Arabidopsis STAY-GREEN, Mendel’s Green Cotyledon Gene, Encodes Magnesium-Dechelatase

• Published in: The Plant cell Vol. 28; no. 9; pp. 2147 - 2160
• Main Authors: Shimoda, Yousuke, Ito, Hisashi, Tanaka, Ayumi
• Format: Journal Article
• Language: English
• Published: United States American Society of Plant Biologists 01.09.2016
• ISSN: 1040-4651; 1532-298X
• DOI: 10.1105/tpc.16.00428

Pheophytin a is an essential component of oxygenic photosynthetic organisms because the primary charge separation between chlorophyll a and pheophytin a is the first step in the conversion of light energy. In addition, conversion of chlorophyll a to pheophytin a is the first step of chlorophyll degradation. Pheophytin is synthesized by extracting magnesium (Mg) from chlorophyll; the enzyme Mg-dechelatase catalyzes this reaction. In this study, we report that Mendel’s green cotyledon gene, STAY-GREEN (SGR), encodes Mg-dechelatase. The Arabidopsis thaliana genome has three SGR genes, SGR1, SGR2, and STAY-GREEN LIKE (SGRL). Recombinant SGR1/2 extracted Mg from chlorophyll a but had very low or no activity against chlorophyllide a; by contrast, SGRL had higher dechelating activity against chlorophyllide a compared with chlorophyll a. All SGRs could not extract Mg from chlorophyll b. Enzymatic experiments using the photosystem and light-harvesting complexes showed that SGR extracts Mg not only from free chlorophyll but also from chlorophyll in the chlorophyll-protein complexes. Furthermore, most of the chlorophyll and chlorophyll binding proteins disappeared when SGR was transiently expressed by a chemical induction system. Thus, SGR is not only involved in chlorophyll degradation but also contributes to photosystem degradation.