Effects of Water Stress and Abscisic Acid on Transpiration Regulation in Wheat

The transpiration response to recurrent light periods was studied'n water-stressed wheat seedlings. Seedlings were stressed by three methods: addition of mannitol to the root medium, root cooling and drving of the roots in air. All three methods induced almost equal effects on transpiration regulation during alternating dark and light intervals. Exogenous abscisic acid supplied to the shoots of excised plants had qualitatively the same effect as water stress. Water stress and ABA increased the time lapse between light-on and the onset of transpiration increase and lowered the amplitude of transpiration increase in light. Weak light introduced before strong light shortened the delay times.     

Metabolism of Abscisic Acid and Its Regulation in Xanthium Leaves during and after Water Stress

Metabolism of abscisic acid was compared in stressed and in rehydrated leaf blades of Xanthium strumarium L. Chicago strain that were either detached or left intact on the plant. Under all conditions, phaseic acid was the major metabolite. The high level of phaseic acid that was observed in intact plants 1 day after recovery from stress declined slowly and had not yet reached the prestress level 1 week later. The glucosyl ester of abscisic acid, β-d-glucopyranosyl abscisate, accumulated at a low rate during periods of prolonged stress. Repeated stress-recovery cycles resulted in a gradual increase in the level of the glucosyl ester, which did not decline following relief of stress for at least 34 days. The level of the glucosyl ester of abscisic acid may therefore serve as a cumulative indicator of the water stresses to which a particular leaf has been exposed.     

Effect of Osmotic Stress on Abscisic Acid Efflux and Compartmentation in the Roots of Two Maize Lines Differing in Drought Susceptibility

Roots of two Zea mays L. lines (drought-resistant Polj 17, and drought-susceptible F-2) were exposed to osmotic stress induced by sorbitol (osmotic potential –1.0 MPa). The following parameters were determined in cortex cells: membrane permeability for abscisic acid (ABA), ABA fluxes across membranes, pH values and ABA content in cytoplasm and vacuole. Osmotic stress induced different distribution of ABA within cell compartments in the investigated lines. ABA transport in the F-2 line occurred according to the intracellular pH gradient and the anion trap concept. In Polj 17, however, osmotic stress did not cause any significant effect on pH gradient and compartmental ABA content, but had a stimulating effect on ABA efflux from cytoplasm to apoplast and than via xylem to the leaf. These findings indicate different mechanisms of ABA transport between the investigated lines in response to osmotic stress.     

Regulation of Soybean Embryogenesis by Abscisic Acid

Abscisic Acid (ABA) stimulates growth and protein accumulationin soybean (Glycine max. L. Merr.) embryos during the earlyphases of embryogenesis. Growth of mid-stage embryos is suppressedby ABA, but protein accumulation is not impaired. Metabolitedistribution studies indicate that ABA alters partitioning ofsucrose in older embryos such that protein accumulation is sustainedat the expense of lipid accumulation. The responses of in vitrocultured embryos to ABA is consistent with the normal patternof ABA accumulation and disappearance that occurs during embryogenesisin situ. A close correlation exists between ABA levels and embryogrowth rates in situ in three cultivars of soybeans. Dependingon the age or stage of the developing embryo, ABA either servesto promote or inhibit embryo growth. Key words: Embryogenesis, ABA, Seeds, Soybean.     

