Enzymes which have been analyzed for a role in disease from one or more plant pathogens include cutinase, α1,4-polygalacturonase, β1,4-xylanase, mixed-linked (β1,3-β1,4)-glucanase, β1,3-glucanase, pectic lyase, cellulase (β-glucosidase, cellobiohydrolase and β1,4-glucanase), pectin methylesterase, lipase and protease.
Pathogenesis-Related (PR) proteins are a category of proteins that accumulate at high levels in plants following a pathogen attack. Their initial discovery in tobacco ( Nicotiana tabacum) following infection by tobacco mosaic virus (TMV) has prompted years of research and characterization of these proteins ( van Loon and Pieterse, 2006). Various PR proteins have been shown to have antimicrobial properties. For instance, some PR proteins have been shown to have chitinase and glucanase activity (Kauffmann et al., 1987; Legrand et al., 1987). Chitins and glucans are well-known components of fugal and oomycete cell walls, respectively. The accumulation of PR proteins is also closely associated with the induction of the hypersensitive response (HR), accumulation of the plant hormone, salicylic acid (SA), and systemic acquired resistance (SAR), a form of long-lasting, broad-spectrum resistance (See below for HR, SA and SAR, Vlot et al., 2009 ).
In uninfected cells, the NPR1 monomer is constantly degraded in the nucleus by Cullin3-based ubiquitin ligase-mediated proteasome degradation, which prevents inappropriate activation of defense genes expression and SAR. In addition, NPR1 is phosphorylated at Ser55 and Ser59, which prevents SUMO3 modification of NPR1.
Examples of plants transformed with genes coding for antipathogen compounds include peanut plants transformed with antifungal genes that reduced the incidence of Sclerotinia blight, caused by Sclerotinia minor, by 36% compared to susceptible nontransgenic plants. Transgenic rose plants expressing a chitinase transgene from rice showed a 13–43% decrease in symptoms caused by the fungus Diplocarpon rosae, the cause of rose blackspot diseas e. Transgenic broccoli plants expressing an endochitinase gene obtained from the biocontrol fungus Trichoderma harzianum had 14–200 times the endochitinase activity of that of controls and showed significantly less severe symptoms than nontransgenic plants. Similarly, transgenic cotton and tobacco plants expressing the glucose oxidase gene obtained from the biocontrol fungus Talaromyces flavus showed significant resistance of seedlings to the Rhizoctonia fungus and partial resistance to Verticillium, but not to Fusarium. The glucose oxidase generates hydrogen peroxide, which, unfortunately, is toxic to both pathogens and plants. Transgenic A. thaliana plants expressing one or both of the genes coding for a cysteine protein inhibitor or, to a smaller extent, a cowpea trypsin (a serine) protein inhibitor, protect plants significantly from infection by several types of nematodes, especially the reniform nematode Rotyenchus reniformis. Similar increases in resistance have been shown for transgenic tobacco plants overexpressing a glutamate decarboxylase gene, which makes the plants resistant to the root knot nematode; for transgenic tobacco and potato expressing the bacterial gene ubiC, which leads to accumulation of toxic 4-hydroxybenzoic acid glucosides; for canola ( Brassica napus) plants expressing an antimicrobial peptide, which makes the plants resistant to the blackleg disease caused by the fungus L. maculans, and for peanut plants transformed with genes coding for antifungal enzymes, which make plants resistant to S. sclerotiorum.
There is emerging evidence that pathogens make different isoenzymes in the plant or in response to plant extracts than they do when grown in culture. Enzymes of the pathogen that can degrade plant cell walls have also received attention as triggers of plant defenses (i.e. elicitors, see section 13.4.2c).
There is no strong evidence that any cell-wall- degrading enzyme has any role in disease specificity, and thus they are considered basic compatibility factors. A number of genes encoding cell-wall-degrading enzymes have been cloned and mutated by targetted gene disruption in plant pathogenic organisms.
The information regarding a protective effect of PR-proteins in interaction of plants and parasitic nematodes is extremely limited and contra dictory. A number of publications point to the absence of PR-protein induction in plants in response to nematode invasion. Other publications note the PR-protein induction in plants in response to nematode invasion, however, the relation of these proteins with resistance or susceptibility has not been revealed.