Planta (2006) 224: 438–448 DOI 10.1007/s00425-005-0208-6
O R I GI N A L A R T IC L E
Iain M. MacKinnon Æ Adriana Sˇturcova´ Keiko Sugimoto-Shirasu Æ Isabelle His Maureen C. McCann Æ Michael C. Jarvis
Cell-wall structure and anisotropy in procuste, a cellulose synthase mutant of Arabidopsis thaliana Received: 28 October 2005 / Accepted: 12 December 2005 / Published online: 11 January 2006 Springer-Verlag 2006
Abstract In dark-grown hypocotyls of the Arabidopsis procuste mutant, a mutation in the CesA6 gene encoding a cellulose synthase reduces cellulose synthesis and severely inhibits elongation growth. Previous studies had left it uncertain why growth was inhibited, because cellulose synthesis was affected before, not during, the main phase of elongation. We characterised the quantity, structure and orientation of the cellulose remaining in the walls of affected cells. Solid-state NMR spectroscopy and infrared microscopy showed that the residual cellulose did not differ in structure from that of the wild type, but the cellulose content of the prc-1 cell walls was reduced by 28%. The total mass of cell-wall polymers per hypocotyl was reduced in prc-1 by about 20%. Therefore, the fourfold inhibition of elongation growth in prc-1 does not result from aberrant cellulose structure, nor from uniform reduction in the dimensions of the cell-wall network due to reduced cellulose or cell-wall mass. Cellulose orientation was quantified by two quantitative methods. First, the orientation of newly synthesised microfibrils was measured in field-emission scanning electron micrographs of the cytoplasmic face of the inner epidermal cell wall. The ordered transverse orientation of microfibrils at the inner face of the cell wall was severely disrupted in prc-1 hypocotyls, particularly in the early growth phase. Second, cellulose orientation distributions across the whole cell-wall thickness, measured by polarised infrared microscopy, I. M. MacKinnon Æ A. Sˇturcova´ Æ I. His Æ M. C. Jarvis (&) Chemistry Department, Glasgow University, G12 8QQ Glasgow, Scotland, UK E-mail:
[email protected] Tel.: +44-141-3304888 Fax: +44-141-3304888 K. Sugimoto-Shirasu Department of Cell and Developmental Biology, John Innes Centre, NR4 7UH Norwich Research Park, Norwich, UK M. C. McCann Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA
were much broader. Analysis of the microfibril orientations according to the theory of composite materials showed that during the initial growth phase, their anisotropy at the plasma membrane was sufficient to explain the anisotropy of subsequent growth. Keywords Anisotropy Æ Arabidopsis Æ Cellulose microfibril Æ Cellulose synthase Æ Elongation Æ Orientation Abbreviations NMR: Nuclear magnetic resonance Æ TEM: Transmission electron microscopy Æ FE-SEM: Field emission scanning electron microscopy Æ FTIR: Fourier transform infrared
Introduction In most current models of plant morphogenesis it is assumed that form, at the level of single cells, arises from growth driven by turgor but constrained in rate and direction by the anisotropic expansion of the cell wall (Green 1980). Cellulose microfibrils make a major contribution to the mechanical properties of the growing cell (Cosgrove 1997), being the most rigid constituents of the hydrated primary cell wall (Foster et al. 1996; Ha et al. 1997). Cell walls are often likened to fibre-reinforced composite materials. This comparison has been made qualitatively but not quantitatively because the properties of the constituent polymers of primary cell walls are not quantitatively well defined. In actively growing vegetative organs of Arabidopsis, the axis of growth is at right angles to the predominant microfibril orientation (Kerstens and Verbelen 2003). However, in some mutants, growth anisotropy is disrupted without altering the transverse orientations of microfibrils (Wiedemeier et al. 2002; Sugimoto et al. 2003). There is often a correlation between the orientation of microfibrils when they are deposited at the inner face of the cell wall and the orientation of cortical
