Plant-parasitism is believed to have evolved at least three times independently (Blaxter et al., 1998). The genes that were evolved from nematode ancestors of contemporary species are one likely possible mechanism for the origin of nematode parasitism genes and the other mechanism may be of horizontal gene transfer (HGT). Recently it was reported that those genes expressed in the esophageal gland cells of plant-parasitic nematodes show strongest similarities to the bacterial genes which strengthened the existing hypothesis that parasitism genes in plant-nematodes may have been acquired, at least in part, by horizontal gene transfer from bacteria and other microorganisms that inhabit the same parasitic environment. The genes mj-cm-1 and mi-cbp-1 shows strongest similarities to the genes of bacteria (Ding et al., 1998: Lambert et al., 1999). The complementation of a bacterial mutant with mj-cm-1 was also used to provide functional analysis of the gene (Lambert et al., 1999). Most of the parasitism genes are found to be highly similar to bacterial sequences thereby suggesting that these parasitism genes could have been acquired from bacteria through horizontal gene transfer. For instance the nematode endo-1,4-ÎÂ²-glucanases from the Tylenchomorpha, which belong to Glycosyl Hydrolase Family GHF5, show less similarity to plant endoglucanases but show resemblance to the bacteria. The genes encoding the cellulases enzymes of both nematode and bacteria may have evolved from an ancient cellulase of a common ancestor of both the bacteria and nematodes. The endoglucanases from nematode shows the highest similarity with the bacterial one, which also point to a HGT from bacteria to an ancestor of the cyst nematode. Some of the tentative cases of HGT between plant, bacteria, and fungi include GHF (Chen et al, 1997: Quillet et al 1995). It is not possible and advisable to provide the conclusive evidence for a horizontal gene transfer (HGT) from one organism to another organism germ line. There are examples of putative cases of horizontal gene transfer from eukaryote to prokaryote, prokaryote to prokaryote and from prokaryote to eukaryote (Smith et al., 1992: Syvanen, 1994). On the other hand, the presence of bacterial symbionts in nematode ancestors, such as the bacterium Wolbachia symbiont found in filarial nematodes (Blaxter et al., 1999), may also represent a source for transfer of genetic material from bacteria to nematodes.
2.13. PLANT CELL WALL STRUCTURE AND COMPOSITION
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Plant cell walls are the complex mixture of carbohydrates, proteins, lignin, water, other substances such as cutin, suberin, and various inorganic compounds that vary from plant to plant species, cell types, and even to the neighboring cells also. This composition and structural variation is further increase due to various developmental events and exposure to various number of abiotic and biotic stresses (Showalter, 1993). The major carbohydrates in the growing plant cell wall consist of cellulose, hemicellulose and pectin. The cellulose microfibrils are linked to hemicellulose forming the network of cellulose-hemicellulose which is embedded in the pectin matrix. Plants cell walls are composed of three types of layers- the middle lamella, the primary cell wall and in some case the secondary cell wall. The middle lamella is rich in pectin and deposited soon after mitosis and connects two adjacent plant cells (Cosgrove, 2005). The major polysaccharides in the primary cell wall are cellulose, pectin and hemicellulose whereas secondary cell walls is composed of cellulose, lignin, and hemicellulose like xylan, glucuronoxylan, arabinoxylan and glucomannan. The most common hemicellulose in the primary cell wall of the plants is xyloglucan. The outer part of the primary plant cell wall epidermis is impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle. Secondary cell walls also contain a wide range of compounds that changes their mechanical properties and permeability. The Structural proteins are also found in most plant cell walls and are classified as Hydroxyproline rich glycoproteins (HRGP), Glycine-rich proteins (GRPs), Arabinogalactan proteins (AGP) and Proline-rich proteins (PRPs). Most of them are glycosylated and contain hydroxyproline. These proteins are found to be concentrated in specialized cells and in cell corners also. Plant cell walls of the epidermis and endodermis may also contain suberin or cutin, two polyester-like polymers that protect the cell from herbivores (Moire et al., 1999). Plant cells walls also contain numerous enzymes, such as hydrolase, peroxidases, a-mannosidases, pmannosidases, p1,3-glucanases, p1,4- glucanases, polygalacturonase, pectin methylesterases phosphatases, invertases, , malate dehydrogenase, arabinosidases, a-galactosidases, pgalactosidases, pglucuronosidases, pxylosidases, proteases, and ascorbic acid oxidase (Varner and Lin, 1989).
