Boron transport has long been believed as a passive process. In the last several years, with the use of molecular genetics, transporters responsible for boron transport have been identified. Transporters responsible for boron uptake, xylem loading, distribution among leaves, efflux from roots are now described and through the characterization of these transporters, it is now revealed that plants regulate these transporters in response to the boron conditions in the environment for boron homeostasis. Further more, with the use of the knowledge of the transport systems, plants tolerant to low- or high-boron stress are generated. These technologies may be useful to design plants/crops tolerant to soils with low or high boron.

Understandings of molecular mechanisms of boron action in plants remain limited to those known in cell walls. Toxicity mechanisms are largely unknown. In order to obtain the insights into mechanisms of boron toxicity, screening of Arabidopsis thaliana mutants sensitive to excess boron was conducted. The screening strategy was to select plants that grow reasonably well under normal boron condition (0.03 mM boric acid), but not under high boron condition (3 mM boric acid). We used root growth as a selection parameter. About 20,000 ethyl methanesulfonate (EMS) mutagenized M2 seeds of Arabidopsis thaliana (ecotype Col-0) were grown on plates containing high levels of boric acid (3 mM). After 2 weeks, plants showed short-root were transferred to media containing normal levels of boric acid (0.03 mM) and those plants that recovered growth were selected. The phenotypes were reevaluated at M3 generation and reselected plants were crossed with Col-0 for genetic analysis. Finally, we isolated 7 recessive mutants. Most of the mutants showed extremely shorter roots than wild type under boron toxicity, but not under the normal condition, indicating its sensitivity to excess boron.

Both species of arsenic (As), arsenate and arsenite are highly toxic to plants. Arsenic contamination is a major problem in Southeast Asia particularly in Bangladesh and West Bengal. In these countries, As-contaminated groundwater is widely used for irrigating rice in dry season that results in elevated As accumulation in soils and in rice grain and straw. So it is important for understanding the accumulation and transport mechanisms of arsenic in rice. We monitored increased arsenic content in rice grain and straw when rice plants were irrigated with heavily As-contaminated shallow tube well water. However, the arsenic content was significantly decreased both in rice grain and straw when the plants were supplied with low As-contaminated water from pond. These results indicate that As accumulation in rice is dependent on its external supply. Now it is urgent to investigate how the As is transported to rice plants. Arsenite transporter in plants has recently been reported. It is also reported in Arabidopsis that arsenate is taken by plants through phosphate transporters. However, it is not well documented in rice plants. There are 13 putative rice phosphate transporters (OsPTs). Based on phylogenetic analysis the OsPTs can be classified into three groups. Some of the members of OsPT family might be involved in arsenate transport in rice. Currently, we are investigating the possible involvement of OsPTs in arsenate transport in rice.

Boron is one of the essential elements for plant growth and development and its deficiency is known to reduce quantity and quality of agricultural products. In this study, we evaluated the effect of boron limitation on the growth and yields of rice plants using hydroponic culture system. Boron limitation did not affect the plant height of young seedlings, but prolonged limitation of boron clearly reduced the plant height under 0.18 µM boron treatments. Moreover, yields of rice grains were severely impaired under 0.18 µM boron treatments, because of inhibited panicle formation and following reduced spikelet numbers. As a next step, we generated transgenic rice plants expressing rice BOR1-GFP to engineer the crop tolerant to boron limited condition. OsBOR1, a boron efflux transporter in rice mediates efficient boron translocation from root to shoot. Thus generated overexpressors were expected to show enhanced root-to-shoot boron transport. We established 11 independent lines and confirmed the expression of introduced gene in several lines. The initial growth of transgenic plants under boron limitation did not differ among the lines and NT. However, boron concentration in xylem sap was higher in some transgenic lines than NT and transgenic lines not expressing introduced gene. These results suggest that the major boron deficient symptom in rice is impaired development of reproductive tissues and up-regulation of boron transporter has the potential in improving boron acquisition into shoots under boron limited condition.