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microtubules (Baskin 2001) but there are notable exceptions, for example, in the mor1 mutant (Sugimoto et al. 2003). Mutants deficient in cellulose synthesis are informative about the role of oriented microfibrils in the determination of plant form (Arioli et al. 1998; Fagard et al. 2000b). A cellulose microfibril is synthesised at the plasma membrane by the co-operative action of a number of proteins, at least some of which are associated with transmembrane terminal complexes (‘rosettes’; Kimura et al. 1999) visible by transmission electron microscopy (TEM) (Brett 2000). Secondary-wall cellulose chains in the xylem of Arabidopsis are synthesised by the physically associated products of a set of three distinct cellulose synthase (CesA) genes (Taylor et al. 2003). Lossof-function mutation in one of these is sufficient to result in secondary walls from which cellulose is absent (Ha et al. 2002). At least two sets of CESA proteins can participate in the synthesis of primary-wall cellulose, operating in different tissues or at different times (Fagard et al. 2000b; Williamson et al. 2001; Beekman et al. 2002). One of these sets consists of CESA1, CESA3 and CESA6. The CESA6 mutant procuste1 (prc-1; Desnos et al. 1996) has a dwarfed phenotype with reduced elongation and radial swelling of the dark-grown hypocotyl, together with reduced cellulose levels in its primary cell walls (Fagard et al. 2000a). Similar phenotypes with reduced elongation and radial swelling of roots and/or hypocotyls result from certain mutations in genes such as KOR that encode other proteins necessary for cellulose synthesis (Nicol et al. 1998). Herbicides such as isoxaben, which inhibit cellulose synthesis, phenocopy these mutants (Scheible et al. 2001; Desprez et al. 2002) while the CESA6 mutant ixr2 (Scheible et al. 2001; Desprez et al. 2002) is isoxaben resistant. It is not immediately evident how reduced cellulose synthesis gives rise to dwarfed, radially swollen phenotypes. Naı¨ vely, lack of cellulose might be expected to result in less cohesive cell walls that would expand faster or burst, instead of showing defects in cell expansion (Beekman et al. 2002) or in the anisotropy of expansion that gives rise to elongation growth (Desnos et al. 1996). Recently Refre´gier et al. (2004) showed that in wild-type Arabidopsis hypocotyls, elongation proceeds in two phases. A slow initial phase is followed by an acropetally spreading rapid phase. In the prc-1 mutant, or after isoxaben treatment of wild-type seedlings, the second, rapid phase was not evident. The sensitivity to isoxaben was limited to the first phase, implying that disruption of cellulose synthesis during the initial, slow elongation phase blocked elongation in the subsequent rapid phase. Refre´gier et al. (2004) suggested that the prc-1 mutation affected elongation in the same way as isoxaben. These observations focus attention on the mechanisms of elongation in the two phases and the relationship between them. A hypothesis consistent with these results is that appropriately ordered orientation of the microfibrils deposited during the initial slow growth phase is essen-
tial for rapid elongation in the subsequent rapid phase, allowing the microfibrils to reorient in an organised fashion towards the axial direction of growth. However, Refre´gier et al. (2004) did not observe any qualitative difference in the transverse orientation of microfibrils newly deposited at the inner face of the epidermal cell wall, either in the prc-1 mutant or in wild-type hypocotyls treated with isoxaben at a level sufficient to impair cellulose synthesis. Refre´gier et al. (2004) concluded that the rapid elongation was inhibited by a different mechanism, perhaps involving interactions of the microfibrils with matrix polymers. In this paper, we use the prc-1 mutant (Fagard et al. 2000a) to explore this and other hypotheses, including that structural defects in the cellulose microfibrils of prc1 might leave them imperfectly functional. We developed procedures to quantify microfibril orientations using both field emission-scanning electron microscopy (FE-SEM) and polarised Fourier transform infrared (FTIR), and a theoretical framework for integrating the biomechanical influences of microfibrils with a broad distribution of orientations. These technical innovations allowed us to re-evaluate the importance of the orientations in which microfibrils are deposited prior to the commencement of rapid elongation growth.
