Auxin regulate nitrogen fixation in rhizobia

Plant growth and development involves the integration of many environmental and endogenous signals that, together with the intrinsic genetic program, determine plant form. Fundamental to this process are several growth regulators collectively called the plant hormones or phytohormones. This group includes auxin, cytokinin, the gibberellins, abscisic acid, ethylene, the brassinosteroids, and jasmonic acid, each of which regulates many aspects of plant growth and development. The majority of root associated bacteria that display beneficial effects on plant growth produce indole-3-acetic acid (IAA). We have described an engineered rhizobium strain, RD64, a derivative of Sinorhizobium meliloti 1021, able to release into liquid growth medium up to 78-fold more IAA than wild-type 1021. We provided evidence that overexpression of IAA in S. meliloti 1021 played a positive role in the adaptation to different environmental conditions, including salt stress and P starvation, both in free-living bacteria and in nodulated Medicago truncatula plants. We also reported that plants nodulated by the IAA overproducing strain (Mt-RD64) exhibited improved nitrogen-fixing ability and a re-modulation of phytohormones, with a higher IAA content in nodules and roots and a decreased IAA levels in shoots, as compared with plants nodulated by the wild type strain (Mt-1021). The modulation of IAA levels in these plants lead to alterations of other important hormones that control plant growth (e.g. cytokinins). Here we show a more detailed elaboration of gene expression data obtained in microarray analysis in which we compared the gene expression pattern of wild type 1021 with that of RD64 and 1021 supplemented with IAA and other four structurally or functionally related molecules [2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-carboxylic acid (ICA), indole (IND) and tryptophan (Trp)]. We found a greater cross-talk between the response processes to IAA and the synthetic auxin 2,4-D than between IAA and the other molecules. This trend was also observed under symbiotic conditions. Indeed, RT-PCR analysis of nitrogen fixation genes revealed that the relative expression levels of nif and fix genes were significantly induced in Mt-RD64 plants and in Mt-1021 plants treated with IAA and the synthetic auxin 2,4-D. This effect seems to be specific to the hormonal activity of IAA since the treatment with structurally similar molecules did not lead to significant alterations in the expression of these genes.