2.14. NEMATODE PLANT CELL WALL DEGRADING ENZYMES
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Nematode parasitism genes encoding the secretory proteins that are expressed in the oesophageal glands of the plant parasitic nematodes plays an important roles in the invasion of nematodes to the host plants thereby establishing the feeding sites and then suppressed the host defences (Huang et al, 2004). An interesting observation is that the known plant parasitic nematode parasitism proteins are not similar with the proteins from Caenorhabditis elegans which is a non-parasitic nematode. However, this parasitism proteins shows similarity in between the plant parasitic nematodes. This parasitism protein also shows similarity to the proteins from bacteria, fungi, plants etc and it is believed that this parasitism protein does not have any functions in nematodes (Baum et al., 2007). As for instance, plant-parasitic nematodes produce enzymes cellulases and pectinases but no substrates for these enzymes are noticed in the nematode. The root-knot and cyst nematodes secretes a mixture of enzymes that soften the plant root cell walls and further help in nematode penetration through the root epidermis and in migration within the host root tissues (Baum et al., 2007)The plant cell wall digesting enzymes cellulase and pectinase genes are already described for root-knot nematode (Huang et al., 2004: Huang et al., 2003: Huang et al., 2005: Rosso et al., 1999) and cyst nematode species (Smant et al., 1998: Gao et al., 2003: Wang et al., 2001: Yan et al., 2001). The first major achievement in parasitism gene discovery was the discovery of cellulase genes from the soybean and potato cyst nematodes. The discovery of cellulase genes was of very important since no cellulase genes had been reported from animals at that time (Smant et al., 1998). The same story for the pectinases also since pectinase was also not reported from animals. The pectinase proteins obtained from nematode was of the type pectate lyase which is found in fungi and bacteria, cyst and root-knot nematodes; (Popeijus et al., 2000: Huang et al., 2003: Huang et al., 2005: de Boer et al., 2002: Doyle & Lambert, 2002) or to the polygalacturonase type of bacteria (Jaubert et al., 2002). The involvement of these enzymes in penetration and migration is well supported by the evidence that these enzymes are produced and released during the nematode penetration and migration and to a smaller extent, or not at all during the later sedentary stages of the nematodes (Huang et al., 2005: Rosso et al., 1999: De Boer et al., 1999: Goellner et al., 2000). These types of cell wall digesting enzymes are also reported from outside the community of sedentary nematode. The enzymes beta-1, 4-endoglucanase genes from Pratylenchus penetrans (Uehara et al., 2001), a migratory parasite that also requires successful means to enter the plant cell-walls was also reported. An enzyme cellulase of the beta-1, 3-endoglucanase type was also recently reported from Bursaphelenchus xenophilus, the pinewood nematode where it is hypothesized of being involved in nematode feeding from the fungal mycelium (Kikuchi et al., 2005).
2.15. OTHER PARASITISM GENES OR PROTEINGS INVOLVED IN PLANT CELL WALL DEGRADATION
There is evident that the potato cyst nematode also secretes a protein that haves the ability to break the non-covalent bonds in plant cell wall in addition to the ability of breaking down the covalent bonds found in plant cell-walls through the enzyme cellulases and pectinase ( Baum et al, 2007) . This type of activity is accomplished by expansin-like protein found in the potato cyst nematode (Qin et al., 2004), which is also the first confirmed report of such protein from outside the plant kingdom. The expansins involves in softening the plant cell-walls by breaking the non-covalent bonds between cell-wall-fibrils, thus allowing the sliding of fibrils past each other. The resultant cell-wall softening could also be demonstrated for the potato cyst nematode expansin parasitism protein (Qin et al., 2004). Till now, no such genes have been reported from root-knot nematodes or other cyst nematodes.
2.15.2. CHORISMATE MUTASE
Chorismate is the precursor for a number of compounds like cellular aromatic amino acids, plant hormone indole-3-acetic acid, plant defence hormone related salicyclic acid and other secondary metabolites (Dewick, 1998). This chorismate-derived compound plays an important role in plant growth and development, in plant defence, and also in interactions with other organisms (Schmid and Amrhein, 1995; Weaver and Hermann, 1997). The enzyme Chorismate mutase catalysed the pericyclic claisen-like rearrangement of chorismate to prephenate in shikimate pathway, a primary metabolic pathway that are found in plants and other micro-organisms (Romero et al, 1995) this enzyme is well characterized from microbes and plants, and not described from any other animals outside the plant-parasitic nematodes (Roberts et al., 1998; Romero et al ., 1995; Schmid and Amrhein, 1995). The first animal Chorismate mutase gene (Mj-cm-1) was cloned from Meloidogyne javanica, the root-knot nematode and found to be expressed in the oesophageal gland cells of the nematode. (Lambert et al ., 1999). The enzyme Chorismate mutases are potentially known to be involved in early development of the feeding sites induced by the plant parasitic nematodes, but how this enzyme alter the development of plant cells is not properly known (Doyle and Lambert, 2003). Chorismate mutase were identified from soybean and potato cyst nematodes recently. (Bekal et al., 2003; Gao et al ., 2003; Jones et al ., 2003).
