Differential expression of ACC oxidase genes during low-pH-induced root hair formation in lettuce (Lactuca sativa L.) seedlings

Root hair formation is induced in lettuce seedlings when the seedlings are transferred from a liquid medium at pH 6.0 to one at pH 4.0. Auxin, ethylene, and light are also required for the induction of root hair formation. To investigate the mechanism by which ethylene production is regulated during root hair formation, we isolated three 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase genes (Ls-ACO1, 2, and 3) from lettuce, each of which exists as a single copy in the genome. Analysis of the deduced amino acid sequences of the three ACO proteins as well as a phylogenetic analysis revealed that Ls-ACO3 was the most divergent among the ACO family. Northern hybridization analyses revealed that the mRNA levels of Ls-ACO2, but not Ls-ACO1 and Ls-ACO3, increased in the primary root after the transfer to a pH 4.0 medium. Addition of ACC or indole-3-acetic acid (IAA) to the pH 6.0 medium induced root hair formation, and a concomitant accumulation of Ls-ACO2 mRNA was observed. In contrast, the mRNA levels of Ls-ACO1 and Ls-ACO3 were unaffected by either ACC or IAA treatment. Furthermore, white light irradiation of dark-grown seedlings following the transfer to pH 4.0 medium induced the accumulation of all three ACO mRNAs. However, accumulation of Ls-ACO2 mRNA was also observed in non-irradiated seedlings, suggesting that the expression of Ls-ACO2 was induced not by light but by low pH. These results suggest that among the differentially regulated ACO genes, Ls-ACO2 plays a key role in ethylene production during low-pH-induced root hair formation in lettuce.