A d-pinitol transporter, LjPLT11, regulates plant growth and nodule development in Lotus japonicus

Functional characterization of polyol/monosaccharide transporter 1 in Lotus japonicus

Polyol/Monosaccharide Transporters (PLTs/PMTs) localized in the plasma membrane have previously been identified in plants. The physiological role and the functional properties of these proteins in legume plants are, however, unclear. Here we describe the functional analysis of LjPLT1, a plasma membrane-localized PLT protein from Lotus japonicus. The LjPLT1 gene was strongly expressed in the vascular tissue of roots, stems and leaves. Expression of the LjPLT1 cDNAs in yeast revealed that the protein functions as a broad-spectrum H+ -symporter for both linear polyols of sorbitol and mannitol, and cyclic polyol myo-inositol. It also catalyzes the transport of different hexoses, including fructose, glucose, galactose and mannose. Overexpression of LjPLT1 (OELjPLT1) results in inhibition of plant growth and a decrease in nodule nitrogenase activity in L. japonicus. The soluble sugars were increased in newly expanded leaves, roots and nodules but decreased in mature leaves in OELjPLT1 plants. In addition, the OELjPLT1 seedlings displayed an increased sensitivity to high content mannitol and boron toxicity, but neither drought nor salinity stresses. Taken together, the present study indicates that the LjPLT1 protein may participate in the translocation of hexoses/polyols to regulate multiple physiological and growth processes in L. japonicus.

Overexpression of LjPLT3 Enhances Salt Tolerance in Lotus japonicus

Intracellular polyols are used as osmoprotectants by many plants under environmental stress. However, few studies have shown the role of polyol transporters in the tolerance of plants to abiotic stresses. Here, we describe the expression characteristics and potential functions of Lotus japonicus polyol transporter LjPLT3 under salt stress. Using LjPLT3 promoter-reporter gene plants showed that LjPLT3 was expressed in the vascular tissue of L. japonicus leaf, stem, root, and nodule. The expression was also induced by NaCl treatment. Overexpression of LjPLT3 in L. japonicus modified the growth rate and saline tolerance of the transgenic plants. The OELjPLT3 seedlings displayed reduced plant height under both nitrogen-sufficient and symbiotic nitrogen fixation conditions when 4 weeks old. The nodule number of OELjPLT3 plants was reduced by 6.7–27.4% when 4 weeks old. After exposure to a NaCl treatment in Petri dishes for 10 days, OELjPLT3 seedlings had a higher chlorophyll concentration, fresh weight, and survival rate than those in the wild type. For symbiotic nitrogen fixation conditions, the decrease in nitrogenase activity of OELjPLT3 plants was slower than that of the wild type after salt treatment. Compared to the wild type, both the accumulation of small organic molecules and the activity of antioxidant enzymes were higher under salt stress. Considering the concentration of lower reactive oxygen species (ROS) in transgenic lines, we speculate that overexpression of LjPLT3 in L. japonicus might improve the ROS scavenging system to alleviate the oxidative damage caused by salt stress, thereby increasing plant salinity tolerance. Our results will direct the breeding of forage legumes in saline land and also provide an opportunity for the improvement of poor and saline soils.

Maintaining osmotic balance in legume nodules

This article comments on: Tian L, Liu L, Xu S, Deng R, Wu P, Jiang H, Wu G, Chen Y. 2022. A d-pinitol transporter, LjPLT11, regulates plant growth and nodule development in Lotus japonicus. Journal of Experimental Botany 73, 351–365.