The Arabidopsis paraquat resistant1 mutant accumulates leucine upon dark treatment

The Arabidopsis paraquat resistant1 (PAR1) was classified as L-type amino acid transporter 4 (LAT4) based on a phylogenetic analysis of selected genes from Saccharomyces cerevisiae, Arabidopsis thaliana (L.) Heynh, and Homo sapiens that clustered LAT4 with four other members as a LAT family in Arabidopsis. In silico analysis of the Arabidopsis LATs identified an amino acid permease domain and motifs that are common in amino acid transporters. However, their role in amino acid transport remained to be studied. A knockout mutant for PAR1/LAT4 gene, reported here as par1-5, showed significantly altered growth compared with wild type on leucine-containing growth medium. Mutant par1-5 seedlings showed reduced biomass compared with wild type on nitrate-containing Murashige and Skoog growth medium, which was further reduced when grown on medium containing nitrate and leucine. Radio-labelled leucine uptake studies using leaf protoplasts and seedlings showed increased accumulation of leucine in par1-5 mutants compared with wild type. Increased accumulation of leucine in par1-5 was detected when seedlings or protoplasts were treated in the dark prior to isotopic feeding. These studies suggest that the PAR1/LAT4 protein, in addition to its ability to mediate paraquat and polyamine transport, possess leucine transport activity that is regulated by physiological conditions such as dark induction.