pronounced peaks (Fig. 1). One stream by placing the cut stems of derooted seedlings in a 100 was coincident with the BA marker, and elution followed by ,uM solution, the pattern of metabolites described above was further TLC confirmed that this peak was in fact largely due to observed again after TLC. This pattern was also obtained when BA. The second prominent peak occurred in a zone just below the petioles of leaves excised at late podfill were placed in [3HJco-chromatographed 9G-BA. The metabolite responsible for this BA solution and the blade extracts subjected to TLC. Hence, radioactivity peak is termed M henceforth. formation of M appears to occur in both young and mature The distribution of 3H in the TLC fractions of the various leaves and regardless of whether BA is supplied via the xylem or extracts are compared in Table I. After application to soybean by external application to the leaf blades. leaves, a considerable proportion of the supplied BA was conPurification and Identification of Metabolite M. By a purifiverted to M, and this metabolite accounted for 23 to 33% of the cation scheme involving sequential partition between ethyl aceextracted 3H at 24 h, while 33 to 39% appeared to be due to tate and water, chromatography on cellulose phosphate, preparunmetabolized BA at this time. In contrast, the radioactivity in ative TLC, HPLC in system A, DCCC, and finally HPLC in the other marker zones was low (Table I) and 9R-5'P-BA, system B, a metabolite with the chromatographic properties of M was isolated from the leaves of young soybean plants (M-1) and also from the leaves of plants just prior to flowering (M-2). M-l and M-2 appeared to be identical. M-l and M-2 exhibited 35~~~~~~~~~~~~~~~ HIidentical RF values during TLC and when the chromatogram was sprayed with a solution of ninhydrin and heated to 95°C, both yielded a purple spot. In the DCI-PI (NH3 gas) mass spectrum of underivatized M-1 and M-2, a molecular ion was not clearly evident, but prominent ions were present at m/z 269 (4% relative intensity), 226 (100%), and 136 (81%). The ions of m/z 226 and 136 indicated that the metabolites contained an BA moiety. M- 1 was permethylated and the principal intact 1 72 product could be purified by TLC, but the yield appeared low. The DCI-PI (NH3 gas) mass spectrum ofthis derivative exhibited OR a probable weak MH+ ion at 369, an intense ion at m/z 254 (base peak) and a prominent ion at 240 (14%). The above observations suggested that M-l could be the 9-alanine conjugate of BA (9Ala-BA,I), a metabolite of BA previously identified in Phaseolus vulgaris leaves (13) and soybean callus tissue (6) Distance (cm) (Scheme 1). The ion of m/z 269 in the DCI of the underivatized FIG. 1. The distribution of radioactivity over a thin-layer chromato- compound mass spectrum could be attributed to loss of CO2 from the protonated molecular ion (MH+) of 9Ala-BA (mol wt gram (layer A) of extract of soybean leaf laminae to which [3H]BA has been applied. The layer was developed twice with solvent A. The positions 312). Attempts to derivatize M-l with N,N-dimethylformamide of co-chromatographed markers are denoted as follows: 1, 9R-SP'-BA; dimethyl acetal, which simultaneously derivatizes both the amino and carboxyl groups of amino acids (25), were unsuccess2, 9G-BA; 3, adenosine; 4, adenine; 5, BA; OR denotes the origin. 35
30
25
10
~H
0
6
2
10
14
18
Table I. Radioactivity Extracted from Regions ofSoybean Leaves Treated with [3HJBA and Its Chromatographic Distribution The extracts were subjected to TLC (layer A, solvent A). Each value is the mean of 4 or 5 replicates. Percent of dpm in TLC Zones Plant Stage and Time ofExtraction adenin BA Adenosine 9R-5'P-BA 9GRadioactivity M BA adenine region region region region region region h Mid podfill
24 48 Late podfill 24 48 24 Early leaf yellowing 48 a Not detected.
