Promotion of Xyloglucan Metabolism by Acid pH

Functional and chemical characterization of XAF: a heat-stable plant polymer that activates xyloglucan endotransglucosylase/hydrolase (XTH)

BACKGROUND AND AIMS

Xyloglucan endotransglucosylase/hydrolase (XTH) proteins that possess xyloglucan endotransglucosylase (XET) activity contribute to cell-wall assembly and remodelling, orchestrating plant growth and development. Little is known about in-vivo XET regulation, other than at the XTH transcriptional level. Plants contain 'cold-water-extractable, heat-stable polymers' (CHPs) which are XTH-activating factors (XAFs) that desorb and thereby activate wall-bound XTHs. Because XAFs may control cell-wall modification in vivo, we have further explored their nature.

METHODS

Material was cold-water-extracted from 25 plant species; proteins were precipitated by heat-denaturation, then CHP was ethanol-precipitated. For XAF assays, CHP (or sub-fractions thereof) was applied to washed Arabidopsis thaliana cell walls, and the enzymes thus solubilized were assayed radiochemically for XET activity. In some experiments, the CHP was pre-treated with trifluoroacetic acid (TFA), alkali (NaOH) or glycanases.

KEY RESULTS

CHP specifically desorbed wall-bound XTHs, but not β-glucosidases, phosphatases or peroxidases. CHP preparations from 25 angiosperms all possessed XAF activity but had no consistent monosaccharide composition. Of 11 individual plant polymers tested, only gum arabic and tamarind xyloglucan were XAF-active, albeit less so than CHP. On gel-permeation chromatography, XAF-active cauliflower CHP eluted with a molecular weight of ~7000-140 000, although no specific sugar residue(s) co-eluted exactly with XAF activity. Cauliflower XAF activity survived cold alkali and warm dilute TFA (which break ester and glycofuranosyl linkages, respectively), but was inactivated by hot 2 m TFA (which breaks glycopyranosyl linkages). Cauliflower XAF activity was remarkably stable to diverse glycanases and glycosidases.

CONCLUSIONS

XAFs are naturally occurring heat-stable polymers that specifically desorb (thereby activating) wall-bound XTHs. Their XAF activity considerably exceeds that of gum arabic and tamarind xyloglucan, and they were not identifiable as any major plant polysaccharide. We propose that XAF is a specific, minor, plant polymer that regulates xyloglucan transglycosylation in vivo, and thus wall assembly and restructuring.

Altered cell wall properties are responsible for ammonium-reduced aluminium accumulation in rice roots

The phytotoxicity of aluminium (Al) ions can be alleviated by ammonium (NH4(+)) in rice and this effect has been attributed to the decreased Al accumulation in the roots. Here, the effects of different nitrogen forms on cell wall properties were compared in two rice cultivars differing in Al tolerance. An in vitro Al-binding assay revealed that neither NH4(+) nor NO3(-) altered the Al-binding capacity of cell walls, which were extracted from plants not previously exposed to N sources. However, cell walls extracted from NH4(+)-supplied roots displayed lower Al-binding capacity than those from NO3(-)-supplied roots when grown in non-buffered solutions. Fourier-transform infrared microspectroscopy analysis revealed that, compared with NO3(-)-supplied roots, NH4(+)-supplied roots possessed fewer Al-binding groups (-OH and COO-) and lower contents of pectin and hemicellulose. However, when grown in pH-buffered solutions, these differences in the cell wall properties were not observed. Further analysis showed that the Al-binding capacity and properties of cell walls were also altered by pHs alone. Taken together, our results indicate that the NH4(+)-reduced Al accumulation was attributed to the altered cell wall properties triggered by pH decrease due to NH4(+) uptake rather than direct competition for the cell wall binding sites between Al(3+) and NH4(+).

