Chlorophyll cycle regulates the construction and destruction of the light-harvesting complexes

Chlorophyll a and chlorophyll b are the major constituents of the photosynthetic apparatus in land plants and green algae. Chlorophyll a is essential in photochemistry, while chlorophyll b is apparently dispensable for their photosynthesis. Instead, chlorophyll b is necessary for stabilizing the major light-harvesting chlorophyll-binding proteins. Chlorophyll b is synthesized from chlorophyll a and is catabolized after it is reconverted to chlorophyll a. This interconversion system between chlorophyll a and chlorophyll b refers to the chlorophyll cycle. The chlorophyll b levels are determined by the activity of the three enzymes participating in the chlorophyll cycle, namely, chlorophyllide a oxygenase, chlorophyll b reductase, and 7-hydroxymethyl-chlorophyll reductase. This article reviews the recent progress on the analysis of the chlorophyll cycle and its enzymes. In particular, we emphasize the impact of genetic modification of chlorophyll cycle enzymes on the construction and destruction of the photosynthetic machinery. These studies reveal that plants regulate the construction and destruction of a specific subset of light-harvesting complexes through the chlorophyll cycle. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Research Highlights
► Chlorophyll b is essential in stabilizing light-harvesting complexes (LHC).
► Chlorophyll b breakdown is requisite for the degradation of LHC.
► Interconversion of chlorophyll a and chlorophyll b refers to the chlorophyll cycle.
► Two out of three enzymes involved in the chlorophyll cycle have been identified.
► The regulatory mechanism of the chlorophyll cycle is partly understood.