Low levels of oxygen promote internodal elongation in floating rice independently of enhanced ethylene production

Submergence induces rapid elongation of internodes in floating rice(Oryza sativa L. cv. Habiganj Aman II). The initial signalfor such internodal elongation has been considered to be the reduced partialpressure of oxygen in submerged internodal cavities, which promotes theelongation of internodes through the enhancement of ethylene synthesis. Weexamined the relationship between low oxygen pressure and ethylene production inthe rapid elongation of floating rice internodes using ethylene biosynthesisinhibitors, aminooxyacetic acid (AOA) and CoCl₂. When floating ricestem segments were incubated in an atmosphere of low O₂, internodalelongation accelerated and ethylene production increased. However, in stemsegments treated with AOA or CoCl₂, low O₂ stillstimulated the elongation of internodes although the ethylene production by theinternodes was less than by those in control stem segments where internodalelongation was not promoted. These results indicate that low O₂ iscapable of causing rapid elongation of internodes of floating rice independentlyof enhanced production of ethylene. In addition to low O₂,submergence, ethylene and gibberellic acid each enhanced the production ofethylene by internodal tissues, suggesting that enhanced ethylene production isa common phenomenon accompanied by the acceleration of internodal elongation infloating rice.     

Interactions between abscisic acid, ethylene and gibberellin in internodal elongation in floating rice: the promotive effect of abscisic acid at low humidity

The enhancement of internodal elongation in floating or deepwater rice (Oryza sativa L. cv. Habiganj Aman II) by treatment with ethylene or gibberellic acid (GA₃) at high relative humidity (RH) is inhibited by abscisic acid (ABA). Here, we examined the interactive effects of ethylene, gibberellin (GA) and ABA at low RH on internodal elongation of deepwater rice stem segments. Although ethylene alone hardly promoted internodal elongation of stem sections at 30% RH, it enhanced the internodal elongation induced by GA₃. Application of ABA alone to stem segments had no effect on internodal elongation. However, in the presence of ethylene and GA₃ at 30% RH, ABA further promoted internodal elongation. This promotive effect of ABA was not found in the internodes of stem segments treated either with ethylene or with GA₃ at 30% RH or in the internodes of stem segments treated with ethylene and/or GA₃ at 100% RH.     

Changes in cell wall polysaccharides in the internodes of submerged floating rice

In excised stem segments of floating rice (Oryza sativa L.), as well as in intact plants, submergence greatly stimulates the elongation of internodes. The differences in the composition of cell wall polysaccharides along the highest internodes of submerged and air-grown stem segments were examined. The newly elongated parts of internodes that had been submerged for two days contained considerably less cellulosic and noncellulosic polysaccharides than air-grown internodes, an indication that the cell walls of the newly elongated parts of submerged internodes are extremely thin. In the young parts of both air-grown and submerged internodes, the relative amounts of noncellulosic polysaccharides were equal to those of -cellulose, whereas the relative amounts of -cellulose were higher than those of noncellulosic polysaccharides in the upper, old parts. In the cell-elongation zones of both air-grown and submerged internodes, glucose was predominant among the noncellulosic neutral sugars of cell wall. The relative amount of glucose in noncellulosic neutral sugars decreased toward the upper, old parts of internodes, whereas that of xylose increased.     

Changes in the structure of xyloglucan during cell elongation

Xyloglucans were isolated by sequential extraction of the cell walls of pea (Pisum sativum L. cv. Alaska) with a xyloglucan-specific endoglucanase and KOH. The xyloglucan content and xyloglucan-oligosaccharide composition were determined for fractions obtained from the elongating and non-elongating segments of pea stems grown in the light and in darkness. The results were consistent with the hypothesis that regulated growth of the cell wall depends on xyloglucan metabolism. Furthermore, the characterization of xyloglucan extracted from leaves of light-grown pea plants indicates that xyloglucan metabolism is tissue specific. Changes in xyloglucan subunit structure observed in elongating stems are consistent with the in muro realization of a metabolic pathway that was previously proposed solely on the basis of the in vitro activities of plant glycosyl hydrolases. Received: 21 May 2000 / Accepted: 7 June 2000     

