The plant cell wall constitutes a barrier against micro-organisms. It consists principally of carbohydrate polymers, such as cellulose, hemicellulose and pectin, wall proteins and, possibly, phenolic compounds. Like bacteria and fungi, PPNs have acquired enzyme systems for the degradation of plant cell walls. The hydrolysis of cellulose by PPN is catalyzed by ß-1,4-endoglucanases. The expression of genes encoding such enzymes has been reported in both cyst nematodes and RKNs (Ledger et al., 2006; Rosso et al., 1999; Smant et al., 1998). These genes were shown to be transcribed and transduced only in the subventral glands of nematodes from the J2 within the egg until sedentary J3 in plants (de Boer et al., 1999). These enzymes bind cellulose and degrade carboxymethylcellulose, suggesting that they may act directly or indirectly on cellulose. The degradation of hemicellulose polymers by RKN has been demonstrated by the characterization of an endo-1,4-p-xylanase (Mitreva-Dautova et al., 2006). Finally, pectate lyases and polygalacturonase enzymes responsible for pectin breakdown have been identified in PPN. The secretion of a pectate lyase into the plant by parasitic J2 has been demonstrated during RKN infections, consistent with the injection of this enzyme via the stylet to facilitate the penetration of root tissues and the intercellular migration of J2 within these tissues (Doyle and Lambert, 2002). Based on their gland cell-specific expression and the presence of predicted secretion signal pep-tides, glucanases secreted by PPN are also thought to play an active role in nematode migration through root tissues (Doyle and Lambert, 2002; Huang et al., 2005a; Jaubert et al., 2002a). The similarities between these genes in PPNs and bacteria suggest their probable acquisition by horizontal gene transfer from bacteria (Jaubert et al., 2002a; Ledger et al., 2006; Scholl and Bird, 2005).
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