AbstractBackgroundPROTEIN PHOSPHATASE 2A (PP2A) expression is crucial for the symbiotic association between plants and various microbes, and knowledge on these symbiotic processes is important for sustainable agriculture. Here we tested the hypothesis that PP2A regulatory subunits, especiallyB’φandB’θ,are involved in signalling between plants and mycorrhizal fungi or plant-growth promoting bacteria.ResultsTreatment of tomato plants (Solanum lycopersicum)with the plant growth-promoting rhizobacteria (PGPR)Azospirillum brasilenseandPseudomonas simiaeindicated a role for the PP2A B’θ subunit in responses to PGPR. Arbuscular mycorrhizal fungi influencedB’θtranscript levels in soil-grown plants with canonical arbuscular mycorrhizae. In plant roots, transcripts ofB’φwere scarce under all conditions tested and at a lower level than all other PP2A subunit transcripts. In transformed tomato plants with 10-fold enhancedB’φexpression, mycorrhization frequency was decreased in vermiculite-grown plants. Furthermore, the highB’φexpression was related to abscisic acid and gibberellic acid responses known to be involved in plant growth and mycorrhization.B’φoverexpressor plants showed less vigorous growth, and although fruits were normal size, the number of seeds per fruit was reduced by 60% compared to the original cultivar.ConclusionsExpression of theB’θgene in tomato roots is strongly influenced by beneficial microbes. Analysis ofB’φoverexpressor tomato plants and established tomato cultivars substantiated a function ofB’φin growth and development in addition to a role in mycorrhization.