Insights from next-generation proteomics of bacterial endophytes response to chickpea root exudates and GacS role on plant growth-promoting traits regulation

Abstract

Besides the beneficial association that legumes establish with rhizobia, these plants are also colonized by other endophytic bacteria. Although it is believed that these bacteria also have an important role on plant fitness, the molecular mechanisms of how non-rhizobial bacteria respond to the host-derived signals are poorly understood compared to the well-characterized N2 -fixing legumes-rhizobia symbioses. Here, a high-throughput proteomic analyses of two endophytic bacteria detected proteins involved in metabolism, cell envelope biosynthetic process, stress response, defense against oxidative stress, chemotaxis, nitrogen metabolic process, type iv secretion system and transmembrane transport as response to chickpea root exudates. One of the genes highly upregulated in cellular proteome of Pseudomonas sp. Q1 after exposition to chickpea root exudates is the gene on locus 4453. This gene (2,754 bp) encodes the GacS (global activator of antibiotic and cyanide synthesis), a membrane-bound sensor hybrid histidine kinase, member of the Gac two-component signal transduction system. To investigate the phenotypic traits under the regulation of the Pseudomonas sp. Q1 GacS protein, a knockout mutant for gacS gene was obtained. The swarming and swimming motility, biofilm production or phosphate solubilization were not affected by gacS gene deletion on ∆gacS mutant strain. On the other hand, the production and secretion of siderophores by ∆gacS mutant was reduced around 8.8% compared with the wild-type strain but the biosynthesis of antimicrobial metabolites in vitro was not affected. Our results showed that GacS of Q1 strain is only partially involved in the regulation of the mainly plant beneficial traits of Pseudomonas sp. Q1, suggesting that in this strain the role of this protein can be fulfilled by one or more additional two-component signal transduction systems. However, further studies are necessary to clarify the role of GacS in the regulation of surface motility, siderophore production and antifungal activity of Q1 strain.