Indole-3-acetic acid catabolism in B. japonicum E109 and its impact on soybean inoculation

TORRES DANIELA; MONGIARDINI, ELIAS; DONOSO RAUL ; DONADIO, FLORENCIA; CASSAN, FABRICIO

Saskatoon

Congreso; Rhizosphere 5; 2019

Bradyrhizobium japonicum E109 has been one of the most used rhizobacteria for soybean inoculation in South America during the last 40 years. The genome analyses of E109 showed that besides nitrogen fixation, these bacteria contain several genes related with phytohormone biosynthesis, particularly indole-3-acetic acid (IAA). However, it is not possible to quantify this compound under any experimental conditions, mainly due to the bacterial capacity to degrade it. The degradation kinetics of E109 showed a higher activity during exponentialgrowth phase with a saturation behavior above to 0.5 mM IAA in YEM liquid culture medium. A second genome analysis revealed the presence of a catabolism cluster similar to that previously identified in Pseudomonas putida 1290, but although the iacA gene product is responsible for IAA degradation in P putida, it is not in B.japonicum E109. Conversely, the homologous iacC of E109 encoded for a 3-phenylpropionate dioxygenase subunit α, the main responsible for IAA degradation in E109 forming a particular cluster together with iacF and two genes with unknown function. The iacC mutation rendered E109 unable to degrade IAA and under in vitro and in vivo conditions, confirming the enzyme role. Besides inducing bacterial catabolism, the addition of 40 μg.ml-1 IAA in YEM liquid medium increased both E109 and mutant ΔiacC viability on soybean seeds 4 hours and 6 days after inoculation. Additionally, under in planta conditions, soybean nodulation and the number of nodules in primary roots increased significantly when adding the hormone to ΔiacC, suggesting a regulatory role of iacC in the symbiosis establishment.