大豆AGB1同源基因在防御反应中起正向调控作用

GmAGB1 plays a positive regulatory role in soybean defense responses

摘要:

由Gα、Gβ和Gγ这3个亚基组成的异源三聚体GTP结合蛋白(简称G蛋白)在真核生物的信号转导中起着重要作用。拟南芥基因组中仅有1个Gβ亚基编码基因AGB1，在免疫反应中起着正调控作用，然而AGB1在大豆防御中的作用尚不清楚。鉴于大豆是四倍体植物，存在基因功能冗余，本研究利用菜豆豆荚斑驳病毒诱导的基因沉默技术同时沉默大豆基因组中同源性高达86%-97%的4个GmAGB1同源基因，以探究其在大豆免疫中的作用。沉默GmAGB1导致大豆植株出现矮化表型，暗示GmAGB1可能参与大豆的生长发育；抗病性分析表明沉默GmAGB1导致大豆对大豆斑疹病菌的抗性降低，而该抗性的降低与GmAGB1沉默植株叶片中病原菌侵染诱导的活性氧的积累量的显著下降以及细菌中保守的鞭毛蛋白N端22个氨基酸小肽flg22诱导的GmMPK3激活程度的显著降低相关联。此外，酵母双杂交实验证明大豆中的GmAGB1可能与GmAGG1存在互作关系。综上所述，GmAGB1是大豆防御反应中的正调控因子，且大豆中G蛋白也可能以进化上保守的异源三聚体的形式发挥作用。

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.