Post harvest diseases fruits and vegetables - Xavier University ...

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Novel Approaches for Enhancing Host Resistance 263 engineering. The creation of such plants involves the introduction, from unrelated plant species, of genes for disease resistance (Boiler, 1993). This technology not only allows for a wider genetic diversity but it may enable us to add multiple, diverse resistance genes to one plant variety. Furthermore, creating plant cultivars with inheritable genes for disease resistance is the most environmentally sound strategy for disease management. Desirable target genes for creating useful transgenic plants are usually isolated from plant viruses, bacteria, fungi or other plants, depending on the traits desired. Widely used genes are those known to be inhibitory to fungal growth, and they are often induced following fungal invasion (Boiler, 1993; Broekaert et al., 1995). These genes were reported to provide a quantitative improvement in resistance when introduced into a transgenic host in greenhouse studies (Jongedijk et al., 1995). However, the selection of beneficial target genes for pathogen resistance requires a thorough understanding of the pathogen, the host and the host-pathogen interactions. This knowledge can come from a multitude of genetic and physiological experiments. 2. SOURCES OF GENES FOR BIOENGINEERING PLANTS Mount and Berman (1994) pointed out several natural compounds known to have antimicrobial activity against pathogenic fungi or bacteria, which could be possible sources for genes valuable for bioengineering of plants. The natural antibiotic compounds present an example of such sources. Transgenic plants with genes for antibiotic production may prevent initial infection of plant tissue by postharvest pathogens. Sources for novel antibiosis genes may come from plants themselves or from the many microorganisms being studied for use as biological control agents. If the antibiosis is due to a compound with isolatable genes, the genes might be suitable for bioengineering of plants. However, the widespread use of antibiotics is not recommended, mainly because of the possibility that the pathogen could rapidly develop resistance to the antibiotic compounds. Chitinases and P-l,3-glucanases are other antifungal compounds which are effective against fungal cell-wall polymers and are believed to be involved in plant defense mechanisms against fungal infection. Once a hydrolase gene that is effective against the pathogen has been identified, a desired transgenic plant can be created through molecular manipulation. The insertion of a chitinase gene into tobacco and canola plants was shown to result in enhanced resistance to Rhizoctonia solani http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 264 Postharvest Diseases of Fruits and Vegetables but not to Cercospora nicotianae, suggesting that other factors besides chitinase, may be involved in disease resistance (BrogUe et al., 1991). A later study describes enhanced resistance to fungal attack in transgenic plants by co-expression of chitinase and glucanase genes (Zhu et al., 1994). Other microbial enzymes normally induced in plants, such as peroxidase and other pathogen-defense-related compounds, might also be exploited for bioengineering (Bowles, 1990; Mohan and Kolattukudy, 1990). The knowledge and understanding of postharvest disease etiology and physiology may be of much help in selecting new genes and developing new transgenic plants, resistant to postharvest diseases. For example, the knowledge that wounding leads to fungal penetration into the host and triggers postharvest disease development (Barkai-Golan and Kopeliovitch, 1981; Brown, G.E. and Barmore, 1983; Eckert and Ratnayake, 1994) may lead to genetic alteration of the plant to make it more physically resistant to wounding or to enhance the production of wound-healing compounds to shorten the time that the wounded tissue is vulnerable to pathogen attack (Mount and Berman, 1994). Similarly, understanding the role of the pathogen cell-wall-degrading enzymes in plant tissue deterioration may lead to the creation of new transgenic plants by which the pathogen polygalacturonase (PG) is suppressed. Following the characterization of PG-inhibiting proteins from pears by Stotz et al. (1993), it was suggested that the pear promoter might affect a fruit-specific expression of the gene, resulting in the inhibition o£ Botrytis cinerea. It was further suggested that the characterization of the protein inhibitor from pears should lead to the expression of PG-inhibiting proteins in transgenic plants and possibly to the inhibition of decay development. In fact, Powell et al. (1994) reported that transgenic tomato fruits expressing the gene of fungal PG-inhibiting glycoproteins of pears, were more resistant to B. cinerea than the control fruits. Using nor (non-ripening) or rin (ripening inhibitor) tomato mutants that block many aspects of the ripening process of the fruit, including softening of the tissues and color development, Barkai-Golan et al. (1986) and Giovannoni et al. (1989) found that the tomato's own PG plays an important part in the total PG activity in the host during pathogenesis, and probably takes a major role in the degradation of cell wall pectic substances. Following these findings, it was further suggested that the suppression of fruit PG might be useful in increasing fruit shelf life (Giovannoni et al., 1989). Bioengineering this enzyme has been accomplished with tomatoes (Kramer et al., 1990; Smith et al., 1988) and http://arab2000.forumpro.fr
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Post harvest diseases fruits and vegetables - Xavier University ...

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