脱落酸对植物气孔运动的调控作用

主要介绍脱落酸对气孔运动的调节作用及其调控气孔运动机制(包括对引起气孔关闭信号转导中第二信使、离子通道、酶活性、膜电压和肌动蛋白细胞合架的调控等)的研究进展。     

影响玉米根中K+、Na+离子向地上部输送的因素（简报）

培养液中含有 1mmol·L-1K 、Na 离子的玉米根运往木质部渗出液中的K /Na 比最大 ;低温不影响渗出液中的K /Na 比 ,但极大地减少了根部向地上部输送的K 、Na 总量 ;钙的加入可以大大提高渗出液中的K /Na 比 ,减少由根部向地上部输送的Na 量。     

Possible Involvement of Abscisic Acid in Increases in Activities of Two Vacuolar H+-Pumps in Barley Roots under Aluminum Stress

Levels of abscisic acid (ABA) in barley roots increased upontreatment with AlCl₃. Treatment with AlCl₃ or ABA increasedboth ATP-dependent and PP_i-dependent H⁺-pumping activities intonoplast-enriched membrane vesicles. Increase in the H⁺-pumpingactivities caused by aluminum stress could result from increasedlevels of ABA. ¹Present address: Department of Botany, Faculty of Science,Hirosaki University, Hirosaki, Aomori, 036 Japan     

Ustilago maydis Produces Cytokinins and Abscisic Acid for Potential Regulation of Tumor Formation in Maize

The infection of maize (Zea mays) by the basidiomycete fungus Ustilago maydis leads to common smut of corn characterized by the production of tumors in susceptible aboveground plant tissues. LC-(ES)MS/MS profiles of abscisic acid (ABA) and 12 different cytokinins (CKs) were determined for infected and uninfected maize tissues over a time course following fungal exposure. Samples were taken at points corresponding to the appearance of disease symptoms. Axenic cultures of haploid and dikaryon forms of U. maydis were also profiled. This study confirmed the capability of Ustilago maydis to synthesize CKs, ABA, and auxin (IAA). It also provided evidence for the involvement of CK and ABA in the U. maydis-maize infection process. Significant quantities of CKs and ABA were detected from axenic cultures of U. maydis as was IAA. CKs and ABA levels were elevated in leaves and stems of maize after infection; notable was the high level of cis-zeatin 9-riboside. Variation among hormone profiles of maize tissues was observed at different time points during infection and between infections with nonpathogenic haploid and pathogenic dikaryon strains. This suggested that CKs and ABA accumulate and are likely metabolized in maize tissue infected with U. maydis. Because U. maydis produced these phytohormones at significant levels, it is possible that the fungal pathogen is a source of these compounds in infected tissue. This is the first study to confirm the production of CKs and document the production of ABA by U. maydis. This study also established an involvement of these phytohormones and a possible functional role for ABA in U. maydis infection of maize.     

Timing of Kernel Development in Water-stressed Maize: Water Potentials and Abscisic Acid Concentrations

Maize (Zea mays L. cv. Pioneer 3925) subjected to post-anthesiswater stress during the first 2 weeks of kernel developmenthad lower leaf-water potentials and higher leaf-ABA concentrationsthan well-watered controls. There was a concomitant rise inABA concentration in kernel tissues 3 and 7 d after pollination(DAP), after which the concentration decreased to control levelsby 13 DAP. Kernel water potential, however, remained unchangedby the water stress. Radiolabelled ABA, fed to a leaf, was translocatedto kernels, where free ABA as well as several ABA metaboliteswere the major labelled fractions. This suggested that the stress-inducedkernel ABA was of maternal origin. Since ABA plays a putativerole in seed maturation of several crop species, and appliedABA or water stress often hastens seed development, we expectedthat a water-stress-induced rise in kernel ABA concentrationearly in grain development may serve to prematurely induce storage-productaccumulation. Zein, starch and several enzymes key to the starchsynthesis pathway followed the same course of induction throughoutthe experiment, with no difference between treatments Henceit was concluded that although water stress increased kernelABA independent of kernel water status, there was no apparenteffect of water stress or ABA on timing of early kernel developmentalprocesses. Zea mays L. cv. Pioncer 3925, maize, water stress, abscisic acid, endosperm development     

Regulation of Leaf Starch Degradation by Abscisic Acid Is Important for Osmotic Stress Tolerance in Plants

