Evidence for a Light-Independent Chlorophyll Biosynthetic Pathway in Angiosperm Seeds Germinated in Darkness

Abstract

It is not generally recognised that during the course of evolution of seed plants, many angiosperms and gymnosperms have retained the capacity for light-independent (dark) chlorophyll (Chl)^* synthesis exhibited by cyanobacteria, algae, bryophytes and pteridophytes (1,2,3). As a consequence the view that when greening or mature angiosperm leaves are transferred from light to darkness, Chl(ide) formation ceases immediately because of the cessation of Pch(lide) photoreduction, is still widely held (4). It is reinforced by the visual contrast between etiolated dark grown angiosperm seedlings and the bright green cotyledons of dark grown gymnosperms. This important phenotypic difference in the two groups of seed bearing plants is usually interpreted in terms of an absolute reqirement of angiosperms for light for Pch(lide) reduction, whereas in gymnosperms Chi is formed in light as well as darkness (5). In this context it is not surprising that early reports of trace amounts of Chi in dark grown angiosperm seedlings (6,7) and the accumulation of Chl by wheat embryos grafted onto pine megagametophytes (8) attracted little attention.