Protein phosphatase 2A catalytic subunit (PP2A-C) has a terminal leucine subjected to methylation, a regulatory mechanism conserved from yeast to mammals and plants. Two enzymes, LCMT1 and PME1, methylate and demethylate PP2A-C, respectively. The physiological importance of these posttranslational modifications is still enigmatic. We investigated these processes in Arabidopsis thaliana by mutant phenotyping, by global expression analysis, and by monitoring methylation status of PP2A-C under different environmental conditions. The lcmt1 mutant, possessing essentially only unmethylated PP2A-C, had less dense rosettes, and earlier flowering than wild type (WT). The pme1 mutant, with 30% reduction in unmethylated PP2A-C, was phenotypically comparable with WT. Approximately 200 overlapping genes were twofold upregulated, and 200 overlapping genes were twofold downregulated in both lcmt1 and pme1 relative to WT. Differences between the 2 mutants were also striking; 97 genes were twofold upregulated in pme1 compared with lcmt1, indicating that PME1 acts as a negative regulator for these genes. Analysis of enriched GO terms revealed categories of both abiotic and biotic stress genes. Furthermore, methylation status of PP2A-C was influenced by environmental stress, especially by hypoxia and salt stress, which led to increased levels of unmethylated PP2A-C, and highlights the importance of PP2A-C methylation/demethylation in environmental responses.

PP2A catalytic subunit C2 is of special importance for light/dark regulation of nitrate reductase activity. The level of unmethylated PP2A catalytic subunits decreases in darkness. Protein phosphatase 2A (PP2A) dephosphorylates and activates nitrate reductase (NR) in photosynthetically active tissue when plants are transferred from darkness to light. In the present work, investigation of Arabidopsis thaliana PP2A mutant lines revealed that one of the five PP2A catalytic subunit genes, e.g., C2, was of special importance for NR activation. Impairment of NR activation was, especially pronounced in the c2c4 double mutant. Though weaker, NR activation was also impaired in the c2 single mutant, and c1c2 and c2c5 double mutants. On the other hand, NR activation in the c4c5 double mutant was as efficient as in WT. The c4 single mutant had low PP2A activity, whereas the c2 single mutant possessed WT levels of extractable PP2A activity. PP2A activity was low in both c2c4 and c4c5. Differences in extracted PP2A activity among mutants did not strictly correlate with differences in NR activation, but underpinned that C2 has a special function in NR activation in vivo. The terminal leucine in PP2A catalytic subunits is generally methylated to a high degree, but regulation and impact of methylation/demethylation is barely studied. In WT and PP2A mutants, the level of unmethylated PP2A catalytic subunits decreased during 45 min of darkness, but did not change much when light was switched on. In leucine carboxyl methyl transferase1 (LCMT1) knockout plants, which possess mainly unmethylated PP2A, NR was still activated, although not fully as efficient as in WT.

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.

Background PROTEIN 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, especially B’φ and B’θ, are involved in signalling between plants and mycorrhizal fungi or plant-growth promoting bacteria. Results Treatment of tomato plants (Solanum lycopersicum) with the plant growth-promoting rhizobacteria (PGPR) Azospirillum brasilense and Pseudomonas simiae indicated a role for the PP2A B’θ subunit in responses to PGPR. Arbuscular mycorrhizal fungi influenced B’θ transcript levels in soil-grown plants with canonical arbuscular mycorrhizae. In plant roots, transcripts of B’φ were scarce under all conditions tested and at a lower level than all other PP2A subunit transcripts. In transformed tomato plants with 10-fold enhanced B’φ expression, mycorrhization frequency was decreased in vermiculite-grown plants. Furthermore, the high B’φ expression was related to abscisic acid and gibberellic acid responses known to be involved in plant growth and mycorrhization. B’φ overexpressor 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. Conclusions Expression of the B’θ gene in tomato roots is strongly influenced by beneficial microbes. Analysis of B’φ overexpressor tomato plants and established tomato cultivars substantiated a function of B’φ in growth and development in addition to a role in mycorrhization.

Background PROTEIN 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, especially B’φ, are involved in signalling between plants and mycorrhizal fungi or plant-growth promoting bacteria. Results Treatment of tomato plants (Solanum lycopersicum) with the plant growth-promoting rhizobacteria (PGPR) Azospirillum brasilense and Pseudomonas simiae indicated a role for the PP2A B’θ subunit in responses to PGPR. Arbuscular mycorrhizal fungi also influenced B’q transcript levels, but only in soil-grown plants with canonical arbuscular mycorrhizae, not in vermiculite-grown plants which had only vesicular mycorrhizae. In plant roots, transcripts of B’φ were scarce under all conditions tested and at a lower level than all other PP2A subunit transcripts. In transformed tomato plants with 10-fold enhanced B’φ expression, mycorrhization frequency was decreased in vermiculite-grown plants. Furthermore, the high B’φ expression was related to abscisic acid and gibberellic acid responses known to be involved in plant growth and mycorrhization. B’φ overexpressor 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. Conclusions Expression of the B’θ gene in plant roots is strongly influenced by beneficial microbes. Expression analysis and phenotype observations of established tomato cultivars and B’φ overexpressor plants substantiate a function of B’φ in growth and development in addition to a role in mycorrhization.