深水稻节间伸长生长的机制

淹水可促进深水稻节间快速伸长生长,其主要受内源赤霉素、乙烯、脱落酸等激素信号分子的调控。淹水能促进深水稻植物株体内乙烯、赤霉素的生物合成、抑制脱落酸的生物合成,外源乙烯、赤霉素会加速深水稻节间伸长,而外源脱落酸抑制淹水节间的伸长,其中赤霉素是直接作用因子,乙烯能降低内源脱落酸水平、增加节地赤霉素的敏感性;还与渗透调节、细胞壁组份如膨胀素等有关,淹水及赤霉素都大大增加了膨胀素基因的表达。并就深水稻的     

Gibberellin-induced elongation and osmoregulation in internodes of floating rice

Internodal elongation in floating rice ( Oryza sativa L. cv. Habiganj Aman II) is known to be enhanced by treatment with ethylene or gibberellic acid (GA₃) at high relative humidity (RH). However, ethylene-induced internodal elongation is inhibited at low RH. while GA₃-induced internodal elongation is hardly affected by humidity. We examined the possible involvement of osmoregulation in the stimulation by GA₃ of the elongation of internodes at low RH. Submergence and treatment with ethylene or GA3₃ at 100% RH increased the osmotic potential in internodes of excised stem segments, while GA₃ at 20% RH maintained the osmotic potential at a low level. In internodes of stem segments that had been treated with GA₃ at 20% RH, the activity of invertase and the level of soluble sugars were almost 2- and 1.5-fold higher, respectively, than those in internodes that had been treated with GA₃ at 100% RH. These results indicate that one of the possible mechanisms by which GA₃ promotes elongation of internodes at low RH involves the osmoregulation that is achieved by promotion of the synthesis of invertase.     

Changes in composition of the outer epidermal cell wall of pea stems during auxin-induced growth

The gross composition of the outer epidermal cell wall from third internodes of Pisum sativum L. cv. Alaska grown in dim red light, and the effect of auxin on that composition, was investigated using interference microscopy. Pea outer epidermal walls contain as much cellulose as typical secondary walls, but the proportion of pectin to hemicellulose resembles that found in primary walls. The pectin and hemicellulose fractions from epidermal peels, which are enriched for outer epidermal wall but contain internal tissue as well, are composed of a much higher percentage of glucose and glucose-related sugars than has been found previously for pea primary walls, similar to non-cellulosic carbohydrate fractions of secondary walls. The epidermal outer wall thus has a composition rather like that of secondary walls, while still being capable of elongation. Auxin induces a massive breakdown of hemicellulose in the outer epidermal wall; nearly half the hemicellulose present is lost during 4 h of growth in the absence of exogenous sugar. The percentage breakdown is much greater than has been seen previously for whole pea stems. It has been proposed that a breakdown of xyloglucan could be the basis for the mechanical loosening of the outer wall. This study provides the first evidence that such a breakdown could be occurring in the outer wall.M.S. Bret-Harte would like to thank Dr. Peter M. Ray, of Stanford University, for helpful discussions and for technical and editorial assistance, Dr. Winslow R. Briggs, of the Camegie Institude of Washington, for the use of experimental facilities and for helpful discussions, Dr. Wendy K. Silk, of the University of California, Davis, for helpful discussions and financial support, Dr. Paul B. Green for financial support, and Drs. John M. Labavitch and L.C. Greve, of the University of California, Davis, for performing the -cellulose analysis on short notice, in response to a request by an anonymous reviewer. This work was supported by a National Science Foundation Graduate Fellowship to M.S. B.-H., National Science Foundation Grant DCB8801493 to Paul B. Green, and the generosity of Wendy K. Silk (Department of Land, Air, and Water Resources, University of California, Davis) during the final writing.     

