Endogenous abscisic acid content in cucumber leaves under the influence of unfavourable temperatures and salinity

Endogenous abscisic acid (ABA) content was measured in leavesof Cucumis sativus L. under the influence of hardening (lowand high) temperatures and salinity. The rise in cold and heatresistance of the seedlings was accompanied by a considerableincrease in the ABA level in the leaves. Chloride salinity alsobrought about a rise in the ABA content. The data indicate thatABA may induce resistance when the plants are exposed to severalstresses. Key words: Cucumis sativus, ABA, cold and heat hardening, salinity     

Functional analysis of TaDi19A, a salt-responsive gene in wheat

A salinity stress upregulated expressed sequence tag (EST) was selected from a suppression subtractive hybridization cDNA library, constructed from the salinity-tolerant wheat cultivar Shanrong No. 3. Sequence analysis showed that the corresponding gene (named TaDi19A ) belonged to the Di19 family. TaDi19A was constitutively expressed in both the root and leaf of wheat seedlings grown under non-stressed conditions, but was substantially up-regulated by the imposition of stress (salinity, osmotic stress and cold), or the supply of stress-related hormones [abscisic acid (ABA) and ethylene]. The heterologous over-expression of TaDi19A in Arabidopsis thaliana increased the plants' sensitivity to salinity stress, ABA and mannitol during the germination stage. Root elongation in these transgenic lines showed a reduced tolerance to salinity stress and a reduced sensitivity to ethophon. The expression of the ABA signal pathway genes ABI1 , RAB18 , ERD15 and ABF3 , and SOS2 (SOS pathway) was altered in the transgenic lines. TaDi19A plays a role in the plant's response to abiotic stress, and some possible mechanisms of its action are proposed.     

外源脱落酸对小麦幼苗抗镉胁迫能力的影响

以小麦(Triticum aestivum L.)为试材,采用水培法研究了100mg/L镉(Cd2+)胁迫条件下施用外源脱落酸(ABA)对小麦幼苗生长及某些生理生化指标的影响。结果表明:(1)100mg/L Cd2+胁迫下,小麦叶片膜脂过氧化产物丙二醛(MDA)含量显著提高,植株生长受到抑制;(2)外源ABA能够明显提高Cd2+胁迫小麦幼苗的根系活力,增加其叶片SOD、CAT和POD活性,促进其脯氨酸积累,降低MDA的含量,并以5.0μmol/L ABA的效果最明显;(3)1.0～5.0μmol/L外源ABA不同程度地缓解Cd2+胁迫对小麦幼苗生长的抑制作用,且5.0μmol/L时效果最明显,其株高、根长、总干重分别比单一Cd2+胁迫处理显著提高6.73%、149%和10.52%,而10.0μmol/LABA反而加重了Cd2+对小麦幼苗生长的伤害。因此,适宜浓度的外源ABA能够通过增加体内保护酶活性和脯氨酸含量来缓解Cd2+胁迫对小麦幼苗生长的抑制作用,增强小麦幼苗的抗Cd2+胁迫能力,并以5.0μmol/L ABA处理效果最好。     

Role of abscisic acid in regulation of wheat seedling growth under salinity stress

Growth of wheat seedlings (Triticum aestivum L. cv. Mehran-89), in hydroponic culture, was affected by abscisic acid (ABA). Using salinity stress and exogenous ABA application (10-6 M) to enhance endogenous ABA level, the growth of roots was more suppressed than the growth of shoots. On the other hand, norflurazon, which inhibits ABA biosynthesis, reduced only the growth of shoots. This revised version was published online in July 2006 with corrections to the Cover Date.     

