Repulsive response of Meloidogyne incognita induced by biocontrol bacteria and its effect on interspecific interactions

Abstract

The aversive behavior of Caenorhabditis elegans is an important strategy that increases their survival under pathogen infection, and the molecular mechanisms underlying this behavior have been described. However, whether this defensive response occurs in plant-parasitic nematodes (PPNs), which have quite different life cycles and genomic sequences from the model nematode, against biocontrol microbes and affects interspecific interactions in ecological environments remains unclear. Here, we showed that Meloidogyne incognita, one of the most common PPNs, engaged in lawn-leaving behavior in response to biocontrol bacteria such as Bacillus nematocida B16 and B. thuringiensis Bt79. Genomic analysis revealed that the key genes responsible for the aversive behavior of C. elegans, such as serotonin-and TGF-β-related genes in canonical signaling pathways, were homologous to those of M. incognita, and the similarity between these sequences ranged from 30% to 67%. Knockdown of the homologous genes impaired avoidance of M. incognita to varying degrees. Calcium ion imaging showed that the repulsive response requires the involvement of the multiple amphid neurons of M. incognita. In situ hybridization specifically localized Mi-tph-1 of the serotonin pathway to ADF/ NSM neurons and Mi-dbl-1 of the TGF-β pathway to AVA neurons. Our data suggested that the repulsive response induced by different biocontrol bacteria strongly suppresses the invasion of tomato host plants by M. incognita. Overall, our study is the first to clarify the pathogen-induced repulsive response of M. incognita and elucidate its underlying molecular mechanisms. Our findings provide new insights into interspecific interactions among biocontrol bacteria, PPNs, and host plants.

Novel Genes and Key Signaling Molecules Involved in the Repulsive Response of Meloidogyne incognita against Biocontrol Bacteria

“…RNA interference (RNAi)-based silencing of the target genes of M. incognita was performed as described in our previous study M.…”

“…incognita, chemotaxis plates were prepared by pouring 4 mL of 23% (w/v) PF-127 (Sigma-Aldrich, St. Louis, USA) into 6 cm Petri dishes at 20 °C, and the gel was allowed to solidify. Then, 5 μL of the chemical/bacterial culture or control solution/uninoculated LB fluid medium was added to opposite sides of the plate as previously described . Then approximately 100 M.…”

“…incognita were generated by PCR using the designed primers (Table ). Details of the protocol used for ISH have been described previously …”

“…The results of our previous study demonstrated that biocontrol bacteria, including Bacillus nematocia B16 and Bacillus thuringiensis Bt79, induced a repulsive response of M. incognita, which potently suppressed their invasion into host plants . Furthermore, two canonical molecular signaling pathways, namely, the serotonin and TGF-β pathways, contribute to the defensive behavior of M.…”

“…Then, 5 μL of the chemical/bacterial culture or control solution/uninoculated LB fluid medium was added to opposite sides of the plate as previously described. 20 Then approximately 100 M. incognita J2s was placed at the center of the plates and covered with lids. The plates were then placed in an incubator for 6 h at 25 °C before counting the number of worms on both sides of the plates.…”