Understanding the molecular response of rice to biotic (brown planthopper) and abiotic (nitrogen deficiency) stress

Abstract

Rice, one of the world’s most important food crops and staple food for more than half of the world’s population, is constantly exposed to severe biotic and abiotic stresses, resulting in significant yield losses of up to 70%. The present study investigated the role of β-1,3-glucanase (Gns5) in host plant susceptibility to brown planthopper (BPH) using RNAi-based approaches (Chapter 2). Knock-down of Gns5 in these RNAi lines (using two different promoter systems) exhibited enhanced resistance in terms of survival, fecundity and developmental rate compared to controls lines (BPHsusceptible TN1; empty vector transformants). Microscopic studies demonstrated that the RNAi lines exhibited higher levels of callose accumulation post BPHinfestation, compared to TN1 or empty vector lines. Electronic Penetration Graphs (EPG) suggested that the probing frequency by BPH was significantly greater (p<0.05), but the duration of phloem ingestion was significantly shorter, on the RNAi lines compared to control lines. This study showed that knockdown of Gns5 prevents callose decomposition and therefore increases the resistance of the commercially grown TN1 cultivar towards BPH. Physiological and molecular responses of rice to nitrogen (N) deficiency and the combination of N deficiency and BPH infestation were also investigated. N stress was shown to cause significant (p<0.05) reductions in shoot height, number of tillers and leaves, leaf area, root length, relative water content and chlorophyll content in a dose-dependent manner in both TN1 (susceptible to BPH) and IR70 (resistant to BPH) (Chapter 3). Further studies were carried out to identify transcriptions factors (TFs) involved in the response to N deficiency. Twelve TFs, previously reported to potentially be involved in the response of rice to BPH were up-regulated in IR70 compared to ten TFs up-regulated in TN1; furthermore, the magnitude of the response was significantly greater in IR70 (Chapter 4). Expression profiles of the combined stress (N + BPH) showed that more TFs were down-regulated in IR70 at different time points post BPH infestation compared to the susceptible TN1 (Chapter 5). Two of these N-responsive TF genes, Os02g0214500 and Os03g0437200 showed an interesting pattern of expression whereby those genes were down-regulated at most time points post BPH infestation in IR70 in response to reduced levels of N, in contrast to TN1 where both these TFs genes were up-regulated. These findings provide a platform for developing stresstolerant rice cultivars.