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

Summary 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 ( NYC 1) and NYC 1‐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 NYC 1. NYC 1 mRNA and NYC 1 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 NYC 1 was greatly increased compared with that of the wild type; however, the amount of NYC 1 mRNA was the same as in the wild type. In contrast, NYC 1 did not accumulate in the mutant without chlorophyll b , even though the NYC 1 mRNA level was high after incubation in the dark. Quantification of the LHC protein showed no strong correlation between the levels of NYC 1 and LHC proteins. However, the level of chlorophyll fluorescence of the dark adapted plant ( F o ) was closely related to the accumulation of NYC 1, suggesting that the NYC 1 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.