Accumulation of the NON‐YELLOW COLORING 1 protein of the chlorophyll cycle requires chlorophyll b in Arabidopsis thaliana

Chlorophyll a and chlorophyll b are interconverted in the chlorophyll cycle. The initial step in the conversion of chlorophyll b to chlorophyll a is catalyzed by the chlorophyll b reductases NON‐YELLOW COLORING 1 (NYC1) and NYC1‐like (NOL), which convert chlorophyll b to 7‐hydroxymethyl chlorophyll a. This step is also the first stage in the degradation of the light‐harvesting chlorophyll a/b protein complex (LHC). In this study, we examined the effect of chlorophyll b on the level of NYC1. NYC1 mRNA and NYC1 protein were in low abundance in green leaves, but their levels increased in response to dark‐induced senescence. When the level of chlorophyll b was enhanced by the introduction of a truncated chlorophyllide a oxygenase gene and the leaves were incubated in the dark, the amount of NYC1 was greatly increased compared with that of the wild type; however, the amount of NYC1 mRNA was the same as in the wild type. In contrast, NYC1 did not accumulate in the mutant without chlorophyll b, even though the NYC1 mRNA level was high after incubation in the dark. Quantification of the LHC protein showed no strong correlation between the levels of NYC1 and LHC proteins. However, the level of chlorophyll fluorescence of the dark adapted plant (F_o) was closely related to the accumulation of NYC1, suggesting that the NYC1 level is related to the energetically uncoupled LHC. These results and previous reports on the degradation of chlorophyllide a oxygenase suggest that the a feedforward and feedback network is included in chlorophyll cycle.