02 Jan 2025
Decoding Leaf Greenness: Biologists Unraveled a Regulatory Module that Ensures Chlorophyll Homeostasis in Land Plants

Molecular control of chlorophyll homeostasis in land plants
Photosynthesis fuels nearly all life on Earth. Chlorophyll is the most abundant photosynthetic pigment that is in charge of light capture and charge separation in photosynthesis. Maintaining stable chlorophyll levels is a prerequisite for efficient photosynthesis and thereby food production. Chlorophyll homeostasis relies on the precise coordination of chlorophyll biosynthesis and degradation. However, the specific mechanism of regulation of chlorophyll metabolism remains unclear. Based on detailed protein interaction results, combined with genetic analysis, this study revealed that the scaffolding protein BCM1 (Balance of Chlorophyll Metabolism 1), the chlorophyll synthesis-related protein GUN4 (Genomes Uncoupled 4) and the chlorophyll decomposition-related protein SGR1 (Mg-dechelatase 1) Carboxyl-terminal (CT) domain-specific interactions. At the same time, the chloroplast protease EGY1 (Ethylene-dependent Gravitropism-deficient and Yellow-green 1) is also involved in the chlorophyll metabolism process mediated by BCM1. This research reveals a specific regulatory mechanism of chlorophyll metabolism in land plants, which synchronises the biosynthesis and decomposition of chlorophyll through the post-translational regulation of GUN4 and SGR1 mediated by BCM1 and EGY1.
The co-evolution of BCM1, GUN4 and SGR1 in chlorophyll metabolism played a crucial role in adapting plants from aquatic to terrestrial. The study found that BCM plays a conserved dual function in regulating chlorophyll metabolism in Arabidopsis and tomato. In Arabidopsis leaves and seeds and tomato leaves and fruits, BCM plays an essential role in maintaining chlorophyll biosynthesis and inhibiting chlorophyll degradation. Further research found that the CT domain of BCM1 is the key to the dual functions of BCM1. The deletion of the CT domain will cause BCM1 to be unable to function in chlorophyll metabolism. The function of the CT domain of BCM1 is highly conserved in Arabidopsis and tomato. It is known that GUN4 protein can activate magnesium chelatase in chlorophyll synthesis to enhance chlorophyll synthesis. Magnesium chelatase encoded by SGR1 is extremely important in the decomposition process of chlorophyll.
This study found that the biological functions of GUN4 and SGR1 depend on their CT domain, and the deletion of the CT domain will lead to the impairment of the biological functions of GUN4 and SGR1. Protein interaction data also indicate that the CT domain is a key in the interaction between GUN4/SGR1 and BCM1. The study further discovered that chloroplast protease EGY1 is an important participant in BCM1-mediated chlorophyll metabolism. EGY1 can interact with BCM1, GUN4 and SGR1, and participate in the BCM1-mediated hydrolysis of SGR1 protein. Genetic and bioinformatics data also indicate that EGY1 and BCM1 jointly regulate the proteostasis of enzymes related to chlorophyll homeostasis.
In summary, this study demonstrates an evolutionary model in which the CT domain of BCM1 interacts with the CT domains of GUN4 and SGR1 to achieve precise regulation of chlorophyll homeostasis. This model further clarifies how plants achieve precise regulation of chlorophyll metabolism during their adaptation to the terrestrial environment.
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About the research paper:
Journal title: “Fu D., Zhou H., Grimm B., and Wang P.* The BCM1-EGY1 module balances chlorophyll biosynthesis and breakdown to confer chlorophyll homeostasis in land plants. Molecular Plant 2024”.
The journal paper can be accessed from here.
Professor Peng WANG
The School of Biological Sciences