Homologs of Caenorhabditis elegans chemosensory genes have roles in behaviour and chemotaxis in the root-knot nematode Meloidogyne incognita

Nematode chemosensation is a vital component of their host-seeking behaviour. The globally important phytonematode, Meloidogyne incognita, perceives and responds (via their sensory organs such as amphids and phasmids) differentially to various chemical cues emanating from the rhizosphere during the course of host finding. However, compared to the free-living worm, Caenorhabditis elegans, the molecular intricacies behind the plant nematode chemotaxis are yet an unexploited territory. In the present study, four putative chemosensory genes of M. incognita, namely, Mi-odr-1, Mi-odr-3, Mi-tax-2 and Mi-tax-4 were molecularly characterized. Mi-odr-1 mRNA was found to be expressed in the cell bodies of amphidial neurons and phasmids of M. incognita. Mi-odr-1, Mi-odr-3, Mi-tax-2 and Mi-tax-4 transcripts were highly expressed in early life stages of M. incognita, consistent with a role of these genes in host recognition. Functional characterization of Mi-odr-1, Mi-odr-3, Mi-tax-2 and Mi-tax-4 via RNAi revealed behavioural defects in M. incognita and perturbed attraction to host roots in Pluronic gel medium. Knockdown of Mi-odr-1, Mi-odr-3, Mi-tax-2 and Mi-tax-4 resulted in defective chemotaxis of M. incognita to various volatile compounds (alcohol, ketone, aromatic compound, ester, thiazole, pyrazine), non-volatiles of plant origin (carbohydrate, phytohormone, organic acid, amino acid, phenolic) and host root exudates in an agar-pluronic gel based assay plate. In addition, ascaroside-mediated signalling was impeded by downregulation of chemosensory genes. This new information that behavioural response in M. incognita is modulated by specific olfactory genes can be extended to understand chemotaxis in other nematodes.