Isolation and characterization of the ACC synthase genes from lettuce (Lactuca sativa L.), and the involvement in low pH-induced root hair initiation.

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.