Arabidopsis Hypocotyl Research Articles

Tensile properties of Arabidopsis cell walls depend on both a xyloglucan cross-linked microfibrillar network and rhamnogalacturonan II-borate complexes.

The mechanical properties of plant organs depend upon anatomical structure, cell-cell adhesion, cell turgidity, and the mechanical properties of their cell walls. By testing the mechanical responses of Arabidopsis mutants, it is possible to deduce the contribution that polymers of the cell wall make to organ strength. We developed a method to measure the tensile parameters of the expanded regions of turgid or plasmolyzed dark-grown Arabidopsis hypocotyls and applied it to the fucose biosynthesis mutant mur1, the xyloglucan glycosyltransferase mutants mur2 and mur3, and the katanin mutant bot1. Hypocotyls from plants grown in the presence of increasing concentrations of dichlorobenzonitrile, an inhibitor of cellulose synthesis, were considerably weakened, indicating the validity of our approach. In order of decreasing strength, the hypocotyls of mur2 > bot1 and mur1 > mur3 were each found to have reduced strength and a proportionate reduction in modulus compared with wild type. The tensile properties of the hypocotyls and of the inflorescence stems of mur1 were rescued by growth in the presence of high concentrations of borate, which is known to cross-link the pectic component rhamnogalacturonan II. From comparison of the mechanical responses of mur2 and mur3, we deduce that galactose-containing side chains of xyloglucan make a major contribution to overall wall strength, whereas xyloglucan fucosylation plays a comparatively minor role. We conclude that borate-complexed rhamnogalacturonan II and galactosylated xyloglucan contribute to the tensile strength of cell walls.

Hypergravity-induced changes in gene expression in Arabidopsis hypocotyls

Under hypergravity conditions, the cell wall of stem organs becomes mechanically rigid and elongation growth is suppressed, which can be recognized as the mechanism for plants to resist gravitational force. The changes in gene expression by hypergravity treatment were analyzed in Arabidopsis hypocotyls by the differential display method, for identifying genes involved in hypergravity-induced growth suppression. Sixty-two cDNA clones were expressed differentially between the control and 300 g conditions: the expression levels of 39 clones increased, whereas those of 23 clones decreased under hypergravity conditions. Sequence analysis and database searching revealed that 12 clones, 9 up-regulated and 3 down-regulated, have homology to known proteins. The expression of these genes was further analyzed using RT-PCR. Finally, six genes were confirmed to be up-regulated by hypergravity. One of such genes encoded 3-hydroxy-3-methylglutaryl-Coenzyme A reductase (HMGR), which catalyzes a reaction producing mevalonic acid, a key precursor ofterpenoids such as membrane sterols and several types of hormones. The expression of HMGR gene increased within several hours after hypergravity treatment. Also, compactin, an inhibitor of HMGR, prevented hypergravity-induced growth suppression, suggesting that HMGR is involved in suppression of Arabidopsis hypocotyl growth by hypergravity. In addition, hypergravity increased the expression levels of genes encoding CCR1 and ERD15, which were shown to take part in the signaling pathway of environmental stimuli such as temperature and water, and those of the α-tubulin gene. These genes may be involved in a series of cellular events leading to growth suppression of stem organs under hypergravity conditions.

The cytoskeleton, elongation and the control of elongation.

