Signaling And Regulation Of Induced Drought Tolerance And Pathogen Defense In Plants

The abscisic acid signaling controls HrpN-induced drought tolerance in ArabidopsisHrpN, a protein produced by the plant pathogenic bacterium Erwinia amylovora, has been shown to stimulate plant growth and resistance to pathogens and insects. Here we report that HrpN activates abscisic acid (ABA) signaling to induce drought tolerance (DT) in Arabidopsis thaliana (L.) plants grown with water stress. Spraying wild-type plants with HrpN promoted stomatal closure, decreased leaf transpiration rate, increased moisture and proline levels in leaves, and alleviated extents in damage to cell membranes and plant drought symptoms caused by water deficiency. In plants treated with HrpN, ABA levels increased; expression of several ABA-signalling regulatory genes and the important effector gene rd29B was induced or enhanced.Cloning of tobacco cDNA involved in constitutive expression of SARTobacco constitutive expresser of systemic acquired resistancel-1 (cesl-1) and ces2-1 mutants have been generated from the variety NC89. The three genotypes varied greatly in growth and resistance to Alternaria alternata. The ces2-l mutant grew better but essl-1 grew worse that the wild type (WT), whereas both mutants were more resistant to the pathogen. Both mutated loci showed to be dominant in heredity. We suppose that CES1 gene positively regulates SAR but negatively regulates growth, while CES2 positively regulates both SAR and growth.To determine genes that are associated with CES1 and CES2 functions, mRNA differential display was applied to cesl-1, ces2-1, and NC89. Thirteen cDNA fragments were obtained selected. The fragment R321 shows higher homology with the rice blast-resistant mRNA; R411 is a part of tobacco mitochondrial genome; R441 shares 88% homology with the auxin-repressed protein gene (APR1) in a 312-bp region; and R461 is 92% homological with one of H₂O₂-inducing tobacco genes in a region of 130 bp. The four fragments were cloned in the vector pBinPLUS-2mβ for further study.Because evidence role of auxin in plant defense has emerged in very recent years, the fragment R441 highly homological with APR1 was analyzed for expression levels in cesl-1, ces2-1, and WT. The fragment transcript had the highest, moderate, and lowest levels incesl-1, WT and ces2-1, respectively. This result is comparable to the growth and resistance phenotypes of the plants. Thus, we hypothesize that the tobacco APRl gene is involved in auxin signaling in the apparent negative regulation of plant growth and positive regulation of defense. This notion would open an avenue to addressing crosstalk between SAR and growth under mediation by auxin.Besides testing the hypothesis for R441, many remain to be determined for the other gene fragments. The high homology of R461 with the H2O2-induced gene in a long region, and the role of H2O2 in plant defense, suggest that the constitutive SAR in cesl-1 and ces2-1 may be subject to H2O2 signaling. Moreover, R321 apparently as a homolog of the rice resistance gene conferring resistance to the blast pathogen may interplay more comprehensively with gene-for-gene resistance.