Investigation of selenium-containing root exudates of Brassica juncea using HPLC-ICP-MS and ESI-qTOF-MS

Abstract

Selenium-containing root exudates were investigated in a known selenium accumulator model plant. Indian mustard (Brassica juncea) plants were grown hydroponically and supplemented with selenite (SeO₃²⁻) in a 25% Hoagland's nutrient solution. Additive concentrations were 0, 1, 5 and 20 µg mL⁻¹ Se with five replicate plants per treatment level. Plants were exposed to the respective Se solutions for two weeks, then placed in deionized water for two more weeks. The hydroponic solutions were collected for analysis after the first two weeks of selenium supplementation (day 14) and twice during the deionized water period (days 21 and 28). Separation by ion-pairing high performance liquid chromatography was followed by inductively coupled plasma-mass spectrometry (ICP-MS) for selenium specific detection. Chromatographic peaks unable to be identified by retention-time matching were collected for analysis by electrospray ionization mass spectrometry (ESI-MS). Additional chemical experiments were performed for structural elucidation. Several selenium-containing compounds were identified in the exudate-containing solution and two were identified as selenocystine and the selenosulfate (SSeO₃²⁻) ion. The presence of dimethylselenide (CH₃SeCH₃) is also observed but cannot be attributed exclusively to plant exudation because plants were not grown in sterile conditions. Further, the incorporation of fortified selenoamino acids into peptide structures was found to occur under neutral pH conditions, suggesting that exuded enzymes might facilitate such a reaction. Finally, physiological differences resulting from selenium supplementations were noted and discussed.