Cell Extension Growth Research Articles

Reduced pectin content of cell walls prevents stress-induced root cell elongation in Arabidopsis.

The primary cell walls of plants provide mechanical strength while maintaining the flexibility needed for cell extension growth. Cell extension involves loosening the bonds between cellulose microfibrils, hemicelluloses and pectins. Pectins have been implicated in this process, but it remains unclear if this depends on the abundance of certain pectins, their modifications, and/or structure. Here, cell wall-related mutants of the model plant Arabidopsis were characterized by biochemical and immunohistochemical methods and Fourier-transform infrared microspectroscopy. Mutants with reduced pectin or hemicellulose content showed no root cell elongation in response to simulated drought stress, in contrast to wild-type plants or mutants with reduced cellulose content. While no association was found between the degrees of pectin methylesterification and cell elongation, cell wall composition analysis suggested an important role of the pectin rhamnogalacturonan II (RGII), which was corroborated in experiments with the RGII-modifying chemical 2β-deoxy-Kdo. The results were complemented by expression analysis of cell wall synthesis genes and microscopic analysis of cell wall porosity. It is concluded that a certain amount of pectin is necessary for stress-induced root cell elongation, and hypotheses regarding the mechanistic basis of this result are formulated.

Ascorbate in the Apoplast: Metabolism and Functions

Ascorbic acid (AA) is one of the most important antioxidants and redox-active substances of plants found in the apoplast. In the form of ascorbate anion, it can be exported to the cell wall through anion channels, where it is further oxidized to dehydroascorbic acid (DHA) and in this form is translocated back into the cytoplasm through as yet unidentified transporters. In the apoplast AA is exposed to both enzymatic and non-enzymatic oxidation, participating in the antioxidant and pro-oxidant processes: detoxification of O3, generation of hydroxyl radicals, reduction of phenoxyl radicals, etc. The intermediate product of the AA oxidation in apoplast, monodehydroascorbic acid (MDHA), serves as an extracellular electron acceptor for a number of redox chains localized at the plasma membrane (PM). All three forms of ascorbate, AA, DHA and MDHA, can potentially be involved in the cell signalling. Released from the cell through anionic channels during PM depolarization, AA can stimulate the production of hydroxyl radicals that activate Ca2+-permeable channels, and thus trigger Ca2+-signalling. Due to the ability to generate hydroxyl radicals and reduce phenoxyl radicals, AA affects cell wall extensibility and extension growth. By reducing Fe3+ to a freely soluble transport form of Fe2+, AA is involved in iron uptake by root cells. Apoplastic AA also plays an important role in a number of other processes and is potentially a unique signal-regulatory molecule functioning in the plant cell walls.

Characterization of a Glycan Exo-Hydrolase that Shows a Biphasic Expression in the Course of an In Vitro Culture of Photoautotrophic Oxybasis rubra Cells

Like plant leaves, suspension cultures of photoautotrophic Oxybasis rubra Fuentes-Bazan (syn. Chenopodium rubrum L.) cells pass through distinct developmental phases when grown under CO2 as the sole carbon source: an initial cell division phase of 4 weeks, a stationary phase of another 4 weeks and an aging phase (3-4 weeks) when the cell senesce and finally die. These phases are reflected by differential gene expression. A gene that was strongly expressed in the course of the stationary phase but much lesser during the exponential growth phase of the cell culture was isolated from a cDNA-library of stationary cells and completed by 5’-RACE. From homology analysis, the gene was tentatively identified as glycan exo-hydrolase (Oxybasis rubra glycan exo-hydrolase, OrGEH). Heterologous expression in E. coli yielded a protein with a preference to hydrolyze the s-D-galactopyranoside, s-D-fucopyranoside and s-D-glucopyranoside of the corresponding artificial p-nitrophenyl substrates. Possible function of the protein in cell-wall metabolism was confirmed by extracellular localization of the GFP-fusion protein in transformed Nicotiana benthamiana leaves, by an N-terminal signal sequence for the secretory pathway and by the positive response of gene expression to auxin. Suspension cultured OrGEH RNAi-lines showed not only strongly reduced cell division activity but also lack of the stationary phase during which the cells usually double their biomass by extension growth. Although the natural substrate of the protein is not known we propose a function of OrGEH in the processing of cell wall components during cell division and cell extension growth.

