Genes involved in small peptide biosynthesis are implicated in water stress responses of grapevine

The hypothesis that small peptides in the CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) family are responsible for stomatal regulation under soil drying was tested in grapevine (Vitis vinifera L.). Potted Cabernet Sauvignon grapevines were subjected to water stress by withholding irrigation over a 10-day period followed by 6 days of holding stress and 10 days of recovery. Soil drying resulted in declines in leaf (Ψ_L) and stem (Ψ_S) water potentials and root and leaf hydraulic conductances. Near-complete stomatal closure occurred within 10 days of withholding water. Extending the duration of dry soil over an additional week had no additional effect on stomatal conductance (g_s), but decreased Ψ_L and Ψ_S. Xylem sap abscisic acid (ABA) increased during peak water stress and declined during extended stress. Transcripts of CLE1, CLE3 and CLE6 followed the same patterns as ABA and its transcript NCED1, increasing as the soil dried and decreasing upon soil rehydration. Gene expression of CLE9 increased in both roots and leaves in response to soil drying, but, in contrast to the other CLE peptides, it remained high in the roots even after the vines recovered from water stress. The results indicate that CLE9 may be a key root-to-shoot signal of soil drying in grapevine. Together with the putative regulation of leaf ABA by CLE peptides, stomatal regulation is suggested to be indirectly controlled by root and leaf CLE9 under soil drying. An overall model of hydraulic and chemical signalling in grapevine under water stress that incorporates the role of CLE peptides is proposed.