The auxins catabolism by Bradyrhizobium japonicum E109 and its importance for the soybean-Bradyrhizobium symbiosis

DONADIO, FLORENCIA; TORRES DANIELA; NIEVAS SOFIA; CASSAN, FABRICIO

Congreso; IV Reunión conjunta de sociedades de biología de la República Argentina; 2020

B. japonicum E109 is one of the most widely used strains for the soybean inoculant formulation in Argentina. This strain is considered a real plant growth-promoting rhizobacteria (PGPR) because its capacity to fix biological nitrogen and to biosynthesize several phytohormones as the indole-3-acetic acid (IAA). The IAA metabolism comprises several mechanisms, such as the biosynthesis, catabolism, conjugation and hydrolysis of auxins conjugates, which globally regulate the IAA status. The IAA catabolism has been partly elucidated in B. japonicum E109 under saprophytic conditions but its importance for the soybean-Bradyrhizobium symbiosis still remains unclear. The aim of this work was to evaluate the catabolism of IAA and its impact on the saprophytic and symbiotic behavior of B. japonicum E109 and B. diazoefficiens USDA110 and its mutants incapable to degrade the hormone. To this ends, aliquots of 40 µg.ml-1 of IAA were added to pure cultures of B. japonicum E109, B. diazoefficiens USDA110 and USDA110 ΔiacA, ΔiacC and E109 ΔiacC mutants during exponential growth phase (1x109 cfu.ml-1). After 24 h incubation, both the saprophytic and symbiotic behavior was evaluated for each strain. The saprophytic performance was evaluated through the biomass production (OD595), cell viability (ufc.ml-1) and IAA concentration in the culture medium (µg.ml-1); while the symbiotic performance was assessed by the evaluation of root nodulation through Burton´s test. For each treatment, 30 seedlings were maintained under growth chamber conditions for 21 d with 16/8 h photoperiod at 28ºC and were irrigated with nitrogen-deficient sterile N-free Hoagland's solution [25 % (v/v)]. Our results demonstrated that the exogenous addition of IAA caused significant changes in the saprophytic behavior of all the evaluated strains. In the case of biomass production (OD595) a 32.1% increase was determined in E109 and USDA110 wild type strains associated with 3.16% increase in the number of viable cells (ufc.ml-1). In the case of the ΔiacC and ΔiacA mutants, they increased around 29.7% the biomass production and 3.3% the number of viable cells (ufc.ml-1). In the case of the soybean seed inoculation, the exogenous treatment with IAA increased the number of Bradyrhizobium viable cells recovered from inoculated seeds after 4 h inoculation (RF%) around 27.4% in comparison with control treatment without addition of IAA. In relation with the symbiotic performance, treatment with IAA increased the percentage of root nodulation of the ΔiacA mutant by 30% and decreased by 55% in the case of USDA110. E109 and ΔiacC maintained a percentage of nodulation around 100%. Finally, the number of total nodules increased significantly in the ΔiacC and ΔiacA mutants by an average 30% by exogenous addition of IAA. These results suggest that the exogenous addition of IAA to E109 and USDA110 or their mutant cultures causes a positive effect on both saprophytic and symbiotic behavior of these bacteria, independently of their ability to degrade the hormone in the culture medium.