Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/28/2006
Publication Date: 5/16/2006
Citation: Barry, C., Giovannoni, J.J. 2006. Ripening in the tomato green-ripe mutant is inhibited by ectopic expression of a novel protein that disrupts ethylene signaling. Proceedings of the National Academy of Sciences. 103:7923-7928. Interpretive Summary: Altered ethylene responsiveness in plant tissues influences development and can compromise the plants ability to respond to environmental stimuli. The mechanisms by which the ethylene signal is perceived and transduced to mediate phenotypic changes is not fully understood although many elegant studies in Arabidopsis have led to the identification of critical components of this signaling pathway. Control of ethylene responsiveness in crop plants is of commercial importance to reduce senescence, over-ripening and post-harvest deterioration of fruit, vegetable and floral crops. Previous research has led to the generation of transgenic horticultural crops with altered ethylene responsiveness to counteract the negative impacts of ethylene on ripening and floral senescence. These studies have successfully achieved their aims but subsequent evaluation of horticultural performance has revealed that constitutive ethylene-insensitivity has deleterious effects on seed germination, seedling vigor and adventitious rooting in tomato and petunia. Here we report cloning of the tomato Green-ripe (GR) gene that inhibits ethylene response with large effects on fruit but minimal influences on other tissues. This differential mediation of ethylene responsiveness by GR may be useful for reducing the impact of the less desirable consequences of ethylene on tissues such as ripe fruit whilst maintaining normal plant vigor.
Technical Abstract: To achieve full development of the ripe phenotype, climacteric fruits, such as tomato require synthesis, perception and signal transduction of the plant hormone ethylene. The non-ripening phenotype of the dominant Green-ripe (Gr) and Never-ripe 2 (Nr-2) mutants of tomato is the result of reduced ethylene responsiveness in fruit tissues. In addition a subset of ethylene responses associated with floral senescence, abscission and root elongation are also impacted in mutant plants but to a lesser extent. Using positional cloning we have identified an identical 334 bp deletion in a gene of unknown biochemical function residing at the Gr/Nr-2 locus. Consistent with a dominant gain of function mutation, this deletion causes ectopic expression of GR/NR-2, which in turn leads to ripening inhibition. A CaMV35::GR transgene recreates the Gr/Nr-2 mutant phenotype but does not lead to a global reduction in ethylene responsiveness suggesting tissue-specific modulation of ethylene responses in tomato. GR/NR-2 encodes a novel evolutionary conserved protein of unknown biochemical function that has not previously been associated with ethylene signaling. Because GR/NR-2 has no sequence homology with the previously described Nr (Never-ripe) ethylene receptor of tomato we now refer to this gene only as GR. Identification of GR expands the current repertoire of ethylene signaling components in plants and provides a tool for further elucidation of ethylene response mechanisms and for controlling ethylene signal specificity in crop plants.