SgR1, Encoding a Leucine-Rich Repeat Containing Receptor-like Protein, Is a Major Aphid (Schizaphis graminum) Resistance Gene in Sorghum

Greenbug, <i>Schizaphis graminum</i>, is one of the important cereal aphid pests of sorghum in the United States and other parts of the world. <i>Sorghum bicolor</i> variety PI 607900 carries the <i>Schizaphis graminum</i> resistance (<i>SgR1</i>) gene that underlies plant resistance to greenbug biotype I (GBI). Now, the <i>SgR1</i> has been determined as the major gene conferring greenbug resistance based on the strong association of its presence with the resistance phenotype in sorghum. In this study, we have successfully isolated the <i>SgR1</i> gene using a map-based cloning approach, and subsequent molecular characterization revealed it encodes a leucine-rich repeat containing receptor-like protein (LRR-RLP). According to DNA sequence analysis, the <i>SgR1</i> gene are conserved among GBI-resistance sorghum accessions but are variable within susceptible lines. Furthermore, an InDel (-965 nt) at its promoter region and a single-nucleotide polymorphism (SNP, 592 nt) in the CDS of the <i>SgR1</i> were detected and they are well conserved within resistant genotypes. When the <i>SgR1</i> gene was cloned and transferred into <i>Arabidopsis</i> plants, the <i>SgR1</i> was activated in the transgenic <i>Arabidopsis</i> plants in response to attack by green peach aphids according to the results of the histochemical assay, and GUS activity was detected in situ in spots around the vasculature of the leaf where the phloem is located, suggesting its biological function in those transgenic <i>Arabidopsis</i> plants. Overall, this study confirms that the <i>SgR1</i> gene coding for an LRR-RLP is the major resistance gene to greenbug, a destructive pest in sorghum and wheat. This represents the first greenbug resistance gene cloned so far and indicates that the simple-inherited GBI resistance gene can be used for sorghum improvement with genetic resistance to GBI via molecular breeding or cross-based conventional breeding technologies.