"B" Regulatory Subunits of PP2A: Their Roles in Plant Development and Stress Reactions

Abstract

Protein phosphatase PP2A is an enzyme complex consisting of C (catalytic), A (scaffold) and B (regulatory) subunits. B subunits are a large family of proteins that regulate activity, substrate specificity and subcellular localization of the holoenzyme. Knowledge on the molecular functions of PP2A in plants is less than for protein kinases, but it is rapidly increasing. B subunits are responsible for the large diversity of PP2A functioning. This paper intends to give a survey on their multiple regulatory mechanisms. Firstly, we give a short description on our current knowledge in terms of "B"-mediated regulation of metabolic pathways. Next, we present their subcellular localizations, which extend from the nucleus to the cytosol and membrane compartments. The next sections show how B subunits regulate cellular processes from mitotic division to signal transduction pathways, including hormone signaling, and then the emerging evidence for their regulatory (mostly modulatory) roles in both abiotic and biotic stress responses in plants. Knowledge on these issues should be increased in the near future, since it contributes to a better understanding of how plant cells work, it may have agricultural applications, and it may have new insights into how vascular plants including crops face diverse environmental challenges.

Keywords: B subunit; abiotic stress; biotic stress; development; metabolism; protein phosphatase PP2A; signal transduction; subcellular localization.

Figure 1

Summary for the known and supposed roles of PP2A/B subunits in Arabidopsis metabolism and development. Abbreviations: CMT—cortical microtubule; ET—ethylene; ETR1—ethylene response 1; NR—nitrate reductase; PM—plasma membrane; PPB—preprophase band; px—peroxisome; TTP—Ton1-TRM-PP2A complex; VIP1—VirE2-interacting protein 1. Functions that are not fully proven are indicated with red characters. Figure created with BioRender.com (accessed on 15 January 2023).

Figure 2

The roles of PP2A/B subunits in the Arabidopsis responses to abiotic and biotic stresses. Abbreviations: ACO3—aconitase 3; AOX—alternative oxidase; BAK1—brassinosteroid insensitive 1–associated kinase 1; BES1—BRI1–EMS–suppressor 1; BRI1—brassinosteroid insensitive 1; BZR1—brassinazole resistant 1; CPK1—calcium-dependent protein kinase 1; ET—ethylene; flg22—flagellin; FLS2—flagellin sensing receptor 2; ICS—isochorismate synthase; mit—mitochondrion; NPR1—nonexpresser of PR1; OST1—open stomata 1; PM—plasma membrane; PRR—pattern recognition receptor; ROS—reactive oxygen species; SA—salicylic acid; SAG1—senescence associated gene 1; SOD—superoxide dismutase; UPR—unfolded protein response. Figure created with BioRender.com (accessed on 15 January 2023).