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 insilenced 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 promotingMPK6 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.

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Receptor-like kinases (RLKs) are a large group of pattern recognition receptors (PRRs) and play a critical role in recognizing pathogens, transducing defense signals, and mediating the activation of immune defense responses. Although extensively studied in the model plant Arabidopsis, studies of RLKs in crops, including soybean, are limited. When a BAK1-interacting receptor-like kinase (BIR1) homolog (referred to as GmBIR1 hereafter) was silenced by the BPMV (Bean pod mottle virus)-induced gene silencing (BPMV-VIGS), it resulted in phenotypes that were reminiscent of constitutively activated defense responses, including a significantly stunted stature with observable cell death on the leaves of the silenced plants. In addition, both SA and H2O2 were over-accumulated in the leaves of the GmBIR1-silenced plants. Consistent with this autoimmune phenotype, GmBIR1-silenced plants exhibited significantly enhanced resistance to both Pseudomonas syringae pv. glycinea (Psg) and Soybean mosaic virus (SMV), two different types of pathogens, compared to the vector control plants. Together, our results indicated that GmBIR1 is a negative regulator of immunity in soybean and the function of BIR1 homologs is conserved in different plant species.