Drought-induced expression of aquaporin genes in leaves of two common bean cultivars differing in tolerance to drought stress

Aquaporin proteins are part of the complex response of common bean (Phaseolus vulgaris L.) to drought which affects the quality and quantity of yield of this important crop. To better understand the role of aquaporins in common bean, drought-induced gene expression of several aquaporins was determined in two cultivars, the more drought tolerant Tiber and the less tolerant Starozagorski ?ern. The two bean cultivars were selected among 16 European genotypes based on the tolerance to drought determined by time needed for plants to wilt after withholding irrigation and yield at harvest. The expression patterns of two plasma membrane intrinsic proteins, PvPIP1;2 and PvPIP2;7, and two tonoplast intrinsic proteins, PvTIP1;1 and PvTIP4;1 in leaves of 21 day old plants were determined by RT-qPCR in both cultivars under three degrees of drought stress, and under rehydration and control conditions. Gene expression of all four examined aquaporins was down-regulated in drought stressed plants. After rehydration it returned to the level of control plants or was even higher. The responses of PvPIP2;7 and PvTIP1;1 during drought and rehydration were particularly pronounced. The gene expression of PvPIP2;7 and PvTIP4;1 during drought was cultivar specific, with greater down-regulation of these two aquaporins in drought tolerant Tiber. Under drought stress the relative water content and water potential of leaves were higher in Tiber than in Starozagorski plants. The differences in these physiological parameters indicate greater prevention of water loss in Tiber during drought, which may be associated with rapid and adequate down-regulation of aquaporins. These results suggest that the ability of plants to conserve water during drought stress involves timely and sufficient down-regulation of gene expression of specific aquaporins.     

<Emphasis Type="Italic">Platycladus orientalis</Emphasis> PoKub3 is involved in abiotic stress responses in transgenic arabidopsis

Ku70-binding proteins associate with Ku70 and their expression levels can affect DSB repair efficiency via the DNA-PK-dependent repair pathway. However, how Ku70-binding proteins in plants exert a regulatory function under abiotic stress is poorly understood. Here, we cloned and characterized a PoKub3 gene from 500-year-old Platycladus orientalis. With increasing age, PoKub3 expression in P. orientalis increased gradually. The PoKub3 expression levels in leaves were upregulated under salt, heat, UV-C and abscisic acid treatments according to qRT-PCR. Moreover, PoKub3 overexpression in Arabidopsis thaliana improved tolerance to salt and drought stress compared with wild-type (WT) and vector control (VC) plants. High RAB18 and DREB2A expression and low JAZ1 and ABI2 expression provided strong evidence that salt tolerance was enhanced in the overexpression plants. Similarly, high RAB18 and DREB2A expression, accompanied by low JAZ1 and LOX1 expression and high DREB1A, CPK10, GSTF6 and APX1 expression, suggested the drought tolerance mechanism was associated with the abscisic acid pathway. In addition, lower malondialdehyde content, electrolyte leakage and stomatal conductance, and higher soluble sugar and relative water contents in PoKub3 overexpression lines than in WT and VC plants demonstrated its role in salt and drought tolerance. Together, these findings show that PoKub3 positively regulates salt and drought tolerance by regulating stress-related genes.     

Nucleotide Diversity of the Coding and Promoter Regions of <Emphasis Type="Italic">DREB1D</Emphasis>, a Candidate Gene for Drought Tolerance in <Emphasis Type="Italic">Coffea</Emphasis> Species

Climate change is posing a major challenge to coffee production worldwide leading to a need for the development of coffee cultivars with increased drought tolerance. In several plant species, the use of DREB genes in crop improvement has achieved promising results to desiccation tolerance engineering. Recent studies reported CcDREB1D specific patterns of expression in Coffea canephora and functional evidence of this gene involvement in drought stress responses. However, knowledge on natural diversity of this gene is largely unknown. In this context, this study aimed at evaluating the sequence variability of the DREB1D gene in several Coffea genotypes. Nucleotide variation in promoters and coding regions of this gene were evaluated in a population consisting of 38 genotypes of C. canephora, C. arabica and C. eugenioides, most of them characterized by different phenotypes (tolerance vs. susceptibility) in relation to drought. The genetic diversity of the loci revealed different haplotypes for the promoter and coding regions. In particular, our findings suggest association between drought tolerance and the genetic variations on DREB1D promoter regions, but not with those from its corresponding coding regions. Gene expression studies revealed up-regulated expression of DREB1D gene upon drought mainly in leaves of drought-tolerant clones of C. canephora, and in response to drought, high, and low temperatures in leaves of C. arabica, suggesting a key role of this gene in coffee responses to abiotic stress.     

Changes in plasma membrane aquaporin gene expression under osmotic stress and blue light in tomato

Divergent abiotic stresses induce osmotic stress on plant cells resulting in an imbalance in water homeostasis which is preserved by aquaporins. Since the plasma membrane aquaporins (PIPs) were shown to be involved in seed development and responses to abiotic stresses, we focused on determining the contribution of mannitol-induced osmotic stress, blue light (BL), and 7B-1 mutation to their gene expression in tomato (Solanum lycopersicum L.) seeds. To assess that, we used a quantitative RT-PCR to determine the expression profiles of genes encoding PIPs. Subsequently, a multiple linear regression analysis was used to evaluate the impact of studied stressors (mannitol and BL) and 7B-1 mutation on PIP gene expressions. We found that mannitol-induced osmotic stress and 7B-1 mutation (conferring the lower responsiveness to osmotic stress- and BL-induced inhibition of seed germination) decreased expression of PIP1;3, PIP2;3 and PIP1;2, PIP2;1 genes, respectively. This might be a way to retain water for radicle elongation and seed germination under the stress conditions. Interestingly, the expression of PIP1;3 gene was downregulated not only by osmotic stress, but also by BL. Altogether, our data indicate the existence of a link between osmotic stress and BL signalling and the involvement of the 7B-1 mutation in this crosstalk.     

Effects of progressive drought stress on the physiology,antioxidative enzymes and secondary metabolites of Radix Astragali

The dried roots of Radix Astragali are widely used in traditional Chinese medicine, and flavonoids present in the root of this herb have been implicated in its bioactivity. We subjected 2-year-old Astragalus membranaceus Bge. var. mongholicus (Bge.) Hsiao to a progressive drought stress over 14 days of water withholding and studied the physiological and biochemical responses and secondary metabolite accumulation. Results indicated that drought stress reduced relative water content, reduced yield, but increased electrolyte leakage, malondialdehyde, proline and soluble sugar content. Mild and moderate drought stress enhanced some antioxidative enzyme activity to protect plants from damaging, but antioxidative enzyme activity was limited by severe stress. The calycosin-7-O-β-d-glycoside and ononin content of plant roots was enhanced with degree of drought stress, whereas calycosin and formononetin levels did not differ significantly until 14 days. These results demonstrate that Radix Astragali can adapt to water stress by up-regulating antioxidant enzymes and accumulation of osmotic agents, and a certain degree of drought stress can enhance accumulation of some flavonoids, potentially facilitating higher yields of pharmacological activity of calycosin-7-O-β-d-glycoside production.