Cortical microtubules and microfibril deposition in the cell wall of root hairs ofEquisetum hyemale

Summary The cell wall of root hairs ofEquisetum hyemale is shown to be composed of three different cell wall textures. The growing cell wall at the tip of the hair is composed of a dispersed texture of microfibrils, which continues along the outside of the whole hair. With increasing distance from the tip an increasing number of helicoidally arranged lamellae is deposited. These findings correspond with the observed isotropism of young hairs in polarized light.Hairs of approximately 4 days old become positive birefringent, indicating that longitudinally oriented layers prevail over layers with a transverse direction. This phenomenon starts at the base of the hair. Full-grown hairs are positive birefringent up to the tip and concordantly show a thick additional inner cell wall layer which forms a helical pattern the length of the hair, with a mean microfibril angle of 25 with the cell axis.Cortical microtubules, subjacent to the dispersed, the helicoidal and the helical wall texture are axially aligned and, thus, not in coalignment with the last deposited microfibrils.Coated and smooth vesicles are present in the cortical cytoplasm of both growing and full-grown hairs. Electron-dense profiles (20 nm in diameter), surrounded by a halo (of 50 nm) were observed on the wall-plasmalemma interface in full-grown hairs only. A relation of these structures with microfibril deposition could not be demonstrated. They might represent channels transporting material to the wall, which, in full-grown hairs, is heavily impregnated with a tawny brown substance.The general hypothesis that cortical microtubule orientation directs microfibril deposition is disputed.     

Secretion of cell wall polysaccharides in Vicia root hairs

Root hairs of hairy winter vetch ( Vicia villosa Roth) synthesize and secrete abundant cell wall matrix polysaccharides, making this an excellent system for the study of secretion during tip growth. Roots with newly formed hairs were preserved by cryofixation and freeze substitution. Cryofixed root hairs showed excellent structural and antigenic preservation. Ultrastructural analyses showed numerous vesicles near the tip and a concentration of Golgi bodies in the subapical region of the hair. The distribution of polygalacturonic acid and xyloglucan in the endomembrane system and cell wall were revealed by immunolabeling, using previously characterized monoclonal antibodies. De-esterified polygalacturonic acid was present on the external surface of the cell wall, but was not detected within the cell, although chemical de-esterification revealed abundant antigen in Golgi bodies and secretory vesicles. Methyl-esterified polygalacturonic acid epitopes were detected within the medial and trans cisternae of Golgi bodies, in secretory vesicles, and throughout the wall, indicating that pectin is secreted in a neutral form and may then be de-esterified in muro . Xyloglucan was also detected within the trans cisternae of Golgi bodies, secretory vesicles and throughout the cell wall. Double labeling experiments demonstrated that both polysaccharides occur simultaneously in the same Golgi bodies, and that secretory vesicles containing both polygalacturonic acid and xyloglucan deliver the polysaccharides to the cell wall at the growing tip.     

A helicoidal cell wall texture in root hairs ofLimnobium stoloniferum

Summary The cell wall of root hairs ofLimnobium stoloniferum is composed of two fibrillar layers: an outer layer with a dispersed texture and an inner layer with a helicoidal texture. In stained oblique sections the helicoidal layer appears as a series of bow-shaped structures. In sections which were shadow-casted after the embedding medium was removed, the following properties of the helicoidal layer can be directly observed. (1) It is build up of superimposed lamellae. (2) Each lamella consists of parallel oriented microfibrils. (3) Going into the helicoidal layer, there is a counter-clockwise discontinuous rotation of the microfibril orientation in successive lamellae. (4) Between adjacent lamellae the average angular displacement of the microfibril orientation is about 23 degrees. The dispersed outer layer is also polylamellated, but with randomly arranged microfibrils in each lamella. Both layers are present in the lateral wall as well as in the apical wall of the root hairs. Observations indicate that in the cell wall of the tip the parallel oriented microfibrils of the outermost helicoidal lamellae become distorted towards a dispersed arrangement. The suggestion is made that the dispersed outer layer is derived from the helicoidal layer.     

