Selenium accumulation, distribution, and speciation in spineless prickly pear cactus: a drought- and salt-tolerant, selenium-enriched nutraceutical fruit crop for biofortified foods.
The organ-specific accumulation, spatial distribution, and chemical speciation of selenium (Se) were previously unknown for any species of cactus. We investigated Se in Opuntia ficus-indica using inductively coupled plasma mass spectrometry, microfocused x-ray fluorescence elemental and chemical mapping (μXRF), Se K-edge x-ray absorption near-edge structure (XANES) spectroscopy, and liquid chromatography-mass spectrometry (LC-MS). μXRF showed Se concentrated inside small conic, vestigial leaves (cladode tips), the cladode vasculature, and the seed embryos. Se K-edge XANES demonstrated that approximately 96% of total Se in cladode, fruit juice, fruit pulp, and seed is carbon-Se-carbon (C-Se-C). Micro and bulk XANES analysis showed that cladode tips contained both selenate and C-Se-C forms. Inductively coupled plasma mass spectrometry quantification of Se in high-performance liquid chromatography fractions followed by LC-MS structural identification showed selenocystathionine-to-selenomethionine (SeMet) ratios of 75:25, 71:29, and 32:68, respectively in cladode, fruit, and seed. Enzymatic digestions and subsequent analysis confirmed that Se was mainly present in a "free" nonproteinaceous form inside cladode and fruit, while in the seed, Se was incorporated into proteins associated with lipids. μXRF chemical mapping illuminated the specific location of Se reduction and assimilation from selenate accumulated in the cladode tips into the two LC-MS-identified C-Se-C forms before they were transported into the cladode mesophyll. We conclude that Opuntia is a secondary Se-accumulating plant whose fruit and cladode contain mostly free selenocystathionine and SeMet, while seeds contain mainly SeMet in protein. When eaten, the organic Se forms in Opuntia fruit, cladode, and seed may improve health, increase Se mineral nutrition, and help prevent multiple human cancers.
Agricultural Research Service, United States Department of Agriculture, Parlier, California 93648, USA. gary.banuelos@ars.usda.gov
Gary S
GS
Bañuelos
Sirine C
SC
Fakra
Spencer S
SS
Walse
Matthew A
MA
Marcus
Soo In
SI
Yang
Ingrid J
IJ
Pickering
Elizabeth A H
EA
Pilon-Smits
John L
JL
Freeman
D000143:Acids; D005591:Chemical Fractionation; D002851:Chromatography, High Pressure Liquid; D002853:Chromatography, Liquid; D018556:Crops, Agricultural / Q000187:drug effects / Q000378:metabolism; D019587:Dietary Supplements; D055864:Droughts; D019286:Endopeptidase K / Q000378:metabolism; D005527:Food, Fortified; D005638:Fruit / Q000187:drug effects / Q000378:metabolism; D013058:Mass Spectrometry; D029747:Opuntia / Q000187:drug effects / Q000378:metabolism*; D009928:Organ Specificity / Q000187:drug effects; D010936:Plant Extracts; D015203:Reproducibility of Results; D055051:Salt-Tolerant Plants / Q000187:drug effects / Q000378:metabolism*; D012643:Selenium / Q000378:metabolism*; D018036:Selenium Compounds / Q000378:metabolism / Q000494:pharmacology; D064588:Selenium Oxides; D012965:Sodium Chloride / Q000494:pharmacology*; D013052:Spectrometry, X-Ray Emission; D013054:Spectrophotometry, Atomic; D056928:X-Ray Absorption Spectroscopy
D000143:Acids; D010936:Plant Extracts; D018036:Selenium Compounds; D064588:Selenium Oxides; D012965:Sodium Chloride; D019286:Endopeptidase K; D012643:Selenium