Ectopic expression of a novel OsExtensin‐like gene consistently enhances plant lodging resistance by regulating cell elongation and cell wall thickening in rice

Plant lodging resistance is an important integrative agronomic trait of grain yield and quality in crops. Although extensin proteins are tightly associated with plant cell growth and cell wall construction, little has yet been reported about their impacts on plant lodging resistance. In this study, we isolated a novel extensin‐like (OsEXTL) gene in rice, and selected transgenic rice plants that expressed OsEXTL under driven with two distinct promoters. Despite different OsEXTL expression levels, two‐promoter‐driven OsEXTL‐transgenic plants, compared to a rice cultivar and an empty vector, exhibited significantly reduced cell elongation in stem internodes, leading to relatively shorter plant heights by 7%–10%. Meanwhile, the OsEXTL‐transgenic plants showed remarkably thickened secondary cell walls with higher cellulose levels in the mature plants, resulting in significantly increased detectable mechanical strength (extension and pushing forces) in the mature transgenic plants. Due to reduced plant height and increased plant mechanical strength, the OsEXTL‐transgenic plants were detected with largely enhanced lodging resistances in 3 years field experiments, compared to those of the rice cultivar ZH11. In addition, despite relatively short plant heights, the OsEXTL‐transgenic plants maintain normal grain yields and biomass production, owing to their increased cellulose levels and thickened cell walls. Hence, this study demonstrates a largely improved lodging resistance in the OsEXTL‐transgenic rice plants, and provides insights into novel extensin functions in plant cell growth and development, cell wall network construction and wall structural remodelling.     

Modification of cell wall polysaccharides during cell elongation in Phaseolus vulgaris hypocotyls

The effect of gibberellic acid (GA) and naphthylacetic acid (NAA) on hypocotyl elongation and cell wall polysaccharides was studied using Phaseolus vulgaris seedlings grown in light condition. The hypocotyl was demarcated into two segments — one near the root was called lower and the one near the cotyledon was called upper. The upper segment showed a typical sigmoidal growth curve while lower segment did not show any growth at all. GA promoted the growth of upper segment while NAA showed clear inhibition in both the segments. Xyloglucan content showed a clear inverse correlation with growth. Pectic polysaccharides did not show a clear trend, though showed an initial inverse correlation with growth. It is concluded that degradation of low and high molecular weight xyloglucans are involved in cell wall loosening which in turn may be responsible for the elongation growth of Phaseolus hypocotyls in light.     

Changes in cellular osmotic potential and mechanical properties of cell walls during light-induced inhibition of cell elongation in maize coleoptiles

The growth rate of maize ( Zea mays L. cv. Cross Bantam T51) coleoptiles in the dark was highest at the basal zone and decreased towards the tip. Growth was strongly inhibited by white fluorescent light (5 W m⁻²), especially in the basal zone of coleoptiles. Light irradiation caused an increase in the values of stress-relaxation parameters, the minimum stress-relaxation time and the relaxation rate and a decrease in the extensibility (strain/stress) of the cell walls at all zones. In addition, during growth, the accumulation of osmotic solutes was strongly inhibited by white light irradiation, resulting in an increased osmotic potential. The influences of white light on the mechanical properties of the cell wall and the osmotic potential of the tissue sap were most prominent in the basal zone. Significant correlations were observed between the increment of coleoptile length and the mechanical properties of the cell walls or the osmotic potential of the tissue sap and osmotic solutes content. Furthermore, light inhibited the outward bending of split coleoptile segments. These facts suggest that white light inhibits elongation of maize coleoptiles by modifying both the mechanical properties of the cell walls and cellular osmotic potential, which control the rate of water uptake.     

Enhancement of transpiration by ethylene is responsible for absence of internodal elongation in floating rice at low humidity

Internodal elongation in floating rice (Oryza sativa) is known to be enhanced by treatment with ethylene or gibberellic acid (GA3) at high relative humidity (RH). However, ethylene-induced internodal elongation is inhibited at low RH, while GA3-induced internodal elongation is hardly affected by humidity. We examined the effects of ethylene and GA3 on the rate of transpiration in stem segments incubated at 30% or 100% RH. Ethylene promoted the transpiration of stem segments at 30% RH, but not at 100% RH, while GA3 had little effect on transpiration at either 30% or 100% RH. We propose that the absence of ethylene-induced internodal elongation at low RH is due, at least in part, to ethylene-induced transpiration.     

