Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2008
Publication Date: 8/7/2008
Citation: Barry, C., Mcquinn, R.P., Chung, M., Besuden, A., Giovannoni, J.J. 2008. Amino acid substitutions in homologs of the STAY-GREEN (SGR) protein are responsible for the green-flesh and chlorophyll retainer mutations of tomato and pepper. Plant Physiology. 147:179-187. Interpretive Summary: Color change is one of the most dramatic events occurring in fleshy fruits as they begin to ripen, serving as a signal to seed dispersing fauna that the fruit is ripe, palatable and nutritious. Fleshy fruits predominantly accumulate carotenoids, anthocyanins and flavanoids and the synthesis of these compounds at the onset of ripening is preceded by, or occurs concomitantly with, the degradation of chlorophyll. While mechanisms of chlorophyll degradation have been elucidated, the regulation and role of chlorophyll degradation to the ripening process and overall quality of fruits remains poorly understood. “Stay green” mutants have been identified in several plant species and classified on the basis of their chlorophyll retention and senescence phenotypes. In this report we describe the molecular identification of a tomato mutation termed green-flesh (gf) via the utilization of a genetic map-based cloning approach. In addition, we have demonstrated that a mutation in a pepper version of the same gene represents the basis for the stay green phenotype of the pepper chlorophyll retainer (cl) mutant which is the basis of brown-fruited pepper varieties.
Technical 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 and pepper, 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. 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, that we have termed SGR-like, has an as yet undefined function.