Starch serves functions that range over a timescale of minutes to years, according to the cell type from which it is derived. In guard cells, starch is rapidly mobilized by the synergistic action of β-AMYLASE1 (BAM1) and α-AMYLASE3 (AMY3) to promote stomatal opening. In the leaves, starch typically accumulates gradually during the day and is degraded at night by BAM3 to support heterotrophic metabolism. During osmotic stress, starch is degraded in the light by stress-activated BAM1 to release sugar and sugar-derived osmolytes. Here, we report that AMY3 is also involved in stress-induced starch degradation. Recently isolated Arabidopsis thaliana amy3 bam1 double mutants are hypersensitive to osmotic stress, showing impaired root growth. amy3 bam1 plants close their stomata under osmotic stress at similar rates as the wild type but fail to mobilize starch in the leaves. ¹⁴C labeling showed that amy3 bam1 plants have reduced carbon export to the root, affecting osmolyte accumulation and root growth during stress. Using genetic approaches, we further demonstrate that abscisic acid controls the activity of BAM1 and AMY3 in leaves under osmotic stress through the AREB/ABF-SnRK2 kinase-signaling pathway. We propose that differential regulation and isoform subfunctionalization define starch-adaptive plasticity, ensuring an optimal carbon supply for continued growth under an ever-changing environment.     

Role of Abscisic Acid in Water Stress-induced Antioxidant Defense in Leaves of Maize Seedlings

Roles of abscisic acid (ABA) in water stress-induced oxidative stress were investigated in leaves of maize ( Zea mays L.) seedlings exposed to water stress induced by polyethylene glycol (PEG 6000). Treatment with PEG at &#109 0.7 MPa for 12 and 24 h led to a reduction in leaf relative water content (RWC) by 7.8 and 14.1%, respectively. Duration of the osmotic treatments is considered as mild and moderate water stress. The mild water stress caused significant increases in the generation of superoxide radical ( O 2 &#109 ) and hydrogen peroxide (H 2 O 2 ), the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and the contents of ascorbate (ASC), reduced glutathione (GSH). The moderate water stress failed to further enhance the capacity of antioxidant defense systems, as compared to the mild water stress. The contents of catalytic Fe, which is critical for H 2 O 2 -dependent hydroxyl radical ( &#148 OH) production, and the oxidized forms of ascorbate and glutathione pools, dehydroascorbate (DHA) and oxidized glutathione (GSSG), markedly increased, a significant oxidative damage to lipids and proteins took place under the moderate water stress. Pretreatment with ABA caused an obvious reduction in the content of catalytic Fe and significant increases in the activities of antioxidant enzymes and the contents of non-enzymatic antioxidants, and then significantly reduced the contents of DHA and GSSG and the degrees of oxidative damage in leaves exposed to the moderate water stress. Pretreatment with an ABA biosynthesis inhibitor, tungstate, significantly suppressed the accumulation of ABA induced by water stress, reduced the enhancement in the capacity of antioxidant defense systems, and resulted in an increase in catalytic Fe, DHA and GSSG, and oxidative damage in the water-stressed leaves. These effects were completely prevented by addition of ABA, which raised the internal ABA content. Our data indicate that ABA plays an important role in water stress-induced antioxidant defense against oxidative stress.     

Effects of Abscisic Acid and Water Stress on Development and Morphology of Wheat

Experiments were conducted to compare the effects of abscisicacid (ABA) and water stress treatments on leaf morphology andfloral development in a spring wheat. In one experiment injectionsof ABA or a control solution were given twice a week into thebase of the main stem for a period of 3 weeks. In a similarexperiment control plants were watered daily and treated plantswere subjected to water stress by watering only once a week.In both experiments the treated plants produced smaller leavesand fewer spikelets per ear. Analysis of epidermal morphologyusing polystyrene imprints of selected leaf blades from themain stem and a tiller of each plant showed that, compared withcontrol plants, both ABA and water stress decreased the meancell size, reduced the number of stomata per leaf, and increasedthe production of trichomes in all the leaves sampled. Datafor stomatal lengths and stomatal indices showed differencesbetween a main stem leaf and a tiller leaf which were consistentfor both experiments. It is concluded that ABA could mediatemany of the responses of wheat plants to prolonged water stress.The possible adaptive value of these responses is discussed.     