Changes in growth,osmotic potential and cell permeability of wheat cultivars under salt stress

10-d-old wheat seedlings were grown hydroponically in presence and absence of 100 mM NaCl for 7 d. Salt stress decreased growth of shoots and roots of both cultivars; fresh mass of sensitive cultivar being more affected. NaCl increased membrane permeability to urea, methylurea and ethylurea and decreased membrane partiality in root cortex cells of sensitive cultivar. Neither parameter changed by NaCl in resistant cultivar. NaCl treatment decreased water permeability and osmotic potential in both cultivars; sensitive cultivar was more affected. The results extends our previous data that cell membrane properties are different in salt sensitive and resistant genotypes and so cell permeability could be a potential trait indicating salt tolerance.     

Suppression of cell wall stiffening along coleoptiles of wheat (Triticum aestivum L.) seedlings grown under osmotic stress conditions

Effects of polyethylene glycol (PEG)-induced osmotic stress on the mechanical properties of cell walls and the levels of their components were investigated along intact wheat (Triticum aestivum L.) coleoptiles. Stress-relaxation analysis showed that the cell walls of stressed coleoptiles were loosened as compared with those of unstressed ones not only in the apical but in the basal regions. The amounts of wall-bound ferulic acid (FA) and diferulic acid (DFA) of stressed coleoptiles were substantially lower than those of unstressed ones in all regions. The cellulose and hemicellulose contents increased toward the coleoptile base. Osmotic stress reduced the cellulose content in the basal region but it slightly affected the hemicellulose content. The molecular weight of hemicellulose in the apical region of stressed coleoptiles was higher than that of unstressed ones, while that in the basal region was almost the same in both coleoptiles. FA, DFA and cellulose contents correlated with the cell wall mechanical property. The amount and molecular weight of hemicellulose, however, did not correlate. These results suggest that the reduced levels of FA and DFA in all regions and cellulose in the basal region of wheat coleoptiles are involved in maintaining the cell wall extensibility under osmotic stress.     

Hydrogen sulfide promotes wheat seed germination under osmotic stress

Effects of NaHS, H₂S donor, on germination and antioxidant metabolism in wheat (Triticum aestivum L.) seeds under osmotic stress were investigated. With the enhancement of osmotic stress, which was mimicked by PEG-6000, the seed germination dropped gradually. NaHS treatment could promote wheat seed germination against osmotic stress in a dose-dependent manner; while Na⁺ and other sulfur-containing components, such as S²⁻, SO₄²⁻, SO₃²⁻, HSO₄⁻ and HSO₃⁻, were not able to improve seed germination as NaHS did, confirming H₂S or HS⁻ derived from NaHS contribute to the protective roles. Further experiments showed that NaHS treatment combined with PEG enhanced the activities of amylase and esterase in comparison to PEG treatment alone. Alternatively, NaHS treatment significantly reduced malondialdehyde and hydrogen peroxide accumulation in seeds. Significant enhancement of catalase and ascorbate peroxidase activities and decrease in lipoxygenase activity were observed in NaHS treated seeds, while peroxidase and superoxide dismutase activities were not affected as compared with the control. Furthermore, the H₂S donor treatment could retain higher levels of endogenous H₂S in wheat seeds under osmotic stress. These data indicated that H₂S played a protective role in wheat seed against osmotic stress.     

Improved germination under osmotic stress of tobacco plants overexpressing a cell wall peroxidase.

The cell wall is a fundamental component in the response of plants to environmental changes. To directly assess the role of the cell wall we have increased the expression and activity of a cell wall associated peroxidase (TPX2), an enzyme involved in modifying cell wall architecture. Overexpression of TPX2 had no effect on wild-type development, but greatly increased the germination rate under high salt or osmotic stress. Differential scanning calorimetry showed that transgenic seeds were able to retain more water available for germination than wild-type seeds. Thermoporometry calculations indicated that this could be due to a lower mean pore size in the walls of transgenic seeds. Therefore, the higher capacity of transgenic seeds in retaining water could result in higher germination rates in conditions where the availability of water is restricted.     

