Molybdenum sequestration in Brassica species. A role for anthocyanins?

To elucidate plant mechanisms involved in molybdenum (Mo) sequestration and tolerance, Brassica spp. seedlings were supplied with molybdate, and the e ffects on plant physiology, morphology, and biochemistry were analyzed. Whe n supplied with (colorless) molybdate Indian mustard (Brassica juncea) seed lings accumulated water-soluble blue crystals in their peripheral cell lave rs. Energy dispersive x-ray analysis showed that Mo accumulated predominant ly 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-acc umulated Mo was different than that for molybdate, indicating complexation with a plant molecule. Because the blue compound was water soluble and show ed a pH-dependent color change, possible involvement of anthocyanins was in vestigated. 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 s upplied with molybdate. In the anthocyanin-containing varieties, the blue c ompound colocalized with anthocyanins in the peripheral cell layers. Mo acc umulation by the three B. rapa varieties was positively correlated with ant hocyanin content. Addition of molybdate to purified B. rapa anthocyanin res ulted 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.