Developmental expression and molecular analysis of two Meloidogyne incognita pectate lyase genes

Proteinaceous secretions from the oesophageal glands of plant-parasitic nematodes have crucial roles in nematode parasitism of plants. Two cDNAs (designated Mi-pel-1 and Mi-pel-2) encoding pectate lyases were isolated from the root-knot nematode, Meloidogyne incognita, oesophageal gland-cell subtractive cDNA libraries, and the corresponding genomic DNAs were subsequently cloned. Southern blot analyses revealed that homologues to these pectate lyase genes were broadly distributed in Meloidogyne species, and present as members of a small multigene family. Mi-pel-1 and Mi-pel-2 encoded, respectively, predicted proteins of 271 and 280 amino acids, each of which was preceded by a signal peptide for secretion. Interestingly, these pectate lyases showed diversity at the amino acid level, with only 31% identity and 49% similarity. These pectate lyases were classified into the same family of pectate lyases with those of other phytoparasitic nematodes that contain four conserved regions characteristic of the class III pectate lyases of microbes. In situ mRNA hybridisation analyses showed the transcripts of Mi-pel-1 and Mi-pel-2 accumulated exclusively within the subventral oesophageal gland cells of M. incognita. RT-PCR analysis confirmed that their transcriptions were strong at the pre-parasitic and early parasitic second-stage juveniles, and not detectable at the late parasitic stages of the nematodes. These results indicated that these pectate lyases, like cellulases, could be secreted into plant tissues to facilitate the penetration and intercellular migration of M. incognita during the early stages of plant parasitism.