Department of Biology, California State University Fresno, Fresno, California, United States of America.
PLoS One. 2012;7(12):e50516. doi: 10.1371/journal.pone.0050516. Epub 2012 Dec 3.
Selenium (Se) hyperaccumulator plants can concentrate the toxic element Se up to 1% of shoot (DW) which is known to protect hyperaccumulator plants from generalist herbivores. There is evidence for Se-resistant insect herbivores capable of feeding upon hyperaccumulators. In this study, resistance to Se was investigated in seed chalcids and seed beetles found consuming seeds inside pods of Se-hyperaccumulator species Astragalus bisulcatus and Stanleya pinnata. Selenium accumulation, localization and speciation were determined in seeds collected from hyperaccumulators in a seleniferous habitat and in seed herbivores. Astragalus bisulcatus seeds were consumed by seed beetle larvae (Acanthoscelides fraterculus Horn, Coleoptera: Bruchidae) and seed chalcid larvae (Bruchophagus mexicanus, Hymenoptera: Eurytomidae). Stanleya pinnata seeds were consumed by an unidentified seed chalcid larva. Micro X-ray absorption near-edge structure (µXANES) and micro-X-Ray Fluorescence mapping (µXRF) demonstrated Se was mostly organic C-Se-C forms in seeds of both hyperaccumulators, and S. pinnata seeds contained ∼24% elemental Se. Liquid chromatography-mass spectrometry of Se-compounds in S. pinnata seeds detected the C-Se-C compound seleno-cystathionine while previous studies of A. bisulcatus seeds detected the C-Se-C compounds methyl-selenocysteine and γ-glutamyl-methyl-selenocysteine. Micro-XRF and µXANES revealed Se ingested from hyperaccumulator seeds redistributed throughout seed herbivore tissues, and portions of seed C-Se-C were biotransformed into selenocysteine, selenocystine, selenodiglutathione, selenate and selenite. Astragalus bisulcatus seeds contained on average 5,750 µg Se g(-1), however adult beetles and adult chalcid wasps emerging from A. bisulcatus seed pods contained 4-6 µg Se g(-1). Stanleya pinnata seeds contained 1,329 µg Se g(-1) on average; however chalcid wasp larvae and adults emerging from S. pinnata seed pods contained 9 and 47 µg Se g(-1). The results suggest Se resistant seed herbivores exclude Se, greatly reducing tissue accumulation; this explains their ability to consume high-Se seeds without suffering toxicity, allowing them to occupy the unique niche offered by Se hyperaccumulator plants.
硒（Se）超积累植物可以将有毒元素硒浓缩到茎（DW）的 1%，这已知可以保护超积累植物免受普通食草动物的侵害。有证据表明，能够以超积累植物为食的抗硒昆虫食草动物是存在的。在这项研究中，研究了在消耗硒超积累物种 Astragalus bisulcatus 和 Stanleya pinnata 豆荚内种子的种象甲和种象甲幼虫中对硒的抗性。在硒超积累植物的栖息地和种子食草动物中收集的种子中，确定了硒的积累、定位和形态。Astragalus bisulcatus 种子被种象甲幼虫（Acanthoscelides fraterculus Horn，鞘翅目：象甲科）和种象甲幼虫（Bruchophagus mexicanus，膜翅目： Eurytomidae）消耗。Stanleya pinnata 种子被一种未鉴定的种象甲幼虫消耗。微束 X 射线吸收近边结构（µXANES）和微束 X 射线荧光图谱（µXRF）表明，在两种超积累植物的种子中，硒主要以有机 C-Se-C 形式存在，并且 S. pinnata 种子含有约 24%的元素硒。Stanleya pinnata 种子中硒化合物的液相色谱-质谱检测到 C-Se-C 化合物 seleno-cystathionine，而之前对 A. bisulcatus 种子的研究检测到 C-Se-C 化合物甲基硒代半胱氨酸和γ-谷氨酰甲基硒代半胱氨酸。微束 XRF 和 µXANES 表明，从超积累植物种子中摄取的硒重新分布到食草动物组织中，部分种子 C-Se-C 被生物转化为硒代半胱氨酸、硒代胱氨酸、硒代二谷胱甘肽、硒酸盐和亚硒酸盐。Astragalus bisulcatus 种子平均含有 5750 µg Se g(-1)，然而从 A. bisulcatus 种子荚中出现的成年甲虫和成年象甲蜂仅含有 4-6 µg Se g(-1)。Stanleya pinnata 种子平均含有 1329 µg Se g(-1)；然而，从 S. pinnata 种子荚中出现的象甲幼虫和成虫含有 9 和 47 µg Se g(-1)。结果表明，抗硒种子食草动物排除了硒，大大减少了组织积累；这解释了它们能够在不遭受毒性的情况下消耗高硒种子的能力，使它们能够占据硒超积累植物提供的独特生态位。
