Rice 7-Hydroxymethyl Chlorophyll a Reductase Is Involved in the Promotion of Chlorophyll Degradation and Modulates Cell Death Signaling

The loss of green coloration via chlorophyll (Chl) degradation typically occurs during leaf senescence. To date, many Chl catabolic enzymes have been identified and shown to interact with light harvesting complex II to form a Chl degradation complex in senescing chloroplasts; this complex might metabolically channel phototoxic Chl catabolic intermediates to prevent oxidative damage to cells. The Chl catabolic enzyme 7-hydroxymethyl Chl reductase (HCAR) converts 7-hydroxymethyl Chl (7-HMC ) to Chl . The rice ( ) genome contains a single homolog ( ), but its exact role remains unknown. Here, we show that an knockout mutant exhibits persistent green leaves during both dark-induced and natural senescence, and accumulates 7-HMC and pheophorbide (Pheo ) in green leaf blades. Interestingly, both rice and Arabidopsis mutants exhibit severe cell death at the vegetative stage; this cell death largely occurs in a light intensity-dependent manner. In addition, 7-HMC treatment led to the generation of singlet oxygen ( O ) in Arabidopsis and rice protoplasts in the light. Under herbicide-induced oxidative stress conditions, leaf necrosis was more severe in plants than in wild type, and -overexpressing plants were more tolerant to reactive oxygen species than wild type. Therefore, in addition to functioning in the conversion of 7-HMC to Chl in senescent leaves, HCAR may play a critical role in protecting plants from high light-induced damage by preventing the accumulation of 7-HMC and Pheo in developing and mature leaves at the vegetative stage.