Biochemical Alterations in Maize Plants induced by Viruses causing Maize Lethal Necrosis and their Relevance for Insect Vectors

Abstract

Maize lethal necrosis (MLN) - a big threat to maize production and food security in Kenya, is caused by co-infection of maize with Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV). In severely affected maize fields, MLN destroys the crop completely with a devastating impact on food security of smallholder households. Since the disease was only recently documented in Kenya, there is limited information on its pathogenesis due to SCMV and MCMV interaction, the effect of the disease on host plant, and consequently on insect vectors. Therefore, this study aimed to generate baseline information on critical biochemical changes in maize plants infected by viruses causing MLN and the consequence of some of these changes to insect vectors (thrips). This included determination of symptom progression and the rate of viral multiplication in single and dual infection by SCMV and/or MCMV and profiling of changes in volatiles from MCMV- and co-inoculated maize plants. Behavioral responses of selected thrips species towards headspace volatiles from MCMV and SCMV/MCMV coinoculated maize plants were also determined. The experiments were conducted in insect-proof screenhouses and laboratories at icipe, Nairobi. Maize plants were artificially inoculated by respective viruses and disease severity scored on a scale of 1-5. Virus titers on inoculated plants were determined by Double Antibody Sandwich-Enzyme Linked Immunosorbent Assay (DASELISA), followed by quantification using real-time Quantitative Reverse Transcription PCR (RT-qPCR). Volatiles were collected by headspace method using portable air entrainment kit and then chemically characterized. On the other hand, behavioral response assays of two species of thrips to the headspace volatiles from both infected and non-infected plants was carried out in a Four-arm olfactometer. All statistical analyses were done using R software version 3.2.3. Results indicate that disease severity in MCMV and SCMV co-infected plants increased concomitantly with MCMV accumulation. Coupled GC-mass spectrometry (GC-MS) analysis of the volatiles compounds showed that uninfected plants produced richer volatile profiles, mainly comprising terpenoids, C5-C6 alcohols, aromatic and aliphatic compounds than infected plants (both singly and co-infected). Additionally, MCMV inoculated plants emitted VOCs more attractive to Frankliniella williamsi and Thrips tabaci. The study provides new information on multi-trophic plant-MLN-vector interactions. Clear understanding of these interactions has the potential to improve integrated disease management strategies against MLN.