The epidermis is often considered to be the growthlimiting cell layer in young, growing herbaceous plants (Niklas, 1992). In his classic work, Green (1980) elaborated this idea further for organ development, and attributed a crucial role to the orientation of cellulose fibrils as indicator of mechanical properties. A factual proof for a relation between the mean cellulose orientation and mechanical anisotropy was reported recently (Kerstens et al., 2001). Cortical microtubules are supposed to guide the cellulose synthesising complexes in the plane of the plasma membrane (Giddings and Staehelin, 1991). Changes in the direction of cell expansion are accompanied by changes in the orientation of the cortical microtubules. A role for microtubule reorientation is also expected during changes in the rate of elongation: microtubules then will guide nascent cellulose microfibrils deposition in the longitudinal direction, which will hamper elongation and facilitate cell expansion in another direction (Taiz and Zeiger, 1998). However, this generally accepted relation between microtubules and microfibrils has also been questioned (e.g. Baskin, 2000; Emons and Mulder, 2000; Sugimoto et al., 2000; Williamson, 1990). In the elongation zone of Arabidopsis roots, microtubules in epidermal cells are transverse to the root axis. Towards the end of elongation, when cells enter the root hair initiation zone, microtubules switch to a slightly oblique orientation in atrichoblasts and to very oblique or longitudinal orientations in trichoblasts. The rate of elongation can be affected by different means. When roots are exposed to ethylene or ACC, the precursor of ethylene, the elongation rate is quickly and severely reduced, in a dose-dependent way (Le et al., 2001). This is the result of cells stopping elongation at a much reduced size. As soon as elongation of a cell ceases, microtubules change their orientation from transverse to longitudinal. This rearrangement is cell position bound. In the early part of the elongation zone, cell elongation up to a length of 30 μm is not affected and microtubules keep predominantly to a transverse orientation. Osmotic stress induces similar effects on elongation and microtubule reorientation. Experiments with microtubule drugs, demonstrate that microtubules are essential for elongation to go on but reorientation of microtubules is not involved and only concomitant with the fast control of elongation rate. We propose that the switch in microtubule orientation is a mechanism used by the cells to consolidate the shorter cell size and to inhibit further elongation. In the Arabidopsis hypocotyl, elongation starts at the base and proceeds in an acropetal direction. In darkness, elongation proceeds at a much higher rate than in the light. In the lower half of the hypocotyls, cells grow to about 2.5 times the size they reach in the light, while the cells at the top of the hypocotyl become 6–7 times longer. This is accompanied by striking differences in microtubule levels. In darkness, microtubules are quite immobile and neatly transverse in elongating cells. In the light, however, they are extremely mobile and transverse but also a variety of other microtubule orientations were found. However, in all plants studied, cells that have stopped elongation have fewer microtubules and these generally have a longitudinal orientation. Therefore, we propose that when the elongation rate is maximal (darkness) the wall synthesis activity at the cell membrane recruits all available microtubules in transverse arrays. When the rate of cell elongation is much lower (light) only a fraction of the microtubules is recruited and the remainder of the microtubules can freely change orientation. This leads to the variety of orientations seen in the light. When cells have no longer the possibility to elongate (aged cell at the base) the * Corresponding author. Tel.: +32-3-820-2277; fax: +32-3-820-2271. Cell Biology International 27 (2003) 287–289 Cell Biology International

Translational nonsense codon suppression as indicator for functional pre-tRNA splicing in transformed Arabidopsis hypocotyl-derived calli.

The transient expression of three novel plant amber suppressors derived from a cloned Nicotiana tRNA(Ser)(CGA), an Arabidopsis intron-containing tRNA(Tyr)(GTA) and an Arabidopsis intron-containing tRNA(Met)(CAT) gene, respectively, was studied in a homologous plant system that utilized the Agro bacterium-mediated gene transfer to Arabidopsis hypocotyl explants. This versatile system allows the detection of beta-glucuronidase (GUS) activity by histochemical and enzymatic analyses. The activity of the suppressors was demonstrated by the ability to suppress a premature amber codon in a modified GUS gene. Co-transformation of Arabidopsis hypocotyls with the amber suppressor tRNA(Ser) gene and the GUS reporter gene resulted in approximately 10% of the GUS activity found in the same tissue transformed solely with the functional control GUS gene. Amber suppressor tRNAs derived from intron-containing tRNA(Tyr) or tRNA(Met) genes were functional in vivo only after some additional gene manipulations. The G3:C70 base pair in the acceptor stem of tRNA(Met)(CUA) had to be converted to a G3:U70 base pair, which is the major determinant for alanine tRNA identity. The inability of amber suppressor tRNA(Tyr) to show any activity in vivo predominantly results from a distorted intron secondary structure of the corresponding pre-tRNA that could be cured by a single nucleotide exchange in the intervening sequence. The improved amber suppressors tRNA(Tyr) and tRNA(Met) were subsequently employed for studying various aspects of the plant-specific mechanism of pre-tRNA splicing as well as for demonstrating the influence of intron-dependent base modifications on suppressor activity.