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Chitinase is a putative parasitism protein and identified from the subventral glands of the soybean cyst nematode (Gao et al., 2002). This parasitism protein has a clearly defined function but no clear role for this function during the production of protein. The occurance of chitin in nematode has been found only in the egg shell (Bird et al., 1991) and the presence of this parasitism protein chitinases has been discussed as having a role in the hatching of nematode (Baum et al., 2007). In situ expression (Gao et al., 2002) and microarray expression studies demonstrate that chitinase is not found to be expressed in the eggs of nematodes but noticed a strong expression peak in the early phases of parasitism after penetration inside the plants (Baum et al., 2007).
Annexin genes represents a group of family that codes for calcium dependent phospholipid binding proteins and has a broad range of reported functions. The mRNA for a secretory isoform of an annexin-like protein was also reported to be expressed from the dorsal-gland of the soybean cyst nematode (Gao et al., 2002). However, no clear confirmation about its role in parasitism can be drawn at this time. This gene is also known to reported from Globodera pallida , the potato cyst nematode (Baum et al., 2007).
A group of parasitism gene candidates was reported when comparing the gene expression patterns among the developmental stages of the potato cyst nematode (Qin et al., 2004). The presence of a small family of genes which codes for secretory proteins that shows high similarity to proteins binding to the small G-protein Ran, called as Ran-Binding Protein in the Microtubule organizing center(RanBPMs) was revealed during futher ananylsis of this gene. Several of these genes were reported to be expressed in the dorsal-gland (Qin, 2001). The exact role and functions of Ran-Binding Protein in the Microtubule organizing center is still not. It is assumed that the potato cyst nematode proteins with similarity to RanBPM may have a possible function in regulating the cell cycle activities as observed in developing syncytia (Qin, 2001)
Calcirecticulin-like proteins are also reported to be secreted from other plant parasitic nematodes and regarded as good candidates for its role in parasite-host interactions (Nakhasi et al., 1998, Pritchard et al., 1999). Calcirecticulin-like protein preceded by a signal peptide was also reported to be secreted from the subventral-glands of a root knot nematode (Jaubert et al., 2002). The confusing array of putative or demonstrated calcirecticulin functions reported ((Nakhasi et al., 1998) make it difficult to confirmed its role in host parasitism by the root-knot nematodes.
2.15.7. SMALL BIOACTIVE PEPTIDES
The most commonly expressed parasitism gene in Heterodera glycines, the soybean cyst nematode (Gao et al., 2003) was first identified as clone HG-SYV46 (Wang et al., 2001) through the secretion of signal peptide of an esophageal gland cell cDNA library. The computational analyses found out that the C-terminal domain of HG-SYV46 is related to the members of the CLAVATA3-ESR-like (CLE) family of signaling proteins in Arabidopsis (Olsen and Skriver, 2003). The CLAVATA3 in Arabidopsis has been found out as a key factor determining the shoot meristem differentiation (Fletcher et al., 1999). The expression of the cDNA of Heterodera glycines CLAVATA3-like peptide in the clavata3 (clv3) Arabidopsis mutant found to restored the wild-type phenotype confirming the first report of ligand mimicry in plant nematode interactions (Wang, et al., 2005). This finding is the first report of a direct regulatory interaction between plant and nematode proteins. It will be interesting to find out if the small C-terminal extension of the cyst nematode ubiquitin extension proteins when considering the importance of this small peptides in signaling roles of plant development and plant nematode interactions (Tytgat et al., 2004; Gao et al., 2003) will have the regulatory functions in plant cell. It is also supported by the fact that the role of small peptides in nematode-plant interactions is also presented by an unknown peptide fraction smaller than 3 kilo Dalton isolated from potato cyst nematode secretions(Baum et al, 2007).