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How Spineless Prickly Pear Cactus Utilize Selenium Print
Monday, 12 March 2012 13:50

Due to drought and limited freshwater supplies, the increased accumulation of naturally occurring salts, boron (B), and selenium (Se) has worsened in some agricultural areas, such as in the west side of the San Joaquin Valley, California. Growing Se-biofortified crops is an emerging method for utilizing these “semiretired lands” because the nutraceutical benefits of enhancing organic Se, an essential micronutrient in crops, is concomitant with the phytoremediation of inorganic Se pollutants.

Researchers from the U.S. Department of Agriculture used ALS Beamline 10.3.2 to study the chemical forms and distribution of Se in the attractive alternative crop Opuntia ficus-indica, an edible spineless prickly pear cactus that tolerates both drought and adverse saline- and B-impacted soil conditions while accumulating and volatilizing organic Se. Micro x-ray fluorescence (mXRF) mapping showed Se concentrated in the tips of the plant’s cladodes (edible pads), cladode vasculature, and seed embryos. Se K-edge x-ray absorption near edge structure (XANES) spectroscopy demonstrated that >90% of total Se in cladodes, fruit juice, fruit pulp, and seeds is in organic form (C-Se-C). Cladode tips contain both inorganic selenate (SeO42–) and C-Se-C. Enzymatic digestion confirmed that Se was mainly present in a “free,” non-proteinaceous form inside cladode and fruit, whereas in the seed, Se was incorporated into proteins associated with lipids.

Inductively coupled plasma mass spectrometry followed by liquid chromatography–mass spectrometry showed the two major C-Se-C organic Se species selenocystathionine (SeCyst) and selenomethionine (SeMet) were present in ratios of 3, 2.45, and 0.47 in Opuntia cladode, fruit, and seed, respectively. Results also show that Opuntia accumulate inorganic Se and metabolize it into organic SeCyst and SeMet. mXRF chemical mapping revealed that when inorganic Se is metabolized and Se reduced, the resulting products—SeCyst and SeMet—accumulate in the cladode tips before being transported to the rest of the cladode.

 

Opuntia ficus-indica. Tricolor-coded mXRF map of seeds, cladode tips and vasculatures in Opuntia. Se K-edge XANES spectra of cladode tips, seeds, juice, and pulp compared to standards. Bicolor-coded chemical map of cladode tips and vasculatures.

 

 


 

 

Work performed on ALS Beamline 10.3.2.

Citation: G.S. Bañuelos, S.C. Fakra, S.S. Walse, M.A. Marcus, S.I. Yang, I.J. Pickering, E.A.H. Pilon-Smits, and J.L. Freeman, “Selenium Accumulation, Distribution, and Speciation in Spineless Prickly Pear Cactus: A Drought- and Salt-Tolerant, Selenium-Enriched Nutraceutical Fruit Crop for Biofortified Foods,”Plant Physiol. 155 (2011). Full Article (PDF)