Study On Mechanism Of Uptake And Translocation Of Selenomethionine By Rice Roots

Selenite is the main selenium form which is absorbed by rice roots. Most selenite is transformed to organic forms which aren’t readily translocated to shoots after being absorbed, as becomes the bottleneck limiting selenium content in brown rice. Analysis of selenium speciation in roots, stems and leaves was performed using HPLC-ICP-MS to investigate the predominate forms of selenite transformation in roots and of selenium transport into leaves. The results showed that seleno-L-methionine and selenite were primarily detected, followed by seleno-L-cysteine, only trace amounts of other Se species such as MeSeMet and an unidentified Se species were detected in roots. Similarly, seleno-L-methionine was also primarily detected, followed by selenoL-cysteine in both stems and leaves, only trace amounts of MeSeMet and selenite were detected in stems. Obviously, seleno-L-methionine was the predominant species present in roots, stem and leaves. It explained clearly that most selenite was transformed to seleno-L-methionine which was transported to shoots after being absorbed by rice roots.Supplied with the equivalent concentration of seleno-L-methionine solutions to rice roots, it was found that seleno-methionine content in roots was gradually increased and reached a maximum and then stabilized with the extension of Se uptake time, indicating that the uptake of seleno-L-methionine by rice roots has reached saturation point. Similarly, treated with different concentrations of seleno-Lmethionine solutions, the uptake rate of seleno-L-methionine was increased with the increases of selenium concentration in the solutions in the same period. After the uptake rate reached a maximum and then became stable, suggesting Se uptake was saturated at high concentrations of seleno-L-methionine.Treated with CCCP and DNP solutions, it was found that both CCCP and DNP could inhibit the uptake of seleno-L-methionine by rice roots to a large extent, indicating that the absorption of seleno-L-methionine was closely related to energy metabolism. In addition, selenium concentration in rice roots was far higher than selenium levels in solution, indicating that seleno-L-methionine entered rice roots against concentration gradient. Thus, it was concluded that seleno-L-methionine was actively absorbed by rice roots. pH affected the uptake of seleno-L-methionine greatly. The results showed that the absorption of seleno-L-methionine by rice roots was gradually increased with increasing pH in the range of p H 3.0-5.0, and achieved the highest at pH 5.0, then declined in the range of pH 5.0-8.0. Different types of amino acids could affect the uptake of seleno-L-methionine to some extent. Methionine, Tyrosine, Phenylalanine and Leucine could inhibit the uptake of seleno-L-methionine to higher levels, followed by Leucine, Serine, Alanine, Valine, Proline, Threonine, and Cysteine.chr1 is a mutant of nitrate transporter which belongs to the NRT family. The absorption of seleno-L-methionine by mutant chr1 was significantly lower than that of wild-type. The gene chr1 was transformed into yeast and the absorption of seleno-Lmethionine by the yeast was significantly increased, indicating that the transporter was responsible for the transport of seleno-L-methionine. Compared with mutant chr1, the rice seedling of wild type had a higher level of selenium in roots, stems and leaves. Similarly,selenium concentration was higher in roots, stems and leaves of rice seedling by overexpressing gene chr1 than that in wild-type plants. Selenium species analysis further revealed that the concentration of seleno-L-methionine in roots, stems and leaves of rice seedling by overexpressing gene chr1 than that in wild-type plants. It strongly demonstrated that CHR1 was responsible for the transport of seleno-Lmethionine. It provides important theoretical basis for breeding selenium-enriched varieties of rice in this study.