Water stress and cell wall polysaccharides in the apical root zone of wheat cultivars varying in drought tolerance

Glycosyl composition and linkage analysis of cell wall polysaccharides were examined in apical root zones excised from water-stressed and unstressed wheat seedlings (Triticum durum Desf.) cv. Capeiti ("drought-tolerant") and cv. Creso ("drought sensitive"). Wall polysaccharides were sequentially solubilized to obtain three fractions: CDTA+Na(2)CO(3) extract, KOH extract and the insoluble residue (alpha-cellulose). A comparison between the two genotypes showed only small variations in the percentages of matrix polysaccharides (CDTA+Na(2)CO(3) plus KOH extract) and of the insoluble residues (alpha-cellulose) in water-stressed and unstressed conditions. Xylosyl, glucosyl and arabinosyl residues represented more than 90mol% of the matrix polysaccharides. The linkage analysis of matrix polysaccharides showed high levels of xyloglucans (23-39mol%), and arabinoxylans (38-48mol%) and a low amount of pectins and (1-->3), (1-->4)-beta-d-glucans. The high level of xyloglucans was supported by the release of the diagnostic disaccharide isoprimeverose after Driselase digestion of KOH-extracted polysaccharides. In the "drought-tolerant" cv. Capeiti the mol% of side chains of rhamnogalacturonan I and II significantly increased in response to water stress, whereas in cv. Creso, this increase did not occur. The results support a role of the pectic side chains during water stress response in a drought-tolerant wheat cultivar.     

The cooperative relation between non-hydraulic root signals and osmotic adjustment under water stress improves grain formation for spring wheat varieties

Non-hydraulic root signals (nHRS) and osmotic adjustment (OA) are two important adaptive responses of plants to water stress. There is little understanding of their relationships during water stress. The threshold range of soil water potential to occurrence of nHRS, the capacity for OA, grain yield and water use efficiency (WUE) were examined in three spring wheat ( Triticum aestivum L.) varieties (two bred after 1975 and one bred before 1900) under water stress conditions. The threshold range of nHRS was significantly correlated with the maintenance rate of grain yield (MRGY) ( r  = 0.99, P  < 0.05) under moderate drought (−0.49 to −0.55 MPa) but not under severe drought (−0.70 to −0.76 MPa). There were similar correlations between OA and the MRGY. However, regulation of nHRS precedes OA during gradual water stress. The threshold range of nHRS and OA was positively correlated ( r  = 0.93, P  < 0.05), suggesting a mechanism for adapting to drought. WUE was higher for modern than for old varieties and was correlated with the root efficiency (full biomass weight including root per root weight, r  = 0.78, P  < 0.05) and the root water uptake efficiency (water consumption per root weight, r  = 0.72, P  < 0.05). However, there was a significant negative correlation between WUE and root weight ( r  = –0.84, P  < 0.01). The cooperative relationship between the threshold range of nHRS and OA under water stress was beneficial for improving grain formation for spring wheat varieties.     

Jasmonates modulate the promotion effects induced by SNP on root development of wheat under osmotic stress through lipoxygenase activation

We investigated promotion effects of exogenous sodium nitroprusside (SNP) on wheat seedling (Triticum aestivum L.) lateral root (LR) and root hair development, and the relationship between endogenous jasmonate (JA) production and activity changes of lipoxygenase (LOX) isoenzymes under osmotic stress generated by 15 % PEG-6000. Our results showed that 25 or 50 μM SNP could significantly increase LR length and number whether or not the seedlings were under PEG stress. When 50 μM cPTIO, 50 μM SHAM or 50 μM NDGA was supplemented, the promotion effects of SNP were blocked. SNP could also induce the production of endogenous JAs in roots, and 25 μM SNP induced the maximum JA content. The effect of SNP on JA production could also be blocked by adding cPTIO, SHAM or NDGA. Furthermore, the activity of lipoxygenase (LOX) in roots was affected by SNP; the maximal activity of LOX also occurred in the roots treated by 25 μM SNP under PEG stress, or 50 μM SNP without PEG stress. LOX isoenzymes in roots were detected by electrophoresis; the results showed that 25 μM SNP could noticeably increase the activities of LOXII and LOXIII under PEG stress. Our results suggest that, under osmotic stress generated by PEG, the promotion effects of exogenous SNP on wheat LR and root hair development could be mediated by endogenous JAs through LOX activation.     