Materials and methods Isolation of cell walls Hypocotyl cell walls were prepared from 7-day-old seedlings of Arabidopsis thaliana (L.) Heyhn ecotype Columbia (wild type) and procuste-1 (pcr-1) mutants, grown from seeds obtained from the Nottingham Arabidopsis Stock Centre (University of Nottingham, UK). The hypocotyls were excised and stored frozen. Several hundred hypocotyls were homogenised in 2% Triton 20, and wet-sieved through stacked stainless steel squaremesh sieves to separate primary-walled and xylem cells (Wilson et al. 1988). Xylem cell walls were collected on an 850-lm sieve and primary cell walls on a 150-lm sieve. The cell walls were dried in a graduated acetone series with steps of 20, 40, 60, 70, 80, 90 and 100%. NMR spectroscopy Cross-polarisation magic angle spinning (CPMAS) 13C NMR spectra were recorded on a Varian Unity Plus spectrometer at 75.430 MHz. Samples were packed in a cylindrical 7 mm diameter Kel-F rotor with fluorocarbon end caps and spun at 3–4 kHz. The proton spin-lock field remained constant during both CP and data acquisition. The Hartmann–Hahn matching condition was optimised by reducing the 13C field to the point at which maximum signal intensity was obtained (Ha et al. 1997). Resonance assignments are based on published data (Sˇturcova´ et al. 2004; Wickholm et al. 1998). Parameters of the pulse
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sequence were contact time 1.0 ms; data acquisition time 14.9 ms; recovery delay 1 s. Preparation of cryo-sections Hypocotyl segments, held in a droplet of water on top of a cryo-pen, were plunged in liquid nitrogen and then sectioned with glass knives at 90C in a Reichert cryomicrotome. The 2 lm cryo-sections were transferred on a loop at room temperature onto a barium fluoride window (Spectroscopy Central, Wigan, UK) for FTIR microscopy. FTIR microspectroscopy FTIR spectra were obtained on a Nicolet Nexus spectrometer equipped with a Nicolet Continuum microscope attachment, a liquid-nitrogen-cooled MCT detector and a single ZnSe wire grid polariser. Between 8 and 12 spectra were obtained from isolated cell walls for each sample and then averaged. For vapour-phase deuteration experiments, the sample was enclosed in a through-flow hydration cell. A stream of nitrogen was passed through either a drying tube filled with phosphorus pentoxide or a bubbling tube filled with D2O. The nitrogen line was arranged to allow switching, without exposure to the air, between the drying and deuteration modules. Spectra were collected at 4 cm1 resolution and with 128 co-added scans. Aperture sizes varied according to the dimensions of the section but were always at least 25 lm in each dimension to minimise distortion of the spectra by scattering effects. Transverse and longitudinal spectra were sampled from the same apertured area of sample. Band assignments are based on published data (Chen et al. 1997; Jarvis and McCann 2000). To determine microfibril orientations by polarised FTIR, spectra were collected from each apertured area with the polariser rotated through 180 in 10 steps. A similar series of background spectra was collected beforehand so that the apertured area did not need to be moved while collecting spectra from the sample. The resulting polarised intensity curves were matched against curves predicted from the theoretical analysis of Sˇturcova´ et al. (2003). Briefly, the degree to which any band in the vibrational spectrum is polarised depends on the angle h between the transition moment of the vibrational mode responsible and the chain axis, and on the angle a between the chain axis and the direction in which the incident radiation is polarised. Modelling the distribution of chain orientations, a, as a Gaussian function allows the mean orientation and the width of the distribution around the mean, denoted by R, to be calculated from any well-polarised band in the FTIR spectrum for which h is known (Sˇturcova´ et al. 2003). Convenient vibrational bands for assessing the orientation of pure deuterated cellulose are at 1,165 cm1
(stretching of the glycosidic linkage) and 3,344 cm1 (overlapping O–H stretching modes, principally O3–H). For the 1,165 cm-1 vibrational mode, h is 28 (Sˇturcova´ et al. 2003). Because a number of overlapping bands contribute to the absorbance at 3,344 cm1, no single value of h can be deduced from bond orientations but a ‘virtual’ h value of 45 is indicated by the polarised spectra of well-oriented cellulose from celery (Apium) collenchyma, for which R has been determined at close to 10 (Sˇturcova´ et al. 2004). A vibrational mode with h=45 shows moderate longitudinal dichroism due to the rotational averaging effect described by Sˇturcova´ et al. (2003). The dichroism at 3,344 cm1 was significantly less than for the 1,165 cm1 band but for experiments on procuste it was found the precision was improved by measuring polarisation at 3,344 cm1, where the absorbance was less subject to interference from non-cellulosic polysaccharides and to baseline errors. Field emission scanning electron microscopy Samples of 7-day-old dark-grown hypocotyls were imaged using a Philips XL30 FESEM (FEI, Eindhoven, The Netherlands) as described previously (Sugimoto et al. 2001, 2003). A 6·4 array of reference points was superimposed on each FE-SEM image and the orientation of the microfibril closest to each reference point was measured relative to the cell axis. One image was measured from close to the centre of each cell. About 20 cells were examined for each genotype.