Vanadate inhibition of auxin-enhanced H secretion and elongation in pea epicotyls and oat coleoptiles

In experiments carried out to investigate the acid secretion theory of auxin action, we utilized sodium orthovanadate, an agent found to be a selective inhibitor of a plasma membrane-associated H(+)-pumping ATPase in Neurospora [Bowman, B. J. & Slayman, C. W. (1979) J. Biol. Chem. 245, 2928-2934]. At 1 mM, vanadate inhibited auxin-enhanced medium acidification by pea epicotyl segments within 5 min, whether added 0.5 or 2.5 hr after auxin. Inhibition of acidification was total after 10-15 min but could be reversed within 10 min after vanadate removal. When given as a 40-min pretreatment, vanadate completely prevented any auxin-enhanced acidification. Vanadate inhibition of medium acidification by oat coleoptile segments was also total and reversible, but both inhibition and reversal occurred after longer lag times than in pea. Inhibitory effects of vanadate on elongation in pea and oat tissue closely paralleled its effects on acidification, and the inhibitory effect of vanadate on elongation could be reversed by an acidic buffer. Vanadate did not inhibit respiration or protein synthesis in pea epicotyl segments, although it strongly inhibited L-[(14)C]leucine uptake. These results indicate the importance of cell wall acidification for short- and long-term auxin-enhanced growth and suggest the participation in wall acidification of a plasma membrane-associated ATPase acting as an H(+) pump.

A beta-galactosidase from pea seeds (PsBGAL): purification, stabilization, catalytic energetics, conformational heterogeneity, and its significance

A basic glycosylated beta-galactosidase (PsBGAL) has been purified from pea seeds by 910-fold with a specific activity of 77.33 mumoL min(-1) mg(-1) protein. The purified enzyme is an electrophoretically homogeneous protein consisting of a single protein band with an apparent M(r) of 55 kDa, while the deglycosylated enzyme has a M(r) of 54.2 kDa on SDS-PAGE under reducing conditions. According to MALDI-TOF measurements of the 55 kDa band, the enzyme showed a homology with BGAL from other sources present in the SWISS-PROT database, while it showed no resemblance to any lectin. The N-terminal sequence of PsBGAL was determined as TIECK and showed a resemblance to BGAL from Arabidopsis thaliana (Q93Z24). The enzyme showed an unique property of multiple banding patterns on SDS-PAGE at 20 mA current, with tryptic digests of all bands having similar m/z values (using MALDI-TOF) while it showed only a single band at 10 mA current. PsBGAL is effectively compartmentalized during seed maturation inside vacuoles (pH approximately 5). The enzyme is capable of hydrolyzing pea seed xyloglucan, and it may be involved in modifying the cell wall architecture during seedling growth and development. The enzyme has a protonated carboxyl group at its active site as observed by ionization constant, thermodynamics, and chemical modification studies.

The role of endoxyloglucan transferase in the organization of plant cell walls

The plant cell wall plays a central role in morphogenesis as well as responsiveness to environmental signals. Xyloglucans are the principal component of the plant cell wall matrix and serve as cross-links between cellulose microfibrils to form the cellulose-xyloglucan framework. Endoxyloglucan transferase (EXGT), which was isolated and characterized in 1992, is an enzyme that mediates molecular grafting reaction between xyloglucan molecules. Structural studies on cDNAs encoding EXGT and its related proteins have disclosed the ubiquitous presence in the plant kingdom of a large multigene family of xyloglucan-related proteins (XRPs). Each XRP functions as either hydrolase or transferase acting on xyloglucans and is considered to be responsible for rearrangement of the cellulose-xyloglucan framework, the processes essential for the construction, modification, and degradation of plant cell walls. Different XRP genes exhibit potentially different expression profiles with respect to tissue specificity and responsiveness to hormonal and mechanical signals. The molecular approach to individual XRP genes will open a new path for exploring the controlling mechanisms by which the plant cell wall is constructed and reformed during plant growth and development.

The relationship between xyloglucan endotransglycosylase and in-vitro cell wall extension in cucumber hypocotyls

It has been proposed that cell wall loosening during plant cell growth may be mediated by the endotransglycosylation of load-bearing polymers, specifically of xyloglucans, within the cell wall. A xyloglucan endotransglycosylase (XET) with such activity has recently been identified in several plant species. Two cell wall proteins capable of inducing the extension of plant cell walls have also recently been identified in cucumber hypocotyls. In this report we examine three questions: (1) Does XET induce the extension of isolated cell walls? (2) Do the extension-inducing proteins possess XET activity? (3) Is the activity of the extension-inducing proteins modulated by a xyloglucan nonasaccharide (Glc4-Xyl3-Gal2)? We found that the soluble proteins from growing cucumber (cucumis sativum L.) hypocotyls contained high XET activity but did not induce wall extension. Highly purified wall-protein fractions from the same tissue had high extension-inducing activity but little or no XET activity. The XET activity was higher a pH 5.5 than at pH 4.5, while extension activity showed the opposite sensitivity to pH. Reconstituted wall extension was unaffected by the presence of a xyloglucan nonasaccharide (Glc4-Xyl3-Gal2), an oligosaccharide previously shown to accelerate growth in pea stems and hypothesized to facilitate growth through an effect on XET-induced cell wall loosening. We conclude that XET activity alone is neither sufficient nor necessary for extension of isolated walls from cucumber hypocotyls.

