Differential expression of two novel members of the tomato ethylene-receptor family

The phytohormone ethylene regulates many aspects of plant growth, developme nt, and environmental responses. Much of the developmental regulation of et hylene responses in tomato (Lycopersicon esculentum) occurs at the level of hormone sensitivity. In an effort to understand the regulation of ethylene responses, we isolated and characterized tomato genes with sequence simila rity to the Arabidopsis ETR1 (ethylene response 1) ethylene receptor. Previ ously, we isolated three genes that exhibit high similarity to ETR1 and to each other. Here we report the isolation of two additional genes, LeETR4 an d LeETR5, that are only 42% and 40% identical to ETR1, respectively. Althou gh the amino acids known to be involved in ethylene binding are conserved, LeETR5 lacks the histidine within the kinase domain that is predicted to be phosphorylated. This suggests that histidine kinase activity is not necess ary for an ethylene response, because mutated forms of both LeETR4 and LeET R5 confer dominant ethylene insensitivity in transgenic Arabidopsis plants. Expression analysis indicates that LeETR4 accounts for most of the putativ e ethylene-receptor mRNA present in reproductive tissues, but, like LeETR5, it is less abundant in vegetative tissues. Taken together, ethylene percep tion in tomato is potentially quite complex, with at least five structurall y divergent, putative receptor family members exhibiting significant variat ion in expression levels throughout development.