Role of Polysaccharide Synthesis in Elongation Growth and Cell Wall Loosening in Intact Rice Coleoptiles

2,6-Dichlorobenzonitrile (DCB) inhibited only increases in levelsof the cellulosic polysac-charides while monensin and galactoseinhibited increases in levels of both the cellulosic and thematrix polysaccharides in intact rice coleoptiles that weresubmerged in water. Elongation growth of rice coleoptiles wassuppressed by DCB at 10^–6 M, by monensin at 10^–7M, and by galactose at 3 ? 10^–3 M and above. Thus, thesynthesis of both the cellulosic and the matrix polysaccharidesis essential for the elongation of intact rice coleoptiles.These inhibitors increased the minimum stress-relaxation timeand the relaxation rate and they decreased the mechanical extensibilityof the cell wall, indicating that they inhibited cell wall loosening.The concentrations of the inhibitors required for inhibitionof cell wall loosening were higher than those for suppressionof elongation. The data suggest that polysaccharides synthesisplays two roles in elongation. It keeps the cell wall in a "loosened"condition by producing new extensible cell walls, while itsother role is probably related to the fixation or extensionof polymers already present in the cell wall. (Received November 15, 1990; Accepted May 23, 1991)     

植物细胞壁松弛蛋白——膨胀素

膨胀素(expansin,也称作扩张素或扩张蛋白)是一种引起植物细胞壁松弛的蛋白质,在植物细胞伸展以及一系列涉及细胞壁修饰的生命活动中起着关键作用。膨胀素由多基因家族编码,目前的研究表明膨胀素超家族由4个基因亚家族构成。膨胀素存在于不同的种属植物中,并克隆了大量的扩张蛋白基因。综述了近年来国内外有关膨胀素基因和蛋白的结构特征及作用机制等方面的研究进展。     

植物细胞壁松弛因子

植物细胞壁的松弛是细胞伸长必需的一个生理过程,发生于植物生长发育的各个阶段。研究发现参与细胞壁松弛的因子有多种,主要包括膨胀素（expansin）、木葡聚糖内转糖苷酶/水解酶（XTH）、糖基水解酶和羟基自由基（·OH）四大类。本文主要对这些细胞壁松弛因子的结构特征、作用机制及其在植物生理过程中的作用等方面的研究进展进行综述。     

Observations on the Structure of the Cell Walls of Valonia ventricosa and of Dictyosphaeria favulosa

The dissection of partially macerated piem of the cell wallof Valonia ventricosa is described. Laminae comprising a veryfew, or only one, of the component lamellae of the wall havebeen readily obtained. The occurrence of three directions offibrillar orientation is confirmed. In Dictyosphaeria favulosa the cell walls show three equallywell developed directions of fibrillar orientation, one forminga set of great circle meridians about the cell, so definingtwo diametrically opposite ‘poles’, and the otherstwo series of spirals, right- and left-handed, lying obliquelyto the meridians at approximately equal angles in the regionof 70°–80°. Single lamellae of Valonia walls show only one direction offibril orientation, which is parallel to the major extinctiondirection when viewed between crossed Nicol prisms. When stretchedtransversely to the fibrils these isolated lamellae open outinto networks in a characteristic fashion, indicating the presenceof some system of lateral linkages between the strands. The strands of such networks are shown by electron microscopyto be composed of aggregates of microfibrils of indefinite lengthand of diameter apparently about 100 Å. This is much lessthan the diameter (300 Å.) of similar fundamental microfibrilspreviously reported; the difference may arise from the differentmethode of preparation of the material. Electron-micrographsdo not reveal any structural basis for the lateral linkagesbetween fibrils. The mechanics of the growth of these multi-lamellate walls isbriefly considered.     

Electrical Properties of the Plasmalemma and Tonoplast in Valonia ventricosa

Studies were made on the electric potentials of the plasmalemma (E_co) and tonoplast (E_vc) in small cells (1-3 mm diameter) of Valonia ventricosa. To measure E_co, microelectrodes with long tapers were inserted into the vacuole with the path of electrode entry off-center. The microelectrode then was pushed across the vacuole and into the cytoplasm on the opposite side of the cell. A reference electrode was placed in the artificial seawater bathing the cell. A similar method was used to measure E_vc except that the reference electrode was placed in the vacuole.     

