Poster: Heavy Metals

Abs # 107: Regulation of iron uptake in Arabidopsis

Presenter:	Connolly, Erin L, erinc@biol.sc.edu
Authors	Connolly, Erin L (A)   Campbell, Nathan  (A)   Guerinot, Mary Lou  (B)   Kerkeb, Loubna  (A)
Affiliations:	(A): Department of Biological Sciences, University of South Carolina (B): Department of Biological Sciences, Dartmouth College

The Arabidopsis FRO2 gene encodes the inducible ferric reductase responsible for reduction of iron at the root surface. Here, we show that the transcript abundances of FRO2 and IRT1 , the major transporter responsible for high affinity metal uptake under iron deficiency, are coordinately regulated. Although FRO2 mRNA levels are elevated in 35S-FRO2 plants regardless of iron status, ferric reductase activity is only elevated in the transgenic plants under conditions of iron deficiency. Thus, overexpression of FRO2 did not result in accumulation of elevated levels of iron as FRO2 is subject to post-transcriptional regulation. Previously, we showed that IRT1 is subject to post-transcriptional regulation; IRT1 protein accumulates only in iron-deficient roots of 35S-IRT1 plants. These results demonstrate that the targeted overexpression of either IRT1 or FRO2 may not be sufficient to confer enhancement of metal accumulation for the purpose of fortification of crop foods. Site-directed mutagenesis of IRT1 is yielding insight into amino acid residues important for IRT1 protein accumulation; mutation of a particular lysine residue in IRT1 results in a protein that is not subject to post-transcriptional regulation, suggesting that IRT1 is subject to metal-induced ubiquitination. Finally, 35S-FRO2 plants display remarkable tolerance to growth on low iron as compared to WT, presumably because these plants have elevated levels of ferric reductase activity when iron is limiting. This demonstrates that reduction of ferric iron to ferrous iron is the rate-limiting step in iron uptake under conditions of low iron availability and suggests that the expression of FRO2 in crops may yield lines capable of thriving on low iron soils.