The phosphorylation state and expression of soybean BiP isoforms are differentially regulated following abiotic stresses

The mammalian BiP is regulated by phosphorylation, and it is generally acce pted that its unmodified form constitutes the biologically active species. In fact, the glycosylation inhibitor tunicamycin induces dephosphorylation of mammalian BiP, The stress-induced phosphorylation state of plant BiP has not been examined, Here, we demonstrated that soybean BiP exists in interc onvertible phosphorylated and nonphosphorylated forms, and the equilibrium can be shift to either direction in response to different stimuli. In contr ast to tunicamycin treatment, water stress condition stimulated phosphoryla tion of BiP species in soybean cultured cells and stressed leaves. Despite their phosphorylation state, we demonstrated that BiP isoforms from water-s tressed leaves exhibit protein binding activity, suggesting that plant BiP functional regulation may differ from other eukaryotic BiPs, We also compar ed the induction of the soybean BiP gene family, which consists of at least four members designated soyBiPA, soyBiPB, soyBiPC, and soyBiPD, by tunicam ycin and osmotic stress. Although all soybean BiP genes were induced by tun icamycin, just the soyBiPA RNA was upregulated by osmotic stress. In additi on, these stresses promoted BiP induction with different kinetics and acted synergistically to increase BiP accumulation. These results suggest that t he soybean BiP gene family is differentially regulated by abiotic stresses through distinct signaling pathways.