HCAR Is a Limitation Factor for Chlorophyll Cycle and Chlorophyll Degradation in Chlorophyll--Overproducing Plants.

The chlorophyll (Chl) cycle is the metabolic pathway for Chl and Chl inter-conversion. In this pathway, Chl is synthesized from Chl by the catalyzing action of chlorophyllide oxygenase (CAO). In contrast, Chl is firstly reduced to produce 7-hydroxymethyl Chl (HMChl) , which is catalyzed by two isozymes of Chl reductase (CBR), non-yellow coloring 1 (NYC1) and NYC1-like (NOL). Subsequently, HMChl is reduced to Chl by HMChl reductase (HCAR). CAO plays a pivotal role in Chl / ratio regulation and plants over-accumulate Chl in CAO-overexpressing plants. NYC1 is more accumulated in Chl--overproducing plants, while HCAR is not changed. To investigate the role of HCAR in Chl cycle regulation, the Chl metabolites of Chl--overproducing plants were analyzed. The results showed that HMChl accumulated in these plants, and it decreased and the Chl / ratio increased by overexpressing HCAR, implying HCAR is insufficient for Chl cycle in Chl--overproducing plants. Furthermore, during dark-induced senescence, the non-programmed cell death symptoms (leaves dehydrated with green color retained) of Chl--overproducing plants were obviously alleviated, and the content of HM pheophorbide (HMPheide) and Pheide were sharply decreased by overexpressing HCAR. These results imply that HCAR is also insufficient for Chl degradation in Chl--overproducing plants during senescence, thus causing the accumulation of Chl metabolites and non-programmed cell death of leaves. With these results taken together, we conclude that HCAR is not well regulated and it is a limiting factor for Chl cycle and Chl degradation in Chl--overproducing plants.