Increased expression of ethylene receptor genes during low pH-induced root hair formation in lettuce (Lactuca sativa L.) seedlings: direct and indirect induction by ethylene and auxin, respectively

Abstract

The plant hormones ethylene and auxin mediate the formation of root hairs on lettuce seedlings that are transferred from pH 6.0 to pH 4.0 medium. To investigate the regulatory mechanism of ethylene, we isolated ethylene receptor genes from lettuce. Three putative transmembrane domains were found in Ls-ERS1 and Ls-ETR1 and four in Ls-ETR2 and Ls-ETR3. Five bacterial histidine kinase motifs were highly conserved in Ls-ERS1 and Ls-ETR1, but not Ls-ETR2 or Ls-ETR3. Phylogenetic analysis supported these similarities among family members. Genomic Southern hybridization revealed that each gene existed as a single copy in the genome. mRNAs of these genes were detected in seedling roots after pre-culture at pH 6.0. After transfer to pH 4.0 medium, Ls-ERS1 and Ls-ETR2 expression and ethylene produc- tion increased and were maintained at higher levels than those found at pH 6.0. The addition of 1-aminocyclopropane-1-carboxylic acid (ACC) to the pH 6.0 medium noticeably induced both ethylene production and Ls-ERS1 and Ls-ETR2 expression. A marked increase in the mRNA level of Ls-ERS1, with a slight increase in Ls-ETR2 mRNA level, was noted with the addition of indole-3-acetic acid (IAA); however, ethylene production was also induced. Simulta- neous treatment with an ethylene biosynthesis inhibitor and IAA markedly inhibited ethylene production and ethylene receptor gene expres- sion. These results suggest that ethylene receptor gene expression is differentially regu- lated among the family members during low pH-induced root hair formation in lettuce seedl- ings, and that the increased expression of Ls-ERS1 and Ls-ETR2 during this process is induced by ethylene rather than by auxin.