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SUMMARY: Soil water uptake by roots is a key component of plant water homeostasis contributing to plant growth and survival under ever‐changing environmental conditions. The water transport capacity of roots (root hydraulic conductivity; Lpr) is mostly contributed by finely regulated Plasma membrane Intrinsic Protein (PIP) aquaporins. In this study, we used natural variation of Arabidopsis for the identification of quantitative trait loci (QTLs) contributing to Lpr. Using recombinant lines from a biparental cross (Cvi‐0 x Col‐0), we show that the gene encoding class 2 Sucrose‐Non‐Fermenting Protein kinase 2.4 (SnRK2.4) in Col‐0 contributes to >30% of Lpr by enhancing aquaporin‐dependent water transport. At variance with the inactive and possibly unstable Cvi‐0 SnRK2.4 form, the Col‐0 form interacts with and phosphorylates the prototypal PIP2;1 aquaporin at Ser121 and stimulates its water transport activity upon coexpression in Xenopus oocytes and yeast cells. Activation of PIP2;1 by Col‐0 SnRK2.4 in yeast also requires its protein kinase activity and can be counteracted by clade A Protein Phosphatases 2C. SnRK2.4 shows all hallmarks to be part of core abscisic acid (ABA) signaling modules. Yet, long‐term (>3 h) inhibition of Lpr by ABA possibly involves a SnRK2.4‐independent inhibition of PIP2;1. SnRK2.4 also promotes stomatal aperture and ABA‐induced inhibition of primary root growth. The study identifies a key component of Lpr and sheds new light on the functional overlap and specificity of SnRK2.4 with respect to other ABA‐dependent or independent SnRK2s. [ABSTRACT FROM AUTHOR]

Summary Mechanisms that regulate water channels in the plant plasma membrane (PM) were investigated in Arabidopsis suspension cells. Cell hydraulic conductivity was measured with a cell pressure probe and was reduced 4-fold as compared to control values when calcium was added in the pipette and in bathing solution. To assess the significance of these effects in vitro , PM vesicles were isolated by aqueous two-phase partitioning and their water transport properties were characterized by stopped-flow spectrophotometry. Membrane vesicles isolated in standard conditions exhibited reduced water permeability (P f ) together with a lack of active water channels. In contrast, when prepared in the presence of chelators of divalent cations, PM vesicles showed a 2.3-fold higher P f and active water channels. Furthermore, equilibration of purified PM vesicles with divalent cations reduced their P f and water channel activity down to the basal level of membranes isolated in standard conditions. Ca2+ was the most efficient with a half-inhibition of P f at 50–100 µm free Ca2+ . Water transport in purified PM vesicles was also reversibly blocked by H+ , with a half-inhibition of P f at pH 7.2–7.5. Thus, both Ca2+ and H+ contribute to a membrane-delimited switch from active to inactive water channels that may allow coupling of water transport to cell signalling and metabolism. [ABSTRACT FROM AUTHOR]

SummaryA plasma membrane (PM) fraction was purified from Arabidopsis thaliana using a standard procedure and analyzed by two-dimensional (2D) gel electrophoresis. The proteins were classified according to their relative abundance in PM or cell membrane supernatant fractions. Eighty-two of the 700 spots detected on the PM 2D gels were microsequenced. More than half showed sequence similarity to proteins of known function. Of these, all the spots in the PM-specific and PM-enriched fractions, together with half of the spots with similar abundance in PM fraction and supernatant, have previously been found at the PM, supporting the validity of this approach. Extrapolation from this analysis indicates that (i) approximately 550 polypeptides found at the PM could be resolved on 2D gels; (ii) that numerous proteins with multiple locations are found at the PM; and (iii) that approximately 80% of PM-specific spots correspond to proteins with unknown function. Among the latter, half are represented by ESTs or cDNAs in databases. In this way, several unknown gene products were potentially localized to the PM. These data are discussed with respect to the efficiency of organelle proteome approaches to link systematically genomic data to genome expression. It is concluded that generalized proteomes can constitute a powerful resource, with future completion of Arabidopsis genome sequencing, for genome-wide exploration of plant function. [ABSTRACT FROM AUTHOR]