Signal Transduction Pathways Involved In Sensing Water-soluble Compounds In Meloidogyne Incognita And Caenorhabditis Elegans

Root-knot nematodes need to accurately sense signal compounds secreted by host plants in order to locate and then successfully infect host plants.Thus,understanding the mechanisms of how root-knot nematodes sense plant signal compounds will provide critical information on the pathogenesis and potential controls against these nematode pests.However,due to limited genetic knowledge about root-knot nematodes,it's difficult to identify the signal transduction pathways involved in their sensing of host plants.The objectives of this thesis are to use Caenorhabditis elegans as the model nematode to identify the signal transduction pathways involved in sensing water-soluble compounds and then through comparative genomics to identify and then RNAi to confirm the function of the homologs in the root-knot nematode Meloidogyne incognita.In this study,We first determined the neurons in C.elegans that are involved in sensing citric acid.Three allelopathy neurons were identified: AWA,AWC(ON)and ASER.Among them,the ASER neuron plays a major role in sensing citric acid through the GCY-1/GCY-5 — c GMP — TAX-2/TAX-4 signaling pathway.Interestingly,though commonly known as an olfactory neuron,AWC(ON)was found to be involved in sensing water-soluble substances through the STR-2 — G protein — ODR-1/ DAF-11— c GMP— TAX-2/TAX-4 signal transduction pathway.We also found that the DEG/ENa C and MAPK pathways were involved in perceptual regulation in both the ASER and AWC(ON)neurons,with their main contribution found in AWC(ON)neuron.The citric acid sensing pathway in the ASER neuron in C.elegans was further found involved in sensing acidic environments in general,with the DEG/ENa C pathway involved in regulating perceptual behavior.The signal transduction pathway is GCY-1/GCY-4—c GMP—TAX-2/TAX-4.However,in other acidic environments,the regulation of MAPK pathway was more complicated,with mpk-1 positively regulates acid sensing in the AWC(ON)neuron but negatively in the ASER neuron.Sequence comparisons between the acid sensing genes identified in C.elegans and the genome sequence of M.incognita revealed 27 possible chemosensory-related genes in M.incognita.RT-q PCR analyses revealed that 20 of the 27 genes showed significantly different expressions between the presence and absence of citric acid.Through ds RNA interference,genes minc3s00025g01614(Mi-gcy-9),minc3s00056g02910(Mi-odr-1),minc3s00187g07094(Mi-gpa-1),and minc3s00022g01386(Mi-gpa-5)were found positively regulated by citric acid.In a general acidic environment,23 of the 27 genes were significantly up-regulated.Consistent with the findings in C.elegans,minc3s02630g30946(Mi-gcy-4)was up-regulated in acidic p H while minc3s00006g00427(Mi-gpa-7)was down-regulated.During infection of host plants,eleven of these genes were also up-regulated while two of these genes were negatively regulated in M.incognita.In this study,we found that the homologs of chemosensory-related genes in C.elegans play overall similar roles in M.incognita.However,several differences were found between the two species.For example,the citric acid receptors found in C.elegans were GCY-1 and GCY-5,while MINC3S00025G01614(Mi-GCY-9)was found in M.incognita.Receptors GCY-1 and GCY-4 were found in C.elegans to sense acidic environment,while only MINC3S02630G30946(Mi-GCY-4)was found in M.incognita.The results in this study provide potential targets for developing control measures against root-knot nematodes in field conditions.