渗透胁迫条件下小麦成熟胚愈伤组织的诱导及植株再生的研究

用不同浓度的PEG6000及NaCl对5个小麦品种的成熟胚组织培养物进行处理,研究在渗透胁迫条件下基因型和激素对成熟胚愈伤组织的诱导及分化的影响。结果表明,小麦整株水平与细胞水平的抗性存在一定相关,不同基因型对干旱与盐胁迫的敏感程度不同,成熟胚愈伤组织的诱导率和植株再生率表现出明显的差异。初步得到了晋麦47、长武134、红芒麦的耐旱愈伤组织以及晋麦47、长武134的耐盐愈伤组织,并获得了晋麦47和长武134具有一定抗性的再生芽。     

Microtubule dynamics is required for root elongation growth under osmotic stress in Arabidopsis

Osmotic stress caused by drought and soil salinity is one of the factors that affect plant root system growth and development. Previous studies have shown that microtubule plays a critical role in plant roots response to osmotic stress, however, the underlying mechanism remains unclear. In the present study, the microtubule orientations in Arabidopsis roots growing under osmotic stress were determined using confocal fluorescence microscopy. The results showed that osmotic stress could significantly inhibit primary root elongation in Arabidopsis, and pharmacological tests confirmed that microtubules were involved in Arabidopsis roots response to osmotic stress. In vivo visualization of microtubule structures with the microtubule-binding domain–green fluorescent protein (GFP) reporter revealed altered microtubule orientation in rhizodermal cells under osmotic stress. These results above indicated that osmotic stress could inhibit the elongation growth of Arabidopsis primary root, and the inhibition effects might result from the changes in microtubule orientation.     

The genetic basis of durum wheat germination and seedling growth under osmotic stress

Durum wheat (Triticum turgidum L. var. durum) is mainly produced under rainfed but often sub-optimal moisture conditions in the Mediterranean basin. A set of 114 durum wheat recombinant inbred lines (RILs) developed from the cross of cultivars Omrabi5 × Belikh2 were tested for the ability to tolerate moisture deficiency at the germination and early seedling growth stage. The stress was imposed by exposing the germinating grain to 12 % polyethylene glycol. It induced a measurable reduction in root length, shoot length, and the percentage of normal seedlings. The germination and seedling growth of Belikh2 were more strongly inhibited than those of Omrabi5, and both parents were outperformed by > 50 % of the RILs. A quantitative trait locus (QTL) analysis was carried out by first assembling a linkage map from 265 informative microsatellites. Composite interval mapping revealed nine QTL spread over seven chromosomes. Five of these were associated with coleoptile length, and one of the five explained nearly 29 % of the relevant phenotypic variance. The coleoptile length was significantly correlated with the seedling growth, plant height, and thousand kernel mass derived from field-grown plants of the same RIL population.     

Photosynthesis under osmotic stress

The reversibility of the inhibition of photosynthetic reactions by water stress was examined with four systems of increasing complexity—stromal enzymes, intact chloroplasts, mesophyll protoplasts, and leaf slices. The inhibition of soluble chloroplast enzymes by high solute concentrations was instantly relieved when solutes were properly diluted. In contrast, photosynthesis was not restored but actually more inhibited when isolated chloroplasts exposed to hypertonic stress were transferred to conditions optimal for photosynthesis of unstressed chloroplasts. Upon transfer, chloroplast volumes increased beyond the volumes of unstressed chloroplasts, and partial envelope rupture occurred. In protoplasts and leaf slices, considerable and rapid, but incomplete restoration of photosynthesis was observed during transfer from hypertonic to isotonic conditions. Chloroplast envelopes did not rupture in situ during water uptake. It is concluded that inhibition of photosynthesis by severe water stress is at the biochemical level brought about in part by reversible inhibition of chloroplast enzymes and in part by membrane damage which requires repair mechanisms for reversibility. Both soluble enzymes and membranes appear to be affected by the increased concentration of internal solutes, which is caused by dehydration.     