Changes in molecular size of previously deposited and newly synthesized pea cell wall matrix polysaccharides : effects of auxin and turgor

Effects of indoleacetic acid (IAA) and of turgor changes on the apparent molecular mass (M(r)) distributions of cell wall matrix polysaccharides from etiolated pea (Pisum sativum L.) epicotyl segments were determined by gel filtration chromatography. IAA causes a two- to threefold decline in the peak M(r) of xyloglucan, relative to minus-auxin controls, to occur within 0.5 hour. IAA causes an even larger decrease in the peak M(r) concurrently biosynthesized xyloglucan, as determined by [(3)H]fucose labeling, but this effect begins only after 1 hour. In contrast, IAA does not appreciably affect the M(r) distributions of pectic polyuronides or hemicellulosic arabinose/galactose polysaccharides within 1.5 hours. However, after epicotyl segments are cut, their peak polyuronide M(r) increases and later decreases, possibly as part of a wound response. Xyloglucan also undergoes IAA-independent changes in its M(r) distribution after cutting segments. In addition, the peak M(r) of newly deposited xyloglucan increases from about 9 kilodaltons shortly after deposition to about 30 kilodaltons within 0.5 hour. This may represent a process of integration into the cell wall. A step increase in turgor causes the peak M(r) of previously deposited xyloglucan (but not of the other major polymers) to increase about 10-fold within 0.5 hour, returning to its initial value by 1.5 hours. This upshift may comprise a feedback mechanism that decreases wall extensibility when the rate of wall extension suddenly increases. IAA-induced reduction of xyloglucan M(r) might cause wall loosening that leads to cell enlargement, as has been suggested previously, but the lack of a simple relation between xyloglucan M(r) and elongation rate indicates that loosening must also involve other wall factors, one of which might be the deposition of new xyloglucan of much smaller size. Although the M(r) shifts in polyuronides may represent changes in noncovalent association, and for xyloglucan this cannot be completely excluded, xyloglucan seems to participate in a dynamic process that can both decrease and increase its chain length, possible mechanisms for which are suggested.

Purification and Characterization of Pectinmethylesterase from Ficus awkeotsang Makino Achenes

Pectinmethylesterase from the pericarp of jelly fig (Ficus awkeotsang) achenes was extracted and purified to a specific activity of 289 micromole proton produced per minute per milligram protein. Pectinmethylesterase, a major protein with high specific activity in the crude extract, was monomeric with a molecular weight of 38,000. The enzyme preparation was stable in distilled water at 4 degrees C for at least 6 months, and at 60 degrees C for at least 10 minutes. This enzyme functioned optimally at pH 6.5 to 7.5 when the assay mixture contained no NaCl or at low NaCl concentration. The pH optimum shifted to lower pH as the NaCl concentration was increased. The K(m) value for pectin was 0.75 milligram per milliliter pectin, corresponding to a V(max) value of 310 micromoles per minute per milligram protein. Inhibition studies with antibodies indicated that jelly fig achene pectinmethylesterase and the two other pectinmethylesterases from orange and tomato were similar in their active site conformation; however, the surface determinants may be very different because no precipitation between anti-jelly fig pectinmethylesterase immune serum and the pectin methylesterase from orange and tomato could be observed in the double immunodiffusion analysis. Specific antisera raised against jelly fig achene pectinmethylesterase in a Western blot experiment also showed low similarity between jelly fig pectinmethylesterase with that from orange and tomato. This observation was also supported by the very low isoelectric point (pH 3.5) of jelly fig pectinmethylesterase, compared with high isoelectric points reported for most of the pectinmethylesterases. Amino acid composition and N-terminal sequence have been obtained. High homology of the N-terminal amino acid residues between jelly fig and tomato pectinmethylesterase (O Markovic, H Jornvall [1986] Eur J Biochem 158: 455-462) was observed. Pectinmethylesterase activity causes the release of protons from the deesterification of pectin such that a low pH environment is created, and this may be related to the cell growth. Pectinmethylesterase is not needed for jelly fig seed germination, however the gel formed from pectin and pectinmethylesterase may insure a water source for the germinating jelly fig seeds.