dpm mg- fresh wt 179.4 216.8 257.8 189.8 292.2 238.6
1.8 1.0 1.3 1.4 2.0 2.4
26.0 36.6 23.0 28.3 33.4 33.8
1.2
0.5 0.7 0.8 0.8
3.5 3.8 3.5 5.7
1.5 4.4
NDa ND
2.0
4.3 3.5 1.5
1.6
32.7 24.5 39.1 35.8 33.5 26.5
337
METABOLISM OF 6-BENZYLAMINOPURINE IN SOYBEAN LEAVES
C-HOH2 NH
NN) OH2
100-
CH-COOR NH-R'
I
247
B
50-
224 181
134
R
II III
H
-(li2-C6F5 -M2-C6F5
H
-M2 C6F5 SCHEME
,.
..
200
.f
,.I
300
250
320
I
.
470
/
1
500 670
FIG. 3. The DCI-PPINI mass spectra of the di-pfBz derivative of metabolite M-2. A, positive ion spectrum; B, negative ion spectrum. It should be noted that the high mass ions in only A have been multiplied by 20. Apart from an ion at m/z 266 (10%) in the negative ion spectrum, there were no ions of intensity above 2% in the m/z ranges not plotted. However a significant high mass ion occurred at m/z 521 (1%) in the positive ion spectrum which was attributable to a C2H5 adduct of the mono-pfBz derivative (i.e. 492 + 29).
-M2-(CH2)2-M3
V
472
m/z
-wH2-C6F5
IV
311
196
I
150
130
H
491
I
4. 0-
,25
100
A
277 3 .0 -313 219
x10
so 1 .0
2Li
238
267 253
~~~~~~~~~369 I 1-1.111 1-11,11 .......j kAI II..l I
220
240
260
280
300
320
340
266
360
m/z FIG. 2. The DISI mass spectrum of metabolite M-2. The intensities of ions are expressed as a percentage of the glycerol dimer peak at m/z 185.
ful. To provide more rigorous structural information, M-2 was examined by procedures different from those employed with M-1. The DISI mass spectrum (Fig. 2) exhibited a clear peak at 313 attributable to MH+; the ions at m/z 277 and 369 are due to glycerol (trimer and tetramer, respectively) which was used as the sample matrix. M-2 was derivatized with pentafluorobenzylbromide by an extractive alkylation procedure with tetrabutylammonium hydrogen sulfate as an ion-pairing reagent. This yielded two derivatives separable by TLC (layer B, solvent E). The derivative of higher RF (0.78) exhibited the DCI-PPINI (CH4 gas) mass spectrum shown in Figure 3, while the other (RF 0.40) gave the following DCI-PPINI spectrum (relative intensities are in parentheses): positive ion m/z 136 (100%), 182 (85), 198 (22), 226 (80), 313 (42), 341 (22), 493 (16); negative ion m/z 134 (5%), 181 (15), 224 (12), 311 (100). The former derivative was concluded to be a di-pfBz derivative (MH+ 673) and successive losses of pfBz moieties with addition of H would yield the ions of m/z 493 and 313 in the positive ion spectrum and 491 and 311 in the negative ion spectrum (Fig. 3). The derivative of RF 0.40 was concluded to be a mono-pfBz derivative of M-2 (MH+ 493). The DISI and the DCI-PPINI spectra in conjunction provided unequivocal evidence that the mol wt of M-2 was 312. Although the molecular ion was not evident in the negative ion spectra, for detection of other high mass ions (i.e. ions formed by loss of pfBz), the sensitivity of the negative ion spectrum was usually about 30 times that of the positive ion DCI mass spectrum. Hence, negative ion CI mass spectrometry may be partic-
447
1457
477
1
1 672
491
5L 450
250
soo
650
690
B
28
11
253
477
447
266
457 450
450
491
I 6, 5
0
500
650
690
m/z FIG. 4. The EI mass spectra of the di-pfBz derivatives of metabolite M-2 (A), and synthetic 9Ala-BA (B). In the m/z regions omitted, there were no peaks with an intensity greater than 1.3%.
