IRON UPTAKE BY LEAF MESOPHYLL-CELLS - THE ROLE OF THE PLASMA MEMBRANE-BOUND FERRIC-CHELATE REDUCTASE

The uptake of Fe-59 from FeCl3, ferric (Fe3+) citrate (FeCitr) and Fe3 +-EDTA (FeEDTA) was studied in leaf mesophyll of Vigna unguiculata (L. ) Walp. Uptake rates decreased in the order FeCl3>FeCitr much greater than FeEDTA, and uptake depended on an obligatory reduction step of Fe 3+ to Fe2+, after which the ion could be taken up independently of the chelator, citrate. Uptake was strongly increased by photosyntheticall y active light (lambda > 630 nm), and kinetic analysis revealed satura tion kinetics with a K(m) (FeCitr) of 80-110 muM. In the presence of a n external Fe2+ scavenger, bathophenanthroline disulfonate, the mesoph yll also reduced external FeCitr with a K(m) of approx. 50-60 muM. The reduction rates for FeCitr were five- to eightfold higher than necess ary for uptake. Purified plasma membranes from leaves revealed an NADH -dependent FeCitr- and FeEDTA-reductase activity, which had a pH optim um of 6.5-6.8 and a K(m) of approx. 20 muM for NADH. Under anaerobic c onditions, a K(m) of 130-170 muM for ferric chelates was obtained, whi le in the presence of oxygen a K(m) (FeCitr) of approx. 100 muM was fo und. It is concluded that the leaf plasma membrane provides a ferric-c helate-reductase activity, which plays a crucial role in iron uptake o f leaf cells. Under in-vivo conditions, however, reactive oxygen speci es or strong (blue) light may also contribute to the obligatory reduct ion of Fe3+ prior to uptake.