Plant root development is highly responsive both to changes in nitrate availability and beneficial microorganisms in the rhizosphere. We previously showed that Phyllobacterium brassicacearum STM196, a plant growth-promoting rhizobacteria strain isolated from rapeseed roots, alleviates the inhibition exerted by high nitrate supply on lateral root growth. Since soil-borne bacteria can produce IAA and since this plant hormone may be implicated in the high nitrate-dependent control of lateral root development, we investigated its role in the root development response of Arabidopsis thaliana to STM196. Inoculation with STM196 resulted in a 50% increase of lateral root growth in Arabidopsis wild-type seedlings. This effect was completely abolished in aux1 and axr1 mutants, altered in IAA transport and signaling, respectively, indicating that these pathways are required. The STM196 strain, however, appeared to be a very low IAA producer when compared with the high-IAA-producing Azospirillum brasilense sp245 strain and its low-IAA-producing ipdc mutant. Consistent with the hypothesis that STM196 does not release significant amounts of IAA to the host roots, inoculation with this strain failed to increase root IAA content. Inoculation with STM196 led to increased expression levels of several IAA biosynthesis genes in shoots, increased Trp concentration in shoots, and increased auxin-dependent GUS staining in the root apices of DR5::GUS transgenic plants. All together, our results suggest that STM196 inoculation triggers changes in IAA distribution and homeostasis independently from IAA release by the bacteria.

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Induced systemic resistance (ISR) is a process elicited by telluric microbes, referred to as plant growth-promoting rhizobacteria (PGPR), that protect the host plant against pathogen attacks. ISR has been defined from studies using Pseudomonas strains as the biocontrol agent. Here, we show for the first time that a photosynthetic Bradyrhizobium sp. strain, ORS278, also exhibits the ability to promote ISR in Arabidopsis thaliana, indicating that the ISR effect may be a widespread ability. To investigate the molecular bases of this response, we performed a transcriptome analysis designed to reveal the changes in gene expression induced by the PGPR, the pathogen alone, or by both. The results confirm the priming pattern of ISR described previously, meaning that a set of genes, of which the majority was predicted to be influenced by jasmonic acid or ethylene, was induced upon pathogen attack when plants were previously colonized by PGPR. The analysis and interpretation of transcriptome data revealed that 12-oxo-phytodienoic acid, an intermediate of the jasmonic acid biosynthesis pathway, is likely to be an actor in the signaling cascade involved in ISR. In addition, we show that the PGPR counterbalanced the pathogen-induced changes in expression of a series of genes.

Using their 1-amino cyclopropane-1-carboxylic acid (ACC) deaminase activity, many rhizobacteria can divert ACC from the ethylene biosynthesis pathway in plant roots. To investigate the role of this microbial activity in plant responses to plant growth-promoting rhizobacteria (PGPR), we analyzed the effects of acdS knock-out and wild-type PGPR strains on two phenotypic responses to inoculation-root hair elongation and root system architecture-in Arabidopsis thaliana. Our work shows that rhizobacterial AcdS activity has a negative effect on root hair elongation, as expected from the reduction of ethylene production rate in root cells, while it has no impact on root system architecture. This suggests that PGPR triggered root hair elongation is independent of ethylene biosynthesis or signaling pathway. In addition, it does indicate that AcdS activity alters local regulatory processes, but not systemic regulations such as those that control root architecture. Our work also indicates that root hair elongation induced by PGPR inoculation is probably an auxin-independent mechanism. These findings were unexpected since genetic screens for abnormal root hair development mutants led to the isolation of ethylene and auxin mutants. Our work hence shows that studying the interaction between a PGPR and the model plant Arabidopsis is a useful system to uncover new pathways involved in plant plasticity.The implication of ethylene signaling in the responses of plants to biotic interactions is well recognized. 1 It is undoubtedly an important piece in plant's armory against pathogen attacks as well as response to beneficial bacteria such as nitrogen fixing rhizobactearia (e.g., ethylene transduction pathway represses nodule formation in legumes 2,3 ). How these bacteria can affect the plant ethylene signaling pathway? Most of them are proteobacteria that harbor in their genome a gene (AcdS) coding for an ACC deaminase. During the plant bacteria interaction, ACDS is thought to metabolize the ethylene precursor ACC. As such, it is generally admitted that much of the ACC produced by the plant ACC synthase (ACS) activity in roots may be exuded in the rhizosphere, where it will be taken up by the rhizobacteria and subsequently hydrolyzed by AcdS. 4 This should decrease ACC content in root and predictably ethylene biosynthesis, hence partially release the negative regulation exerted by this gaseous hormone on root development and ultimately plant growth. 4 It should also diminish defense mechanisms and thus favors the interaction of bacteria with plants. This is an attractive hypothesis which started to get some momentum since either introducing an AcdS gene or removing it, stimulates or respectively affects the interaction with the plant. [5][6][7][8][9] The in vitro inoculation of plant mineral media with efficient PGPR strains leads to characteristic morphogenetic responses of the root system. Probably the most extensively described of these phenotypes consists of more numerous and/or longer lateral roots. [10][11][12] However, even ...