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

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Host Protection and Defense Mechanisms 93 Peroxidases are another group of PR proteins whose activity has been correlated with plant resistance against pathogens. Plant peroxidases, which are glycoproteins that catalyze the oxidation by peroxide of many organic and inorganic substrates, have been implicated in a wide range of physiological processes, such as ethylene biosynthesis, auxin metabolism, respiration, lignin formation, suberization, growth and senescence. Findings of correlations between deposition of cell wall strengthening materials, such as lignin, suberin and extensin, and peroxidase activities are consistent with a role for this enzyme in defense through wall-strengthening processes (Chitoor et al., 1999). The importance of peroxidase lies in the fact that the host cell wall constitutes one of the first lines of defense against pathogens, and peroxidase is a key enzyme in the wall-building processes. Such processes include the accumulation of lignin and phenolic compounds, which has been correlated with enhanced resistance in various host-pathogen interactions, and suberization, which leads to enhanced resistance by healing wounds with a layer of phenolic and aliphatic compounds (Mohan and Kolattukudy, 1990). However, the resistance against pathogens may also be related to the highly reactive oxygen species such as H2O2 or oxygenase, which are likely to be toxic to pathogens and which are formed by peroxidase activity during the deposition of cell wall compounds (Goodman and Novacky, 1994). In a recent study Ray and Hammerschmidt (1998) found that the activity of peroxidase in potato tubers increased greatly following infection with Fusarium sambucinum, the cause of potato dry rot. Such an infection induces several isoforms of peroxidase whose role has not been identified. Lignin levels also increased but the lignified zones could be breached, allowing the infection to develop further into the tuber. The increased peroxidase levels in infected tubers were expected to increase their potential to synthesize lignin and so enhance their resistance to infection. However, infection of tubers of transgenic potatoes following introduction of a foreign peroxidase gene from cucumbers, which was associated with an induced resistance response, had no effect on disease caused by several potato pathogens (Ray et al., 1998). These results indicated that the incorporation of a putative defense-associated peroxidase and the formation of transgenic potato plants does not necessarily result in the enhancement of resistance. It has been suggested, however, that understanding the function of peroxidase and determining the availability of the substrates needed for the enzyme activity are important for using transgenic studies to evaluate the role of peroxidase in defense responses (Ray et al., 1998, Chitoor et al., 1999). http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE CHAPTER 7 PHYSIOLOGICAL AND BIOCHEMICAL CHANGES FOLLOWING INFECTION Postharvest infection of fruits and other plant organs may induce a number of alterations in their physiological and biochemical processes or in the host tissue constituents, as a result of host-pathogen interactions. Process changes may include acceleration of ethylene evolution, stimulation of the respiratory enzymes, enhanced pectolytic activity, altered protein synthesis or polyamine-synthesis enzyme activity, etc.; and tissue changes may include increased cell-wall soluble pectin, and changed organic acid and sugar contents. The infection, or sometimes an attempted infection, may also induce the accumulation of low-molecular-weight defense chemicals - the phytoalexins - or enhance the production of preformed antimicrobial compounds in the host. The latter changes are dealt with in the chapter on Host Protection and Defense Mechanisms. A. CHANGES IN FRUIT RESPIRATION AND ETHYLENE EVOLUTION Studies carried out over the last few decades have found enhanced respiratory activity and increased ethylene production following injury or exposure to other stresses, such as low temperature, gamma irradiation, etc. Invasion by pathogens, including fungi, bacteria and viruses, seems to be no different in this respect from other plant stresses. Both respiration and ethylene evolution are important indicators of the physiological state of the fruit, and their enhancement following infection highlights its effects on the postharvest ripening and senescence processes. Stimulation of respiration and ethylene evolution processes in citrus fruits infected by Penicillium digitatum, was first recorded at the beginning of the 1940s (Biale, 1940; Biale and Shepherd, 1941; Miller et al., 1940), and subsequent studies of various citrus fruits inoculated with divers postharvest fungi confirmed that this was a general post-infection phenomenon (Schiffmann-Nadel, 1974). Similarly to the fungi, pathogenic bacteria such as Pseudomonas syringae can also enhance respiratory http://arab2000.forumpro.fr
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