Abstract
MAPK cascades play an important role in induction of PAMPs-triggered defense responses. We have isolated a PAMPs-responsive MAPKK, OsMKK4 in rice, and demonstrated that OsMKK4 activates two MAPK, OsMPK3 and OsMPK6. The activation of this MAPK cascade induces the expression of multiple defense-related genes, cell death, diterpenoid phytoalexin biosynthesis, and phenylpropanoid biosynthesis. To identify signaling cascade leading to a series of defense response downstream of OsMKK4-OsMPK6, we focused on TFs which regulate phenylpropanoid biosynthesis. Tobacco NtMYB2 TF regulates a phenylpropanoid biosynthesis gene under PAMPs signal. And the expression of the NtMYB2 gene is regulated by GATA-type TF, AGP1. In rice, the regulatory mechanism of PAMPs-triggered phenylpropanoid biosynthesis is still unknown. Expression of OsMYB55, which is an ortholog of NtMYB2, is up-regulated by OsMKK4-OsMPK6. And OsGATA3, which is an ortholog of AGP1, is phosphorylated by OsMKK4-OsMPK6 in vitro. These results show a novel pathway in which OsMKK4-OsMPK6 activates OsGATA3 and OsGATA3 promotes the expression of OsMYB55 to induce phenylpropanoid biosynthesis genes.