施用有机肥环境下盐胁迫小麦幼苗长势和内源激素的变化

在盆栽条件下,研究了不同浓度NaCl处理下,底施不同施用量有机肥小麦品种豫麦49-198幼苗的生长变化,在此基础上,选择出合适的NaCl处理浓度和有机肥施用量区间,并对此情况下小麦幼苗苗和根中内源激素含量和比例的变化进行了测定,以揭示其耐盐差异机制。结果表明,15000-35000 kg/hm²施用量有机肥处理明显减轻NaCl浓度为150 mmol/L的盐胁迫,其中25000 kg/hm²有机肥处理效果最明显;45000 kg/hm²以上的有机肥处理对幼苗生长抑制无明显缓解作用;当NaCl浓度为450 mmol/L时,各种施用量的有机肥处理均不能减轻盐胁迫对幼苗生长的抑制。150 mmol/L NaCl胁迫下,不同施用量有机肥处理,分别为对照(不施肥)、低施用量(15000 kg/hm²)、中施用量(25000 kg/hm²)和高施用量(35000 kg/hm²)的有机肥,土壤盐度的增加量随有机肥用量增加而上升,对小麦幼苗生长的抑制作用得到缓解,以25000 kg/hm²有机肥处理缓解作用最强。有机肥处理下盐胁迫小麦幼苗苗和根中ABA含量的增加得到显著缓解,IAA和GAs的含量比不施有机肥的对照有不同程度的提高, 说明盐胁迫下有机肥处理小麦幼苗具有较高IAA和GAs合成量。盐胁迫下有机肥处理使苗中ZR的含量较高而根中则较低,说明抗盐性较强的有机肥处理可迅速将根部合成的ZR向苗中转移,促进苗的生长。盐胁迫下有机肥处理的IAA/ABA、GAs/ABA、ZR/ABA的比值也有不同程度提高。在盐胁迫下,有机肥处理尤其是在25000 kg/hm²施用量时,小麦幼苗协调自身激素平衡的能力较强可能是其生长受抑制较小的重要原因。     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

Abscisic Acid Control of Lectin Accumulation in Wheat Seedlings and Callus Cultures : Effects of Exogenous ABA and Fluridone

Wheat germ agglutinin is found in wheat embryos and a similar lectin is present in the roots of older plants. We report here that 10 micromolar abscisic acid (ABA) produces an average two to three-fold enhancement in the amount of lectin in the shoot base and the terminal portion of the root system of hydroponically grown wheat seedlings. Although ABA stunts seedling growth, a similar growth inhibition produced by ancymidol is not accompanied by elevated lectin levels. To further clarify the role of ABA, wheat callus cultures were employed. Callus derived from immature embryos was grown on growth medium containing various combinations of ABA and 2,4-dichlorophenoxyacetic acid. Those grown in the presence of 10 micromolar ABA exhibit the largest increases in lectin compared to material grown on other regimes. The involvement of ABA in lectin accumulation was further probed with fluridone, an inhibitor of carotenoid synthesis which has also been linked to depressed levels of endogenous ABA. Wheat seedlings grown in the presence of 1 or 10 milligrams per liter fluridone have few or no carotenoids, and wheat germ agglutinin levels in the shoot base and roots are lower compared to controls. The greatest effect (a 39% reduction in the shoot base) is produced at an herbicide concentration of 10 milligrams per liter. Exogenous 10 micromolar ABA greatly stimulates lectin accumulation in the presence of fluridone, but the levels are not as high as those produced by ABA alone. These results indicate that lectin synthesis is under ABA control in both wheat embryos and adult plants.     

Stress-induced accumulation of wheat germ agglutinin and abscisic Acid in roots of wheat seedlings

Wheat germ agglutinin (WGA) levels in roots of 2-day-old wheat seedlings increased up to three-fold when stressed by air-drying. Similar results were obtained when seedling roots were incubated either in 0.5 molar mannitol or 180 grams per liter polyethylene glycol 6000, with a peak level of WGA after 5 hours of stress. Longer periods of osmotic treatment resulted in a gradual decline of WGA in the roots. Since excised wheat roots incorporate more [³⁵S]cysteine into WGA under stress conditions, the observed increase of lectin levels is due to de novo synthesis. Measurement of abscisic acid (ABA) levels in roots of control and stressed seedlings indicated a 10-fold increase upon air-drying. Similarly, a five- and seven-fold increase of ABA content of seedling roots was found after 2 hours of osmotic stress by polyethylene glycol 6000 and mannitol, respectively. Finally, the stress-induced increase of WGA in wheat roots could be inhibited by growing seedlings in the presence of fluridone, an inhibitor of ABA synthesis. These results indicate that roots of water-stressed wheat seedlings (a) contain more WGA as a result of an increased de novo synthesis of this lectin, and (b) exhibit higher ABA levels. The stress-induced increase of lectin accumulation seems to be under control of ABA.     

