Homologs of SD6 and ICE2 from rice may be involved in regulation of ABA in woody perennial buds

Abstract

The availability of ABA, a central component in the regulation of the dormancy cycle in grapevine buds, is controlled by coordinated and opposite regulation of NCED and ABA8OX expression, as shown during natural dormancy release and following treatment with Hydrogen Cyanamide (HC). This implies the existence of a shared regulatory entity, which serves as an upstream switch. A molecular switch for integrated and opposite regulation of NCED and ABA8OX was recently described in rice, involving a pair of bHLH transcription factors (OsSD6-OsICE2) that directly regulate ABA8OX3 expression and indirectly regulate NCED2 expression, by direct regulation of the expression of the NCED repressor OsbHLH048. Here, we tested whether expression of the Vitis homologs of the rice SD6 and ICE2 are regulated by dormancy release stimuli, and whether the direction of regulation agrees with that of ABA8OX. Treatment with two independent stimuli of bud break (HC and hypoxia), as well as natural dormancy release, resulted in upregulation of OsSD6 homologs and down regulation of OsICE2 homologs, in agreement with the rice model. In unexpected contrast, the homolog of OsbHLH048 was down-regulated. Our results suggest a grapevine model in which (1) the homologs of OsSD6 and OsICE2 act as direct activators and repressors of ABA8OX3 expression, as for rice, (2) they have opposed effects on the expression of an OsbHLH048 homolog, which serves as direct activator of NCED expression, as for Arabidopsis, and (3) together they act as a switch that allows removal of ABA repression, followed by meristem reactivation and bud break. HIGHLIGHTA molecular switch for integrated and opposite regulation of NCED and ABA8OX in rice seeds, operated by three bHLH transcription factors, is conserved in grapevine buds and regulates dormancy release