Regulation of Pro-Apoptotic Phosphorylation of Kv2.1 K+ Channels

Caspase activity during apoptosis is inhibited by physiological concentrations of intracellular K⁺. To enable apoptosis in injured cortical and hippocampal neurons, cellular loss of this cation is facilitated by the insertion of Kv2.1 K⁺ channels into the plasma membrane via a Zn²⁺/CaMKII/SNARE-dependent process. Pro-apoptotic membrane insertion of Kv2.1 requires the dual phosphorylation of the channel by Src and p38 at cytoplasmic N- and C-terminal residues Y124 and S800, respectively. In this study, we investigate if these phosphorylation sites are mutually co-regulated, and whether putative N- and C-terminal interactions, possibly enabled by Kv2.1 intracellular cysteine residues C73 and C710, influence the phosphorylation process itself. Studies were performed with recombinant wild type and mutant Kv2.1 expressed in Chinese hamster ovary (CHO) cells. Using immunoprecipitated Kv2.1 protein and phospho-specific antibodies, we found that an intact Y124 is required for p38 phosphorylation of S800, and, importantly, that Src phosphorylation of Y124 facilitates the action of the p38 at the S800 residue. Moreover, the actions of Src on Kv2.1 are substantially decreased in the non-phosphorylatable S800A channel mutant. We also observed that mutations of either C73 or C710 residues decreased the p38 phosphorylation at S800 without influencing the actions of Src on tyrosine phosphorylation of Kv2.1. Surprisingly, however, apoptotic K⁺ currents were suppressed only in cells expressing the Kv2.1(C73A) mutant but not in those transfected with Kv2.1(C710A), suggesting a possible structural alteration in the C-terminal mutant that facilitates membrane insertion. These results show that intracellular N-terminal domains critically regulate phosphorylation of the C-terminal of Kv2.1, and vice versa, suggesting possible new avenues for modifying the apoptotic insertion of these channels during neurodegenerative processes.     

Water Stress Induced Proline Accumulation in Contrasting Wheat Genotypes as Affected by Calcium and Abscisic Acid

Proline accumulation and mobilization in roots of 7-d-old seedlings of wheat genotypes varying in sensitivity towards water stress were compared. Water stress was induced by polyethylene glycol (PEG-6000; osmotic potential −1.5 MPa) in the presence of 0.1 mM abscisic acid (ABA), 1 mM calcium chloride, 0.5 mM verapamil (Ca²⁺ channel blocker), 0.5 mM fluridone (inhibitor of ABA biosynthesis). While both the genotypes did not differ in total proline accumulation, rate of proline accumulation and utilization was higher in tolerant genotype C 306 as compared to susceptible genotype HD 2380. The treatment with ABA and CaCl₂ caused further increase in proline accumulation during stress and reduced its mobilization during recovery. The membrane stability and elongation rate of roots was observed to be higher at ABA and calcium treatment in both the genotypes under stress. As was evident from inhibitor studies, the tolerant genotype was more responsive to ABA and the susceptible one to calcium. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural Requirements of Abscisic Acid (ABA) and its Impact on Water Flow During Radial Transport of ABA Analogues Through Maize Roots

