The role of cell wall-modifying proteins in plant penetration and feeding site proliferation by the potato cyst nematode Globodera rostochiensis

The main objective of this thesis was to investigate two distinct groups of proteins involved in plant cell walls modifications in the parasitism of the potato cyst nematode, Globodera rostochiensis , namely pectate lyases and expansins. Plant parasitism of potato cyst nematode proceeds through two main stages i.e. mobile and sessile. During the migratory phase, potato cyst nematode uses cell wall degrading enzymes and mechanical force of the stylet protruding from nematode's buccal cavity to disrupt cell walls in host plant tissue. In order to degrade plant cell wall components, nematodes produce a range of cell wall degrading and modifying proteins. Plant parasitic nematodes were the first animals for which it was shown that they are able to degrade cell wall polymers (i.e. cellulose microfibrils) without the help of symbiotic microorganisms. This first finding of an endoglucanase produced by G. rostochiensis turned out to be only a tip of an iceberg. To date, similar evidence is found in plant parasitic nematodes for the presence of enzymes capable of depolymerising the pectin matrix in plant cell walls (i.e. pectate lyase and exopolygalacturonase). In Chapter II, a novel putative pectate lyase gene from G. rostochiensis is described including a functional assay using Agrobacterium mediated leaf infiltration and site directed mutagenesis. In Chapters III and IV, we showed that potato cyst nematode is producing and secreting expansins, which are likely to aid the activity of cell wall degrading enzymes. Contrary to pectinolytic enzymes expansins are a relatively newly described group of proteins and there are still a lot of unresolved issues that need to be addressed with regard to their mode of action, biological functions and classification. Since their discovery in 1992, expansins were thought to be specific for land plants. In this thesis, we present the first evidence of a functional expansin produced outside of the plant kingdom. We demonstrated that invasive juveniles of G. rostochiensis secrete b-expansin, which most likely facilitates the disruption of cell walls in the root tissues of a host by making the structural cell wall polymers more accessible to cell wall degrading enzymes. The cell wall modifications occurring during feeding site formation by potato cyst nematode, described in the fourth chapter of this thesis, are part of the elaborate changes in the plant cell wall architecture during syncytium induction and maintenance. In contrast to the rigorous breakdown of cell walls and subsequent collapse of the protoplast during nematode migration, the modifications during feeding site formation aim at a subtle remodeling of the cell wall, including both cell wall dissolution and synthesis. The uniform structure of nematode induced syncytia suggests that these cell wall modifications are precisely controlled. A key role for the recruitment of plant enzymes (e.g. cellulases and pectin methyl esterases), by nematodes to modify plant cell wall structure to their advantage has been pointed at in several papers. The data presented in Chapter V shows that these cell wall modifications in a nematode-induced syncytium are mediated by enzymes and -expansins and expansin-like proteins of the host plant. This is the first report that links morphological changes in the cell walls of dicotes plants to the expression of b - expansin genes.