Plants encode a large number of proteases in activating intracellular signaling through proteolytic cleavages of various protein substrates. One type of the substrates is proligands, including peptide hormones, which are perceived by cell surface-resident receptors. The peptide hormones are usually first synthesized as propeptides, and then cleaved by specific proteases for activation. Accumulating evidence indicates that the protease-mediated cleavage of proligands can be triggered by environmental stresses and subsequently activates plant stress signaling. In this perspective, we highlight several recent publications and provide an update about stress-induced cleavage of propeptides and receptor-associated components by proteases in the activation of cell surface-resident receptor signaling in plants. We also discuss some questions and future challenges in the research of protease functions in plant stress response.
Plants sense the presence of pathogens or pests through the recognition of evolutionarily conserved microbe- or herbivore-associated molecular patterns or specific pathogen effectors, as well as plant endogenous danger-associated molecular patterns. This sensory capacity is largely mediated through plasma membrane and cytosol-localized receptors which trigger complex downstream immune signaling cascades. As immune signaling outputs are often associated with a high fitness cost, precise regulation of this signaling is critical. Protease-mediated proteolysis represents an important form of pathway regulation in this context. Proteases have been widely implicated in plant–pathogen interactions, and their biochemical mechanisms and targets continue to be elucidated. During the plant and pathogen arms race, specific proteases are employed from both the plant and the pathogen sides to contribute to either defend or invade. Several pathogen effectors have been identified as proteases or protease inhibitors which act to functionally defend or camouflage the pathogens from plant proteases and immune receptors. In this review, we discuss known protease functions and protease-regulated signaling processes involved in both sides of plant–pathogen interactions.
The mechanisms by which Ca 2+ -store-release channels and Ca 2+ -entry channels are coupled to receptor activation are poorly understood. Modification of Ca 2+ signals by 2-aminoethoxydiphenyl borate (2-APB), suggests the agent may target entry channels or the machinery controlling their activation. In DT40 B-cells and Jurkat T-cells, complete Ca 2+ store release was induced by 2-APB (EC 50 10–20 µM). At 75 µM, 2-APB emptied stores completely in both lymphocyte lines, but had no such effect on other cells. In DT40 cells, 2-APB mimicked B-cell receptor (BCR) cross-linking, but no effect was observed in mutant DT40 lines devoid of inositol 1,4,5-trisphosphate (InsP 3 ) receptors (InsP 3 Rs) or phospholipase C-γ2 (PLC-γ2). Like the BCR, 2-APB activated transfected TRPC3 (canonical transient receptor potential) channels, which acted as sensors for PLC-γ2-generated diacylglycerol in DT40 cells. The action of 2-APB on InsP 3 Rs and TRPC3 channels was prevented by PLC-inhibition, and required PLC-γ2 catalytic activity. However, unlike BCR activation, no increased InsP 3 level could be measured in response to 2-APB. Also, calyculin A-induced cytoskeletal reorganization prevented 2-APB-induced InsP 3 R and TRPC3-channel activation, but not that induced by the BCR. 2-APB still activated TRPC3 channels in DT40 cells with fully depleted Ca 2+ stores, indicating its action was not via Ca 2+ release. Significantly, 2-APB-induced InsP 3 R and TRPC3 activation was prevented in DT40 knockout cells devoid of the BCR- and PLC-γ2-coupled adaptor/kinases, Syk, Lyn, Btk or BLNK. The results suggest that 2-APB activates Ca 2+ signals in lymphocytes by initiating and enhancing coupling between components of the BCR–PLC-γ2 complex and both Ca 2+ -entry and Ca 2+ -release channels.