B regulatory subunit of phosphatase 2A (PP2A) involved in salt stress response (PubMed:31221736). Under salt stress conditions, required for the catalytic activity of PP2A and the dephosphorylation of SIT1, a negative regulator of salt tolerance (PubMed:31221736). Dephosphorylation of SIT1 turns off salt-induced SIT1 activity directly, which has a positive effect on salt tolerance (PubMed:31221736). PP2A consists of a common heteromeric enzyme, composed of a catalytic subunit (subunits C), a constant regulatory subunit (subunit A), and a variety of regulatory subunits such as subunits B (the R2/B/PR55/B55, R3/B''/PR72/PR130/PR59 and R5/B'/B56 families) (By similarity). Interacts with SIT1 (PubMed:31221736). The partial localization to the plasma membrane may be due to its association with SIT1. Expressed in root stele and epidermal cells. Induced by salt stress (at protein level). Phosphorylated at THR-476, THR-493, SER-502 and THR-508 by SIT1. No visible phenotype under normal growth conditions, but mutant plants exhibit increased sensitivity to salt stress. Belongs to the phosphatase 2A regulatory subunit B56 family.