The light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) 
is the most abundant membrane protein in green plants, and its degradation is a 
crucial process for the acclimation to high light conditions and for the 
recovery of nitrogen (N) and carbon (C) during senescence. However, the 
molecular mechanism of LHCII degradation is largely unknown. Here, we report 
that chlorophyll b reductase, which catalyzes the first step of chlorophyll b 
degradation, plays a central role in LHCII degradation. When the genes for 
chlorophyll b reductases NOL and NYC1 were disrupted in Arabidopsis thaliana, 
chlorophyll b and LHCII were not degraded during senescence, whereas other 
pigment complexes completely disappeared. When purified trimeric LHCII was 
incubated with recombinant chlorophyll b reductase (NOL), expressed in 
Escherichia coli, the chlorophyll b in LHCII was converted to 7-hydroxymethyl 
chlorophyll a. Accompanying this conversion, chlorophylls were released from 
LHCII apoproteins until all the chlorophyll molecules in LHCII dissociated from 
the complexes. Chlorophyll-depleted LHCII apoproteins did not dissociate into 
monomeric forms but remained in the trimeric form. Based on these results, we 
propose the novel hypothesis that chlorophyll b reductase catalyzes the initial 
step of LHCII degradation, and that trimeric LHCII is a substrate of LHCII 
degradation.