Abstract
The chlorophyll (Chl) cycle is the metabolic pathway for Chl a and Chl b inter-conversion. In this pathway, Chl b is synthesized from Chl a by the catalyzing action of chlorophyllide a oxygenase (CAO). In contrast, Chl b is firstly reduced to produce 7-hydroxymethyl Chl (HMChl) a, which is catalyzed by two isozymes of Chl b reductase (CBR), non-yellow coloring 1 (NYC1) and NYC1-like (NOL). Subsequently, HMChl a is reduced to Chl a by HMChl a reductase (HCAR). CAO plays a pivotal role in Chl a/b ratio regulation and plants over-accumulate Chl b in CAO-overexpressing plants. NYC1 is more accumulated in Chl-b-overproducing plants, while HCAR is not changed. To investigate the role of HCAR in Chl cycle regulation, the Chl metabolites of Chl-b-overproducing plants were analyzed. The results showed that HMChl a accumulated in these plants, and it decreased and the Chl a/b ratio increased by overexpressing HCAR, implying HCAR is insufficient for Chl cycle in Chl-b-overproducing plants. Furthermore, during dark-induced senescence, the non-programmed cell death symptoms (leaves dehydrated with green color retained) of Chl-b-overproducing plants were obviously alleviated, and the content of HM pheophorbide (HMPheide) a and Pheide b were sharply decreased by overexpressing HCAR. These results imply that HCAR is also insufficient for Chl degradation in Chl-b-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 b degradation in Chl-b-overproducing plants.
Highlights
Chlorophyll (Chl) is the key pigment responsible for harvesting solar energy and producing charge separation and electron transport during photosynthesis in green plants
We investigated the role of HMChl a reductase (HCAR) in the Chl cycle by analyzing plants that were overexpressing chlorophyllide a oxygenase (CAO), and the results implied that HCAR activity is insufficient for catalyzing HMChl a reduction in Chl-b-overproducing plants
Our results demonstrate that HCAR is a limiting factor for the Chl cycle and Chl b degradation in Chl-b-overproducing plants
Summary
Chlorophyll (Chl) is the key pigment responsible for harvesting solar energy and producing charge separation and electron transport during photosynthesis in green plants. Once it is synthesized during greening, Chl must be incorporated into its target apo-proteins to build functional photosynthetic proteins [1]. Chl should be degraded in coordination with its apo-proteins during senescence. This is because free Chl can produce reactive oxygen species, causing cell death [2]. The accumulation of pheophorbide a (Pheide a) induces non-programmed cell death (non-PCD), shown by leaves dehydrated but with green color retained in both light-dependent and light-independent manners [3,4,5]
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