Protein phosphatase 2A as a regulator and mediator of beneficial plant-microbe interactions

Abstract

Protein phosphatase 2A (PP2A) is involved in the regulation of stress response, growth and hormone signalling during the plant life cycle. A variety of plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhiza fungi (AMF) stimulate plant growth and improve plant health, but the contribution of PP2A to the beneficial plant-microbe interactions is still obscure. The treatment of Arabidopsis wild type and PP2A (related) mutants, exhibiting either low or high PP2A activity, with the PGPR Azospirillum brasilense (wild-type strain Sp245 and auxin-deficient strain FAJ0009) and Pseudomonas simiae (WCS417r) revealed the essential role of the PP2A catalytic subunits from subfamily I in a positive response to PGPR. Moreover, the role of the regulatory subunit B’θ may also be important in this regard. Furthermore, the positive effect on the fresh weight of roots and shoots was especially pronounced in Arabidopsis mutants with low PP2A activity. The treatment of tomato plants with the above-mentioned PGPR confirmed at the molecular level that the PP2A catalytic subunit belonging to subfamily I and the regulatory subunit B’θ may be important mediators in the interaction between plants and PGPR. The role of the regulatory subunit B’φ, previously characterized as a regulator of AMF colonization, was also investigated. For this purpose, transgenic tomato plants were engineered to overexpress the B’φ subunit derived from Solanum lycopersicum cv. Heinz under control of the cauliflower mosaic virus 35S promoter. The resulting plants were characterized by slowed development with significantly decreased root and shoot biomass, and a threefold reduction in seed number. Semi-quantitative reverse-transcription PCR analysis of the transgenic plants showed a significant disturbance in the homeostasis of abscisic acid (ABA) and gibberellins (GAs), and a striking reduction in the content of bioactive GAs in roots.