ularly useful for quantitation of pfBz derivatives of compounds such as M-2. Since the metabolites reacted with ninhydrin, possessed a mol wt of312, and contained an intact BA moiety, they were probably 9Ala-BA. Accordingly, M-2 and synthetic 9Ala-BA (DL-form) were compared critically. Like M-2, 9Ala-BA yielded mono- and di-pfBz derivatives (II and III, respectively). The two derivatives of 9Ala-BA could not be distinguished from those of M-2 by TLC (layer B, solvent E) or by DCI-PPINI spectra. The di-pfBz derivatives exhibited identical EI mass spectra (Fig. 4). These spectra showed an intense ion at m/z 238 which would not be given by an a-alanine conjugate of BA and were therefore consistent with a ,B-alanine structure for M-2. Both spectra exhibited a prominent ion at m/z 447 attributable to loss of
ZHANG ET AL.
338
COOpfBz radical and an analogous cleavage is evident in the EI mass spectra of the N-pfBz ethyl and n-butyl esters of 9AlaBA (see below). 9Ala-BA and M-2 co-chromatographed during TLC in the following systems: layer A, solvents A and F; layer B, solvent C (RF 0.34) and G (RF 0.58); layer C, solvent A; layer B/P, solvent B (RF 0.28). Both compounds exhibited the same retention times during HPLC in systems A and C (13.3 and 6.8 min, respectively). Furthermore, when [3H]M prepared for use as a chromatographic marker (see "Materials and Methods") was mixed with authentic 9Ala-BA and subjected to DCCC and HPLC, radioactivity and UV absorption were coincident. The di-pfBz derivative of 9Ala-BA was converted to ethyl (IV) and n-butyl (V) esters by transesterification. The esters exhibited the following El mass spectra (relative intensities in parentheses): ethyl ester, m/z 520 (M+, 3), 447 (20), 339 (6), 325 (31), 266 (7), 253 (23), 239 (17), 238 (23), 226 (47), 225 (51), 224 (27), 181 (100), 106 (98); n-butyl ester, m/z 548 (M+,2), 447 (26), 367 (6), 353 (23), 311 (9), 266 (5), 253 (28), 239 (20), 238 (24), 226 (54), 225 (43), 224 (27), 181 (65), 106 (100). These esters co-chromatographed with the corresponding esters derived from the di-pfBz derivative of M-2. Finally, 9Ala-BA, M-1, and M-2 exhibited identical UV spectra (.ma 270 nm) in 50% (v/v) methanol containing 0.2 N acetic acid. A summation of the above evidence establishes that M is the 9-alanine conjugate of BA (I), i.e. 9AlaBA. Inhibition of Formation of 9Ala-BA. When applied to soybean leaves, BA was found to retard senescence markedly. However, 9Ala-BA exhibited only very slight senescence-retarding activity and therefore could be an inactivated form of BA in soybean leaves as it is in a number of bioassays (14). Hence suppression of 9Ala-BA formation might enhance the senescence retarding activity of BA. It was therefore relevant to determine whether compounds known to inhibit purified #-(9-cytokinin)alanine synthase (22), the enzyme responsible for formation of alanine conjugates of cytokinins (7), would also suppress conversion of BA to 9Ala-BA in soybean leaves. Three potent inhibitors of the enzyme, namely, 2,4-D, 5,7-dichloro-IAA, and N-benzyl-N'phenylurea, were first tested with intact leaves. The two auxins both suppressed 9Ala-BA formation markedly (Table II) and 2,4-D also elevated the level of free BA appreciably. Inhibition of 9Ala-BA formation was accompanied by a greater degree of metabolism to adenine and adenosine, especially when 2,4-D was supplied. However, although N-benzyl-N'-phenylurea inhibited the purified enzyme, it had no effect on 9Ala-BA formation in intact soybean leaves (Table II). In excised leaf discs, 2,4-D was again strongly inhibitory and also elevated the level of free BA, but NAA and GA3 were essentially inactive (Table III). a
-
Plant Physiol. Vol. 83, 1987
However, 2,4-D induced slight yellowing of the leaf discs even at 25 ,uM, but this effect was not evident when 2,4-D was applied to the intact leaves (Table II).