Preparation, conformational analysis, and biological evaluation of 6a-carbabrassinolide and related compounds

In order to probe the ligand recognition sites of brassinosteroid (BR)-receptors, B-ring modified brassinolides (BLs), i.e. 6a-carbaBL ( 3 ), 6a-carba-B-homoBL ( 4 ), the C-5 epimers of compounds 3 and 4 ( 6 and 7 , respectively), and 6-deoxo-6a-oxo-6a-carbaBL ( 8 ), were prepared from castasterone ( 2 ) via homologation with trimethylsilyldiazomethane and boron trifluoride etherate. Biological evaluation of these compounds using the rice lamina inclination (RLI) test gave the following order of potency (percent activity): BL ( 1 ) (100%)> 2 (13%)> 3 (0.3%)≫ 4 , 6 , 7 , 8 (0%). Using the rescue test of hypocotyl length of the BR deficient mutant, Arabidopsis det2, the corresponding order was: 1 (100%)> 8 (44%)> 2 (28%)> 3 (4%)> 6 (0.5%)> 4 (0.1%), and 7 (inhibition). The markedly different activities of 8 in the two assay systems provide an evident experimental proof that the BR-receptors involved in RLI and in Arabidopsis hypocotyl elongation recognize BR molecules in different structure manners, at least in the B-ring region.

Secondary xylem development in Arabidopsis: a model for wood formation.

Our understanding of the molecular controls regulating the identity of the vascular cambium and the development of secondary xylem and phloem have not yet benefited much from the use of Arabidopsis as a genetic system. Under appropriate growth conditions Arabidopsis undergoes extensive secondary growth in the hypocotyl, with the development of both a vascular and a cork cambium. The secondary xylem of the hypocotyl develops in two phases, an early phase in which only vessel elements mature and a later stage in which both vessel elements and fibres are found. During this second phase the secondary xylem of Arabidopsis closely resembles the anatomy of the wood of an angiosperm tree, and can be used to address basic questions about wood formation. The development of the vascular cambium and secondary growth in Arabidopsis hypocotyl is described and its utility as a model for wood formation in trees is considered.

Disruption of the Actin Cytoskeleton Results in the Promotion of Gravitropism in Inflorescence Stems and Hypocotyls of Arabidopsis

The actin cytoskeleton is hypothesized to play a major role in gravity perception and transduction mechanisms in roots of plants. To determine whether actin microfilaments (MFs) are involved in these processes in stem-like organs, we studied gravitropism in Arabidopsis inflorescence stems and hypocotyls. Localization studies using Alexa Fluor-phalloidin in conjugation with confocal microscopy demonstrated a longitudinally and transversely oriented actin MF network in endodermal cells of stems and hypocotyls. Latrunculin B (Lat-B) treatment of hypocotyls caused depolymerization of actin MFs in endodermal cells and a significant reduction of hypocotyl growth rates. Actin MFs in Lat-B-treated inflorescence stems also were disrupted, but growth rates were not affected. Despite disruption of the actin cytoskeleton in these two organs, Lat-B-treated stems and hypocotyls exhibited a promotion of gravitropic curvature in response to reorientation. In contrast, Lat-B reduced gravitropic curvature in roots but also reduced the growth rate. Thus, in contrast to prevailing hypotheses, our results suggest that actin MFs are not a necessary component of gravitropism in inflorescence stems and hypocotyls. Furthermore, this is the first study to demonstrate a prominent actin MF network in endodermal cells in the putative gravity-perceiving cells in stems.

Disruption of the actin cytoskeleton results in the promotion of gravitropism in inflorescence stems and hypocotyls of Arabidopsis.

The actin cytoskeleton is hypothesized to play a major role in gravity perception and transduction mechanisms in roots of plants. To determine whether actin microfilaments (MFs) are involved in these processes in stem-like organs, we studied gravitropism in Arabidopsis inflorescence stems and hypocotyls. Localization studies using Alexa Fluor-phalloidin in conjugation with confocal microscopy demonstrated a longitudinally and transversely oriented actin MF network in endodermal cells of stems and hypocotyls. Latrunculin B (Lat-B) treatment of hypocotyls caused depolymerization of actin MFs in endodermal cells and a significant reduction of hypocotyl growth rates. Actin MFs in Lat-B-treated inflorescence stems also were disrupted, but growth rates were not affected. Despite disruption of the actin cytoskeleton in these two organs, Lat-B-treated stems and hypocotyls exhibited a promotion of gravitropic curvature in response to reorientation. In contrast, Lat-B reduced gravitropic curvature in roots but also reduced the growth rate. Thus, in contrast to prevailing hypotheses, our results suggest that actin MFs are not a necessary component of gravitropism in inflorescence stems and hypocotyls. Furthermore, this is the first study to demonstrate a prominent actin MF network in endodermal cells in the putative gravity-perceiving cells in stems.

