HSG1, a grape Bcl-2-associated athanogene, promotes floral transition by activating CONSTANS expression in transgenic Arabidopsis plant.

HSG1 (Heat shock-induced gene 1) is one of the high-temperature-induced proteins in grapevine. Sequence analysis has demonstrated that HSG1 is a grape Bcl-2-associated athanogene (BAG) that contains IQ and BAG domains in its sequence. HSG1 expression was not detected in leaves without heat treatment, while tendrils, stems, flowers, and berries expressed HSG1 even without heat treatment. By heat treatment at 45°C for 60 min, HSG1 expression was upregulated in both leaves and berries. HSG1-overexpressing Arabidopsis plant remained viable even after exposure to extremely high temperatures. Although no morphological changes were observed, the HSG1-overexpressing Arabidopsis plant exhibited faster floral transition than the control plant. Analysis of the transcription profile of HSG1-overexpressing seedling by cDNA microarray and quantitative RT-PCR demonstrated that the gene expression of the flowering promoter, CONSTANS (CO), in the photoperiod pathway was up-regulated in the seedling. Importantly, the overexpression of HSG1 increased CO expression by ~400% compared with the control seedling. Meanwhile, heat treatment upregulated grape CO expression in grape leaves. Our study provides the possibility that plant BAG protein may be related to the regulation of floral transition by activating CO expression in the photoperiod pathway.