Minisymposium 1: Nutrient Biology

Abs # 11003: 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

Iron, an essential nutrient, can be toxic if it accumulates to high levels in cells. Thus, iron uptake and accumulation are highly regulated processes. Strategy I plants, including the dicots and non-grass monocots, induce expression of a set of genes in response to iron deprivation. The Arabidopsis FRO2 gene encodes the inducible ferric chelate 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. FRO2 and IRT1 are induced together following the imposition of iron starvation, and are coordinately repressed following iron resupply. Although FRO2 mRNA levels are elevated in 35S-FRO2 plants regardless of iron status, ferric chelate 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 in roots or shoots of plants 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. Thus, expression of both IRT1 and FRO2 is controlled at a minimum of two levels. Site-directed mutagenesis of IRT1 is yielding insight into amino acid residues important for IRT1 protein accumulation; mutation of two particular lysine residues in IRT1 results in a protein that is not subject to post-transcriptional regulation; this result suggests that IRT1 is subject to metal-induced ubiquitination. Interestingly, mutation of a set a histidine residues that comprise a putative intracellular metal binding domain in IRT1 does not affect either iron transport by IRT1 or IRT1 protein accumulation. Finally, 35S-FRO2 plants display tolerance to growth on low iron as compared to WT. Currently, we are working to determine whether the low iron tolerance phenotype of the 35S-FRO2 plants is due to enhanced reduction of iron at the root surface when plants are grown on low iron, or to enhanced storage of iron in the seed, or both. Our results suggest 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.