Selenium accumulation in flowers and the associated implications for ecology, evolution and fortified foods

Background/Question/Methods

Selenium, both an essential micronutrient and a toxin, is hyperaccumulated by some plants up to 1% of the plants’ dry weight.  The functional significance of this rare phenomenon appears to be elemental defense against herbivores and pathogens.  In this study, the first of its kind, we investigate Se distribution and speciation in flowers and the effects of floral Se on pollen quality and plant-pollinator interactions.  The objectives of this study were to determine: 1) Se distribution and speciation in the flowers of Stanleya pinnata and Brassica juncea, 2) how elevated Se levels affect pollen viability, 3) how floral Se affects visitation by honey bees and other potential pollinators, 4) Se distribution and speciation in pollinators collected from hyperaccumulators in the field, and 5) whether honey collected from Northern Colorado, a seleniferous area, has elevated levels of Se.

Results/Conclusions

The Se hyperaccumulator S. pinnata preferentially allocated Se to flowers, particularly to the sex parts, possibly to protect these valuable tissues from herbivores and pathogens.  In contrast, the related non-hyperaccumulator B. juncea had higher Se levels in leaves than flowers, and within flowers Se was evenly distributed.  S. pinnata flowers accumulated non-toxic methyl-selenocysteine (MeSeCys), while B. juncea flowers contained a mixture of MeSeCys and the more toxic Se-cystine, selenate and selenite.  For B. juncea, 2,200 mg Se g^-1 DW impaired pollen germination, however, the more ecologically relevant concentration of 370 mg Se g^-1 DW did not.  In S. pinnata, Se concentrations as high as 4,400 mg g^-1 DW did not impair pollen germination.  Pollinators showed no visitation preference between high and low Se plants of either plant species.  Indeed, pollinators collected from Se-rich S. pinnata contained measurable Se, up to 270 mg g^-1.  Interestingly, native bumble bees contained 10-fold higher Se levels than introduced honey bees, and a higher percentage of their Se was non-toxic MeSeCys (96% compared to 58%).  Honey from seleniferous areas contained 0.4-1 mg Se g^-1 fresh weight (FW), concentrations that could provide human health benefits. 

This study is the first to shed light on the possible cost of evolving Se hyperaccumulation and provides a foundation for future experiments on the ecological and evolutionary aspects of floral hyperaccumulation.  In addition, these results have important implications for the management of seleniferous habitats and for the cultivation of Se-rich plants for phytoremediation or as Se-fortified food.