Use of a pH-response curve for growth to predict apparent wall pH in elongating segments of maize coleoptiles and sunflower hypocotyls

To determine the relationship between apparent pH of the wall solution and shoot segment elongation, curves for the initial growth rates as a function of pH of the external solution were determined for maize (Zea mays L.) coleoptiles and sunflower (Helianthus annuus L.) hypocotyls and used to predict apparent wall pH in segments responding to indole-3-acetic acid (IAA) and fusicoccin (FC). When a solution having a pH predicted for walls of coleoptile segments responding to IAA was applied to the segments in the presence of IAA, this pH was not maintained. However, when the same was done for coleoptile segments responding to FC, the predicted pH was maintained in the external solution. Sunflower hypocotyl tissue did not maintain the external pH at the predicted value in the presence of either IAA or FC. The results indicate that wall loosening in coleoptiles caused by IAA may not be solely controlled by pH in the wall, yet growth (wall loosening) caused by FC apparently is directly related to wall pH. In sunflower the growth response to neither IAA nor FC appears to be directly correlated with wall pH.

Auxin and Fusicoccin Enhancement of beta-Glucan Synthase in Peas : An Intracellular Enzyme Activity Apparently Modulated by Proton Extrusion

Fusicoccin (FC), like indoleacetic acid (IAA), causes Golgi-localized beta-1,4-glucan synthase (GS) activity to increase when applied to pea third internode segments whose GS activity has declined after isolation from the plant. This suggests that GS activity is modulated by H(+) extrusion; in agreement, vanadate and nigericin inhibit the GS response. The GS response is not due to acidification of the cell wall. Treatment of tissue with heavy water, which in effect raises intracellular pH, mimics the IAA/FC GS response. However, various treatments that tend to raise cytoplasmic pH directly, other than IAA- or FC-induced H(+) extrusion, failed to increase GS activity, suggesting that cytoplasmic pH is not the link between H(+) extrusion and increased GS activity. Although FC stimulates H(+) extrusion more strongly than IAA does, FC enhances GS activity at most only as much as, and often somewhat less than, IAA does. This and other observations indicate that GS enhancement is probably not due to membrane hyperpolarization, stimulated sugar uptake, or changes in ATP level, but leave open the possibility that GS is controlled by H(+) transport-driven changes in intracellular concentrations of ions other than H(+).

The roles of cell-wall acidification and proton-pump stimulation in auxin-induced growth: studies using monensin

The carboxylic ionophore, monensin, rapidly induced cell-wall acidification and a decrease in cytosolic pH when added to maize coleoptiles at low external pH and Na(+) concentration. Elongation growth at rates equivalent to those obtained with indole-3-acetic acid was induced for about 1 h. Stimulation of the outwardly directed proton pump apparently occurred, since under the same conditions monensin induced membrane hyperpolarization of maize root rhizodermis cells. When the external pH was high (>8) and Na(+) present, monensin treatment caused only minimal changes in membrane potential and cytosolic pH. Although the ionophore transported protons out of the cell, resulting in cell-wall acidification, no elongation growth occurred. However, under identical conditions, indole-3-acetic acid dit induce growth. The data indicates that stimulation of the outwardly directed electrogenic proton pump rather than the subsequent acidification of the cell wall is vital for the induction of elongation growth.

