DIFFERENTIAL REGULATION OF THE TOMATO ETR GENE FAMILY THROUGHOUT PLANT DEVELOPMENT

Ethylene perception in plants is co-ordinated by multiple hormone rece ptor candidates sharing sequence commonalties with prokaryotic environ mental sensor proteins known as two-component regulators. Two tomato h omologs of the Arabidopsis ethylene receptor ETR1 were cloned from a r oot cDNA library. Both cDNAs, termed LeETR1 and LeETR2, were highly ho mologous to ETR1, exhibiting similar to 90% deduced amino acid sequenc e similarity and 80% deduced amino acid sequence identity. LeETR1 and LeETR2 contained all the major structural elements of two-component re gulators, including the response regulator motif absent in LeETR3, the gene encoding tomato NEVER RIPE (NR). Using RNase protection analysis , the mRNAs of LeETR1, LeETR2 and NR were quantified in tissues engage d in key processes of the plant life cycle, including seed germination , shoot elongation, leaf and flower senescence, floral abscission, fru it set and fruit ripening. LeETR1 was expressed constitutively in all plant tissues examined. LeETR2 mRNA was expressed at low levels throug hout the plant but was induced in imbibing tomato seeds prior to germi nation and was down-regulated in elongating seedlings and senescing le af petioles. IVR expression was developmentally regulated in floral ov aries and ripening fruit. Notably, hormonal regulation of NR was highl y tissue-specific. Ethylene biosynthesis induced NR mRNA accumulation in ripening fruit but not in elongating seedlings or in senescing leav es or flowers. Furthermore, the abundance of mRNAs for all three LeETR genes remained uniform in multiple plant tissues experiencing marked changes in ethylene sensitivity, including the cell separation layer t hroughout tomato flower abscission.