Plant must adapt to changing environmental factors and coordinate their growth and development accordingly. This is accomplished with effective cell communication that includes signaling molecules and receptors to perceive the signal. Plasma membrane proton pumps are key plant proteins that are involved in various physiological adaptations to the environment. An array of hormones and signaling peptides are involved in regulation of the proton pumping activity. The aim of this study is to understand how the PSY family of signaling peptides is regulated, how cells respond to PSY1, and what the potential physiological effects of the PSYs are.
PSY1 is perceived by a receptor kinase PSY1R. PSY1R was found to interact with the major
isoforms of proton pumps in Arabidopsis, AHA1 and AHA2, both in vivo and in vitro. The receptor kinase phosphorylated T881 of AHA1/2, a modification that caused activation of the pump. Plants exposed to exogenously applied PSY1 showed an increased level of phosphorylated T881 as well as rapid proton efflux from roots. The physiological effect of this proton pump activation was demonstrated to initiate cell elongation. The activation mechanism of PSY1R involved multiple transphosphorylation events, homodimerization and heterodimerization with SERK co-receptors.
The very C-terminal domain of the kinase is essential for the function, as a shortened version of PSY1R kinase did not autophosphorylate, form dimers or interact with AHA1/2. The importance of the C-terminal domain represents a novel kinase regulatory mechanism.
Signaling peptides are regulated at the level of expression, maturation and secretion. Members of the PSY family showed distinct expression patterns indicating that they are involved in other physiological processes besides cell elongation. As an example, enhanced expression of PSY2 and PSY3 in guard cells indicated that they regulate stomata, which is an organ responsible for gas exchange in the leaf that is necessary for photosynthesis.
To visualize the maturation and secretion of PSY1 in living tissue a PSY1-GFP fusion protein was expressed in tobacco leaves. In cells that overproduced PSY1-GFP accumulations of ER structures occurred, and cells that expressed PSY1-GFP in lower levels had a signal too vague for detection in the apoplast.
In growth analyses of seedlings, PSY1 and PSY2 had a positive effect on elongation in roots and hypocotyls, whereas PSY3 had a negative effect. Future studies will reveal the molecular mechanisms behind this, and reveal if all members of the PSY family bind to the same receptor and affect the plasma membrane proton pump.