The MAPK Kinase Kinase GmMEKK1 Regulates Cell Death and Defense Responses

Abstract

MAPK signaling pathways play critical roles in plant immunity. Here, we silenced multiple genes encoding MAPKs using virus-induced gene silencing mediated by <i>Bean pod mottle virus</i> to identify MAPK genes involved in soybean (<i>Glycine max</i>) immunity. Surprisingly, a strong hypersensitive response (HR) cell death was observed when soybean <i>MAPK KINASE KINASE1</i> (<i>GmMEKK1</i>), a homolog of Arabidopsis (<i>Arabidopsis thaliana</i>) <i>MEKK1</i>, was silenced. The HR was accompanied by the overaccumulation of defense signaling molecules, salicylic acid (SA) and hydrogen peroxide. Genes involved in primary metabolism, translation/transcription, photosynthesis, and growth/development were down-regulated in <i>GmMEKK1-</i>silenced plants, while the expression of defense-related genes was activated. Accordingly, <i>GmMEKK1</i>-silenced plants were more resistant to downy mildew (<i>Peronospora manshurica</i>) and <i>Soybean mosaic virus</i> compared with control plants. Silencing <i>GmMEKK1</i> reduced the activation of GmMPK6 but enhanced the activation of <i>Gm</i>MPK3 in response to flg22 peptide. Unlike Arabidopsis MPK4, <i>Gm</i>MPK4 was not activated by either flg22 or SA. Interestingly, transient overexpression of <i>GmMEKK1</i> in <i>Nicotiana benthamiana</i> also induced HR. Our results indicate that <i>Gm</i>MEKK1 plays both positive and negative roles in immunity and appears to differentially activate downstream MPKs by promoting <i>Gm</i>MPK6 activation but suppressing GmMPK3 activation in response to flg22. The involvement of <i>Gm</i>MPK4 kinase activity in cell death and in flg22- or SA-triggered defense responses in soybean requires further investigation.