Molecular approaches to understand plant-insect interactions to enhance pest control

Abstract

Background: Phloem-feeding insects cause significant crop damage worldwide, but despite this little is understood about how plants protect and defend themselves from these threats. Phloem-feeding insects are very specific in their mode of feeding and present a unique stress on plant fitness. Not only do these insects feed for long periods of time on host plants, but they also act as vectors for plant viruses. The Brown planthopper (BPH)-Rice and Aphid-Arabidopsis systems provide good models for studying the induced responses in plants to phloem-feeding insects. Results: In BPH-rice interaction studies, the results showed that 29% of differentially expressed genes in response to BPH feeding were involved in stress responses in plants. Of particular interest was the differential expression of genes encoding the pathogen related proteins β-1,3-glucanase 1, 2 and 5 genes and genes encoding callose synthase 1, 3 and 5. QRT-PCR results have shown that genes encoding callose synthase 1 and 5 (GSL1 and GSL5) were highly expressed in both the moderately resistant IR64 and the resistant IR70 rice cultivars; they were however down regulated in the BPH susceptible cultivar TN1. Similarly, genes encoding the GTP binding protein were expressed to higher levels in cultivars IR64 and IR70 in response to BPH feeding, compared to TN1. In contrast, genes involved in callose degradation, namely β-1,3-glucanase genes 1, 2 and 5 (Gns1/Osg1, Gns2 and Gns5) were highly expressed in the susceptible cultivar in response to BPH feeding; Osg1 and Gns2 were not expressed in either IR64 or IR70, while β- Gns5 was down regulated in both resistant cultivars, compared to the susceptible cultivar (TN1). This differential gene expression in response to BPH feeding might suggest an important role for these genes in plant defence against phloem-feeding insects. Further studies demonstrated that the exogenous application of hydrogen peroxide to the susceptible rice cultivar TN1 improved resistance of this cultivar to BPH to moderate. GTP binding protein, Callose synthase GSL1 and GSL5 genes were up-regulated, while β-1,3-glucanase genes Gns1, 2, 3 and 5 were down-regulated in response to BPH feeding, suggesting that reactive oxygen species generated under hydrogen peroxide treatment might play a role in bringing about the responses leading to resistance. In aphid-Arabidopsis interaction studies, aphid bioassays showed that oxidative signal inducible protein kinases (Oxi1 serine-threonine MAPKs), β-1,3-glucanase Gns1, Gns2 and Gns3 mutants were resistant to aphid feeding and they could survive until the seeding stage when infested. However, Camta3-1, Camta3-2 (calmodulin binding transcription activators), and the Oxi1 null mutant (oxidative signal inducible with no-function) died in response to aphid infestation before reaching the seeding stage. Furthermore, Col-0 (Columbia) and WS2 (Wisconsin) wild type backgrounds for Oxi1 and Oxi1 null mutant respectively, died quickly under aphid feeding. Gene expression analysis using QRT-PCR on the aphid resistant Oxi1 mutant and the susceptible parental line demonstrated that transcripts for callose synthase gene GSL5 were expressed at a higher level in the Oxi1 mutant compared to Col-0. Whilst β-1,3-glucanase Gns1, 2, 3 and 5 genes were down-regulated in the Oxi1 mutant in response to aphid feeding, β-1,3-glucanase Gns2 gene was induced in Col-0 to high levels in response to aphid feeding. Application of hydrogen peroxide putatively induced the oxidative inducible signalling (Oxi1 serine-threonine) MAPKs. Induction of Oxi1 stimulated callose production probably via a Ca2+ signalling pathway. Application of hydrogen peroxide to Col-0 improved the resistance level of this susceptible line in response to aphid feeding. Transcript expression analysis demonstrated that GSL5 was expressed at high levels in response to aphid feeding, while β-1,3-glucanase Gns2 gene was down-regulated in response to hydrogen peroxide treatment. In addition Gns1, 3 and 5 genes were not expressed in response to aphid feeding. Interestingly, hydrogen peroxide increased the susceptibility of the Oxi1 mutant to aphid attack. Conclusion: β-1,3-glucanase Gns2 gene might play an important role in plant susceptibility to phloem feeding insects in both monocots and dicots. Evidence from the present study suggests that callose synthase GSL5 plays an important role in plant defence against insects and may be a key gene in insect/wound response in plants. The application of hydrogen peroxide induces Oxi1 serine-threonine MAPKS and increased callose production via a Ca2+ signalling pathway and caused a down-regulation of β-1,3-glucanase Gns 1, 2, 3 and 5 genes. Over expression as well as down-regulation of Oxi1 may increase plant susceptibility to phloem feeding (BPH-aphids) insects suggesting that specific levels of Oxi1 are required.