Structure of the Primary Cell Walls of Suspension-Cultured Rosa glauca Cells: II. Multiple Forms of Xyloglucans

Xyloglucans, characteristic hemicellulosic polysaccharides of plant primary walls, have been isolated from Rosa glauca suspension-cultured cells. The cell wall material was fractionated by two sequences of extraction based on solubilization of the hemicelluloses in alkaline and organic solvent systems, respectively. In both cases, only a part (about 50%) of the total xyloglucan could be extracted, the rest remaining tightly associated with cellulose and necessitating the use of acid to be solubilized. Purification of xyloglucans was effected by formation of a gel in appropriate mixtures of dimethyl sulfoxide and water. Further fractionation could be achieved on a cellulose column eluted with chaotropic solvents. This demonstrated the heterogeneity of xyloglucans in the primary cell walls. Analytical data show that all fractions are constituted with the same sugars: l-arabinose, l-fucose, d-galactose, d-xylose, and d-glucose, but their relative proportions differ, particularly the ratio of glucose to xylose which varies from 1.2 to 2 within the different xyloglucans. The structure of these hemicelluloses was established by methylation analysis and shown to consist of a (1 --> 4)-linked glucan backbone which carries substituents on the O-6 of glucose. Here again, the multiple forms of xyloglucans was suggested by the various patterns of substitutions found on the different fractions. The configuration of the linkages were established by (13)C nuclear magnetic resonance spectroscopy and shown to be beta for the glucan backbone, alpha for the xylosyl and fucosyl substituents, and beta for the galactosyl substituents. These configurations agree with the specific rotation of the xyloglucan.

Ethylene-induced lateral expansion in etiolated pea stems : the role of Acid secretion

Ethylene-induced inhibition of elongation and promotion of lateral expansion in the stems of etiolated pea (Pisum sativum L. var Alaska) seedlings is not associated with any alteration of auxin-stimulated proton extrusion. Indeed, lateral expansion in response to ethylene apparently requires an acidified wall since it is prevented by strong neutral buffers and by the ATPase inhibitor orthovanadate. Ethylene treatment reduces the capacity of live and frozen-thawed sections to extend in the longitudinal direction in response to acid. The effect of ethylene on lateral acid growth capacity is more complicated. Ethylene-treated internodes do not exhibit acid-induced lateral expansion. Ethylene-treated segments which have been frozen-thawed do show an enhanced capacity to extend in the transverse direction at acid pH, but only when the inner tissues have been removed by coring. We conclude that two of the factors which control the directionality of expansion during ethylene treatment are a decrease in the sensitivity of the walls to acid longitudinally and an increase in the sensitivity of the outer cortical parenchyma walls to acid in the transverse direction.

Effect of IAA on Growth and Soluble Cell Wall Polysaccharides Centrifuged from Pine Hypocotyl Sections

Auxin-induced elongation and cell wall polysaccharide metabolism were studied in excised hypocotyl sections of ponderosa pine (Pinus ponderosa) seedlings. Sections excised from hypocotyls of ponderosa pine elongate in response to the addition of auxin. The neutral sugar composition of the extracellular solution removed from hypocotyl sections by centrifugation was examined. In cell wall solution from freshly excised sections, glucose, galactose, xylose, and arabinose make up more than 90% of the neutral sugars, while rhamnose, fucose, and mannose are relatively minor components. The neutral sugar composition of the polysaccharides of the pine cell wall solution is both qualitatively and quantitatively similar to that of pea. Following auxin treatment of pine hypocotyls, the neutral sugar composition of the cell wall changes; glucose, xylose, rhamnose, and fucose increase by nearly 2-fold relative to controls in buffer without auxin. These changes in neutral sugars in response to auxin treatment are similar to those found in pea, with the exception that in pea, rhamnose levels decline in response to auxin treatment.

Soluble Cell Wall Polysaccharides Released from Pea Stems by Centrifugation : I. EFFECT OF AUXIN

