Characterization of two G-type half-size ABC transporter genes from the lipid-secreting medicinal plant Lithospermum erythrorhizon.

Terrestrial plants can secrete various lipophilic metabolites to facilitate environmental adaptation. Transporters belonging to the G subfamily of ATP-binding cassette (ABCG) proteins are known to be involved in the secretion of lipophilic metabolites from plant cells. The medicinal plant Lithospermum erythrorhizon secretes lipophilic specialized metabolites, shikonin derivatives, and echinofurans, as well as triacylglycerol, a major storage lipid. However, the underlying molecular mechanism responsible remains largely uncharacterized. In this study, we identified two genes encoding ABCG transporter proteins from L. erythrorhizon, LeABCG1 and LeABCG2, by transcriptome analysis. Phylogenetically, LeABCG1 is a class A ABCG containing the members that are involved in suberin and exine deposition and jasmonic acid signaling, whereas LeABCG2 is a class D ABCG involved in cuticle formation. The genomic structures of LeABCG1 and LeABCG2 are conserved among the orthologs identified in Arabidopsis and tomato. LeABCG1 is mainly expressed in above-ground tissues, and its expression in root bark is higher than that in peeled roots. In contrast, LeABCG2 did not show organ-specific gene expression patterns. Exogenous treatment of hairy roots with intracellular and extracellular metabolites of L. erythrorhizon slightly affected the expression of these genes. Both the LeABCG1 and LeABCG2 proteins localized to the plasma membrane in plant cells, while LeABCG2 was also distributed in punctate structures on the plasma membrane. Furthermore, LeABCG1 and LeABCG2 are the first two ABCG transporter proteins that are actually expressed in L. erythrorhizon, suggesting that they are involved in the secretion of lipophilic metabolites across the plasma membrane.