The effect of calcium on the production of ethylene and abscisic acid by fungus Fusarium culmorum and by wheat seedlings infected with that pathogen

Wheat seedlings infected and non-infected with Fusarium culmorum were grown on mediums with different content of calcium (0, 2, 4, 8 mM). It was found that the higher the content of calcium in the medium, the greater the amounts of ethylene produced in both non-infected and infected wheat seedlings, whereas the level of ABA in their tissues was decreased. Taking into consideration the fact that ethylene inhibits, whereas ABA stimulates the growth and development of Fusarium culmorum, it may be assumed that the production of ethylene increased under the influence of calcium and the decreased level of ABA in wheat seedlings causes the reduction growth and development of pathogen and as a result it lowers the degree of infection of wheat seedlings by this fungus. Thus, on the base of the obtained results it may be concluded that the inhibiting influence of calcium on injurying the wheat seedlings by Fusarium culmorum may be caused by the influence of this cation on the hormone balance in the plant.     

WGA reduces the level of oxidative stress in wheat seedlings under salinity

Protective effect of exogenous wheat germ agglutinin (WGA) on wheat seedling (Triticum aestivum L.) during salinity stress was studied. In particular, we examined the state of pro- and antioxidant systems as well as the level of peroxide oxidation of lipids and electrolyte leakage under control conditions and when stressed with NaCl. Generation of superoxide anions and activity of both superoxide dismutase (SOD) and peroxidase increased during saline stress. Accumulation of O₂ ^·− resulted in peroxide oxidation of lipids and electrolyte leakage in response to stress. The injurious effect of salinity on root growth of seedlings was manifested by a decreased mitotic index (MI) in apical root meristem. This study show that WGA pretreatment decreased salt-induced superoxide anion generation, SOD and peroxidase activities, levels of lipid peroxidation and electrolytes leakage as well as correlating with a reduction in the inhibition of root apical meristem mitotic activity in salt-treated plants. This suggests that exogenous WGA reduced the detrimental effects of salinity-induced oxidative stress in wheat seedlings. Thus WGA effects on a balance of reactive oxygen species (ROS) and activities of antioxidant enzymes may provide an important contribution to a range of the defense reactions induced by this lectin in wheat plants.     

Hormonal Status in Wheat and Rice Seedlings under Anoxia

A comparative analysis of the effects of anoxia on growth, fresh weight gain, and phytohormones in wheat (Triticum aestivum L.) and rice (Oryza sativa L.) seedlings was performed. In both plant species, a total cessation of root growth occurred during the initial hours of anoxia. In an anaerobic environment, the fresh weight of wheat seedlings decreased. An increase in the shoot length and weight under the stress conditions was found only in rice seedlings. During the initial hours of anoxia, the level of free ABA in wheat and rice tissues increased manifold, and the accumulation of a free ABA form occurred at the expense of the hydrolysis of its bound forms. The IAA content in plant tissues also increased. In wheat, the accumulation of IAA was short, but in rice, a high hormone level was retained during the entire experiment, and, as a result, its concentration exceeded that of ABA. A level of cytokinins in the tissues of both plant species was affected by anoxia to a lesser extent than that of other phytohormones. This level somewhat decreased under anoxia similarly to the level in darkness under aeration. It is suggested that IAA accumulation in hypoxia-tolerant rice seedlings under anoxia favors maintenance of shoot growth and simultaneous inhibition of root growth. At the same time, in the hypoxia-sensitive wheat, an increase in the ABA level resulted in growth cessation.     

