Specific PP2A Catalytic Subunits Are a Prerequisite for Positive Growth Effects in Arabidopsis Co-Cultivated with Azospirillum brasilense and Pseudomonas simiae

Plant growth-promoting rhizobacteria (PGPR) stimulate plant growth, but the underlying mechanism is poorly understood. In this study, we asked whether PROTEIN PHOSPHATASE 2A (PP2A), a regulatory molecular component of stress, growth, and developmental signaling networks in plants, contributes to the plant growth responses induced by the PGPR Azospirillum brasilense (wild type strain Sp245 and auxin deficient strain FAJ0009) and Pseudomonas simiae (WCS417r). The PGPR were co-cultivated with Arabidopsis wild type (WT) and PP2A (related) mutants. These plants had mutations in the PP2A catalytic subunits (C), and the PP2A activity-modulating genes LEUCINE CARBOXYL METHYL TRANSFERASE 1 (LCMT1) and PHOSPHOTYROSYL PHOSPHATASE ACTIVATOR (PTPA). When exposed to the three PGPR, WT and all mutant Arabidopsis revealed the typical phenotype of PGPR-treated plants with shortened primary root and increased lateral root density. Fresh weight of plants generally increased when the seedlings were exposed to the bacteria strains, with the exception of catalytic subunit double mutant c2c5. The positive effect on root and shoot fresh weight was especially pronounced in Arabidopsis mutants with low PP2A activity. Comparison of different mutants indicated a significant role of the PP2A catalytic subunits C2 and C5 for a positive response to PGPR.