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Ripening in the tomato Green-ripe mutant is inhibited by ectopic expression of a protein that disrupts ethylene signaling.
To achieve full ripening, climacteric fruits, such as tomato require synthesis, perception and signal transduction of the plant hormone ethylene. The nonripening 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 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 an evolutionary conserved protein of unknown biochemical function that we associate here 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.
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