The calcium-dependent protein kinase CPK7 acts on root hydraulic conductivity

Abstract

The hydraulic conductivity of plant roots (Lpr) is determined in large part by the activity of aquaporins. Mechanisms occurring at the post-translational level, in particular phosphorylation of aquaporins of the plasma membrane intrinsic protein 2 (PIP2) subfamily, are thought to be of critical importance for regulating root water transport. However, knowledge of protein kinases and phosphatases acting on aquaporin function is still scarce. In the present work, we investigated the Lpr of knockout Arabidopsis plants for four Ca2+-dependent protein kinases. cpk7 plants showed a 30% increase in Lpr because of a higher aquaporin activity. A quantitative proteomic analysis of wild-type and cpk7 plants revealed that PIP gene expression and PIP protein quantity were not correlated and that CPK7 has no effect on PIP2 phosphorylation. In contrast, CPK7 exerts a negative control on the cellular abundance of PIP1s, which likely accounts for the higher Lpr of cpk7. In addition, this study revealed that the cellular amount of a few additional proteins including membrane transporters is controlled by CPK7. The overall work provides evidence for CPK7-dependent stability of specific membrane proteins. Plant aquaporins that are critical regulators of root water transport are regulated by phosphorylation. However the knowledge of protein kinases acting on aquaporin function is still scarce. In the present work, by combining physiological measurements of root water transport with quantitative proteomics we identified CPK7, a member of calcium-dependent protein kinases family as an unexpected negative regulator of the cellular abundance of a family of aquaporins (PIP1, Plasma membrane Intrinsic Protein 1). In addition, this study revealed that the cellular amount of a few additional membrane transporters is controlled by CPK7. Thus this work provides evidence for a new CPK7-dependent mechanism involved in the stability of specific membrane proteins, with effects on water transport and possibly other root functions, opening new perspectives in membrane protein research.