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.