The ability ofGA3 and NAA to inhibit degradative metabolism of the natural cytokinin ZR in soybean leafdiscs was also assessed by the same method. In these leaves ZR is rapidly degraded to adenine and adenosine and is also conjugated as O-glucosides (18). However, 50 gM NAA and 100 ,uM GA3 had no effect on metabolism of [3H]ZR (7.5 Mm) as evidenced by two-dimensional TLC.
DISCUSSION In the present study, 9Ala-BA was identified as the principal metabolite of BA in soybean leaves. Rapid metabolism of BA to 9Ala-BA was confined to the first 24 h after application (Table I) and may be associated with uptake of the supplied BA; the remaining BA may be on the leaf surface or in a subcellular compartment where it is stable. This metabolism, which occurs in soybean leaves of widely differing maturity, contrasts with that found for ZR (18); this natural cytokinin was rapidly metabolized in soybean leaves to a diversity of compounds, but principally to adenine, adenosine and O-0-D-glucosyl metabolites. The alanine conjugate of zeatin, lupinic acid, was a very minor metabolite if present at all (18). Conversion of BA to adenine and adenosine was prominent when formation of 9AlaBA was inhibited by auxin (Table II) suggesting that alanine conjugation and N6-benzyl cleavage are alternative mechanisms for BA inactivation in soybean leaves. 9Ala-BA was purified from soybean leaves by a sequence of steps, one of which was DCCC (12). This appears to be the first reported use of this technique in cytokinin metabolite purification, although it has been used for gibberellins (1). One of the two previous identifications of 9Ala-BA as a metabolite of BA was based principally on cocrystallization to constant specific activity and cochromatography involving derivatives of 9Ala-BA (13); the other relied mainly on nuclear magnetic resonance (6). In the present study, 9Ala-BA was identified principally by use of mass spectrometric methods in a complementary manner. The molecular ion ofthe underivatized compound was detected only in the DISI mass spectrum, and hence this technique involving 'soft ionization' (23) may be useful for characterization of other cytokinin-alanine conjugates. These compounds present derivatization problems and the pfBz derivative of 9Ala-BA used in the present study appears to have advantages for determining direct-probe mass spectra over pertrimethylsilyl and permethyl derivatives and also other derivatives (e.g. N-perfluoroacylesters) commonly used for derivatization of amino acids. pfBz derivatives are formed by a single step
Table II. Effects of Some Compounds on [3HJBA Metabolism in Intact Soybean Leaves Areas of leaves were treated with 50 Mm [3H]BA plus 0.25 mm compound and adjacent areas with [3H]BA only to serve as control. Leaf extracts were subjected to two dimensional TLC (layer A, solvent A followed by solvent F). Extracted
Percent of dpm which Cochromatographed with Markers 9G-BA Adenosine 9R-BA Adenine BA
Radioactivity 9Ala-BA dpm mg-' fresh wt
2,4-D Control
5,7-Dichloro-IAA Control N-Benzyl-N'-phenyl-
6.0 2.5
216.8 271.3 214.8 188.0 268.2
8.9 34.9 16.7 31.3 34.3
3.1 2.0 1.6
6.2 2.2 3.9
2.8 2.0 2.8
NDa 1.6
2.1
2.0
4.5
2.7
4.9 2.8 3.6
240.4
35.1
ND
3.0
1.7
5.7
urea
Control a Not detected.