Embryonic control of epidermal cell patterning in the root and hypocotyl of Arabidopsis.

A position-dependent pattern of epidermal cell types is produced during the development of the Arabidopsis seedling root and hypocotyl. To understand the origin and regulation of this patterning mechanism, we have examined the embryonic expression of the GLABRA2 (GL2) gene, which encodes a cell-type-specific transcription factor. Using in situ RNA hybridization and a sensitive GL2::GFP reporter, we discovered that a position-dependent pattern of GL2 expression is established within protodermal cells at the heart stage and is maintained throughout the remainder of embryogenesis. In addition, we show that an exceptional GL2 expression character and epidermal cell pattern arises during development of the root-hypocotyl junction, which represents an anatomical transition zone. Furthermore, we find that two of the genes regulating seedling epidermal patterning, TRANSPARENT TESTA GLABRA (TTG) and WEREWOLF (WER), also control the embryonic GL2 pattern, whereas the CAPRICE (CPC) and GL2 genes are not required to establish this pattern. These results indicate that position-dependent patterning of epidermal cell types begins at an early stage of embryogenesis, before formation of the apical meristems and shortly after the cellular anatomy of the protoderm and outer ground tissue layer is established. Thus, epidermal cell specification in the Arabidopsis seedling relies on the embryonic establishment of a patterning mechanism that is perpetuated postembryonically.

Nucleotides Provide a Voltage-sensitive Gate for the Rapid Anion Channel of Arabidopsis Hypocotyl Cells

The rapid anion channel of Arabidopsis hypocotyl cells is highly voltage-dependent. At hyperpolarized potentials, the channel is closed, and membrane depolarization is required for channel activation. We have previously shown that channel gating is regulated by intracellular nucleotides. In the present study, we further analyze the channel gating, and we propose a mechanism to explain its regulation by voltage. In the absence of intracellular nucleotides, closure at hyperpolarized voltages is abolished. Structure-function studies of adenyl nucleotides show that the apparent gating charge of the current increases with the negative charge carried by nucleotides. We propose that the fast anion channel is gated by the voltage-dependent entry of free nucleotides into the pore, leading to a voltage-dependent block at hyperpolarized potentials. In agreement with this mechanism in which intracellular nucleotides need to be recruited to the channel pore, kinetic analyses of whole-cell and single-channel currents show that the rate of closure is faster when intracellular nucleotide concentration is increased, whereas the rate of channel activation is unchanged. Furthermore, decreasing the concentration of extracellular chloride enhances the intracellular nucleotide block. This result supports the hypothesis of a mechanism in which blocking nucleotides and permeant anions interact within the channel pore.

The auxin signal for protoplast swelling is perceived by extracellular ABP1.

Protoplasts of corn coleoptiles and Arabidopsis hypocotyls respond to the plant hormone auxin with a rapid change in volume. We checked the effect of antibodies directed against epitopes of auxin-binding protein 1 from Arabidopsis thaliana (AtERabp1) and Zea mays (ZmERabp1), respectively. Antibodies raised against the C-terminus of AtERabp1 inhibited the response to auxin, while antibodies raised against a part of box a, the putative auxin-binding domain, induced a swelling response similar to that caused by auxin treatment. Synthetic C-terminal oligopeptides of ZmERabp1 also caused a swelling response. These effects occurred regardless of whether the experiments were carried out with homologous (anti-AtERabp1 antibodies on Arabidopsis protoplasts or anti-ZmERabp1 antibodies in maize protoplasts) or heterologous immunological tools. The results indicate that the auxin signal for protoplast swelling is perceived by extracellular ABP1.

The effects of exogenous proline and proline analogues on in vitro shoot organogenesis in Arabidopsis

In vitro shoot organogenesis from Arabidopsis hypocotyl explants was stimulated by 1 mMproline, and to a lesser extent by 5 mM proline, butinhibited by inclusion of 10 mM proline in thehormonally-supplemented regeneration medium. Theability of low concentrations of the prolineanalogues, azetidine-2-carboxylate and thioproline toovercome the stimulatory effect of 1 mM proline, anda slight increase in the stimulative effects of 1 mMproline by D-proline, are consistent with an importantrole for the interconversions of proline and itsprecursors in regulating cell division anddifferentiation.