Results and discussion Content and structure of crystalline cellulose Solid-state 13C NMR is the technique most widely used to characterise the structure of cellulose from higher plants, where the diameter of the crystalline units is too small for crystallography (Atalla and VanderHart 1984; Smith et al. 1998; Sˇturcova´ et al. 2004). Solid-state 13C NMR spectra are particularly indicative of chain conformation, ordered or otherwise (Horii et al. 1983; Wickholm et al. 1998; Vie¨tor et al. 2002). The 13C NMR spectra of cell walls isolated from 7day-old wild-type and prc-1 seedlings are compared in Fig. 1. The difference spectrum shows that prc-1 cell walls had 20–30% less cellulose than wild type, comprising both surface and interior chains. The ratio of surface to interior chains was similar to that in the wildtype cell walls. There was no evidence of increased disorder or change in the size of the crystalline units (surface-to-volume ratio). The prc-1 phenotype, with reduced cellulose content, is readily identifiable by FTIR screening of hypocotyl cells (Fagard et al. 2000a; Mouille et al. 2003). We used more advanced FTIR methods (Sˇturcova´ et al. 2004) to probe the crystalline structure of the cellulose in situ in
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Fig. 1 Solid-state 13C NMR spectra of prc-1 and wild type cell walls. The difference spectrum (wild type—prc-1) is scaled x5 and shows the following peaks assignable to cellulose: 105 ppm (C-1); 89 ppm (C-4, interior chains); 84 ppm (C-4, surface chains); 75 ppm (C-5); 72 ppm (C-2/C-3); 65 ppm (C-6, interior chains); 62 ppm (C-6, surface chains)
the cell walls. Vapour-phase deuteration of plant cell walls converts free –OH groups to –OD in non-cellulosic polysaccharides and in the surface chains of cellulose, whereas the –OH groups of the crystalline interior chains of cellulose are too tightly hydrogen bonded to exchange. The residual –OH stretching bands in the FTIR spectrum are therefore derived from the interior chains of cellulose, and their pattern is a sensitive indicator of the hydrogen bonding that holds the crystalline interior of the microfibrils together (Liang and Marchessault 1959; Mare´chal and Chanzy 2000; Sˇturcova´ et al. 2004). Using vapour-phase deuteration in conjunction with FTIR microscopy, cellulose structure can be elucidated at the single-cell level (Sˇturcova´ et al. 2004). It is also possible to measure the orientation of cellulose within cell walls using polarised FTIR microscopy (Sˇturcova´ et al. 2003). In principle this can be done in the same experiment, but we found it preferable to determine cellulose orientation on hypocotyl cryo-sections and cellulose structure on either intact hypocotyls or cell walls isolated from them. Thin sections are advantageous for avoiding non-linear (saturation) effects in the measurement of FTIR dichroism, and the orientation of the squat prc-1 cells was not always evident in preparations of isolated cell walls. Non-polarised deuteration-FTIR spectra of intact prc-1 hypocotyls are compared with the wild type in Fig. 2. The difference spectrum shows characteristic positive peaks from cellulose and both positive and negative peaks in the carbonyl stretching region. These
Fig. 2 FTIR spectra of intact, deuterated prc-1 and wild-type hypocotyls dried directly onto the lower window of the deuteration cell. Deuteration converted –OH and –NH groups (stretching frequencies 3,200–3,500 cm1) to –OD and –ND (stretching frequencies 2,300–2,600 cm1) except for the inaccessible –OH groups of crystalline cellulose and some –NH groups in the interior regions of protein molecules. The difference spectrum (wild type—prc-1) shows positive features characteristic of crystalline cellulose in the 3,200–3,400 cm1 and 980–1,430 cm1 regions, together with both positive and negative features characteristic of carboxyl groups, of pectins, in the 1,600–1,750 cm1 region
