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Abstract

To achieve full development of the ripe phenotype, climacteric fruits, such as tomato, apple and banana, 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 by 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 membrane localized 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.