Identification of Polyamine Transporters in Plants: Paraquat Transport Provides Crucial Clues

Polyamine (PA) transport as well as PA biosynthesis, degradation and conjugation plays a vital role in the regulation of intracellular PA levels, which are essential for cell growth. Generally, PA uptake activity is elevated in rapidly proliferating cells. Previous studies showed that PA uptake in plant cells occurred via energy-dependent, protein-mediated transport systems. Numerous lines of evidence suggest that paraquat (PQ), one of the most widely used herbicides, is transported by the PA transport system in diverse organisms including plants. The PA/PQ transport interactions are proposed to be due to specific structural similarities between PA and PQ. The understanding of PA transport mechanisms has progressed in parallel with that of PQ transport, but the molecular identity of the plant PA/PQ transporter has remained an enigma. Recently, independent studies identified the L-type amino acid transporter (LAT) family transmembrane proteins as transporters of both PA and PQ. Arabidopsis LAT family proteins showed different subcellular localization properties, which suggested that these transporters were involved in intracellular PA trafficking and PA uptake across the plasma membrane. The identification of plant PA transporters is an important step in understanding the mechanism of PA homeostasis in plant cells. In this review, we highlight recent advances in the study of PA transport systems that are linked to the understanding of PQ translocation.