Ethylene Insensitivity Conferred by theGreen-ripeandNever-ripe 2Ripening Mutants of Tomato

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 (Solanum lycopersicum) 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 nonethylene (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. Our data suggest that this inhibition is due to ethylene insensitivity in mutant fruit. Further investigation of ethylene responses in Gr and Nr-2 plants also revealed weak ethylene insensitivity during floral senescence and abscission and, during inhibition of root elongation, a phenotype associated with the triple response. However, ethylene-induced inhibition of hypocotyl elongation and petiole epinasty are normal in Gr and Nr-2, suggesting that these loci regulate a subset of ethylene responses. We have mapped both dominant mutations to a 2-cM overlapping region of the long arm of chromosome 1 of tomato, a region not previously linked to any known ethylene signaling loci. The phenotypic similarity and overlapping map location of these mutations suggest Gr and Nr-2 may be allelic and may possibly encode a novel component of the ethylene response pathway.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. Therefore, the study of fruit ripening is of importance to both basic plant biology and agriculture.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 (Seymour et al., 1993). These data suggest that the genetic mechanisms that regulate fruit ripening are likely conserved. Little is known of the identities of these genetic pathways, although experiments, largely performed using tomato (Solanum lycopersicum), have led to the hypothesis that ripening in climacteric fruits such as tomato, banana, apple, and stone fruits is regulated by ethylene dependent and independent pathways (Adams-Phillips et al., 2004a;Giovannoni, 2004).The involvement...

Differential Control of Ethylene Responses byGREEN-RIPEandGREEN-RIPE LIKE1Provides Evidence for Distinct Ethylene Signaling Modules in Tomato

“…The Gr mutant and CaMV35S::SlGR overexpression lines display a reduced response to ethylene-induced floral abscission that lies between that observed in control and Nr plants (Barry et al, 2005;Barry and Giovannoni, 2006). Similarly, the CaMV35S::SlGRL1 lines have reduced rates of ethylene-induced floral abscission, comparable to those of the Gr mutant (Fig.…”

“…Characterization of the Gr mutant and overexpression of SlGR in tomato indicated that this gene influences a subset of ethylene responses in tomato (Barry et al, 2005;Barry and Giovannoni, 2006). However, overexpression of SlGRL1 in tomato causes inhibition of a subset of ethylene responses that overlap but are distinct from those observed in the Gr mutant and CaMV35S::SlGR lines (Fig.…”

“…The parental tomato (Solanum lycopersicum) line Ailsa Craig, the ethyleneinsensitive mutants Nr and Gr, and CaMV35S::SlGR transgenic lines were described previously (Lanahan et al, 1994;Barry et al, 2005;Barry and Giovannoni, 2006). Plants were grown in peat-based compost supplemented with fertilizer in greenhouses equipped with heating and cooling systems and supplemental lighting either at Cornell University or Michigan State University.…”

“…Experiments to evaluate the triple-response phenotype in dark-grown tomato seedlings and floral abscission were performed as described previously (Barry et al, 2005) with the exception that seedlings were measured at 8 d after sowing and flowers were induced to abscise by treatment with ethylene at a concentration of 2 mL L 21 . Tomato plants for investigating ethylene responses during petiole epinasty were grown in Jiffy-7 Peat Pellets (http://www.hummert.com/) for 4 weeks under 16-h-light/8-h-dark cycles at 28°C and 65% relative humidity.…”

“…fleshy fruit development), and several studies have revealed novel phenotypes associated with the manipulation of ethylene biosynthesis or signaling components that would not be amenable to study in Arabidopsis (Oeller et al, 1991;Xu et al, 2006;Boualem et al, 2008;Penmetsa et al, 2008;Hattori et al, 2009). The Green-ripe (Gr) mutant of tomato (Solanum lycopersicum) displays reduced ethylene responsiveness in a subset of tissues, resulting in plants with impaired fruit ripening, petal senescence, floral abscission, and an altered seedling triple response (Barry et al, 2005). Sequence analysis of the Gr locus identified a 334-bp deletion in the 59-flanking region of a tomato homolog of AtRTE1 that results in the ectopic expression of SlGR in the Gr mutant but does not alter the sequence of the proteincoding region (Barry and Giovannoni, 2006).…”