Involvement of cell wall-bound phenolic acids in decrease in cell wall susceptibility to expansins during the cessation of rapid growth in internodes of floating rice

The cell walls in the elongating zone of submerged floating rice internodes show high susceptibility to expansins. When internode sections corresponding to such an elongation zone were incubated for 24 h under osmotic stress conditions produced by treatment with 100 mM polyethylene glycol 4000 (PEG), the cell wall susceptibility to expansins remained at its initial level, while the susceptibility of internode sections incubated under unstressed conditions decreased considerably during the same period. The contents of polysaccharides and phenolic acids as ferulic, diferulic and p-coumaric acids in the cell walls of internode sections increased substantially under unstressed conditions, but the increases were almost completely prevented by osmotic stress. Ferulic acid applied to internode sections under osmotic stress reduced the susceptibility of the cell walls to expansins and increased the levels of ferulic and diferulic acids in the cell walls, with little effect on the accumulation of polysaccharides. In contrast, applied p-coumaric acid increased the level of p-coumaric acid in the cell walls without a change in the levels of ferulic and diferulic acids but did not reduce the susceptibility to expansins. These results suggest that the deposition of ferulic and diferulic acids is a primary determinant in regulating the reduction of the susceptibility of cell walls to expansins in floating rice internodes.     

PhEXPA1, a Petunia hybrida expansin,is involved in cell wall metabolism and in plant architecture specification

Expansins are wall-loosening proteins that induce wall stress relaxation and irreversible wall extension in a pH-dependent manner. Despite a substantial body of work has been performed on the characterization of many expansins genes in different plant species, the knowledge about their precise biological roles during plant development remains scarce. To yield insights into the expansion process in Petunia hybrida, PhEXPA1, an expansin gene preferentially expressed in petal limb, has been characterized. The constitutive overexpression of PhEXPA1 significantly increased expansin activity, cells size and organ dimensions. Moreover, 35S::PhEXPA1 transgenic plants exhibited an altered cell wall polymer composition and a precocious timing of axillary meristem development compared with wild-type plants. These findings supported a previous hypothesis that expansins are not merely structural proteins involved in plant cell wall metabolism but they also take part in many plant development processes. Here, to support this expansins dual role, we discuss about differential cell wall-related genes expressed in PhEXPA1 expression mutants and gradients of altered petunia branching pattern.     

Change in XET activities,cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (_w=–0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low _w. The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-_w vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.     

Changes in osmotic pressure and cell wall properties during auxin- and ethylene-induced growth of intact coleoptiles of rice

Indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid, the precursor of ethylene, stimulated elongation of coleoptiles of seedlings of intact rice ( Oryza sativa L. cv. Sasanishiki) submerged in buffer solution with constant air-bubbling. The osmotic pressure of the cell sap decreased during elongation of coleoptiles. In the presence of 30 μ M aminooxyacetic acid, an inhibitor of ethylene biosynthesis, in-dole-3-acetic acid at 30 μ M accelerated the decrease in the osmotic pressure in the early stage of growth. 1-Aminocyclopropane-1-carboxylic acid at 30 μ M did not influence the decrease in the osmotic pressure.
Both indole-3-acetic acid and 1-aminocyclopropane-1-carboxyIic acid decreased the minimum stress-relaxation time and the relaxation rate of the cell wall, suggesting that both auxin and ethylene induce elongation of rice coleoptiles by stimulating cell wall loosening. These growth regulators caused an increase in the level of glucose in hemicelluloses in the early stage of growth and a decrease in the level in the subsequent last growth phase. Indole-3-acetic acid decreased the hydroxyproline and glucosamine levels per unit dry weight of the cell wall. These changes in the level of cell wall components may be associated with the changes in the mechanical properties of the cell walls caused by auxin and ethylene.     

Changes in apoplastic peroxidase activity and cell wall composition are associated with cold‐induced morpho‐anatomical plasticity of wheat leaves

