Project topics and materials
Get Complete Project Material File(s) Now! »
General Fungal Invasion Strategy
Plants make use of a preformed defence system such as the cell wall, resident pathogen repellents like polyphenols and tannins for protection against pathogens. They also utilise induced defence systems like the hypersensitive responses for this purpose (Walton, 1997). Nonetheless, pathogens have evolved mechanisms for entering and colonising plant tissues (Lawton, 1997). In order to ensure their survival, pathogens must adapt to the prevailing apoplastic or cellular conditions, overcome the existing physical or biochemical defences, employ mechanisms of obtaining plant nutrients and must circumvent the plant’s inducible defence responses (Lawton and Lamb, 1987; Cook et al., 1999).
Fungal pathogens must penetrate their hosts in order to establish infection (Walton, 1997, Schafer, 1994). Although some penetrate through natural openings such as stomata or wounds, many fungi invade by direct penetration through the plant surface (Agrios, 1988). Penetration of the plant cell can be achieved by mechanical forces or by enzymatic degradation (Schafer, 1994). In cases where penetration is not by mechanical force, enzymatic degradation is usually used. Even after penetration, CWDEs are utilised for derivation of nutrients from the plant cell wall (Walton, 1994). Therefore, for survival the pathogen must be able to recognise, become associated with, exploit the nutrient reserves of, and combat defence responses of its host (Herron et aI., 2000).
Secretion of CWDEs is generally the way that pathogens circumvent the physical barrier that is presented by the plant cell wall (Walton, 1994). Most CWDEs are glycoside hydrolases that degrade cellulose and pectate matrices by the addition of water to break the glycoside bond (Herron et al., 2000). The release of CWDEs has previously been shown to occur in a time scale where some enzymes are secreted much earlier than others (Albersheim and Anderson, 1971).
The Role of EndoPGs in Pathogenesis
The ability of a plant to respond defensively against an invading pathogen depends on its perception (recognition) of the pathogen. This information then must be transmitted from the infected cells to adjacent plant cells (Cervone et al., 1997). These cells then elicit biochemical changes that act cooperatively to limit invasion of the pathogen (De Lorenzo and Cervone, 1997; De Lorenzo et al., 1994). EndoPGs degrade the pectin structure of the plant cell wall. In this process they weaken the cell wall and allow the fungal pathogen to invade the rest of the plant tissue (Cervone et al., 1989). However, the link between secretion of endoPGs and pathogenicity is not absolute. In some cases the link has been clearly demonstrated (Weeds et al., 1999; Ten Have et al., 1998) and in others, it appears to be less significant (Gao et al., 1996; Scott-Craig et al., 1990). This implies that investigations on the role of individual endoPGs should be carried out in a case-by-case manner and generalisations for different plant pathogen interactions should be made with great care.
The first molecular evidence to show that endoPGs have an important function in degradation of cell walls was first reported by Capran and co-workers (1996). In their study, sitedirected mutagenesis was used to show that histidine 234 residue of the endoPG of Fusarium moniliforme is critical for enzymatic and macerating activity, and not for binding to PGIP. This histidine residue is conserved in endoPGs. These results focussed on the specific function of a single amino acid residue (H234) in degrading the pectic constituent of the plant cell.
EndoPGs have been reported to have two opposing roles in fungal pathogenesis (Cervone et aI., 1997). Firstly, they are utilised by fungi as efficient tools of aggression. This is achieved by degrading the plant cell wall structure allowing a pathogen to penetrate its host. Secretion of additional endoPGs results in further penetration and enhances the survival ofthe pathogen in the host (De Lorenzo and Cervone, 1997; Cervone et al., 1989). Secondly, they act as potential preelicitors of plant defence signal molecules. This is an advantage to the host plant, since the plant then defends itself from the invading pathogen (Cervone et al., 1997). EndoPGs initiate the production of elicitors for signal transduction known as oligogalacturonides (OGAs) from degradation ofhomogalacturonan polymer of pectin (Cervone et ai., 1989, 1997). Degradation of pectin by endoPGs in the presence of PGIPs gives rise to the production of elicitor-active OGAs of 10-15 residues in size. Furthermore, the presence ofPGIPs increases the residence time of such molecules to act as signalling molecules. The early production of these endoPGs is compatible with both roles (De Lorenzo and Cervone, 1997; Cervone et al., 1997).
EndoPGs from different fungal species are different in their substrate degradation capacity and they also differ in their susceptibility to PGIP inhibition (Cook et al., 1999). This observation has two implications. Firstly, it means that successful pathogens should have the capacity to degrade the pectin walls of their hosts more rapidly than non-virulent pathogens. Secondly, they should secrete endoPGs that are not easily inhibited by the host PGIPs.
The pathogenicity of fungi like Botrytis cinerea on tomatoes and apples is controlled by the rate of endoPG production by that fungus (Ten Have et al., 1998; Weeds et al., 1999). In more pathogenic isolates of B. cinerea, higher levels of endoPGs are produced than in less pathogenic isolates. This has important implications in designing control measures against this pathogen. Genetic transformation of plants with a superior PGIP that strongly inhibits these endoPGs in B. cinerea would be an attractive option. In Leptosphaeria maculans, the causal agent of blackleg of canola, endoPGs are secreted at the initiation of disease. EndoPGs from L. maculans are inhibited by extracts from the stems of canola (Annis and Goodwin, 1997). Resistant canola cultivars have higher inhibition efficiency to the endoPGs from L. maculans than the susceptible cultivars. This shows that the PGIPs in canola interact with the endoPGs of L. maculans to give rise to resistance to blackleg. The pectic fragments obtained after digestion of bean cell walls with endoPGs of the bean pathogen Colletotrichum lindemuthianum, resulted in differential elicitation of defence responses in bean seedlings (Nuss et al., 1996; Boudart et al., 1998). C. lindemuthianum secretes endoPGs to degrade the cell walls of the host. Boudart and co-workers (1998) showed that between two near isogenic lines of bean, resistant and susceptible to C. lindemuthianum, the pectic fragments from the resistant lines elicited higher levels of pathogen-related (PR) proteins than those from the susceptible lines. Sizes of fragments that are produced from endoPG digestion of the pectic polymer determine the extent of defence signal elicitation. In the presence of the PGIP inlnbitors, the length of the OGAs is between 10-15 residues, which are elicitor-active (Lafitte et aZ., 1984). ‘This is a classical example of an interaction between endoPGs and PGIPs that results in triggering PR proteins whose function is to enhance resistance of the host plant against a pathogen.
CHAPTER 1
Literature Review
CHAPTER 2
Molecular relatedness ofpolygalacturonase-inlnbiting protein genes in Eucalyptus species
CHAPTER 3
Molecular analysis of an endopolygalacturonase gene from Cryphonectria cubensis, a Eucalyptus canker pathogen
CHAPTER 4
Production of polygalacturonases in isolates of Cryphonectria cubensis of differing pathogenicity
CHAPTER 5
Inhibition of fungal polygalacturonases from four tree pathogens by stem extracts from two Eucalyptus grandis clones
CHAPTER 6
Cloning and sequence analysis of the endopolygalacturonase gene from the pitch canker fungus, Fusarium circinatum
SUMMARY (ENGLISH)
