RELATIVE EXPRESSION OF NITROGEN FIXATION DROUGHT TOLERANCE GENES AND RECOVERY CAPACITY AFTER WATER STRESS IN SOYBEAN

The success of soybean cultivation in Brazil is attributable, among other factors, to the ability to form symbiosis with N2-fixing bacteria. However, the occurrence of drought is limiting to the biological nitrogen fixation (BNF), being the main environmental factor that affects the symbiotic efficiency, with negative consequences to crop yield. The objective of this study was to evaluate some traits associated to BNF and the expression of genes related to the symbiosis under drought in two soybean genotypes. The experiment was conducted in greenhouse conditions, in a completely randomized design with 2 × 2 factorial arrangement, two soybean genotypes (PI471938 - with drought tolerance; and BRR14-89152 – progeny from PI471938) and two water conditions: well watered and drought stress applied during the beginning of the reproductive growth stage (R1 - R2) per one week. The seeds were inoculated with a mixture of Bradyrhizobium japonicum and B. diazoefficiens, as recommended in commercial inoculants in Brazil, just before sowing, using a broth containing 1 × 109 viable cells mL−1. After the stress period, some traits related to BNF were evaluated, such as N content in the shoots and ureides in petioles. The expression of genes related to the drought-tolerant symbiosis via RT-qPCR was analyzed in roots with nodules. The plant capacity to recover after drought was based on yield components. PI471938 showed better symbiosis efficiency, with greater accumulation of N in shoots and ureides content in the petioles, under both water conditions, compared with the progeny. However, problems were observed in the translocation of N compounds in the plants under drought stress, with an increase by 90% in the ureides concentration in the petioles, considering the average between the two genotypes. Comparing with plants well watered, the gene encoding for Aquaporin, Nodulin, Glutathione peroxidase, Glutamine synthetase and Heat shock proteins 21 were up-regulated in roots with nodules under drought stress in both genotypes, representing mechanisms of plant protection and the BNF under stress. BRR14-89152 showed a better recovery capacity after drought stress, with maintenance of yield when compared with plants well watered, whereas PI471938 had a reduction by 15% in grain yield when plants were submitted to drought stress. The progeny BRR14-89152 is a promising candidate for soybean breeding programs, because promotes the BNF protection via genetic regulation, and keep the grain yield after drought stress at the reproductive period.