Molecular and cellular characterization of GA-Stimulated Transcripts GASA4 and GASA6 in Arabidopsis thaliana.

GA and ABA play antagonistic roles in numerous cellular processes essential for growth, development, and stress responses. GASA4 and GASA6 belong to a family of GA-Stimulated transcripts in Arabidopsis, known as GA-inducible and ABA-repressible. We have found that GASA4 and GASA6 expression is likely mediated through a repressor of GA responses, GA INSENSITIVE (GAI) protein. Moreover, GASA4 and GASA6 are in general up regulated by growth hormones (auxin, BR, cytokinin, and GA) and down regulated by stress hormones (ABA, JA, and SA), indicating a role of GASA4 and GASA6 in hormone crosstalk. Genetic analyses show that suppression of both GASA4 and GASA6 causes late flowering, while over-expression of GASA6 causes early flowering in Arabidopsis. GASA family members encode small polypeptides sharing common structural features: an N-terminal signal peptide, a highly divergent intermediate region, and a conserved C-terminal domain containing 12 conserved cysteines. Despite the presence of a signal peptide, it has not been determined whether or not GASA4 and GASA6 can be processed in vivo. By using imaging and immunological analyses, we show that the N-terminal signal peptide is cleaved as predicted, and the cleavage is important for proper sub-cellular localization of GASA4 and GASA6.