Mendel's green cotyledon gene encodes a positive regulator of the chlorophyll-degrading pathway

*Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657, Japan;
^†Institute of Radiation Breeding, National Institute of Agrobiological Sciences, Hitachi-ohmiya 219-2293, Japan; and
^‡National Institute of Floricultural Sciences, National Agriculture and Food Research Organization, Tsukuba 305-8519, Japan

Communicated by June B. Nasrallah, Cornell University, Ithaca, NY, July 18, 2007 (received for review May 15, 2007)

Abstract

Mutants that retain greenness of leaves during senescence are known as “stay-green” mutants. The most famous stay-green mutant is Mendel's green cotyledon pea, one of the mutants used in determining the law of genetics. Pea plants homozygous for this recessive mutation (known as i at present) retain greenness of the cotyledon during seed maturation and of leaves during senescence. We found tight linkage between the I locus and stay-green gene originally found in rice, SGR. Molecular analysis of three i alleles including one with no SGR expression confirmed that the I gene encodes SGR in pea. Functional analysis of sgr mutants in pea and rice further revealed that leaf functionality is lowered despite a high chlorophyll a (Chl a) and chlorophyll b (Chl b) content in the late stage of senescence, suggesting that SGR is primarily involved in Chl degradation. Consistent with this observation, a wide range of Chl–protein complexes, but not the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit, were shown to be more stable in sgr than wild-type plants. The expression of OsCHL and NYC1, which encode the first enzymes in the degrading pathways of Chl a and Chl b, respectively, was not affected by sgr in rice. The results suggest that SGR might be involved in activation of the Chl-degrading pathway during leaf senescence through translational or posttranslational regulation of Chl-degrading enzymes.

Footnotes

^§To whom correspondence should be addressed at:
Laboratory of Plant Breeding and Genetics, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657, Japan.
E-mail: akusaba{at}mail.ecc.u-tokyo.ac.jp
Author contributions: Y.S. and M.K. designed research; Y.S., R.M., M.N., H.Y., and M.K. performed research; Y.S. and M.K. analyzed data; and Y.S. and M.K. wrote the paper.
The authors declare no conflict of interest.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database [accession nos. AB303331 (PsSGR; JI4), AB303332 (PsSGR; JI2775), AAW82959 (GmSGR1), AAW82960 (GmSGR2), AAY98500 (LeSGR), AY850161 (AtSGR1), AY699948 (AtSGR2), AAW82958 (SbSGR), AAW82956 (ZmSGR1), AAW82957 (ZmSGR2), AAW82954 (OsSGR), AAW82955 (HvSGR), BAF49740 (NYC1), BAF49741 (NOL), AP008216 (OsCHL), and AK120554 (OsPaO)].
This article contains supporting information online at www.pnas.org/cgi/content/full/0705521104/DC1.
Abbreviations:

CDS,

coding sequence;

Chide,

chlorophyllide;

Chl,

chlorophyll;

Chlase,

chlorophyllase;

DDI,

days of dark incubation;

Fv/Fm,

ratio of variable to maximum chlorophyll florescence;

LHCI,

light-harvesting complex I;

LHCII,

light-harvesting complex II;

PaO,

Pheide a oxygenase;

Pehide,

pheophorbide;

PSI,

photosystem I;

PSII,

photosystem II;

RCC,

red chlorophyll catabolite;

RFP,

red fluorescent protein;

Rubisco,

ribulose-1,5-bisphosphate carboxylase/oxygenase.