Selenium Accumulation in Flowers and its Effects on Pollination

Authors: QUINN, C - Colorado State University; PRINS, C - Colorado State University; GROSS, A - Colorado State University; HUNTZIS, L - Colorado State University; REYNOLDS, R - Colorado State University; FREEMAN, J - Fresno State University; YANG, S - University Of Saskatchewan; COVEY, P - Colorado State University; Banuelos, Gary

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2011
Publication Date: 7/15/2011
Citation: Quinn, C.F., Prins, C.N., Gross, A.M., Huntzis, L.J., Reynolds, R.J., Freeman, J.L., Yang, S., Covey, P.A., Banuelos, G.S. 2011. Selenium Accumulation in Flowers and its Effects on Pollination. New Phytologist. p. 1469-8137.

Interpretive Summary: Selenium (SE) serves no known essential function in plants. Plants take up, however, and metabolize Se via the sulfur assimilation pathway. So called Se hyperaccumulating plants concentrate Se to levels greater than 1,000 mg/kg dry weight. Non-hyperaccumulator plants may suffer Se toxicity when grown on high concentrations of Se because they non-specifically incorporate organic forms of Se into proteins, causing toxicity. In contrast, Se hyperaccumlators concentrate Se primarily as methylselenocysteine (MeSeCys) which is not incorporated into proteins and therefore does not cause toxicity. The objectives of this study were to determine 1) Se distribution and speciation in flowers of S. pinnata and B. juncea 2) does 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; 5) does honey contain elevated levels of Se. Results from this study reveal differences in Se distribution and speciation between flowers of the S. pinnata and B. juncea., S. pinnata contained less toxic form of Se and preferentially allocated it to its reproductive parts. High Se levels reduced pollen viability in B. juncea but not for S. pinnata. This study gives us insight into differences between the floral ecologies and the impact if Sein Se hyperaccumulator and non-hyperaccumulator plants.

Technical Abstract: Selenium, both an essential micronutrient and a potential toxin, is hyperaccumulated by some plants up to 1% of dry weight. The functional significance of this rare phenomenon may be an elemental defense against herbivores and pathogens. In this first of its kind study, we investigate Se distribution and speciation in flowers and the effects of floral Se pollen quality and plant pollinator interactions. The Se hyperaccumulator plant S. pinnata-preferentially allocated Se to flowers, particularly to the sex parts, possibly to protect these valuable tissues from herbivoces and pathogens. S. pinnata flowers accumulated Se primarily as non-toxic methyl-selenocysteine (MeSeCys). As a result, Se concentrations even as high as 4,400 µg/g DW did not impair pollen germination (when tested), while a non-hyperaccumulator-Brassica juncea- had more Se in its leaves than flowers, in which Se was evenly distributed as a mixture of MeSeCys and the more toxic Se-cystine, selenate and selenite. Selenium distribution and speciation were determined using micro-focused x-ray fluorescence mapping (µXRF) and x-ray absorption near edge structure (µXANES) spectroscopy, respectively. We observed that pollinators showed no visitation preference between high and low Se containing plants. Honey and bumble bees visiting plants were analyzed for Se concentrations. Selenium was distributed through the body of the bumble bee with a high concentration of Se on the rear legs (most likely present in pollen baskets). Se concentrations were found to be 0.4 and 1.0 mg µ/kg FW in honey. The results from this study reveal differences in Se distribution and speciation between flowers of the Se hyper accumulator and the secondary accumulator plant. S. pinnata contained a less toxic form of Se and preferentially allocated it to its reproductive parts, including pollen and ovules. High Se levels reduced pollen viability for B. juncea but not for S. pinnata. Lastly, our Se-rich plants did not deter pollinator visilation. Hence, future studies on bee and Se sensitivity would be beneficial to conduct when producing plants high in Se from Se-tainted growing regions.