The effect of (+) (ABA) and (?)-abscisic acid and nine ABA metabolites, precursors or derivatives on radial water movement through maize roots, was investigated using a suction technique (Freundl and others 1998). (+)-ABA, (+)- and (?)-abscisyl aldehyde, (+)-8?-hydroxymethyl ABA, (+)-8?-methylene, and (+)-8?-acetylene ABA stimulated radial water transport. (?)-ABA, phaseic acid, and (+)-8?-acetylene methyl ABA were ineffective. ELISA analysis for ABA detected and apparent increase of free ABAxyl in xylem sap of excised root systems that were perfused with either (+)-abscisyl aldehyde, (+)-8?-methylene, (+)8?-acetylene-ABA, or ABA-glucose ester. The analogues (+)-8?-hydroxymethyl ABA and (?)-abscisyl aldehyde passed the cortex of maize roots without changing the ABAxyl. The data from this study permit conclusions about the structural requirements for hormonal regulation of hydraulic conductivity.     

Alleviation of Negative Effects of Water Stress in Two Contrasting Wheat Genotypes by Calcium and Abscisic Acid

The individual and interactive role of calcium and abscisic acid (ABA) in amelioration of water stress simulated by polyethylene glycol (PEG) 6000 was investigated in two contrasting wheat genotypes. PEG solution (osmotic potential –1.5 MPa) was applied to 10-d-old seedlings growing under controlled conditions and changes in photosynthetic rate, activities of ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxylase, water potential and stomatal conductance were observed in the presence of 0.1 mM ABA, 5 mM calcium chloride, 1 mM verapamil (Ca²⁺ channel blocker), and 1 mM fluridone (inhibitor of ABA biosynthesis). ABA and calcium chloride ameliorated the effects of water stress and the combination of the two was more effective. The two genotypes varied for their sensitivity to ABA and Ca²⁺ under stress. As was evident from application of their inhibitors, ABA caused more alleviation in C 306 (drought tolerant) while HD 2380 (drought susceptible) was more sensitive to Ca²⁺.     

Abscisic Acid Metabolism in Relation to Water Stress and Leaf Age in Xanthium strumarium

Intact plants of Xanthium strumarium L. were subjected to a water stress-recovery cycle. As the stress took effect, leaf growth ceased and stomatal resistance increased. The mature leaves then wilted, followed by the half expanded ones. Water, solute, and pressure potentials fell steadily in all leaves during the rest of the stress period. After 3 days, the young leaves lost turgor and the plants were rewatered. All the leaves rapidly regained turgor and the younger ones recommenced elongation. Stomatal resistance declined, but several days elapsed before pre-stress values were attained.

Abscisic acid (ABA) and phaseic acid (PA) levels rose in all the leaves after the mature ones wilted. ABA-glucose ester (ABA-GE) levels increased to a lesser extent, and the young leaves contained little of this conjugate. PA leveled off in the older leaves during the last 24 hours of stress, and ABA levels declined slightly. The young leaves accumulated ABA and PA throughout the stress period and during the 14-hour period immediately following rewatering. The ABA and PA contents, expressed per unit dry weight, were highest in the young leaves. Upon rewatering, large quantities of PA appeared in the mature leaves as ABA levels fell to the pre-stress level within 14 hours. In the half expanded and young leaves, it took several days to reach pre-stress ABA values. ABA-GE synthesis ceased in the mature leaves, once the stress was relieved, but continued in the half expanded and young leaves for 2 days.

Mature leaves, when detached and stressed, accumulated an amount of ABA similar to that in leaves on the intact plant. In contrast, detached and stressed young leaves produced little ABA. Detached mature leaves, and to a lesser extent the half expanded ones, rapidly catabolized ABA to PA and ABA-GE, but the young leaves did not. Studies with radioactive (±)-ABA indicated that in young leaves the conversion of ABA to PA took place at a much lower rate than in mature ones. Leaves of all ages rapidly conjugated PA to PA-glucose ester. Furthermore, when half expanded leaves were stressed on the intact plant, their rate of ABA catabolism was enhanced, an effect not observed in the young leaves.

In conclusion, young leaves on intact Xanthium plants produce little stress-induced ABA themselves, but due to import and a low rate of catabolism accumulate more ABA and PA than mature leaves.