Light-Activated H+ Transport into the Vacuole of Valonia ventricosa

Using glass capillary microelectrodes for the measurement ofpotential differences (PD) and antimony microelectrodes forthe measurement of pH, we investigated the light-induced changesof PD between the central vacuole and the external medium, ofpH in the vacuole (pH_v), as well as of pH in the external medium(pH_o) of the green marine alga Valonia ventricosa. PD in thedark was about +30 to +40 mV (vacuole positive), pH_v 6.3, andthe resistance of the protoplast (cell wall-plasmalemma-tonoplast)17.8 kOhm cm². Illumination caused an increase of the positivePD (after a few oscillations) up to +80 to +100 mV, acidificationof the vacuolar sap, alkalinization of the external medium,and a decrease in the resistance of the protoplast to 7.6 kOhmcm². The kinetics of the changes of PD, pH_v, and pH_o were similarto each other. It is concluded that a light-stimulated activeH⁺ flow occurs from the external medium into the central vacuoleof Valonia ventricosa as a result of the onset of photosyntheticactivity.     

Flux Studies and Compartmentation Analysis of Gibberellin A1 in Oat Coleoptiles

Both uptake and loss of GA₁ consist of two distinct transportcomponents. The distribution of GA₁ within oat coleoptile cellsmay thus be in at least two pools, which might arise from compartmentation,localization, or binding of GA₁. There is a net uptake of GA₁,supplied at physiological levels (1.16 x 10^–7 M), intothe slowly exchanging pool and an immobile metabolite of GA₁(probably GA₈) is produced. This is treated as a third poolwithin the cells.     

Turgor Pressure Regulation in Valonia utricularis: Effect of Cell Wall Elasticity and Auxin

The electrical membrane resistance rho(0) of the marine alga Valonia utricularis shows a marked maximum in dependence on the turgor pressure. The critical pressure, P(c), at which the maximum occurs, as well as its absolute value, rho(0) (max), are strongly volume-dependent. Both P(c) and rho(0) (max), increase with decreasing cell volume. It seems likely, that these relationships reflect the elastic properties of the cell wall, because the volumetric elastic modulus, epsilon, is also volume-dependent, increasing hyperbolically with cell volume. Both P(c) and rho(0) (max) can be affected by external application of indole-3-acetic acid at concentrations of 2.10(-7)m to 2 .10(-5)m. The critical pressure is shifted by 1.2 to 6 bars toward higher pressures and the maximum membrane resistance increased up to 5.6-fold. During the course of the experiments (up to 4 hours), however, IAA had no effect on the volumetric elastic modulus, epsilon.The maximum in membrane resistance is discussed in terms of a pressure-dependent change of potassium fluxes. The volume dependence of P(c) and rho(0) (max) suggests that not only turgor pressure but also epsilon must be considered as a regulating parameter during turgor pressure regulation. On this basis a hypothesis is presented for the transformation of both, a pressure signal and of changes in the elastic properties of the cell wall into alterations of ion fluxes. It is assumed that the combined effects of tension and compression of the membranes as well as the interaction between membrane and cell wall opposingly change the number of transport sites for K(+) providing a turgor-sensing mechanism that regulates ion fluxes. The IAA effects demonstrated are consistent with this view, suggesting that the basic mechanisms for turgor pressure regulation and growth regulation are similar.Any relation connecting growth rate with turgor pressure should be governed by two parameters, i.e. by a yielding pressure, at which cell growth starts, and by the critical pressure, at which it ceases again.     

The Effect of Indoleacetic Acid on Cell Wall Extensibility in Avena Coleoptiles

The extensibility of cell walls in segments of living Avenacoleoptiles has been determined after treatment with IAA bythe use of a microextensometer. It has been demonstrated thatIAA has an effect both on the plastic and elastic extensibility,which is more marked if the segments are allowed to grow, exceptat temperatures nearing 0° C. The auxin-induced increasein extensibility is therefore not a direct effect on the walland metabolic factors must be involved. The results are interpreted,along cunent lines of thought, as indicating effects on thelinkage between protopectin or hemicellulose chains themselvesand between these and the microfibrillar network of cellulose.The bearing of these interpretations on the normal growth processin cell walls is briefly discussed.     

