Cloning and functional characterization of LjPLT4, a plasma membrane xylitol H⁺- symporter from Lotus japonicus

Abstract

Polyols are compounds that play various physiological roles in plants. Here we present the identification of four cDNA clones of the model legume Lotus japonicus, encoding proteins of the monosaccharide transporter-like (MST) superfamily that share significant homology with previously characterized polyol transporters (PLTs). One of the transporters, named LjPLT4, was characterized functionally after expression in yeast. Transport assays revealed that LjPLT4 is a xylitol-specific H⁺-symporter (K _m, 0.34 mM). In contrast to the previously characterized homologues, LjPLT4 was unable to transport other polyols, including mannitol, sorbitol, myo-inositol and galactitol, or any of the monosaccharides tested. Interestingly, some monosaccharides, including fructose and xylose, inhibited xylitol uptake, although no significant uptake of these compounds was detected in the LjPLT4 transformed yeast cells, suggesting interactions with the xylitol binding site. Subcellular localization of LjPLT4-eYFP fusions expressed in Arabidopsis leaf epidermal cells indicated that LjPLT4 is localized in the plasma membrane. Real-time RT-PCR revealed that LjPLT4 is expressed in all major plant organs, with maximum transcript accumulation in leaves correlating with maximum xylitol levels there, as determined by GC-MS. Thus, LjPLT4 is the first plasma membrane xylitol-specific H⁺-symporter to be characterized in plants.

Keywords::

• Lotus japonicus
• legumes
• monosaccharide transport
• polyols
• xylitol

Acknowledgements

This work was co-funded by the European Social fund and Greek national resources – project PYTHAGORAS II (funding of research groups in Agricultural University of Athens) (EF), and by the bi-lateral French-Greek program, Platon (Molecular and biochemical characterization of monosaccharide transporters in root nodules) (RL and EF). The work was also supported, in part, by the Max Planck Society (MKU).

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.