Actin cytoskeleton rearrangements during the gravitropic response of Arabidopsis roots

Gravitropic response is a plant growth response against changing its position relative to the gravity vector. In the present work we studied actin cytoskeleton rearrangements during Arabidopsis root gravitropic response. Two alternative approaches were used to visualize actin microfilaments: histochemical staining of fixed roots with rhodamine-phalloidin and live imaging of microfilaments in GFP-fABD2 transgenic plants. The curvature of actin microfilaments was shown to be increased within 30-60 min of gravistimulation, the fraction of auxially oriented microfilaments decreased with a concomitant increase in the fraction of oblique and transversally oriented microfilaments. Methodological issues of actin cytoskeleton visualization in the study of Arabidopsis root gravitropic response, as well as the role of microfilaments at the stages of gravity perception, signal transduction and gravitropic bending formation are discussed. It is concluded that the actin cytoskeleton rearrangements observed are associated with the regulation of basic mechanisms of cell extension growth by which the gravitropic bending is formed.

CLONING AND EXPRESSION ANALYSIS OF BETAINE ALDEHYDE DEHYDROGENASE FROM Pseudomonas fluorescens

Pseudomonas fluorescens are gramnegative rod shaped bacteria that improve plant health and nutrition (Rodriguez and Pfender, 1997; de Bruijn et al., 2007). Most studies describe P. fluorescens as a psychrotrophic bacterium unable to grow at temperatures greater than 32C and therefore as an avirulent bacterium to humans (Naseby et al., 2001). It has many positive effects on the plant health as it enhances the production of plant growth hormones, it boosts the suppression of pathogens (especially fungi and oomycetes), and it directs the elicitation of plant defense responses (Haas and Defago, 2005). Many micro-organisms use glycine betaine (GB) as a sole carbon, nitrogen, and energy source. GB is an efficient osmoprotectant, which is accumulated to counteract drought and high salinity environments, and to maintain a positive cell turgor, needed for cell extension growth. Betaine aldehyde dehydrogenase (BADH) catalyzes the final step of glycine betaine from choline (Chen and Murata, 2002; Hanson et al., 1985; He et al., 2004). Several reports have demonstrated the purification of BADHs from several species (Arakawa et al., 1987; Hibino et al., 2001; Ishitani et al., 1993). Overexpression of BADH may protect intracellular enzymes and organelles against the elevation of intracellular ionic strength or temperature, which results in increase of tolerance of salt and osmotic stresses in many organisms (Kempf and Bremer, 1998; Welsh, 2000; Li et al., 2006). Beside its function in the production of glycine betaine, BADH is a key enzyme for the growth of many pathogens (Velasco-Garci´a et al., 2000). The growth arrest of the pathogen can be accomplished by blocking choline degradation, abolishing synthesis of the osmoprotectant glycine betaine, and accumulating the BADH substrate, betaine aldehyde, which is highly toxic to the pathogen (Boch et al., 1996; Sage et al., 1997). In this investigation, the full length of betB gene, PfBADH, coding for BADH enzyme was isolated from Pseudomonas fluorescens, cloned in Escherichia coli and identified by studying the gene sequence and its expression analysis. Cloning the full length gene of PfBADH is a starting point for the strategic improvement of commercial crops grown under biotic and abiotic stress conditions using gene transfer techniques.

Changes in expansin gene expression, IAA content, and extension growth of leaf cells in maize plants subjected to salinity

Effects of salinity of nutrient solution on expression of ZmEXPA1 expansin gene and leaf growth were studied on maize plants (Zea mays L.). Rapid activation of the gene transcription was shown to precede the resumption of extension growth in leaf cells under water deficit induced by NaCl salinity. Auxins were found to accumulate in leaves during salinity treatment, the accumulation being faster than activation of ZmEXPA1 transcription. In addition, exogenous IAA was shown to enhance the gene expression. Our results indicate that the hormone is involved in regulation of cell extension growth at high salinity through the expression of expansin gene.

