Plant Physiol, August 2001, Vol. 126, pp. 1391-1402

Molybdenum Sequestration in Brassica Species. A Role for Anthocyanins?¹

Department of Biology, Anatomy/Zoology Building, Colorado State University, Fort Collins, Colorado 80523 (K.L.H., S.M.S., E.A.H.P.-S.); Agriculture and Environment Division, Institute of Arable Crops Research-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, United Kingdom (S.P.M., E.L.); Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, California 94270 (N.T.); and Stanford Synchrotron Research Laboratory (SSRL), Stanford Linear Accelerator Center, P.O. Box 20450, Stanford, California 94309 (I.J.P., G.N.G.)

To elucidate plant mechanisms involved in molybdenum (Mo) sequestration and tolerance, Brassica spp. seedlings were supplied with molybdate, and the effects on plant physiology, morphology, and biochemistry were analyzed. When supplied with (colorless) molybdate Indian mustard (Brassica juncea) seedlings accumulated water-soluble blue crystals in their peripheral cell layers. Energy dispersive x-ray analysis showed that Mo accumulated predominantly in the vacuoles of the epidermal cells. Therefore, the blue crystals are likely to be a Mo compound. The x-ray absorption spectrum of the plant-accumulated Mo was different than that for molybdate, indicating complexation with a plant molecule. Because the blue compound was water soluble and showed a pH-dependent color change, possible involvement of anthocyanins was investigated. An anthocyanin-less mutant of Brassica rapa ("fast plants") was compared with varieties containing normal or high anthocyanin levels. The anthocyanin-less mutant did not show accumulation of a blue compound when supplied with molybdate. In the anthocyanin-containing varieties, the blue compound colocalized with anthocyanins in the peripheral cell layers. Mo accumulation by the three B. rapa varieties was positively correlated with anthocyanin content. Addition of molybdate to purified B. rapa anthocyanin resulted in an in vitro color change from pink to blue. Therefore, Mo appears to be sequestered in vacuoles of the peripheral cell layers of Brassica spp. as a blue compound, probably a Mo-anthocyanin complex.