GmAGB1 plays a positive regulatory role in soybean defense responses

Abstract

Heterotrimeric GTP-binding protein (G-proteins) complex, which consists of Gα, Gβ and Gγ subunits, plays critical roles in defense signaling. Arabidopsis genome contains only a single Gβ-encoding gene, AGB1. Loss function of AGB1 in Arabidopsis results in enhanced susceptibility to a wide range of pathogens. However, the function of soybean AGB1 in immunity has not been previously interrogated. Bioinformatic analysis indicated that there are four GmAGB1 homologous genes in soybean genome, sharing homology of 86%-97%. To overcome the functional redundancy of these GmAGB1 homologs, virus-induced gene silencing (VIGS) mediated by the bean pod mottle virus (BPMV) was used to silence these four genes simultaneously. As expected, these four GmAGB1 homologous genes were indeed silenced by a single BPMV-VIGS vector carrying a conserved fragments among these four genes. A dwarfed phenotype was observed in GmAGB1s-silenced soybean plants, suggesting that GmAGB1s play a crucial role in growth and development. Disease resistance analysis indicated that silencing GmAGB1s significantly compromised the resistance of soybean plants against Xanthomonas campestris pv. glycinea (Xag). This reduced resistance was correlated with the decreased accumulation of pathogen-induced reactive oxygen species (ROS) and the reduced activation of GmMPK3 in response to flg22, a conserved N-terminal peptide of flagellin protein. These results indicate that GmAGB1 functions as a positive regulator in disease resistance and GmAGB1 is indispensable for the ROS production and GmMPK3 activation induced by pathogen infection. Yeast two hybrid assay showed that GmAGB1 interacted with GmAGG1, suggesting that an evolutionary conserved heterotrimeric G protein complex similarly functions in soybean.