The metabolism of polysaccharides by pea stem segments treated with and without auxin was investigated using a centrifugation technique for removing solution from the free space of the cell wall. Glucose is the predominant sugar in both the ethanol-soluble and ethanol-insoluble fractions of the cell wall solution extracted with water. In the water-soluble, ethanol-insoluble polysaccharides, arabinose, xylose, galactose, and glucose make up 9.5, 23.8, 23.9, and 39.9%, respectively, of the neutral sugars, while rhamnose, fucose, and mannose are present at concentrations between 0.5 and 2.0%.Auxin treatment enhances the levels of xylose and glucose in ethanol-insoluble polysaccharides relative to controls, and this difference can be detected within 30 minutes of auxin treatment. Cellulose-binding experiments show that the enhanced levels of xylose and glucose are in a polymer having the cellulose-binding properties of xyloglucan. (3)H-glucose labeling experiments confirm the auxin-enhanced metabolism of the xyloglucan fraction; however, increased labeling of arabinose is also observed in auxin-treated sections. Auxin treatment also causes a marked increase in the level of uronic acids centrifuged from pea internode sections. Thus, after 3 hours of incubation in indoleacetic acid, the level of uronic acids in the ethanol-insoluble polysaccharides which can be recovered by centrifugation is increased 2- to 3-fold over sections incubated in water. These auxin-enhanced changes in xylose, glucose, and uronic acids are correlated with enhanced rates of section growth.Incubation of excised pea internode sections in acidic buffers also enhances the rate of xyloglucan and polyuronide metabolism. This acid-enhanced metabolism of xyloglucan and polyuronide is inhibited by low temperature, suggesting that it is enzyme-mediated.Extraction of the cell wall solution with CaCl(2) increases the yield of all neutral sugars. Arabinose and mannose are increased 4- and 3-fold, respectively, and xylose and glucose by about 20%, while galactose levels are 40% higher in cell wall solution extracted with CaCl(2) than in that extracted with water. Although calcium increases the amount of neutral sugars extracted, it does not affect the auxin-induced changes in neutral sugars. Extraction of the cell wall solution with ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'tetraacetic acid enhances the yield of uronic acids and also increases the difference due to auxin treatment.

pH-Dependent Interactions between Pea Cell Wall Polymers Possibly Involved in Wall Deposition and Growth

In an effort to detect a pH-dependent release of polymers such as xyloglucans, thought to be involved in auxin-induced cell wall expansion during growth, radioactively labeled cell walls from pea stem tissue were incubated at different pH values, and changes in water-soluble, ethanol- or trichloroacetic acid-insoluble components were determined. This revealed the occurrence, at neutral pH, of a time- and pH-dependent binding of soluble pectin, in the walls, to a heat-labile, presumably protein, wall component, yielding a trichloroacetic acid-insoluble pectin-protein complex. This reaction, which can also be observed between polymers in water extracts of cell walls, is inhibited at low pH and by Ca(2+), and appears to be of a physical, possibly lectin-like, nature. Progressive binding of pectin or of the pectin-protein complex to the insoluble wall structure is also observed. These reactions may be involved in wall assembly during its deposition, and may participate in, or be analogous to pH-dependent physical interactions that participate in, wall extension during cell growth.

Effect of salt on auxin-induced acidification and growth by pea internode sections

The capacity of excised internode sections of pea to grow and secrete protons in response to indoleacetic acid (IAA) and Ca(2+) and K(+) treatments was examined. By incubating unpeeled and unabraded sections in rapidly flowing solutions, it was shown that acidification of the external medium in the presence or absence of IAA is dependent on the presence of Ca(2+) and K(+). Similar results were obtained when unpeeled and unabraded sections were incubated in dishes with shaking. When peeled or abraded sections were incubated with shaking in IAA, H(+) release was also dependent on the presence of Ca(2+) and K(+). The release of H(+) from sections incubated in Ca(2+) and K(+) is not caused by displacement of H(+) from binding sites in the cell wall. Rather, the release of protons from sections is temperature dependent, and it is concluded that this is a metabolically linked process. Although Ca(2+) and K(+) are essential for the release of H(+) from isolated stem sections of peas, these cations do not influence elongation. Despite the large increase in proton release induced by Ca(2+) and K(+) either in the presence or absence of auxin, growth in the presence of these ions was never greater than it was in their absence. Furthermore, cations do not affect the neutral sugar or uronic acid composition of the solution which can be centrifuged from isolated sections. As is the case for growth, an increase in the neutral sugar and uronide composition of the cell wall solution is dependent only on IAA. It is concluded that IAA-induced growth of pea stem sections is independent of the secretion of protons.

Cell Wall Metabolism in Ripening Fruit: II. CHANGES IN CARBOHYDRATE-DEGRADING ENZYMES IN RIPENING ;BARTLETT' PEARS

Mature ;Bartlett' pear (Pyrus communis) fruits were ripened at 20 C. Fruits at different stages of ripeness were homogenized, and extracts of the low speed pellet (crude cell wall) were prepared. These extracts contained polygalacturonase, pectin esterase, and activity against seven p-nitrophenyl glycoside substrates. Polygalacturonase, alpha-galactosidase, and alpha-mannosidase increased in activity as the fruit ripened. Cellulase and activities against pear wall xylan and arabinan were absent from the extracts.

