Chlorophyll is a deleterious molecule that generates
reactive oxygen species and must be converted to nontoxic molecules during plant senescence. The degradation pathway of
chlorophyll a has been determined; however, that of
chlorophyll b is poorly understood, and multiple pathways of
chlorophyll b degradation have been proposed. In this study, we found that
chlorophyll b is degraded by a single pathway, and elucidated the importance of this pathway in avoiding cell death. In order to determine the
chlorophyll degradation pathway, we first examined the substrate specificity of
7-hydroxymethyl chlorophyll a
reductase.
7-hydroxymethyl chlorophyll a
reductase reduces
7-hydroxymethyl chlorophyll a but not 7-hydroxymethyl
pheophytin a or 7-hydroxymethyl
pheophorbide a. These results indicate that the first step of
chlorophyll b degradation is its conversion to
7-hydroxymethyl chlorophyll a by
chlorophyll b reductase, although
chlorophyll b reductase has broad substrate specificity. In vitro experiments showed that
chlorophyll b reductase converted all of the
chlorophyll b in the light-harvesting
chlorophyll a/b
protein complex to
7-hydroxymethyl chlorophyll a, but did not completely convert
chlorophyll b in the core antenna complexes. When plants whose core antennae contained
chlorophyll b were incubated in the dark,
chlorophyll b was not properly degraded, and the accumulation of 7-hydroxymethyl
pheophorbide a and
pheophorbide b resulted in cell death. This result indicates that
chlorophyll b is not properly degraded when it exists in core antenna complexes. Based on these results, we discuss the importance of the proper degradation of
chlorophyll b.