;Root hair formation is induced when lettuce seedlings are transferred from pH 6.0 to pH 4.0. Ethylene, auxin and light are essential to this process. To investigate the role of ethylene in root hair initiation, we isolated two 1-aminocyclopropane-1-carboxylic acid (ACC) synthase genes (Ls-ACS1 and Ls-ACS2). Seven motifs of known ACS proteins were highly conserved in Ls-ACS1 and Ls-ACS2. The Ls-ACS1 and Ls-ACS2 mRNA levels were constant at pH 6.0, which were lower than that in seedlings at pH 4.0. Ls-ACS1 and Ls-ACS2 transcripts accumulated at pH 4.0 and reached peak levels at 1 h and 30 min after acidification, respectively. Indole-3-acetic acid (IAA) induced the accumulation of both Ls-ACS1 and Ls-ACS2 transcripts, whereas ACC induced only Ls-ACS1 mRNA. These results suggest that acidification-induced auxin accumulations increase the Ls-ACS2 levels, which together with Ls-ACS2-induced ethylene raise the levels of Ls-ACS1. Furthermore, blue and white light gave the highest levels of both Ls-ACS1 mRNA and ethylene production. Darkness was less effective, and red light had an intermediate effect. The different light conditions had no effect on the levels of Ls-ACS2 mRNA. These observations support the involvement of Ls-ACS1 in the production of ethylene, which is crucial for root hair initiation.
_____________________________________________________________________________________________________________________________________________________________________________Takahashi H, Nakamura A, Harigaya W, Fujigasaki R, Iwasa T, Inoue Y 2010 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. Plant Root 4:53-64. 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 production 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. Simultaneous 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 regulated among the family members during low pH-induced root hair formation in lettuce seedlings, and that the increased expression of Ls-ERS1 and Ls-ETR2 during this process is induced by ethylene rather than by auxin.
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