Role of arabidopsis MYC and MYB homologs in drought- and abscisic acid-regulated gene expression.

TLDR

In Arabidopsis, the dehydration‑responsive rd22 gene is induced by abscisic acid and requires protein synthesis, and a 67‑bp promoter fragment containing MYC and MYB recognition sites drives this ABA‑dependent expression. The authors dissected the 67‑bp rd22 promoter in transgenic tobacco, identified MYC and MYB cis‑elements, isolated the MYC‑related DNA‑binding protein rd22BP1, and demonstrated that both rd22BP1 and the MYB protein ATMYB2 activate the promoter in Arabidopsis protoplasts. The study found that rd22BP1 specifically binds the first MYC site, its expression is induced by dehydration and ABA before rd22, and together with the MYB protein ATMYB2, both act as transcriptional activators of the rd22 gene.

Abstract

In Arabidopsis, the induction of a dehydration-responsive gene, rd22, is mediated by abscisic acid (ABA) and requires protein biosynthesis for ABA-dependent gene expression. Previous experiments established that a 67-bp DNA fragment of the rd22 promoter is sufficient for dehydration- and ABA-induced gene expression and that this DNA fragment contains two closely located putative recognition sites for the basic helix-loop-helix protein MYC and one putative recognition site for MYB. We have carefully analyzed the 67-bp region of the rd22 promoter in transgenic tobacco plants and found that both the first MYC site and the MYB recognition site function as cis-acting elements in the dehydration-induced expression of the rd22 gene. A cDNA encoding a MYC-related DNA binding protein was isolated by DNA-ligand binding screening, using the 67-bp region as a probe, and designated rd22BP1. The rd22BP1 cDNA encodes a 68-kD protein that has a typical DNA binding domain of a basic region helix-loop-helix leucine zipper motif in MYC-related transcription factors. The rd22BP1 protein binds specifically to the first MYC recognition site in the 67-bp fragment. RNA gel blot analysis revealed that transcription of the rd22BP1 gene is induced by dehydration stress and ABA treatment, and its induction precedes that of rd22. We have reported a drought- and ABA-inducible gene that encodes the MYB-related protein ATMYB2. In a transient transactivation experiment using Arabidopsis leaf protoplasts, we demonstrated that both the rd22BP1 and ATMYB2 proteins activate transcription of the rd22 promoter fused to the beta-glucuronidase reporter gene. These results indicate that both the rd22BP1 (MYC) and ATMYB2 (MYB) proteins function as transcriptional activators in the dehydration- and ABA-inducible expression of the rd22 gene.