1a. Objectives (from AD-416):
In order to protect from insect attack, plants developed different defense mechanisms including constitutive and inducible defenses. Induced defense was first evidenced in plants that expressed proteinase inhibitors to interfere with insect digestion in response to insect damage. Our recent studies of investigating the aphid-induced transcriptome in sorghum have provided novel insights into the plant molecular responses to greenbug attack. The studies also resulted in the identification of several candidate genes that are potentially responsible for induced plant defense against aphids. Work in our lab is in progress to define both the roles of resistance genes in host defense and regulatory factors controlling these genes and gene products. The specific objectives of this cooperative research project are (1) to clone and characterize these aphid-induced sorghum genes to establish the link between resistance phenotypes and candidate genes, and (2) to evaluate the candidate genes for their effects on improving aphid resistance in sorghum plants. The overall goal of the project is to improve our understanding of the genetic mechanisms underlying host plant defense against aphids, leading to the opportunities to manipulate the responses and defense of crop plants to prevent the damage by aphid pest.
1b. Approach (from AD-416):
The identified candidate genes through analysis of gene expression profiling provide excellent opportunities to establish the link between phenotype and genotype. In order to accomplish the research objectives of this project, we will first take the candidate gene approach to link the observed resistance and candidate sequences. Sequence variation of these candidate genes will be compared between susceptible and resistant lines. In this case, allelic variants or sequence mutation of such genes may be the causal for the host ability to combat with aphid attack. Secondly, the function of the candidate genes will be determined by over-expression or gene knock-out using transgenic approach. Furthermore, molecular experiments will be conducted to analyze and decipher the relationship of signal networks with host responses to aphid attack. It is expected that the outcome of this project will help to determine the extent to which of these genes are responsible for or influence aphid resistance in sorghum.
3. Progress Report:
Based on the QTL mapping data, the loci for greenbug resistance were mapped to the region flanked by markers Starssbnm 93-Starssbnm 102 on chromosome 9. A fine map was constructed for the region with high resolution. As a result, several candidate genes were identified in the chromosomal region, including receptor–like Xa21 binding protein 3-like, map kinase phosphatase, a putative uncharacterized protein, and inorganic pyrophosphatase. These four candidate genes were subjected to differential gene expression analysis using real-time PCR. Relative quantification of gene expression in two parental lines was performed to assess the mean fold change upon greenbug feeding. Validation of gene function will be the next step of this research.