The hypocotyl chloroplast plays a role in phototropic bending of Arabidopsis seedlings: developmental and genetic evidence

Chloroplasts of guard cells and coleoptiles have been implicated in the sensory transduction of blue light. The present study was aimed at establishing whether the chloroplast of the hypocotyl from Arabidopsis, another blue light-responding organ, has similar characteristics to that of sensory-transducing guard cell and coleoptile chloroplasts. Results showed that the phototropic curvature and arch length induced by blue light in Arabidopsis seedlings matched the distribution of mature chloroplasts in the bending hypocotyl. The bending arch consistently included the region of the hypocotyl containing mature chloroplasts, and never extended beyond that region. Manipulation of the extent of greening of dark-grown hypocotyls by varying red light pretreatments elicited blue light-stimulated curvatures and arch lengths that depended on the duration of the red light pretreatment and on the distribution of mature chloroplasts in the hypocotyl. Albino psd2 mutants of Arabidopsis, which lack mature chloroplasts, are devoid of phototropic sensitivity under conditions in which wild-type seedlings show large curvatures. The star mutant of Arabidopsis has a delayed greening and a delayed phototropic response as compared with wild type. Measurements of photosynthetic oxygen evolution and carbon fixation, dark respiration, and light-dependent zeaxanthin formation in the hypocotyl showed features similar to those of guard cells and coleoptiles, and distinctly different from those of mesophyll tissue. These results indicate that the hypocotyl chloroplast has characteristics similar to those associated with guard cell and coleoptile chloroplasts, and that phototropic bending of Arabidopsis hypocotyls appears to require mature chloroplasts.

PROCUSTE1 Encodes a Cellulose Synthase Required for Normal Cell Elongation Specifically in Roots and Dark-Grown Hypocotyls of Arabidopsis

PROCUSTE1 Encodes a Cellulose Synthase Required for Normal Cell Elongation Specifically in Roots and Dark-Grown Hypocotyls of Arabidopsis

PROCUSTE1 encodes a cellulose synthase required for normal cell elongation specifically in roots and dark-grown hypocotyls of Arabidopsis.

Mutants at the PROCUSTE1 (PRC1) locus show decreased cell elongation, specifically in roots and dark-grown hypocotyls. Cell elongation defects are correlated with a cellulose deficiency and the presence of gapped walls. Map-based cloning of PRC1 reveals that it encodes a member (CesA6) of the cellulose synthase catalytic subunit family, of which at least nine other members exist in Arabidopsis. Mutations in another family member, RSW1 (CesA1), cause similar cell wall defects in all cell types, including those in hypocotyls and roots, suggesting that cellulose synthesis in these organs requires the coordinated expression of at least two distinct cellulose synthase isoforms.

Hormonal interactions in the control of Arabidopsis hypocotyl elongation.

The Arabidopsis hypocotyl, together with hormone mutants and chemical inhibitors, was used to study the role of auxin in cell elongation and its possible interactions with ethylene and gibberellin. When wild-type Arabidopsis seedlings were grown on media containing a range of auxin concentrations, hypocotyl growth was inhibited. However, when axr1-12 and 35S-iaaL (which have reduced auxin response and levels, respectively) were grown in the same conditions, auxin was able to promote hypocotyl growth. In contrast, auxin does not promote hypocotyl growth of axr3-1, which has phenotypes that suggest an enhanced auxin response. These results are consistent with the hypothesis that auxin levels in the wild-type hypocotyl are optimal for elongation and that additional auxin is inhibitory. When ethylene responses were reduced using either the ethylene-resistant mutant etr1 or aminoethoxyvinylglycine, an inhibitor of ethylene synthesis, auxin responses were unchanged, indicating that auxin does not inhibit hypocotyl elongation through ethylene. To test for interactions between auxin and gibberellin, auxin mutants were grown on media containing gibberellin and gibberellin mutants were grown on media containing auxin. The responses were found to be the same as wild-type Arabidopsis seedlings in all cases. In addition, 1 microM of the auxin transport inhibitor 1-naphthylphthalmic acid does not alter the response of wild-type seedlings to gibberellin. Double mutants were made between gibberellin and auxin mutants and the phenotypes of these appear additive. These results indicate that auxin and gibberellin are acting independently in hypocotyl elongation. Thus auxin, ethylene, and gibberellin each regulate hypocotyl elongation independently.

