Regulation of Chlorophyll Biosynthesis in Angiosperms

Light controls plant growth and development in a pleiotropic manner. For example, light dictates a pattern of gene expression that leads to photomorphogenesis (Kendrick and Kronenberg, 1994). In the absence of light, an alternative developmental process, skotomorphogenesis, takes place. Photomorphogenesis and skotomorphogenesis are controlled by several different photoreceptors, such as Pr and Pfr and blue-light-absorbing photoreceptors. During skotomorphogenesis, the germinating seedling utilizes all of the nutrients contained in the seed to establish conditions that allow the plant to harvest even traces of light. The hypocotyl elongates dramatically just to place the cotyledons above the soil. Within the cotyledons, proplastids differentiate into etioplasts, and a large supply of the tetrapyrrole pigment precursor Pchlide is built up in the prolamellar bodies of the etioplasts. Together with NADPH, Pchlide is bound to the PORA. This ternary complex is photoactive and thus able to immediately photoreduce the Pchlide to Chlide as soon as the cotyledons are exposed to light (for review, see Ryberg and Sundqvist, 1991). During photomorphogenesis, hypocotyl elongation is suppressed, but cotyledon unfolding and expansion proceed normally. Chloroplast development as well as Pchlide reduction occur in parallel and collectively lead to the rapid greening of the plant. Pioneering experiments performed in Granick's group almost 40 years ago shed some light on the regulatory mechanisms that govern Chl biosynthesis in higher plants. When angiosperm plants were germinated in the dark, their cotyledons appeared pale yellow. If such etiolated seedlings were incubated with ALA, a common precursor -of all tetrapyrrole pigments, they turned greenish (Granick, 1959). This apparent greening was not caused by the synthesis of Chl but was due to the accumulation of excess Pchlide, the immediate precursor of Chlide. Similar to leaf tissues, isolated plastids were found in later studies to accumulate excess Pchlide when they were fed ALA in the dark (Kannangara and Gough, 1977). Taken together, these basic experiments showed that all of the enzymes necessary for the conversion of ALA to Pchlide must be present in dark-grown angiosperm plants, that the entire pathway is likely to operate in the plastid, and that the only known light-requiring reaction for the synthesis of Chl is the reduction of Pchlide to Chlide. It is the aim of this review to summarize our current knowledge on key regulatory steps of Chl biosynthesis in angiosperms.