17.3 11.7 18.0
16.1 13.8
14.7
339 METABOLISM OF 6-BENZYLAMINOPURINE IN SOYBEAN LEAVES study, the two last-mentioned compounds, but not NAA or GA3, Table III. Effects of Compounds on 9Ala-BA Formation in Soybean were also found to inhibit conversion of BA to 9Ala-BA in Leaf Discs Supplied with [3HJBA soybean leaf tissue. In earlier studies, exogenous IAA has been to were extracts and leaf 7.5 subjected [3H]BA was supplied at gM reported to enhance cytokinin metabolism while exogenous cyTLC (layer A, solvent A). tokinins were found to have diverse effects on auxin metabolism Percent of dpm (see references in (22). Hence it is becoming evident that cytoExtracted 3H Compound kinins and auxins interact in a complex manner to control their BA 9Ala-BA metabolism and levels in tissue. dpm disc-' In this study, 9Ala-BA, an inactivated form of BA, was iden16.3 43.1 907 Control ([3H]BA only) tified as the principal metabolite of BA formed in soybean leaves. 13.5 47.3 964 Hence, derivatives of BA with a substituent at N-9 which is GA3 (100 M) 12.1 43.5 892 NAA (50 Mm) slowly cleaved to yield free BA could be more effective than BA 14.3 44.5 955 itself in retarding soybean leaf senescence. This proved to be NAA (5 Mm) 33.8 12.9 711 correct as is discussed in the next paper in this series. 2,4-D (100l M) 22.5
21.4
714
2,4-D(25 Mm)
LITERATURE CITED 1. BEARDER JR, J MACMILLAN 1980 Separation of gibberellins and related compounds by droplet counter-current chromatography. In JR Lenton, ed,
CH2 NH +H
C6F5 m/
NN
N
N
+N-
-
224
(b). +2H
225 226
238, +H 239
CH2 CH2 -NH -CH- COO-CH2-C6F5
- *CH2C6F5
Mt
- *COOCH2C6F5
447
(m/z
m/z 266
672)
m/z 253
e
-
b radical (-
224)
m/z 477
FIG. 5. A rationalization of the fragmentation seen in the El mass spectrum of the di-pfBz derivative of9Ala-BA. The ion [COOpfBz]+ may also contribute to the peak at m/z 225 (see "Discussion").
derivatization reaction,
are
stable during TLC and HPLC and
can be detected with high sensitivity by negative-ion CI MS. The EI mass spectrum of the di-pfBz derivative of 9Ala-BA (Fig. 4)
revealed diagnostic fragmentations which are rationalized in Figure 5. The base peak in the spectrum is at m/z 225 and this could be due to two ions, [COOpfBz]+ and [BA]+. Since other esters (IV and V) also show an intense 225 ion (see "Results"), the [BA]+ ion must at least make a significant contribution to the spectrum of di-pfBz 9Ala-BA. Hence a very prominent feature of the El mass spectra of the three esters of N-pfBz 9AlaBA (III-V) appears to be cleavage of the derivatized amino acid moiety with hydrogen transfer to the charged purine fragment. IAA has been found to act as a competitive inhibitor of,-(9cytokinin)alanine synthase purified from lupin seed (22), and more pronounced inhibition was exhibited by certain synthetic auxins, including 5,7-dichloro-IAA and 2,4-D (22). In the present
British Plant Growth Regulator Group, Monograph 5, pp 25-30 2. COWLEY DE, CC DUKE, AJ LIEPA, JK MACLEOD, DS LETHAM 1978 The structure and synthesis of cytokinin metabolites. I. The 7- and 9-jP-Dglucofuranosides and pyranosides of zeatin and 6-benzylaminopurine. Aust JChem 31: 1095-1111 3. DYBING CD, C LAY 1981 Field evaluations of morphactins and other growth regulators for senescence delay of flax, soybean, wheat and oats. Crop Sci 21: 879-884 4. DYBING CD, CLAY 1981 Yields and yield components of flax, soybean, wheat, and oats treated with morphactins and other growth regulators for senescence delay. Crop Sci 21: 904-908 5. EAGLES J, WM LAIRD, R SELF, RLM SYNGE 1974 