The Arabidopsis cysteine-rich protein GASA4 promotes GA responses and exhibits redox activity in bacteria and in planta.

Although the gibberellin (GA) signaling pathway has been elucidated, very little is known about the steps linking first transcriptional activation to physiological responses. Among the few identified GA-induced genes are the plant-specific GAST1-like genes, which encode small proteins with a conserved cysteine-rich domain. The role of these proteins in plant development and GA responses is not yet clear. The Arabidopsis GAST1-like gene family consists of 14 members, GASA1-14. Here we show that over-expression of the GA-induced GASA4 gene in Arabidopsis promoted GA responses such as flowering and seed germination. Suppression of several GASA genes using synthetic microRNA (miR(GASA) ) also promoted seed germination. This was probably caused by suppression of GASA5, which acts as a repressor of GA responses. Previously, we proposed that GAST1-like proteins are involved in redox reactions via their cysteine-rich domain. The results of this study support this hypothesis, as over-expression of GASA4 suppressed ROS accumulation and the transgenic seeds were partially resistant to the NO donor sodium nitroprusside (SNP). Moreover, Escherichia coli expressing intact GASA4 or a truncated version containing only the cysteine-rich domain were resistant to SNP. Mutated GASA4, in which conserved cysteines were replaced by alanines, lost its redox activity and the ability to promote GA responses, suggesting that the two functions are linked. We propose that GA induces some GAST1-like genes and suppresses others to regulate its own responses. We also suggest that the encoded proteins regulate the redox status of specific components to promote or suppress these responses.