features of the difference spectrum imply a moderate reduction in cellulose content together with a relative increase in pectin content and altered pectin esterification, as reported previously (Fagard et al. 2000a). However, it was difficult to draw conclusions on cellulose crystalline structure from the O–H stretching region (3,200–3,500 cm1) of the spectra from intact hypocotyls, due to interference from cytoplasmic protein. The N–H stretching bands of internal (non-deuterable) amino acid residues give rise to a shoulder around 3,300 cm1 (Fig. 2) which varied in intensity (data not shown) and also interfered with the quantification of crystalline cellulose from its non-deuterable O–H stretching intensity. Deuteration-FTIR spectra were obtained from cell walls isolated from prc-1 and wild-type hypocotyls (Fig. 3). Isolation of the cell walls minimised absorbances contributed by cytoplasmic protein. The relative intensity of the residual hydroxyl stretching region, after deuteration of the hydroxyl groups of surface cellulose and non-cellulosic polysaccharides, is a measure of the crystalline cellulose content of the cell walls (Ha et al. 2002). In primary cell walls of prc-1, the crystalline cellulose content estimated in this way was reduced by 28% relative to the wild type (Table 1). The hydroxyl stretching region of the spectra revealed no difference between the hydrogen-bonding pattern of the remaining crystalline cellulose and that of the wild type. The data are thus consistent with a moderate reduction in cellulose content in the cell walls, without change in microfibril structure.
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The calculation of relative cell-wall area per hypocotyl is simplified by the fact that growth in elongating Arabidopsis hypocotyls is exclusively by cell expansion and cell numbers remain constant (Desnos et al. 1996). Thus, for mean cell lengths and diameters
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Fig. 3 FTIR spectra of deuterated cell walls isolated from prc-1 and wild type hypocotyls. Isolation of cell walls removed most of the interference from protein observed in Fig. 2. The integrated absorbance of the hydroxyl stretching region in the difference spectrum (rescaled x3 above for clarity) is a measure of the deficit of crystalline cellulose in prc-1. The pattern of overlapping bands in this region of the difference spectrum resembles that of both the wild type and prc-1, indicating that the crystal structure of prc-1 cellulose does not differ from wild type
It is possible that a reduction in cellulose synthesis might inhibit growth if feedback mechanisms operate between cellulose synthesis and the synthesis and secretion of other wall polymers thereby reducing the total amount of cell wall synthesised per hypocotyl. Therefore, we estimated the relative amount of cell-wall material per hypocotyl by calculating the cell-wall area from the tissue dimensions in light and transmission electron micrographs, and by measuring the polymer mass per unit area of cell wall by FTIR microscopy. As a parameter of relative amount of cell walls, polymer mass per unit area was preferred to cell-wall thickness because primary cell walls typically contain 60–80% water and swell or shrink, almost exclusively in thickness, when water is absorbed or removed (Jarvis 1992), but their mechanical strength comes from the polymers. The measured thickness of the cell wall is therefore very dependent on the degree of hydration and is not related to the mechanical properties in the way expected of other materials.
