Identification of Lysine Histidine Transporter 2 as an 1-Aminocyclopropane Carboxylic Acid Transporter in by Transgenic Complementation Approach.

1-Aminocyclopropane-1-carboxylic acid (ACC), a biosynthetic precursor of ethylene, has long been proposed to act as a mobile messenger in higher plants. However, little is known about the transport system of ACC. Recently, our genetic characterization of an ACC-resistant mutant with normal ethylene sensitivity revealed that lysine histidine transporter 1 (LHT1) functions as a transporter of ACC. As amino acid transporters might have broad substrate specificity, we hypothesized that other amino acid transporters including paralogs might have the ACC-transporter activity. Here, we took a gain-of-function approach by transgenic complementation of mutant with a selected set of amino acid transporters. When we introduced transgene into the mutant, the transgenic expression of , but not of or , restored the ACC resistance phenotype of the mutant. The result provides genetic evidence that some, if not all, amino acid transporters in Arabidopsis can function as ACC transporters. In support, when expressed in oocytes, both and exhibited ACC-transporting activity, inducing inward current upon addition of ACC. Interestingly, the transgenic expression of , but not of or , could also suppress the early senescence phenotypes of the mutant. Taking together, we propose that plants have evolved a multitude of ACC transporters based on amino acid transporters, which would contribute to the differential distribution of ACC under various spatiotemporal contexts.