A set of genes differentially expressed between avirulent and virulent Meloidogyne incognita near-isogenic lines encode secreted proteins.

A cDNA-amplification fragment length polymorphism (AFLP)-based strategy has been used to identify genes differentially expressed between two pairs of near-isogenic lines (NIL) of the root-knot nematode Meloidogyne incognita either avirulent or virulent against the tomato Mi resistance gene. Gene expression profiles from infective second-stage juveniles (J2) were compared, and 22 of the 24,025 transcript-derived fragments (TDF) generated proved to be differential, i.e., present in both avirulent NIL and absent in both virulent NIL. Fourteen of the TDF sequences did not show any significant similarity to known proteins, while eight matched reported sequences from nematodes and other invertebrates. The differential expression of nine genes was confirmed by reverse transcription-polymerase chain reaction (RT-PCR) experiments. In situ hybridization conducted with five of the sequences showed that two were specifically expressed in the intestinal cells (HM10 and PM1), one in the subventral esophageal glands (HM1), and two in the dorsal esophageal gland of J2 (HM7 and HM12). Analysis of full-length cDNA sequences revealed the presence of a signal peptide for HM1, HM10, and HM12, indicating that the encoded proteins are putatively secreted. Since secreted products in general and esophageal gland secretions in particular are thought to be among the main M. incognita pathogenicity factors, this result suggests a possible dual role for some of the genes encoding such secretions, i.e., they could be involved in both pathogenicity and virulence or avirulence of these biotrophic parasites.