Azospirillum brasilense Sp245 lipopolysaccharides induce target of rapamycin signaling and growth in Arabidopsis thaliana

The Target of Rapamycin (TOR) protein kinase plays a pivotal role in metabolism and gene expression, which enables cell proliferation, growth and development. Lipopolysaccharides (LPS) are a class of complex glycolipids present in the cell surface of Gram-negative bacteria and mediate plant-bacteria interactions. In this study, we examined whether LPS from Azospirillum brasilense Sp245 affect Arabidopsis thaliana growth via a mechanism involving TOR. A. thaliana plants were treated with LPS and plant growth and development were analyzed in mature plants. Morphological and molecular changes as well as TOR expression and activity were analyzed in root tissues. LPS increased total fresh weight, root length and TOR::GUS expression in the root meristem. Phosphorylation of S6k protein, a downstream target of TOR, increased following LPS treatment, which correlated with increased or decreased expression of CycB1;1::GUS protein upon treatment with LPS or TOR inhibitor AZD-8055, respectively. Long term LPS treatment further increased the rosette size as well as the number of stems and siliques per plant, indicating an overall phytostimulant effect for these signaling molecules. Taken together, the results suggest that A. brasilense LPS play probiotic roles in plants influencing TOR-mediated processes.

Keywords: Arabidopsis thaliana; Azospirillum brasilense; Lipopolysaccharides; Root meristem; Target of rapamycin.