DCPTA对盐胁迫下玉米苗期根系生长、渗透调节及膜透性的影响

黑龙江省西部地区盐碱土面积约有66.7×104hm2,制约着黑龙江省粮食生产水平的进一步提高。叔胺类活性物质2-(3,4-二氯苯氧基)-乙基-二乙胺(DCPTA)具有提高作物抗逆性作用,为探讨DCPTA提高玉米耐盐性机制,采用营养液水培试验,在前期试验基础之上,选取"先玉335"(耐盐性强)和"丰禾1号"(耐盐性弱)2个品种,研究DCPTA(15mg·L-1)对Na Cl胁迫下(150 mmol·L-1)玉米幼苗根系生长、渗透调节及质膜透性的影响。结果表明:DCPTA能增加盐胁迫下幼苗的根长、根表面积、根体积、根鲜重和根干重,缓解盐胁迫对根系生长的抑制;DCPTA维持了根系水分平衡,提高了盐胁迫下幼苗根系相对含水量,"先玉335"和"丰禾1号"分别提高3.6%和6.4%;DCPTA通过提高根系可溶性糖含量和可溶性蛋白含量,降低幼苗根系脯氨酸含量,增强了盐胁迫下玉米幼苗根系的渗透调节能力,并通过降低根系丙二醛(MDA)含量和相对电导率,保护了根系质膜结构和功能;与盐胁迫相比,"先玉335"丙二醛含量和相对电导率分别降低21.6%和24.2%,"丰禾1号"分别降低28.1%和30.4%;DCPTA缓解盐胁迫对根系伤害的效果表现为"丰禾1号""先玉335"。     

Rapid deposition of wheat cell wall structural proteins in response to Fusarium-derived elicitors

Two novel cell wall structural proteins of spring wheat (Triticum aestivum L. em Thell.). undergo rapid deposition in the cell wall matrix in a H(2)O(2)-dependent reaction after the elicitation of cultures with Fusarium graminearum (L.)-derived elicitor. The amino acid compositions of these proteins were remarkably similar and indicated that they were highly acidic (pI 3.8). These proteins contained 13--17% each of Gly, Glx and Ser with lesser amounts (6--8%) of Ala, Asx and Thr, and it has been suggested that they are known as glycine- and serine-rich proteins (GSRPs). SELDI-MS ionization spectra demonstrated that these proteins have low molecular masses of 8590 and 4292 Da. These results are discussed in relation to the possible role of these novel proteins in rapid, cell wall defensive reactions to pathogenic attack.     

Molecular mapping of genomic regions associated with wheat seedling growth under osmotic stress

A quantitative trait loci (QTL) approach was applied to dissect the genetic control of the common wheat seedling response to osmotic stress. A set of 114 recombinant inbred lines was subjected to osmotic stress from the onset of germination to the 8^th day of seedling development, induced by the presence of 12 % polyethylene glycol. Root, coleoptile and shoot length, and root/shoot length ratio were compared under stress and control conditions. In all, 35 QTL mapping to ten chromosomes, were identified. Sixteen QTL were detected in controls, 17 under stressed conditions, and two tolerance index QTL were determined. The majority of the QTL were not stress-specific. In regions on five chromosome arms (1AS, 1BL, 2DS, 5BL and 6BL) the QTL identified under stress co-mapped with QTL affecting the same trait in controls, and these were classified as seedling vigour QTL, in addition to those expressed in controls. Tolerance-related QTL were detected on four chromosome arms. A broad region on chromosome 1AL, including five QTL, with a major impact of the gene Glu-A1 (LOD 3.93) and marker locus Xksuh9d (LOD 2.91), positively affected root length under stress and tolerance index for root length, respectively. A major QTL (LOD 3.60), associated with marker locus Xcdo456a (distal part of chromosome arm 2BS) determined a tolerance index for shoot length. Three minor QTL (LOD < 3.0) for root length and root/shoot length ratio under osmotic stress were identified in the distal parts of chromosome arms 6DL (marker locus Xksud27a) and 7DL (marker locus Xksue3b). Selecting for the favourable alleles at marker loci associated with the detected QTL for growth traits may represent an efficient approach to enhance the plants’ ability to maintain the growth of roots, coleoptile and shoots in drought-prone soils at the critical early developmental stages.     

不同CO2浓度下渗透胁迫对小麦膜伤害的影响

研究了常规CO