Permethylation for mass spectrometry: rearrangements for ester linkages and use of potassium t-butoxide. Biomed Mass Spectrom 1: 43-48 6. ELLIOTT DC, MJ THOMPSON 1982/1983 The identity of the major metabolite of benzylaminopurine in soybean cultures and the inhibition of its formation by aminophylline. Plant Sci Lett 28: 29-38 7. ENTscH B, CW PARKER, DS LETHAM 1983 An enzyme from lupin seeds forming alanine derivatives of cytokinins. Phytochemistry 22: 375-381 8. ENTSCH B, CW PARKER, DS LETHAM, RE SUMMONS 1979 Preparation and characterization using HPLC of an enzyme forming glucosides of cytokinins. Biochim Biophys Acta 570: 124-139 9. GARRISON FR, AM BRINKER, LD NOODtN 1984 Relative activities of xylemsupplied cytokinins in retarding soybean leaf senescence and sustaining pod development. Plant Cell Physiol 25: 213-224 10. GORDON ME, DS LETHAM, CW PARKER 1974 Regulators of cell division in plant tissues. XVII. The metabolism and translocation of zeatin in intact radish seedlings. Ann Bot 38: 809-825 11. GYLLEDHAAL 0, H EHRSSON 1975 Determination of sulphonamides by electron-apture gas chromatography. J Chromatogr 107: 327-333 12. HOSTETTMANN K 1980 Droplet counter-current chromatography and its application to the preparative scale separation of natural products. Planta Med 39: 1-18 13. LETHAM DS, RE SUMMONS, CW PARKER, JK MAcLEOD 1979 Regulators of cell division in plant tissues. XXVII. Identification of an amino-acid conjugate of 6-benzylaminopurine formed in Phaseolus vulgaris seedlings. Planta 146: 71-74 14. LETHAM DS, LMS PALNI, GQ TAO, BI GOLLNOW, CM BATES 1983 Regulators of cell division in plant tissues. XXIX. The activities of cytokinin glucosides and alanine conjugates in cytokinin bioassays. J Plant Growth Regul 2: 103115 15. LINDOO SJ, LD NOODEN 1978 Correlation of cytokinins and abscisic acid with monocarpic senescence in soybean. Plant Cell Physiol 19: 997-1006 16. NOODEN LD 1980 Regulation of senescence. In FT Corbin, ed, World Soybean Research Conference II: Proceedings. Westview Press, Boulder, CO, pp 139152 17. NOODEN LD, DS LETHAM 1984 Translocation of zeatin riboside and zeatin in soybean explants. J Plant Growth Regul 2: 265-279 18. NOODEN LD, DS LETHAM 1986 Monocarpic senescence in soybean: role of cytokinin. In M Bopp ed, Plant Growth Substances 1985. Springer-Verlag, Heidelberg, pp 324-332 19. NOODEN LD, GM KAHANAK, Y OKATAN 1979 Prevention of monocarpic senescence in soybeans with auxin and cytokinin: an antidote for selfdestruction. Science 206: 841-843 20. PARKER CW, DS LETHAM 1974 Regulators of cell division in plant tissues. XVIII. Metabolism of zeatin in Zea mays seedlings. Planta 115: 337-344 21. PARKER CW, DS LETHAM, BI GOLLNOW, RE SUMMONS, CC DUKE, JK MACLEOD 1978 Regulators of cell division in plant tissues. XXV. Metabolism of zeatin in lupin seedlings. Planta 142: 239-251 22. PARKER CW, B ENTCH, DS LETHAM 1986 Inhibitors of cytokinin metabolism. I. Inhibitors of two enzymes which metabolise cytokinins. Phytochemistry 25: 303-310
340
ZHANG ET AL.
23. SMITH DE 1981 Discharge ionization/secondary ion mass spectrometry and direct exposure probe analysis of peptides. Technical Report 8142. Finnigan MAT, Sunnyvale, CA, pp 1-7 24. TAO G-Q, DS LETHAM, LMS PLANI, RE SUMMONS 1983 Cytokinin biochemistry in relation to leaf senescence. I. The metabolism of 6-benzylaminopurine and zeatin in oat leaf segments. J Plant Growth Regul 2: 89-102 25. THENOT JP, EC HORNING 1972 Amino acid Ndimethylaminomethylene alkyl
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