The MAPK Kinase Kinase GmMEKK1 Regulates Cell Death and Defense Responses.

MAPK signaling pathways play critical roles in plant immunity. Here, we silenced multiple genes encoding MAPKs using virus-induced gene silencing mediated by to identify MAPK genes involved in soybean () immunity. Surprisingly, a strong hypersensitive response (HR) cell death was observed when soybean (), a homolog of Arabidopsis () , 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 silenced plants, while the expression of defense-related genes was activated. Accordingly, -silenced plants were more resistant to downy mildew () and compared with control plants. Silencing reduced the activation of GmMPK6 but enhanced the activation of MPK3 in response to flg22 peptide. Unlike Arabidopsis MPK4, MPK4 was not activated by either flg22 or SA. Interestingly, transient overexpression of in also induced HR. Our results indicate that MEKK1 plays both positive and negative roles in immunity and appears to differentially activate downstream MPKs by promoting MPK6 activation but suppressing GmMPK3 activation in response to flg22. The involvement of MPK4 kinase activity in cell death and in flg22- or SA-triggered defense responses in soybean requires further investigation.