Auxin-induced H Secretion in Helianthus and Its Implications

We have examined the ability of Helianthus hypocotyl segments as well as segments from a variety of other species to elongate in response to H(+) and to secrete H(+) in response to auxin and fusicoccin. In all cases a positive response was obtained when the cuticular barrier was abraded with carborundum. Removal of the cuticular barrier by "peeling" prevented detection of both auxin-induced elongation and H(+) secretion. Fusicoccin-induced growth and acid secretion are not prevented by peeling. These results suggest considerable tissue selectivity with respect to auxin action but considerably less specificity with respect to fusicoccin. It seems likely that in many dicots auxin-enhanced proton secretion and elongation are controlled by the epidermis and/or closely associated cell layers. The data presented in this paper provide further support for the acid growth theory of auxin action.

Phytochrome-controlled Hydrogen Ion Excretion by Avena Coleoptiles

A red light-induced, far red reversible stimulation of proton efflux from apical segments of etiolated Avena sativa L. cv. Victory coleoptiles was observed. The acidification responses to red light and also to auxin were not the consequence of respired CO(2). The response to red light was strongly inhibited by cycloheximide and carbonyl cyanide, m-chlorophenyl hydrazone, but mannitol had a stimulatory effect. Red light and auxin applied together yielded a greater than additive response, in comparison to the effects of the two stimuli applied separately.

Osmotic Shock Inhibits Auxin-stimulated Acidification and Growth

Cells of oat coleoptiles (Avena sativa L. cv. "Garry") have been osmotically shocked in order to observe the effect of alterations of the plasma membrane on some auxin responses. When coleoptile sections were treated sequentially with 0.5 m mannitol and 1 mm Na-phosphate (pH 6.4) at 4 C, polar auxin transport and acidification by 1 mM CaCl(2) were unaffected, but auxin-stimulated acidification and growth were eliminated. Shock treatment also had no effect on acid-stimulated growth or on freezing point depression by the cytoplasm. It is suggested that osmotic shock modifies a portion of the plasma membrane which interacts with auxin and eventually leads to growth.

Rapid Auxin-induced Decrease in Free Space pH and Its Relationship to Auxin-induced Growth in Maize and Pea

A pH microelectrode has been used to investigate the auxin effect on free space pH and its correlation with auxin-stimulated elongation in segments of pea (Pisum sativum) stem and maize (Zea mays var. Bear Hybrid) coleoptile tissue. Auxin induces a decrease in free space pH in both tissues. In maize coleoptiles, free space pH begins to fall within about 12 minutes of exposure to auxin and decreases by about 1 pH unit by approximately 30 minutes. In pea, pH begins to decrease within an average of 15 to 18 minutes of exposure to auxin and falls by about 0.9 pH unit by approximately 40 minutes. Auxin-stimulated elongation, measured in the same two tissues similarly prepared, appears in maize at the earliest 18 minutes after auxin application, while in pea it appears at the earliest 21 to 24 minutes after auxin application. The auxin analogs p-chlorophenoxyisobutyric acid and phenylacetic acid do not stimulate elongation above control levels in maize or pea tissue segments and do not cause a decrease in free space pH in either tissue. These findings are consistent with the acid secretion theory of auxin action.

Some properties of carbohydrate-binding proteins (lectins) solubilized from cell walls of Phaseolus aureus

From the cell walls of nongrowing segments of mung bean hypocotyls two protein fractions can be solubilized which show carbohydrate binding properties as measured with the hemagglutination assay. The extracts show appreciable binding activity only if the walls are treated with boiled cytoplasmic supernatant prior to extraction; Mn(2+) is only partly effective in a similar way. Hydrolase activity in the extracts is strongly diminished by sequential washing of the walls with sodium dodecylsulphate; extensive washings, however, result in inactivation of the hemagglutination activity. The carbohydrate binding activity of both extracts is strongly diminished towards acidic pH-values and is inhibited by D-galactose and γ-D-galactonolactone. The possible involvement of the binding proteins in extension growth is discussed.