Early responses of rice (Oryza sativa L.) seedlings to inoculation with beneficial diazotrophic bacteria are dependent on plant and bacterial genotypes

Abstract

Rice cultivars, in combination with diazotrophic bacteria, can obtain variable contributions from Biological Nitrogen Fixation (BNF). Plant genetic controls and the genotype of the bacterial strains might regulate in particular ways the plant responses during these associations. Some of this regulation is likely to occur during the first stages of bacterial infection and plant colonization, and some of the early plant responses might involve ethylene signaling, as previously reported for sugarcane. The aim of this work was to investigate whether rice early responses to inoculation with beneficial diazotrophic bacteria and the expression of ethylene receptors (ERs) by the host are dependent on plant and bacterial genotypes. Bacterial colonization, lateral root development and expression of ERs were measured in seedlings of two rice cultivars, IR42 and IAC4440, which, respectively, are known to have higher and lower BNF capabilities. Inoculation experiments were performed with two strains of bacteria: Azospirillum brasilense sp245 and Burkholderia kururiensis M130. Cultivar IR42, which has a relatively high BNF capability, was more colonized by the bacteria early after inoculation in comparison with cv. IAC 4440, which has a lower BNF capability. However, although both cultivars showed a significant increase in lateral root numbers 10 days after inoculation with A. brasilense sp245, plants inoculated with B. kururiensis M130 did not. In addition, a differential ER expression pattern was observed between IR42 and IAC4440 in response to inoculation with the two strains. The expression of ERs was higher in the more BNF-effective cultivar IR42, than in the less BNF-effective cv. IAC4440, especially when cv. IR42 was inoculated with A. brasilense sp245. Moreover, plants associated with B. kururiensis M130 showed a different ER expression profile compared to those inoculated with A. brasilense sp245, with the latter exhibiting greater increases of ER mRNA levels three days after bacterial inoculation. We have shown that two rice genotypes contrasting in their BNF capability responded differentially to early inoculation with different strains of diazotrophic bacteria. The two rice genotypes were colonized with different bacterial numbers during their early association with two strains of diazotrophic bacteria, and the two strains had different effects on the promotion of lateral root development early after inoculation. They also triggered distinct ER expression patterns in the two rice cultivars. These data thus suggest that rice ethylene responses to beneficial diazotrophic bacteria are controlled by both plant and bacterial genotypes.