Phosphoinositide 5-Phosphate and Phosphoinositide 4-Phosphate Trigger Distinct Specific Responses of Arabidopsis Genes

Phosphoinositide phosphates, PtdInsP, are important components of the cell lipid pool that can function as messengers in diverse cellular processes. Lack of information on downstream targets, however, has impeded our understanding of the potential of lipid-signaling to influence gene activity. Our goals here were to identify genes that altered expression in the presence of two isomeric monophosphate lipid messengers (Phosphoinositide 5-Phosphate, PtdIns(5)P, and Phosphoinositide 4-Phosphate, PtdIns(4)P) and to establish whether the two lipids influence distinct or overlapping gene-sets. Our results indicated that PtdIns(5)P and PtdIns(4)P affected genes within shared gene-families but that each messenger influenced the expression of different members within the same family. These results suggested that PtdIns(5)P and PtdIns(4)P participate in separate pathways that, ultimately, may control gene expression. The pathways may have points of convergence but may also counteract each other's effects. A significant fraction (∼40%) of the PtdIns(5)P-stimulated genes belong to various families of wall-modifying genes. Wall-modifying activities are recognized as factors affecting cell extension and plant growth. Elevated PtdIns(5)P concentration influenced stem growth and the effects were different from those triggered by PtdIns(4)P. The data allow insights into plants' response to two related PtdInsP at whole-plant/genome-wide levels and demonstrate that PtdIns(5)P-and PtdIns(4)P-involving mechanisms are distinct, selective and specific.

Evidence for the involvement of cell wall peroxidase in the generation of hydroxyl radicals mediating extension growth.

Hydroxyl radicals (*OH), produced in the cell wall, are capable of cleaving wall polymers and can thus mediate cell wall loosening and extension growth. It has recently been proposed that the biochemical mechanism responsible for *OH generation in the cell walls of growing plant organs represents an enzymatic reaction catalyzed by apoplastic peroxidase (POD). This hypothesis was investigated by supplying cell walls of maize ( Zea mays L.) coleoptiles and sunflower ( Helianthus annuus L.) hypocotyls with external NADH, an artificial substrate known to cause *OH generation by POD in vitro. The effects of NADH on wall loosening, growth, and *OH production in vivo were determined. NADH mediates cell wall extension in vitro and in vivo in an H2O2-dependent reaction that shows the characteristic features of POD. NADH-mediated production of *OH in vivo was demonstrated in maize coleoptiles using electron paramagnetic resonance spectroscopy in combination with a specific spin-trapping reaction. Kinetic properties and inhibitor/activator sensitivities of the *OH-producing reaction in the cell walls of coleoptiles resembled the properties of horseradish POD. Apoplastic consumption of external NADH by living coleoptiles can be traced back to the superimposed action of two enzymatic reactions, a KCN-sensitive reaction mediated by POD operating in the *OH-forming mode, and a KCN-insensitive reaction with the kinetic properties of a superoxide-producing plasma-membrane NADH oxidase the activity of which can be promoted by auxin. Under natural conditions, i.e. in the absence of external NADH, this enzyme may provide superoxide (O2*-) (and H2O2 utilized by POD for) *OH production in the cell wall.

Evidence that hydroxyl radicals mediate auxin-induced extension growth.

Reactive oxygen intermediates, i.e. the superoxide radical (O*-)(2), hydrogen peroxide (H2O2) and the hydroxyl radical (*OH), are generally regarded as harmful products of oxygenic metabolism causing cell damage in plants, animals and microorganisms. However, oxygen radical chemistry may also play a useful role in polymer breakdown leading to wall loosening during extension growth of plant cells controlled by the phytohormone auxin. Backbone cleavage of cell wall polysaccharides can be accomplished in vitro by (*OH) produced from H2O2 in a Fenton reaction or in a reaction catalyzed by peroxidase supplied with O2 and NADH. Here, we show that coleoptile growth of maize seedlings is accompanied by the release of reactive oxygen intermediates in the cell wall. Auxin promotes release of (O*-)(2) and subsequent generation of (*OH)when inducing elongation growth. Experimental generation of (*OH) in the wall causes an increase in wall extensibility in vitro and replaces auxin in inducing growth. Auxin-induced growth can be inhibited by scavengers of (O*-)(2), H2O2 or (*OH), or inhibitors interfering with the formation of these molecules in the cell wall. These results provide the experimental background for a novel hypothesis on the mechanism of plant cell growth in which (*OH), produced from (O*-)(2) and H2O2 by cell wall peroxidase, acts as a wall-loosening agent.

Ecological significance of compatible solute accumulation by micro-organisms: from single cells to global climate.

