Antisense inhibition of the Nr gene restores normal ripening to the tomatoNever-ripe mutant, consistent with the ethylene receptor-inhibition model

The hormone ethylene regulates many aspects of plant growth and development , including fruit ripening. In transgenic tomato (Lycopersicon esculentum) plants, antisense inhibition of ethylene biosynthetic genes results in inhi bited or delayed ripening. The dominant tomato mutant, Never-ripe (Nr), is insensitive to ethylene and fruit fail to ripen. The Nr phenotype results f rom mutation of the ethylene receptor encoded by the NX gene, such that it can no longer bind the hormone. NR has homology to the Arabidopsis ethylene receptors. Studies on ethylene perception in Arabidopsis have demonstrated that receptors operate by a "receptor inhibition" mode of action, in which they actively repress ethylene responses in the absence of the hormone, an d are inactive when bound to ethylene. In ripening tomato fruit, expression of NX is highly regulated, increasing in expression at the onset of ripeni ng, coincident with increased ethylene production. This expression suggests a requirement for the NX gene product during the ripening process, and imp lies that ethylene signaling via the tomato NR receptor might not operate b y receptor inhibition. We used antisense inhibition to investigate the role of NR in ripening tomato fruit and determine its mode of action. We demons trate restoration of normal ripening in Nr fruit by inhibition of the mutan t Nr gene, indicating that this receptor is not required for normal ripenin g, and confirming receptor inhibition as the mode of action of the NR prote in.