Abstract
Color changes often accompany the onset of ripening, leading to brightly colored fruits that serve as attractants to seed-dispersing organisms. In many fruits, including tomato (Solanum lycopersicum) and pepper (Capsicum annuum), there is a sharp decrease in chlorophyll content and a concomitant increase in the synthesis of carotenoids as a result of the conversion of chloroplasts into chromoplasts. The green-flesh (gf) and chlorophyll retainer (cl) mutations of tomato and pepper, respectively, are inhibited in their ability to degrade chlorophyll during ripening, leading to the production of ripe fruits characterized by both chlorophyll and carotenoid accumulation and are thus brown in color. Using a positional cloning approach, we have identified a point mutation at the gf locus that causes an amino acid substitution in an invariant residue of a tomato homolog of the STAY-GREEN (SGR) protein of rice (Oryza sativa). Similarly, the cl mutation also carries an amino acid substitution at an invariant residue in a pepper homolog of SGR. Both GF and CL expression are highly induced at the onset of fruit ripening, coincident with the ripening-associated decline in chlorophyll. Phylogenetic analysis indicates that there are two distinct groups of SGR proteins in plants. The SGR subfamily is required for chlorophyll degradation and operates through an unknown mechanism. A second subfamily, which we have termed SGR-like, has an as-yet undefined function.
Highlights
Color changes often accompany the onset of ripening, leading to brightly colored fruits that serve as attractants to seeddispersing organisms
Pheophorbide a oxygenase (PAO) converts pheophorbide a into red chlorophyll catabolite, which is in turn converted into fluorescent chlorophyll catabolite by red chlorophyll catabolite reductase (RCCR), a novel enzyme that is distantly related to bilin reductases (Wuthrich et al, 2000)
With the aim of refining the map position of the gf locus, we generated two separate F2 populations segregating for tomato and Solanum pennellii alleles on chromosome 8 through crosses between a homozygous gf/gf mutant (LA3534) and two chromosome 8 S. pennellii introgression lines, IL8-2 (GF/GF; LA4074) and IL8-3 (GF/GF; LA4076) that span the long arm of chromosome 8 (Eshed and Zamir, 1994)
Summary
Color changes often accompany the onset of ripening, leading to brightly colored fruits that serve as attractants to seeddispersing organisms. One group of class C mutants identified in Festuca pratensis, pea (Pisum sativum), rice, and Arabidopsis has reduced PAO activity and stable pigment-protein complexes within the chloroplast (Thomas and Howarth, 2000; Park et al, 2007; Ren et al, 2007; Sato et al, 2007) These loci have been shown to encode a family of novel chloroplasttargeted proteins that may promote chlorophyll degradation via destabilization of protein-pigment complexes in the thylakoid membranes through an as-yet undefined mechanism (Armstead et al, 2006, 2007; Jiang et al, 2007; Park et al, 2007; Ren et al, 2007; Sato et al, 2007). These phenomena appear not to be caused by a general inhibition of senescence, but rather as a result of inhibition of chlorophyll degradation because senescence-associated marker genes appear to display normal expression patterns in gf (Akhtar et al, 1999)
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