Yield response of common bean to co-inoculation with Rhizobium and Pseudomonas endophytes and microscopic evidence of different colonised spaces inside the nodule

Microbial inoculants are gaining prominence in technologically advanced agri-systems due to the need for alternatives to the most pollutant agricultural inputs. The objective of this work was to improve the agronomic performance of the rhizobial inoculants for common bean, based on the superior native strain Rlp-LCS0306 of Rhizobium leguminosarum bv. phaseoli (R), through co-inoculation with non-rhizobial partners, namely the autochthonous isolate RVPB2-2 from Pseudomonas brassicacearum subsp. neoaurantiaca (P) and the type strain of Azotobacter chroococcum. It has been reported that co-inoculation improves nodulation, nodule functions and plant growth, although there is a lack of field testing in technologically advanced agri-systems. This work bridges this gap. In the field trial which was carried out in two different environments, the consortium R + P was the most successful, because it increased the N-2 fixation by 51.7 kg ha-1 (87 %) and the yield by 1337 kg ha-1 (59 %), compared with the uninoculated and unfertilised control. In addition, the increased yield observed following inoculation with the above indicated consortium was 16.7 %, compared with the single rhizobia inoculation, and this increase was also superior to that observed with other consortia. The superiority of the R + P consortium could partially be explained because in this study, there was an increased tendency for improved nodule biomass and function following co-inoculation. While this increase was not deemed to be statistically significant, it is noteworthy that nodule biomass increased by 25 % in average and N-fixed by more than 20 %, which, in turn, could be explained by the indole-3-acetic acid (IAA) production and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity of the P strain. However, further delineation of the system is required in order to explain the yield improvement exerted by the consortium. Here, we observed, i) the strong plant growth-promoting potential displayed by the P strain; ii) the colonisation of the nodules by the P strain; and iii) the strategy of colonisation of complementary spaces inside the nodules by P (intercellular) and the rhizobia (intracellular), by confocal microscopy.