Differential gene expression in gall midge susceptible rice genotypes revealed by suppressive subtraction hybridization (SSH) cDNA libraries and microarray analysis

Results revealed up-regulation of genes related to primary metabolism, nutrient relocation, cell organization and DNA synthesis. Concomitantly, defense, secondary metabolism and signaling genes were suppressed. Further, real-time PCR validation of a selected set of 20 genes, in three susceptible rice varieties (TN1, Kavya and Suraksha) during the interaction with the respective virulent gall midge biotypes, also revealed variation in gene expression in Kavya as compared to TN1 and Suraksha.These studies showed that virulent insects induced the plants to step up metabolism and transport nutrients to their feeding site and suppressed defense responses. But Kavya rice mounted an elevated defense response during early hours of virulent gall midge infestation, which was over-powered later, resulting in host plant susceptibility.Plant galls are defined as aberrant plant cells, tissues or organs formed as a result of stimulation by various parasites ranging from fungi and bacteria to insects and mites (Harris et al. 2003). Insects that induce plants to form galls are reported from diverse orders such as Diptera (gall midges), Hymenoptera (gall wasps), Homoptera (gall-forming aphids), Hemiptera (gall-forming psyllids) and Thysanopetra (thrips) (Saltzmann et al. 2008). The gall is the cumulative expression of a suite of adaptations in the host plant for accommodating herbivorous insects (Raman et al. 2011). Gall-inducing insects have profound effects on their hosts. These insects live within the plant tissues and induce tumor-like growths that provide them with food, shelter, and protection from natural enemies (Raman et al. 2005). While, tumors induced by bacteria, viruses and fungi have amorphous growth, galls induced by insects have symmetrical structures and display novel patterns of differentiation which do not occur during normal morphogenesis of the organ (Meyer 1987).Gall-forming insects are also known to manipulate their host plants and induce changes in source-sink