Peroxidases (EC 188.8.131.52) have many physiological roles in several primary and secondary metabolic processes, such as scavenging of peroxide, participation in lignification, regulation of cell growth and differentiation, hormonal signaling, plant defense, indol-3-yl-acetic acid (IAA) catabolism, oxidation of toxic compounds, and ethylene biosynthesis (Campa, 1991). The plant cell wall is a very dynamic structure which controls both cell shape and cell elongation. Various enzymatic processes cleave and re-assemble the cell-wall constituents during cell extension. Changes in the cell-wall architecture can be achieved by class III peroxidases through their two catalytic cycles: peroxidative and hydroxylic (Passardi et al., 2004). They can stop elongation by forming bonds within the cell wall or favor it by regulating the local concentration of H2O2 or by generating reactive oxygen species (ROS), which break cell-wall bonds (Passardi et al., 2004). Indirectly, peroxidases can also control the cell elongation through their auxin oxidase activity. IAA can be oxidized by following two different mechanisms: 1) a conventional hydrogen peroxide-dependent pathway and 2) one that is hydrogen peroxide-independent and requires oxygen (Savitsky et al., 1999). By this way, peroxidases might locally regulate auxin concentration. In parallel, peroxidase expression levels are dependent on the endogenous auxin concentrations (Gaspar et al., 1996). Investigators have found multiple forms of peroxidase in numerous plants including several species of dwarf plants (Evans and Alldridge, 1965; Kamerbeek, 1956; Loy, 1967; McCune, 1961). Several species of genetically dwarf plants have been reported to have a higher peroxidase activity than their nondwarf or normal counterparts (Evans and Alldridge, 1965; Kamerbeek, 1956; McCune and Galston, 1959; Van Overbeek, 1935). It has also been shown that several peroxidase isozymes are quantitatively altered by gibberellic acid application to dwarf plants (Birecka and Galston, 1970; McCune, 1961; Shanan, 1976). These observations, together with the known catalytic properties of peroxidase, led to their implication in the physiology of dwarfing (Loy, 1972; Van Overbeek, 1935). In a comparison of the total peroxidase activities and isozyme patterns between the bush and vine types of Cucurbita pepo L. and Cucurbita maxima Duchesne (squash), Cucumis melo L. (muskmelon), and Citrullus vulgaris Shrad. (watermelon), Loy (1967) found no significant differences in the peroxidase activities or isozyme patterns between bush and vine types. The results of Loy (1972) revealed that the overall genotype of squash cultivars could influence the relative expression of peroxidase activity between bush and vine forms. However, peroxidase differences between bush and vine plants in C. moschata were not investigated.
Wu et al. (2007) selected a bush-type plant from C. moschata. Bush plants were characterized by shorter internodes, earlier flowering, a higher ratio of female to male flowers, and smaller fruit than vine plants. Genetic analysis indicated monogenic inheritance with the bush genotype dominant. Furthermore, a study by Cao et al. (2005) revealed that the bush plant in C. moschata reported by Wu et al. (2007) was a gibberellin-responsive mutant and that the bushy appearance was the result of inhibition of cell elongation. The aim of this study was to compare the total peroxidase activities and peroxidase isozymes as well as the protein profiles between bush and vine-type phenotype in a near-isogenic line of C. moschata bush plants.
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