RPT2: A Signal Transducer of the Phototropic Response in Arabidopsis

The blue light receptor NPH1 (for nonphototropic hypocotyl) has been considered to be the only UV-A/blue light receptor that induces a phototropic response by the hypocotyl and root of Arabidopsis. By analysis of root phototropism (rpt) mutants, we show, however, the involvement of another blue light receptor as well as the existence of two separate signaling pathways working downstream of these receptors in the phototropic response. A newly isolated gene, RPT2, controls one of these pathways. The RPT2 gene is light inducible; encodes a novel protein with putative phosphorylation sites, a nuclear localization signal, a BTB/POZ domain, and a coiled-coil domain; and belongs to a large gene family that includes the recently isolated NPH3 gene. From genetic, physiological, and biochemical evidence, we propose a genetic model of the signaling pathways that induce the phototropic response in Arabidopsis.

Characterization of a nitrate-permeable channel able to mediate sustained anion efflux in hypocotyl cells from Arabidopsis thaliana.

We have characterized a new anionic current in Arabidopsis hypocotyl cells. This current, activated by membrane depolarization, has slow activation and deactivation kinetics in the 10 sec range. It presents many distinct properties from the rapid-type anion current already described on the same membrane. The slow-type channel is highly permeable to nitrate with a PNO3-/PCl- close to 20, but totally impermeable to sulphate. Activation of the channel requires cytosolic ATP and the slow current is partially inhibited by staurosporin, suggesting that channel regulation involves protein phosphorylation. The slow anion channel displays a unique pharmacological profile different from that of the rapid channel: the slow channel is inhibited by DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid) with an IC50 of 26 microM. The slow and rapid anion channels are probably dedicated to specific functions: the first is able to mediate sustained anion efflux, while the second is a good candidate to be involved in fast electrical signalling.

Circadian dysfunction causes aberrant hypocotyl elongation patterns in Arabidopsis.

Many endogenous and environmental signals control seedling growth, including several phototransduction pathways. We demonstrate that the circadian clock controls the elongation of the Arabidopsis hypocotyl immediately upon germination. The pattern of hypocotyl elongation in constant light includes a daily growth arrest spanning subjective dawn and an interval of rapid growth at subjective dusk. Maximal hypocotyl growth coincides with the phase during which the cotyledons are raised, in the previously described rhythm of cotyledon movement. The rhythm of hypocotyl elongation was entrained by light-dark cycles applied to the imbibed seed and its period was shortened in the toc1-1 mutant, indicating that it is controlled by a similar circadian system to other rhythmic markers. The daily groth arrest is abolished by the early flowering 3 (elf3) mutation, suggesting that this defect may cause its long-hypocotyl phenotype. Mutations that affect the circadian system can therefore cause gross morphological phenotypes, not because the wild-type gene functions pleiotropically in several signalling pathways, but rather because the circadian clock exerts wide-spread control over plant physiology.

Petit1, a conditional growth mutant of Arabidopsis defective in sucrose-dependent elongation growth.

The hypocotyl of Arabidopsis is well suited for the analysis of cell elongation because it elongates without cell division. We have isolated a new class of recessive mutants, petit1 (pet1), which are defective in aspects of hypocotyl elongation. The short-hypocotyl phenotype of pet1 is caused by shortened cells. The cells of the elongation zone of the hypocotyl are often deformed. pet1 also shows defects in elongation of the roots, flower stalk, leaves, petals, pedicels, and siliques, and these defects cannot be repaired by the application of auxin, gibberellin, brassinolide, or an inhibitor of ethylene biosynthesis. The short-hypocotyl phenotype of pet1 is pronounced only in growth medium supplemented with sucrose, which has promotive effects on hypocotyl elongation. In pet1 this effect is much reduced, causing the sucrose-dependent short-hypocotyl phenotype of pet1. pet1 accumulates more soluble sugars than the wild type and also shows more intensive iodo-starch staining in the cotyledon and hypocotyl. These results indicate that PETIT1 is involved in a sugar-dependent elongation process that may include correct assembly of expanding cell wall architecture.