Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/6/2005
Publication Date: 11/3/2005
Citation: Barry, C., Thompson, A., Seymour, G., Grierson, D., Giovannoni, J.J. 2005. Ethylene insensitivity conferred by the green-ripe (gr) and never-ripe 2 (nr-2) ripening mutants of tomato. Plant Physiology. 138:267-275. Interpretive Summary: The ripening of a fruit represents the culmination of a series of biochemical processes that have evolved as a mechanism of seed dispersal. In the case of fleshy fruits the changes that occur during ripening impart desirable characteristics to the fruit such as bright colors, softening and sugar and volatile accumulation that attract animals and birds to aid dispersal. Fruits also form an essential component of the human diet providing sources of sugars, fiber, vitamins, minerals and antioxidants. Although there is great diversity in fruit anatomy and phenotypes, the biochemical changes that occur during ripening are conserved in many plant species. For example, research carried out on diverse species indicates that at the onset of ripening there is often a coordinated increase in gene expression and enzyme activity of many proteins involved in cell wall metabolism, pigment synthesis and sugar metabolism. These data suggest that the genetic mechanisms that regulate fruit ripening are likely conserved. Little is known of the identities of these genetic pathways. In this study we have examined the physiological basis of ripening inhibition in two mutants of tomato, Green-ripe (Gr) and Never-ripe 2 (Nr-2), and conclude that their reduced ripening results from decreased ethylene sensitivity. The majority of ethylene response functions defined to date in Arabidopsis are global in effect. While relatively few have been demonstrated to be limited to certain tissues, none has yet been shown to be specific to fruit.
Technical Abstract: The ripening of a fleshy fruit represents the summation of an array of biochemical processes that are regulated by interactions between developmental programs and environmental inputs. Analysis of tomato mutants and inhibitor studies indicate that ethylene is necessary for full development of the ripening program of climacteric fruit such as tomato, yet ethylene alone is not sufficient. This suggests that an interaction between ethylene and non-ethylene (or developmental) pathways mediates ripening. In this study we have examined the physiological basis for ripening inhibition of the dominant Green-ripe (Gr) and Never-ripe 2 (Nr-2) mutants of tomato (Solanum lycopersicum). Our data suggests that this inhibition is due to ethylene insensitivity in mutant fruit. Further investigation of ethylene responses in Gr and Nr-2 plants also revealed reduced responsiveness to ethylene-induced abscission but normal triple response and petiole epinasty phenotypes suggesting that these loci regulate a subset of ethylene responses in tomato that are apparently limited to inflorescence tissues. We have mapped both dominant mutations to a 2 cM overlapping region of the long arm of chromosome 1 of tomato. Taken together with their phenotypic similarity, the map positions of these mutations suggest Gr and Nr-2 may be allelic. We propose that the Gr and Nr-2 loci may represent novel regulators of ethylene responses in tomato that mediates fruit ripening and floral abscission.