T-DNA activation tagging in rice results in a variable response to Meloidogyne graminicola infection

Abstract

Plant-parasitic nematodes (PPNs) are known to be devastating pests of agricultural crops; the rice root knot nematode Meloidogyne graminicola being the most damaging in rice. Variability in response to any trait is important not only to understand plant-stress interactions, but also its utility in stress mitigation. In this direction, the focus of the study was to exploit activation tagging, a well-established functional genomics approach in rice as a plausible strategy to assess variability in the phenotypic and molecular response to M. graminicola infection. T-DNA insertional mutants were developed in rice (acc. JBT 36/14) using an apical meristem-targeted in planta transformation strategy. A panel of 30 mutants that demonstrated molecular evidences for T-DNA integration and portraying a robust phenotype were chosen for the study. Vigorous phenotyping of these mutants against M. graminicola second stage juveniles both under in vitro pluronic gel medium and soil conditions depicted variable response based on four disease scoring parameters and the derived nematode reproductive factor. Among 30 mutants that were evaluated, five showed a resistant phenotype while 25 depicted a susceptible reaction. The resistant plants clearly demonstrated a reduction in all the disease parameters both under in vitro as well as greenhouse conditions. Quantification of transcripts of 25 genes associated with host defence responses belonging to salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) pathways also indicated corroboration with the phenotype exhibited by the mutants. Distinct pattern of gene expression profiles was observed not only between the susceptible and resistant mutants, but also amongst the resistant ones, demonstrating the possibility of lesser known defense pathways being operated in these plants, apparently depending upon the T-DNA integration loci. The variability in the response to nematode challenge demonstrated in this study can be a step towards deciphering of compatible/incompatible relationships that can form a knowledge base towards not only understanding plant-nematode interactions but also extrapolation of their utility in rice improvement programmes.