GmMEKK2 Disrupts the MKK1/2-MPK4 Cascade to Amplify Immune Signalling and Confer Enhanced Resistance to Soybean Mosaic Virus.

Mitogen-activated protein kinase kinase kinase (MAPKKK) assumes a pivotal position within the MAPK cascade, converting external stimuli into intracellular responses and mediating plant stress resistance. However, there are limited reports investigating its function in regulating resistance to soybean mosaic virus (SMV). Here, a MAPKKK2-like gene GmMEKK2 (LOC100798607) was identified from an SMV-resistant cultivar, with an amino acid sequence containing the typical conserved G(T/S)Px(W/Y/F)MAPExV domain, showing homology to AtMEKK1, AtMEKK2 (SUMM1) and OsMAPKKK5. Overexpression of GmMEKK2 in soybean reduced SMV accumulation and the disease index, mitigated the yield loss after SMV inoculation and improved yield traits. In contrast, silencing GmMEKK2 via virus-induced gene silencing (VIGS) significantly enhanced SMV susceptibility, with increased disease index and viral content. Transcriptionally, defence responce genes such as PR1s and RPM1s were pre-activated in the overexpression lines before SMV infection, conferring soybean with preventive resistance. While non-transgenic plants showed severe down-regulation of basic metabolic genes and strong induction of genomic repair genes at 14 days post-inoculation (dpi), overexpression lines exhibited minimal changes. Despite lacking kinase activity, GmMEKK2 bound with GmMKK1 and GmMPK4A to block the MKK1/2-MPK4 cascade, thereby indirectly enhancing the salicylic acid-induced defence responses. Additionally, GmMEKK2 overexpression elevated basal reactive oxygen species (ROS) levels to trigger autoimmunity, while maintaining ROS homeostasis via the antioxidant enzyme system in soybean. This study clarifies the function and molecular mechanism of GmMEKK2 in SMV resistance, providing a strategy for improving soybean SMV resistance.