The osmoadaptation of most micro-organisms involves the accumulation of K(+) ions and one or more of a restricted range of low molecular mass organic solutes, collectively termed 'compatible solutes'. These solutes are accumulated to high intracellular concentrations, in order to balance the osmotic pressure of the growth medium and maintain cell turgor pressure, which provides the driving force for cell extension growth. In this review, I discuss the alternative roles which compatible solutes may also play as intracellular reserves of carbon, energy and nitrogen, and as more general stress metabolites involved in protection of cells against other environmental stresses including heat, desiccation and freezing. Thus, the evolutionary selection for the accumulation of a specific compatible solute may not depend solely upon its function during osmoadaptation, but also upon the secondary benefits its accumulation provides, such as increased tolerance of other environmental stresses prevalent in the organism's niche or even anti-herbivory or dispersal functions in the case of dimethylsulfoniopropionate (DMSP). In the second part of the review, I discuss the ecological consequences of the release of compatible solutes to the environment, where they can provide sources of compatible solutes, carbon, nitrogen and energy for other members of the micro-flora. Finally, at the global scale the metabolism of specific compatible solutes (betaines and DMSP) in brackish water, marine and hypersaline environments may influence global climate, due to the production of the trace gases, methane and dimethylsulfide (DMS) and in the case of DMS, also couple the marine and terrestrial sulfur cycles.

The phytohormone-mediated action of the synthetic regulators on cell extension growth in higher plants

It was shown that the action of synthetic growth activators on growth and development of isolated «поп-decapitated» embryonic axes was analogous to action on embryonic axes of the intact haricot bean seeds; for the 72-hours period size increase of the embryonic axes depended on incubative medium composition: distilled water lutidine N-oxide - 6-methylthiouracil < (IAA). In the experiments with embryonic axes in both of which one cotyledon was preserved ( «monocotyledonous seedlings») size increase was identical to size increase of embryonic axes in intact haricot bean seeds, as in the case of «поп-decapitated» and «decapitated» embryonic axes, and it also depended on the incubative medium composition: distilled water < lutidine N-oxide - 6-methylthiouracil < (IAA). Investigation of the fractional composition of new synthesized [XAC] impulse labelled proteins using the method of one-dimensional PAG-electrophoresis and gel fluorography approach showed that lutidine N-oxide intensifies the synthesis of all cellular proteins fractions with predominating increase of the synthesis of some high-molecular and low-molecular polypeptides, and 6-methylthiouracil intensifies the synthesis of only one polypeptide with molecular weight of 30 kDa. The data obtained are examined accordingly as evidence in favour of ideas, developing in our investigation about phytohormone-mediated action of the synthetic plant growth regulators and differentdirection mechanisms growth activating action of lutidine N-oxide and 6-methylthiouracil The scheme of phytohormone-mediated action of the synthetic regulators on cell extension growth in higher plants is proposed.

Different effects of vanadate on net proton secretion in the fern regnellidium diphyllum and the dicotyledon nymphoides peltata: Relevance to cell growth

Summary The semi-aquatic fern Regnellidium diphyllum differs from higher plants in showing no acid-growth response although it shares with the semi-aquatic dicotyledon Nymphoides peltata a similar increase in elongation with auxin or ethylene. Unlike Nymphoides , the fern maintains a permanendy low apoplastic pH and shows no increase in cell extension with fusicoccin, despite considerable enhancement of net proton secretion. We now show that the p-ATPase inhibitor, sodium vanadate, has different effects on net proton secretion in the two species. In Nymphoides , vanadate inhibits cell extension induced by auxin or fusicoccin and effectively blocks net proton secretion in the short term. In Regnellidium , there is a similar inhibition of IAA-induced growth but vanadate reduces net proton secretion only slighdy after several hours with IAA or fusicoccin and not at all in their absence. Our evidence indicates that a direct link between plasma membrane ATPases, acidification of the wall and IAA-induced cell extension does not operate in the fern. In view of the unusual physiological attributes of Regnellidium and other primitive plants already reported, we suggest that IAA-induced cell extension growth may not be determined by the same mechanisms in higher and lower plants.