Cell length ðprc - 1Þ Hypocotyl length ðprc - 1Þ ¼ ¼ KL Cell length ðWTÞ Hypocotyl length ðWTÞ and Celldiameterðprc - 1Þ Hypocotyldiameterðprc - 1Þ ¼ ¼ KD CelldiameterðWTÞ HypocotyldiameterðWTÞ For each cell, the area of the four longitudinal cell walls is 4 (cell length · cell diameter). The area of the two transverse cell walls is 2 (cell diameter)2. Thus, the ratio of the longitudinal cell-wall areas (prc-1)/(WT) is KLKD either for a single cell or for all the cells in the hypocotyl. Similarly the ratio of the transverse cell-wall areas (prc-1)/(WT) is K2D. For 7-day-old seedlings, KL is 0.25 and KD is 1.6 approximately (Desnos et al. 1996; Fagard et al. 2000a), and transverse cell walls account for about 10% of the total cell-wall area in the wild type. The longitudinal cell-wall area is therefore reduced in prc-1 to about 0.4 of that of the wild type while the transverse cell-wall area is increased by a factor of 2.6, giving a reduction of about 40% in total cell-wall area per hypocotyl. Polymer mass per unit area of cell wall can be estimated from the FTIR absorbance integrated over a representative region of the spectrum. Calculations of relative polymer mass per unit area using the C–H stretching region, corrected for a small contribution from cuticular hydrocarbons, are shown in Table 1. (The integrated absorbance across the 1,000–1,300 cm1 region can also be used but in our spectra this region was more influenced by variation in cell-wall composition and more subject to baseline errors.) On this basis, the mean mass of polymer per unit area of cell wall was greater in prc-1 by a factor of about 1.3. The increased polymer mass per unit area in prc-1 cell walls therefore almost compensated for the decrease in cell-wall area. The estimated reduction in cell-wall polymer mass per hypocotyl is approximately 20%, much less than the reduction in length. It follows that the dwarf phenotype is not simply the result of less cell-wall material. That is, dwarfing is not the result of an overall, isotropic reduction in the dimensions of the cell-wall network due
Table 1 Residual intensity in the –OH stretching region of the FTIR spectra of procuste and wild-type hypocotyl cell walls, a measure of the content of crystalline cellulose. The integrated –OH stretching intensity (3,100–3,650 cm1) is normalised against the integrated –CH stretching intensity (2,800–3,040) excluding the hydrocarbon (cutin) C–H stretching bands at 2,840–2,870 and 2,910–2,940 cm1 Procuste
CH intensity OH intensity OH/CH
Wild type
Mean
SEM (n=14)
Mean
SEM (n=9)
0.0160 0.0337 2.14
0.0042 0.0084 0.07
0.0126 0.0365 2.98
0.0016 0.0031 0.11
P60 (prc-1) for the standard deviation in chain orientation, R
(data not shown). The degree to which the 3,344 cm1 band was polarised in the epidermal spectra is shown in Table 2, and Fig. 6 shows typical polarisation curves as the direction of polarisation was rotated with respect to the cell axis. Using the polarisation relationships described by Sˇturcova´ et al. (2003) allowed the calculation of R (Table 2) assuming that the angular distribution was described by the Gaussian model and was centred on approximately 90 to the cell’s long axis: the fitted curves in Fig. 6 indicate that this model was appropri-
ate. The polarisation curves demonstrated that the distribution of microfibril orientations around the 90 average was so broad as to be close to random, even in the wild type. A similar data set was obtained for pure Apium cellulose with a high degree of orientation (R=10) in the longitudinal axis. The spectra obtained from Apium cellulose (Fig. 7) showed strong polarisation and differ conspicuously from polarisation observed in the Arabidopsis hypocotyls. Comparison of the polarisation difference spectra of wild-type Arabidopsis (Fig. 6) and Apium (Fig. 7) confirmed that the polarised peak at 3,344 cm-1 could be assigned to cellulose and that the non-cellulosic matrix in Arabidopsis hypocotyl cell walls was much less oriented than the cellulose. Polarisation of the carbonyl stretching bands suggested a small amount of anisotropy in the pectic polymers, in both procuste and the wild type, but this was inconsistent in direction (data not shown).
Table 2 Distributions of microfibril orientations M, relative to the cell axis, in the growing zone of prc-1 and wild-type hypocotyls prc-1
Wild type
FTIR data: microfibril orientations throughout the cell-wall thickness Dichroic ratio 1.02 (0.02) 0.86 (0.02) P >80 59 (2) Æsin2Mæ 0.50 0.56 2 0.49 0.44 Æcos Mæ Æsin2Mæ/Æcos2Mæ 1.0 1.3 FE-SEM data: microfibril orientations at the inner face of the cell wall 0.74 (0.08) 0.95 (0.01) Æsin2Mæ Æcos2Mæ 0.26 (0.08) 0.05 (0.01) 3 19 Æsin2Mæ/Æcos2Mæ The averaged values of cos2M (denoted by Æcos2Mæ) predict the extent to which the orientation of the microfibrils will allow radial expansion of the cells, while Æsin2Mæ predicts the extent to which the orientation of the microfibrils will allow cell elongation. Figures in brackets are standard error of the mean. The FTIR data shown were calculated from the dichroic ratio of the hydroxyl stretching band at 3,344 cm1, which differed significantly (P
