PEP7 acts as a peptide ligand for the receptor kinase SIRK1 to regulate aquaporin-mediated water influx and lateral root growth.

Plant receptors constitute a large protein family that regulates various aspects of development and responses to external cues. Functional characterization of this protein family and the identification of their ligands remain major challenges in plant biology. Previously, we identified plasma membrane-intrinsic sucrose-induced receptor kinase 1 (SIRK1) and Qian Shou kinase 1 (QSK1) as receptor/co-receptor pair involved in the regulation of aquaporins in response to osmotic conditions induced by sucrose. In this study, we identified a member of the elicitor peptide (PEP) family, namely PEP7, as the specific ligand of th receptor kinase SIRK1. PEP7 binds to the extracellular domain of SIRK1 with a binding constant of 1.44 ± 0.79 μM and is secreted to the apoplasm specifically in response to sucrose treatment. Stabilization of a signaling complex involving SIRK1, QSK1, and aquaporins as substrates is mediated by alterations in the external sucrose concentration or by PEP7 application. Moreover, the presence of PEP7 induces the phosphorylation of aquaporins in vivo and enhances water influx into protoplasts. Disturbed water influx, in turn, led to delayed lateral root development in the pep7 mutant. The loss-of-function mutant of SIRK1 is not responsive to external PEP7 treatment regarding kinase activity, aquaporin phosphorylation, water influx activity, and lateral root development. Taken together, our data indicate that the PEP7/SIRK1/QSK1 complex represents a crucial perception and response module that mediates sucrose-controlled water flux in plants and lateral root development. SIRK1 is a receptor kinase and, together with the co-receptor QSK1, activates the channel activity of aquaporins. In this work, the authors identify the small peptide PEP7 as a ligand of the SIRK1-QSK1 receptor complex. Using combined methods of biochemistry, biophysics and physiology, the authors show that PEP7 binds to the extracellular domain of SIRK1 and can activate SIRK1 in the presence of QSK1, resulting in activated activity of aquaporins to promote lateral root development. [ABSTRACT FROM AUTHOR]