A sorghum genome-wide association study (GWAS) identifies a WRKY transcription factor as a candidate gene underlying resistance to sugarcane aphid (Melanaphis sacchari) - Heterologous expression of SbWRKY86 in Arabidopsis

Crop plant resistance to insect pests is based on genetically encoded traits which often display variability across diverse germplasm. In a comparatively recent event, a predominant sugarcane aphid (SCA: Melanaphis sacchari) clone has become a significant agronomic pest for grain sorghum (Sorghum bicolor) production. In an effort to uncover candidate genes underlying SCA resistance, we used a forward genetics approach combining the genetic diversity present in the Sorghum Association Panel (SAP) and the Bioenergy Association Panel (BAP) for a Genome Wide Association Study, employing an established SCA damage rating. Statistically significant single nucleotide polymorphisms were found on Chromosome 9 within the WRKY transcription factor 86 (SbWRKY86). Transcripts encoding SbWRKY86 were previously identified as upregulated in SCA-resistant germplasm and the syntenic ortholog in maize accumulates following Rhopalosiphum maidis infestation. Analyses of SbWRKY86 transcript levels displayed elevated levels in additional SCA resistant sorghum lines. Heterologous expression of SbWRKY86 in both tobacco (Nicotiana benthamiana) and Arabidopsis resulted in reduced population growth of green peach aphid (Myzus persicae). Comparative RNA-Seq analyses of Arabidopsis lines expressing 35S:SbWRKY86-YFP identified changes in expression for a small network of genes associated with carbon-nitrogen metabolism and callose deposition, both contributing factors to defense against aphids. As a test of altered plant responses, 35S:SbWRKY86 Arabidopsis lines were activated using the flagellin epitope elicitor, flg22, and displayed significantly increased callose deposition. Our findings indicate that both heterologous and increased native expression of the transcription factor SbWRKY86 is associated with reduced aphid levels in diverse plant models.

作物对虫害的抗性源于遗传编码的性状，这类性状在不同种质资源（germplasm）中往往存在变异。近年来，优势甘蔗蚜（Sugarcane Aphid，简称SCA: Melanaphis sacchari）的一个克隆株系已成为高粱（Sorghum bicolor）籽粒生产中的重大农业害虫。为挖掘调控甘蔗蚜抗性的候选基因，本研究采用正向遗传学策略，结合高粱关联群体（Sorghum Association Panel，SAP）与生物能源关联群体（Bioenergy Association Panel，BAP）的遗传多样性开展全基因组关联研究（Genome Wide Association Study），并采用已建立的甘蔗蚜危害评级体系。研究在第9号染色体上的WRKY转录因子86（WRKY Transcription Factor 86，SbWRKY86）区域内，鉴定到具有统计学显著性的单核苷酸多态性位点（Single Nucleotide Polymorphisms）。此前研究表明，编码SbWRKY86的转录本在抗甘蔗蚜种质资源中呈上调表达，且玉米中的共线性同源基因在玉米蚜（Rhopalosiphum maidis）侵染后会出现表达积累。对SbWRKY86转录本水平的检测显示，其余抗甘蔗蚜高粱品系中该基因的转录本丰度均显著升高。将SbWRKY86异源表达于本氏烟草（Nicotiana benthamiana）与拟南芥（Arabidopsis）后，桃蚜（Myzus persicae）的种群增长受到显著抑制。对表达35S:SbWRKY86-YFP的拟南芥株系开展比较RNA测序（RNA-Seq）分析，发现参与碳氮代谢与胼胝质沉积（callose deposition）的小型基因网络的表达发生显著改变，这两类过程均为抵御蚜虫的重要防御机制。为验证植物防御响应的改变，研究使用鞭毛蛋白表位激发子（flagellin epitope elicitor）flg22处理表达35S:SbWRKY86的拟南芥株系，结果显示其胼胝质沉积水平显著升高。本研究结果表明，无论是异源表达还是提升内源表达SbWRKY86转录因子，均可在多种植物模型中降低蚜虫种群数量。