Growth, Gas Exchange, Abscisic Acid, and Calmodulin Response to Salt Stress in Three Poplars

In the present study, we Investigated the effects of Increaslng sallnlty on growth, gas exchange, absclslc acld （ABA）, calmodulln （CAM）, and the relevance to salt tolerance In seedllngs of Populus euphratlca Ollv. and cutUngs of P. ＂pupularls 35-44＂ （P. popularls） and P. x euramerlcana cv. 1-214 （P. cv. Itallca）. The relatlve growth rates of shoot helght （RGR,） for P. cv. Itallca and P. popularls were severely reduced by Increaslng salt stress, whereas the growth reductlon was relatlvely less in P. euphratica. Slmllarly, P. euphratlca malntalned hlgher net photosynthetlc rates （Pn） and unlt transplration rotes （TRN） than P. cv. Itallca and P. popularls under condltlons of hlgher sallnlty. Sallnity caused a slgnlficant increase In leaf ABA and CaM In the three genotypes after the onset of stress, but NaCl-induced ABA and CaM accumulatlon was more pronounced In P. euphratlca, suggeeUng that P. euphratlca plants are more sensitlve in sensing soil salinlty than the other two poplars. Furthermore, P. euphratica maintained relatively higher ABA and CaM concentrations under conditions of high salinity. The higher capacity to synthesize stress signals, namely ABA and CaM, In P. euphratica and the contrlbuUon of this to the salt resistance of P. euphratica are discussed.     

Control of abscisic acid catabolism and abscisic acid homeostasis is important for reproductive stage stress tolerance in cereals

Drought stress at the reproductive stage causes pollen sterility and grain loss in wheat (Triticum aestivum). Drought stress induces abscisic acid (ABA) biosynthesis genes in anthers and ABA accumulation in spikes of drought-sensitive wheat varieties. In contrast, drought-tolerant wheat accumulates lower ABA levels, which correlates with lower ABA biosynthesis and higher ABA catabolic gene expression (ABA 8'-hydroxylase). Wheat TaABA8'OH1 deletion lines accumulate higher spike ABA levels and are more drought sensitive. ABA treatment of the spike mimics the effect of drought, causing high levels of sterility. ABA treatment represses the anther cell wall invertase gene TaIVR1, and drought-tolerant lines appeared to be more sensitive to the effect of ABA. Drought-induced sterility shows similarity to cold-induced sterility in rice (Oryza sativa). In cold-stressed rice, the rate of ABA accumulation was similar in cold-sensitive and cold-tolerant lines during the first 8 h of cold treatment, but in the tolerant line, ABA catabolism reduced ABA levels between 8 and 16 h of cold treatment. The ABA biosynthesis gene encoding 9-cis-epoxycarotenoid dioxygenase in anthers is mainly expressed in parenchyma cells surrounding the vascular bundle of the anther. Transgenic rice lines expressing the wheat TaABA8'OH1 gene under the control of the OsG6B tapetum-specific promoter resulted in reduced anther ABA levels under cold conditions. The transgenic lines showed that anther sink strength (OsINV4) was maintained under cold conditions and that this correlated with improved cold stress tolerance. Our data indicate that ABA and ABA 8'-hydroxylase play an important role in controlling anther ABA homeostasis and reproductive stage abiotic stress tolerance in cereals.     

Production of Polyamines Is Enhanced by Endogenous Abscisic Acid in Maize Seedlings Subjected to Salt Stress

It is known that salt stress and exogenously applied abscisic acid （ABA） can enhance the polyamine content in plants and that salt stress itself can lead to an increase in endogenous ABA production. In the present study, the relationships between salt-induced ABA and polyamine accumulation were inves- tigated using ABA-deficient mutant （vp5/vp5） maize （Zea mays L.） seedlings and ABA and polyamine biosynthesis inhibitors. The results show that reduced endogenous ABA levels, as a result of either the mutation or by using a chemical inhibitor （sodium tungstate）, also reduced the accumulation of polyamines in salt-stressed leaves of maize seedlings. The polyamine synthesis inhibitors D-arginine and α- difluoromethylornithine also reduced the polyamine content of the leaves of maize seedling under salt stress. Both ABA and polyamine enhanced the dry weight accumulation of salt-stressed seedlings and also increased the activities of the two dominant tonoplast membrane enzymes, H^＋-ATPase and H^＋-PPase, when plants were under salt stress. The results suggest that salt stress induces an increase in endogenous ABA levels, which then enhances polyamine synthesis. Such responses may increase a plant＇s tolerance to salt.