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Peroxidases, one of the key antioxidant enzymes, are widely distributed in nature and catalyze oxidation of various electron donor substrates concomitant with the decomposition of H 2 O 2. The non-animal plant peroxidases (class III peroxidase) are involved in various essential physiological processes of plant growth and development throughout their life cycle. In view of the capability of peroxidases to catalyze the redox reaction for a wide range of substrates, they are considered as one of the important enzyme from the point of view of their various medicinal, biochemical, immunological, biotechnological and industrial applications. They have been successfully used for biopulping and biobleaching in the paper and textile industries. Peroxidases have also been used in organic synthesis, bioremediation, as well as various analytical applications in diagnostic kits, ELISA. Peroxidase based biosensors find application in analytical systems for determination of hydrogen peroxide, glucose, alcohols, glutamate, and choline etc. Thus, in view of array of physiological functions as well as industrial applications, the peroxidases have conquered a dominant position in research groups and become one of the most extensively studied enzymes. In this direction, the present review embodies the classification, mechanism of action, major physiological functions as well as industrial applications of plant peroxidases.

Ripening of fleshy fruit is a differentiation process involving biochemical and biophysical changes that lead to the accumulation of sugars and subsequent changes in tissue texture. Also affected are phenolic compounds, which confer color, flavor/aroma, and resistance to pathogen invasion and adverse environmental conditions. These phenolic compounds, which are the products of branches of the phenylpropanoid pathway, appear to be closely linked to fruit ripening processes. Three key enzymes of the phenylpropanoid pathway, namely phenylalanine ammonia lyase, Omethyltransferase, and cinnamyl alcohol dehydrogenase (CAD) have been reported to modulate various end products including lignin and protect plants against adverse conditions. In addition, peroxidase, the enzyme following CAD in the phenylpropanoid pathway, has also been associated with injury, wound repair, and disease resistance. However, the role of these enzymes in fruit ripening is a matter of only recent investigation and information is lacking on the relationships between phenylpropanoid metabolism and fruit ripening processes. Understanding the role of these enzymes in fruit ripening and their manipulation may possibly be valuable for delineating the regulatory network that controls the expression of ripening genes in fruit. This review elucidates the functional characterization of these key phenylpropanoid biosynthetic enzymes/genes during fruit ripening processes.

An aspen lignin-specific O-methyltransferase (bi-OMT; S-adenosyl-L-methionine: caffeic acid/5-hydroxyferulic acid 3/5-O-methyltransferase, EC 2.1.1.68) antisense sequence in the form of a synthetic gene containing the cauliflower mosaic virus 35S gene sequences for enhancer elements, promoter and terminator was stably integrated into the tobacco genome and inherited in transgenic plants with a normal phenotype. Leaves and stems of the transgenes expressed the antisense RNA and the endogenous tobacco bi-OMT mRNA was suppressed in the stems. Bi-OMT activity of stems was decreased by an average of 29% in the four transgenic plants analyzed. Chemical analysis of woody tissue of stems for lignin building units indicated a reduced content of syringyl units in most of the transgenic plants, which corresponds well with the reduced activity of bi-OMT. Transgenic plants with a suppressed level of syringyl units and a level of guaiacyl units similar to control plants were presumed to have lignins of distinctly different structure than control plants. We concluded that regulation of the level of bi-OMT expression by an antisense mechanism could be a useful tool for genetically engineering plants with modified lignin without altering normal growth and development.