• Temperate grasses, such as wheat, become compact plants with small thick leaves after exposure to low temperature. These responses are associated with cold hardiness, but their underlying mechanisms remain largely unknown. Here we analyse the effects of low temperature on leaf morpho‐anatomical structure, cell wall composition and activity of extracellular peroxidases, which play key roles in cell elongation and cell wall thickening, in two wheat cultivars with contrasting cold‐hardening ability.
• A combined microscopy and biochemical approach was applied to study actively growing leaves of winter (ProINTA‐Pincén) and spring (Buck‐Patacón) wheat developed under constant warm (25 °C) or cool (5 °C) temperature.
• Cold‐grown plants had shorter leaves but longer inter‐stomatal epidermal cells than warm‐grown plants. They had thicker walls in metaxylem vessels and mestome sheath cells, paralleled with accumulation of wall components, predominantly hemicellulose. These effects were more pronounced in the winter cultivar (Pincén). Cold also induced a sharp decrease in apoplastic peroxidase activity within the leaf elongating zone of Pincén, and a three‐fold increase in the distal mature zone of the leaf. This was consistent with the enhanced cell length and thicker cell walls in this cultivar at 5 °C.
• The different response to low temperature of apoplastic peroxidase activity and hemicellulose between leaf zones and cultivar types suggests they might play a central role in the development of cold‐induced compact morphology and cold hardening. New insights are presented on the potential temperature‐driven role of peroxidases and hemicellulose in cell wall dynamics of grasses.

     

Phytochrome inhibits the effectiveness of gibberellins to induce cell elongation in rice

Red light controls cell elongation in seedlings of rice (Oryza sativa L.) in a far-red-reversible manner (Nick and Furuya, 1993, Plant Growth Regul. 12, 195–206). The role of gibberellins and microtubules in the transduction of this response was investigated in the rice cultivars Nihon Masari (japonica type) and Kasarath (indica type). The dose dependence of mesocotyl elongation on applied gibberellic acid (GA₃) was shifted by red light, and this shift was reversed by far-red light. In contrast, coleoptile elongation was found to be independent of exogenous GA₃. Nevertheless, it was inhibited by red light, and this inhibition was reversed by far-red light. The content of the active gibberellin species GA₁ and GA₄ was estimated by radio-immunoassay. In the mesocotyl, the gibberellin content per cell was found to increase after irradiation with red light, and this increase was far-red reversible. Conversely, the cellular gibberellin content in japonica-type coleoptiles did not exhibit any significant light response. Microtubules reoriented from transverse to longitudinal arrays in response to red light and this reorientation could be reversed by subsequent far-red light in both the coleoptile and the mesocotyl. This movement was accompanied by changes in cell-wall birefringence, indicating parallel reorientations of cellulose deposition. The data indicate that phytochrome regulates the sensitivity of the tissue towards gibberellins, that gibberellin synthesis is controlled in a negative-feedback loop dependent on gibberellin effectiveness, and that at least two hormone-triggered signal chains are linked to the cytoskeleton in rice.Abbreviations D darkness - FR far-red light - GA₃ gibberellic acid - GC-SIM gas chromatography-selected ion monitoring - R red light This work was supported by a grant of the Human Frontier Science Organization to P.N. Advice and organizational support by Prof. M. Furuya (Hitachi Advanced Research Laboratory, Hatoyama, Japan) and Prof. N. Murofushi (Department of Agricultural Chemistry, University of Tokyo, Japan) is gratefully acknowledged. Seeds of both rice cultivars were kindly provided by Dr. O. Yatou (Institute for Radiation Breeding, Hitachi-Ohmiya, Japan), and the antiGA₁ Me-antiserum for the radio-immunoassays by Dr. I. Yamaguchi (Department of Agricultural Chemistry, University of Tokyo, Japan).     

Cell wall synthesis during growth and maturation of Nitella internodal cells

An improved ¹³C-density-labeling method was used to study cell wall synthesis in rapidly expanding, slowly expanding and recently mature internodes of Nitella translucens var axillaris (A.Br.) R.D.W. As cells matured, the rate of wall synthesis slowed and the deposition of cellulose microfibrils changed from a predominantly transverse direction in the primary wall of rapidly expanding internodes to a helicoidal array in the secondary wall of mature internodes. The secondary wall was characterized by relatively higher rates of cellulose synthesis and lower rates of pectin synthesis than the primary wall. The synthesis of xyloglucan also decreased markedly at the transition to secondary wall synthesis, while the synthesis of mannose-rich hemicellulose increased. Even though structural differences were striking between the primary and secondary walls of Nitella, compositional differences between the two types of wall were quantitative rather than qualitative. The authors appreciate the assistance of Martin Yousef with the electron microscopy.