小麦黄化胚芽鞘的细胞壁钙调素和钙调素结合蛋白

小麦黄化胚芽鞘经苯基琼脂糖亲和层析提取和纯化,其细胞壁CaM在有钙和缺钙时SDS电泳呈现不同的迁移率;依赖Ca~(2 )与苯基疏水结合;在紫外吸收光谱上具有五个特征峰;对PDE的激活剂量反应曲线和从非活性状态向活性状态转变时所需的Ca~(2 )浓度均和胞内CaM相同,说明细胞壁CaM和胞内CaM具有相同的基本理化特性。采用CaM琼脂糖亲和层析,发现在小麦细胞壁中存在CaM结合蛋白,其中以分子量为40.7 kD的 CaM结合多肽为主。细胞壁CaM结合蛋白不具有过氧化物酶、ATP酶或酸性磷酸酯酶的活性。     

Transcellular delta -mu H+ in Valonia ventricosa and its effect on delayed fluorescence

The possible effect of the illumination-induced transcellular H+-gradient between the central vacuole and the external medium, on both the intensity and the kinetics of delayed fluorescence was studied by measuring both the membrane potentials and H+ fluxes across the plasmatic membranes, and the millisecond component of delayed fluorescence in single cells of the marine alga Valonia ventricosa. The kinetics of the formation of transcellular delta -mu H+ was shown to correlate with the induction kinetics of the millisecond component of delayed fluorescence. Disturbances of transcellular delta -mu H+ by electrical breakdown of the cell or by acidification of the external medium resulted in a decrease of the steady-state level of the light emission and in a decline of the minute oscillations observed in intact cells. The possible involvement of the membrane connections between the chloroplasts and the plasmatic membranes in the formation of transcellular delta-mu H+ is discussed.     

Role of Expansin in Cell Enlargement of Oat Coleoptiles (Analysis of Developmental Gradients and Photocontrol)

Expansins are wall proteins that mediate a type of acid-induced extension in isolated plant cell walls (S. McQueen-Mason, D.M. Durachko, D.J. Cosgrove [1992] Plant Cell 4: 1425-1433). To assess the role of these proteins in the process of cell enlargement in living tissues, we compared the spatial and temporal growth patterns of oat (Avena sativa L.) coleoptiles with four wall properties related to expansin action. These properties were (a) the ability of isolated walls and living segments to extend in acidic buffer, (b) the ability of heat-inactivated walls to extend upon application of expansins, (c) the amount of immunologically detectable expansin in wall protein extracts, and (d) the extractable expansin activity of walls. Growth rate was maximal in the apical half of dark-grown coleoptiles and negligible in the basal region. This growth pattern correlated with properties a and b; in contrast, the amount and activity of extractable expansin (properties c and d) were reduced only in the most basal region. Upon exposure to white light, coleoptiles abruptly ceased elongation at 8 to 10 h after start of irradiation, and this cessation correlated with reductions in properties a to c. The growth cessation at 8 to 10 h also coincided with the loss of growth response to exogenous auxin and fusicoccin in excised coleoptile segments. These results lend correlative support to the hypothesis that expansin action is important for growth responses of living oat coleoptiles (e.g. responses to acidic buffers, auxin, fusicoccin, aging, and light). Our results suggest that changes in the susceptibility of the wall to expansin action, rather than changes in expansin activity, may be a key determinant of the growth patterns in oat coleoptiles.     

Possible Role of Hexosamine-Containing Cell Wall Component in Growth Regulation of Rice Coleoptiles

The cell wall of rice coleoptile was found to contain severalhundred microgram hexosamine per gram dry wt with the pectic,hemicellulosic, and -cellulose fractions containing 50%, 40%,and 10%, respectively. The cell wall hexosamine content increasedseveralfold with coleoptile growth and was higher in air-typecoleoptiles (grown on the surface of water) than water-typeones (grown under water). Rice coleoptiles were cultured in glucosamine, NH₄⁺, glutamine,or asparagine solution and growth was inhibited at 10^–4M and above. Coleoptile growth capacity in glucosamine or NH₄⁺solution correlated inversely with the cell wall hexosaminecontent. Both of these solutions also inhibited elongation ofsubmerged air-type coleoptile sections. Azaserine promoted thegrowth of both intact and excised coleoptiles at 10^–6to 10^–5 M and halved the cell wall hexosamine contentof intact ones. 6-Diazo-5-oxo-L-norleucine promoted the elongationof sections. These results suggest that the hexosamine-containingcell wall component is an important growth suppression factorin rice coleoptiles. (Received April 25, 1983; Accepted August 30, 1983)     

Physiological Evidence for the Accumulation of Restrained Wall Loosening Potential on the Growth-Inhibited Side of Graviresponding Rye Coleoptiles1

Abstract: Subsequent to inversion, horizontally pre-gravistim-ulated coleoptiles exhibit an increased gravicurvature capacity as compared to vertically pre-gravistimulated and subsequently horizontally gravistimulated coleoptiles. This indicates that gravistimulated inhibition of growth of the upper organ flank during gravicurvature is mediated wo the retention of wall loos ening potential. After inversion, this potential contributes to en hanced cell extension on the then physiologically lower side.