Down-regulation of plant V-type H +-ATPase genes after light-induced inhibition of growth

Cell extension growth in the mesocotyl tip of dark-grown Zea mays L. seedlings is dependent on vacuole enlargement and massive flux of ER and Golgi vesicles. Water flow into the expanding vacuole is driven by ion accumulation, which in turn is energized by the vacuolar H +-ATPase (V-ATPase). The V-ATPase energizes the secondary ion transport into the expanding vacuole. As light exposure leads to a strong inhibition of extension growth, the effect of light on transcript levels for subunits A and c of the V-ATPase was analyzed. Partial homologous cDNAs for subunit A and two isoforms of subunit c were cloned by RT-PCR. In dark-grown seedlings transcript levels for both subunits were much higher in the growing mesocotyl tip than in the fully differentiated mesocotyl tissue. Only in the tip region did light exposure lead to a strong and coordinate down-regulation of both mRNAs whereas in the differentiated mesocotyl only a slight decrease was observed. The results indicate that expression of the ‘housekeeping’ V-type H +-ATPase is strongly regulated in response to growth rate.

AnUstilago maydisMutant Partially Blocked in P45014DMActivity Is Hypersensitive to Azole Fungicides

AnUstilago maydisergosterol biosynthesis mutant (A14) which is partially blocked in sterol 14α-demethylase (P45014DM) activity is described. This mutant accumulated the abnormal 14α-methyl sterols, eburicol, 14α-methylfecosterol, and obtusifoliol, along with significant amounts of ergosterol. Although the A14 mutant grew nearly as well as the wild type, it was impaired in cell extension growth, which indicated a dysfunction in apical cell wall synthesis. The mutant was also found to be hypersensitive to the azole fungicides penconazole and tebuconazole.

Phytochrome-Mediated Mitotic Activity and Induction of two-Dimensional Growth in Gametophytes of Anemia phyllitidis (L.) Sw. *

Summary In non-growing protonemata of Anemia phyllitidis mitotic activity can be induced with a single s-min red light pulse that is abolished by brief far red light, indicating phytochrome control. After repeatedly raising the Pfr to a high level with red light pulses (e.g. one pulse every 4 days) some 10 % of gametophytes respond with transition to 2-D growth, which is first apparent in a drastic reduction of the L/B ratio of apical cells ( fr level with brief far red light immediately following red light not only totally prevents a reduction of cell growth but causes a considerable promotion of cell extension growth, which eventually leads to transverse cell divisions. Induction of 2-D growth with red light pulses also occurs in non-growing cells derived after R-pulsed (instead of continuous red light) induction of spore germination; the light treatment of spores must only be so sufficient as to establish a high enough number of protonemata for subsequent experimentation. The slow escape reaction from far red reversibility of red light-induced mitotic activity suggests mediation by a stable form of phytochrome, which appears to be different from that controlling spore germination.

Gradients of Turgor, Osmotic Pressure, and Water Potential in the Cortex of the Hypocotyl of Growing Ricinus Seedlings

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe. Because the osmotic pressure of individual cells (pi(i)) was also determined, the water potential (psi) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of well-watered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor approximately pi(i) and psi approximately 0 across the cortex, there were also no gradients of psi across the tissue. Gradients of cell turgor and pi(i) increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cutting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCl(2) solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.

In vitro molecular weight increase in xyloglucans by an apoplastic enzyme preparation from epicotyls of Vigna angularis

In order to gain insight into the mechanism of cell extension growth, enzymic processes involved in structural modification of cell wall xyloglucans were investigated, using an apoplastic enzyme preparation from epicotyls of dark grown Vigna angularis Ohwi et Ohashi cv. Takara and purified xyloglucans derived from cell walls of Vigna. The reaction of Vigna xyloglucan (mass average molecular weight=420 kDa) with the apoplastic enzyme preparation gave three fractions: (1) a waterinsoluble high molecular weight (820 kDa) xyloglucan fraction (WI), (2) a watersoluble low molecular weight (149 kDa) xyloglucan fraction (WS), and (3) an 80% ethanol‐soluble monosaccharide fraction (ES). WI and WS were chiefly composed of t‐galactosyl‐, t‐xylosyl‐, 2‐xylosyl‐, 4‐glucosyl‐ and 4,6‐glucosyl residues, whereas ES was composed of fucose, galactose, glucose and xylose monomers. The data indicate that WI is generated by the linking of xyloglucan molecules by some alkali stable linkages, probably of glycosidic nature. The optimal pH for the WI‐producing activity of the apoplastic enzyme preparation was 5.4. Higher WI‐producing activity was detected in the upper juvenile than in the lower non‐elongating regions of the epicotyl. Our data suggest the possible involvement of a transglycosylation reaction in the structural changes of the xyloglucans that are responsible for cell extension growth of the Vigna angularis epicotyl. The data are also consistent with the idea that the enzymic processes are regulated by hydrogen ions in the apoplastic space.

