Phenylpropanoid Metabolism in Ripening Fruits

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, O-methyltransferase, 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.