Flavonoid evolution: an enzymic approach.

Flavonoid evolution in land plants is discussed from an enzymic point of view, based on the present day distribution of the major subgroups of flavonoids in bryophytes, lower and higher vascular plants. The importance of varied functions in the origin of pathways with a series of sequential steps leading to end-products is considered; it is argued that the initial function is that of an internal regulatory agent, rather than as a filter against ultraviolet irradiation. The basic synthases, hydroxylases, and reductases of flavonoid pathways are presumed to have evolved from enzymes of primary metabolism. A speculative scheme is presented of flavonoid evolution within a primitive group of algae derived from a Charophycean rather than a Chlorophycean line, as a land environment was invaded. Flavonoid evolution was preceded by that of the phenylpropanoid and malonyl-coenzyme A pathways, but evolved prior to the lignin pathway.