Environmental alterations challenge the plant growth and reproduction in nature and in crop fields. Plants harbour acclimatory and defensive mechanisms that become activated under unfavourable conditions to ensure the plant survival. Recognition of stress factors triggers signalling cascades in plant cell that activate changes in gene expression, induce hormonal signals and modulate plant metabolism via posttranslational modification of enzymes. Reversible protein phosphorylation, carried out by counteracting pairs of protein kinases and phosphatases, presents a major mechanism in signal transduction as well as in the control of metabolic enzymes in plant defence and stress acclimation. 

Protein phosphatase 2A (PP2A) is a trimeric phosphatase composed of a scaffold subunit A, catalytic subunit C and regulatory subunit B, all of which are encoded by multiple genes in the model plant Arabidopsis thaliana. PP2A regulatory subunit B’γ (PP2A-B’γ) has been previously identified as a negative regulator of salicylic acid associated defence and cell death in Arabidopsis leaves. In this PhD thesis, PP2A-B’γ target proteins and its role in regulation of plant acclimation and metabolism were further studied. 

PP2A-B’γ and its closest homologue PP2A-B’ζ were found to modulate the plant growth and stress acclimation under normal growth conditions and under severe abiotic stress. Moreover, PP2A-B’γ was shown to regulate enzymes in plant primary and secondary metabolism. PP2A-B’γ interacted with ACONITASE 3 (ACO3) and controlled its phosphorylation. ACO3 phosphorylation was further connected to accumulation of ACO3 protein. Both PP2A-B’γ and ACO3 abundance were discovered to affect the accumulation of mitochondrial alternative oxidase at post-translational level thus contributing to the control of cell redox balance. In addition to modulation of primary metabolism, PP2A-B’γ was found to influence the formation of 4-methoxyindole- 3-yl-methly glucosinolate (4MO-I3M GSL), a defence compound with antimicrobial activities. Moreover, PP2A-B’γ interacted with activated methyl cycle (AMC) enzymes linked to production of 4MO-I3M GSL. PP2A-B’γ regulated the protein complex formation of AMC enzymes exerting its control over the cell methylation capacity. 

These findings provide new information of plant acclimation under abiotic stress and regulation of stress associated adjustments in plant metabolism. Detailed knowledge of plant acclimatory and defensive mechanisms and stress induced adjustments in plant metabolism is valuable in the development of more tolerant and nutritious crops.