Diferulic and ferulic acid in the cell wall of Avena coleoptiles—Their relationships to mechanical properties of the cell wall

Alkaline hydrolysis liberated ferulic and diferulic acid from polysaccharides of the Avena coleoptile (Avena sativa L. cv. Victory I) cell walls. The amount of the two phenolic acids bound to cell walls increased substantially at day 4–5 after sowing, when the growth rate of the coleoptile started to decrease. The level of these acids was almost constant from the tip to base in 3‐day‐old coleoptiles, but increased toward the basal zone in 4‐ and 5‐day‐old ones. The ratio of diferulic acid to ferulic acid was almost constant irrespective of coleoptile age and zone. An increase in the amount of ferulic and diferulic acids bound to cell wall polysaccharides correlated with a decrease in extensibility and with an increase in minimum stress‐relaxation time and relaxation rate of the cell wall. The level of lignin in the cellulose fraction increased as coleoptiles aged, but this increase did not correlate with changes in mechanical properties of the cell walls. These results suggest that ferulic acid, ester‐linked to cell wall polysaccharides, is oxidized to give diferulic acid, which makes the cell wall mechanically rigid by cross‐linking matrix polysaccharides and results in limited cell extension growth. In addition, it is probable that the step of feruloylation of cell wall polysaccharides is rate‐limiting in the formation of in‐termolecular bridges by diferulic acid in Avena coleoptile cell walls.

Partial Purification and Characterization of Exo-and-Endo-1,3- β-DGlucanases from Atriplex Leaf Tissues, their Involvement in Cell Extension Growth and the Effects of Salinity

Summary β -D-glucanase activity from Atriplex leaf tissues is represented by an endo-1,3- ]sB -D-glucanase(EC 3.2.1.39) and an exo-1,3- β -D-glucanase (EC 3.2.1.58). The latter form is always associated with a β -glucosidase activity, suggesting that it is a bifunctional enzyme. The exo-enzyme, extractable only with a high-salt concentration buffer is confined to walls whereas the endo form which is weakly bound, could have multiple sites, i.e. walls and membranes of endoplasmatic reticulum. During leaf development of plants grown without sodium chloride, exo-glucanase activity is closely correlated to the lamina thickening rate. In leaves from plants grown in solutions containing 340 mM NaCl the lamina thickening is increased while exo-1,3- β -glucanase activity is the same as that in plants cultivated without salt. Under both culture conditions endo-1,3- β -glucanase activity seems to remain more or less constant during lamina thickening. Gel filtration shows at least two forms of endo-glucanases: EG-1 and EG-2. Following ion exchange chromatography EG 1 is detected in the void volume while some of the EG-2 is eluted with 0.28 M NaCl (EG-2B) and the majority with 0.5 M NaCl (EG-2A). These various endoforms have the same affinity for laminarin but have different molecular weights, heat stabilities, and sodium chloride sensitivities in vitro . Culture at the highest salinity level induces a change in EG-1: EG-2 ratio due to a significant increase of EG-1 activity. The role of exo-1,3- β -glucanase in cell extension growth via cell wall remodeling is considered. The adaptative significance of endo-1,3- β -glucanase activity in salt-rich culture is discussed.

Acid pH-Induced Structural Changes in Cell Wall Xyloglucans in Vigna Angularis Epicotyl Segments

Summary Acid pH, like indole-3-acetic acid (IAA), induced a decrease in the mass average molecular weight (MW) of xyloglucans in the hemicellulose fraction when it induced cell wall loosening and cell extension of epicotyl segments of light-grown Vigna angularis Ohwi et Ohashi cv. Takara seedlings, The effect of acid pH on the xyloglucan appeared without a lag period and the molecular weight decreased linearly with time in acidic pH solution. This effect of acid pH on the xyloglucan was completely reversed by subsequent treatment with neutral pH solution. These findings are consistent with the acid-growth theory of auxin action and provide further evidence that the decrease in the mass-average MW of the cell wall xyloglucans is involved in the critical process responsible for the auxin-induced cell extension growth.