Degradation of the plant cell wall by nematodes

Evidence from as early as the 1950’s that phytoparasitic nematodes could secrete enzymes to facilitate penetration of the stylet through the host cell wall or to promote nematode migration within plant tissues was confirmed in 1998 with the report of the first genes encoding endogenous endo-1,4-β-glucanases isolated from animals, the phytoparasitic cyst nematodes. Expressed gene analyses and recent genome sequencing efforts have since identified genes encoding cellulases, hemicellulases, pectinases, and chitinases in multiple species of phytoparasitic nematodes. A cellulose-binding motif present in some nematode endoglucanases also exists independently of the hydrolytic domain in some species and has promoted the discovery of the first expansins outside of the plant kingdom. Database homologies and phylogenetic analyses suggest the potential for independent ancient horizontal gene transfer events of genes encoding a number of nematode cell wall-modifying proteins from microbes present in the nematode’s ecological niche, including soil bacterial sources for root-feeding nematodes and fungal sources for shoot and fungal-feeding nematodes. Subsequent gene duplication and domain shuffling activity has apparently led to the evolutionary expansion of families of genes encoding cell wall-modifying proteins within nematode genomes. In almost all cases, expression of these cell wall-modifying proteins is restricted and active in the nematode esophageal gland cells for secretion from the stylet during migratory stages of the nematode parasitic cycle.