Chloroplast Fe(III) chelate reductase activity is essential for seedling viability under iron limiting conditions

Jeeyon Jeong1, Christopher Cohu2, Loubna Kerkeb3, Marinus Pilon2, Erin L. Connolly3 and Mary Lou Guerinot1

1Department of Biological Sciences, Dartmouth College, Hanover, NH 03755 2Department of Biology, Colorado State University, Fort Collins, CO 80523 3Department of Biological Sciences, University of South Carolina, Columbia, SC 29208

Photosynthesis, heme biosynthesis and Fe-S cluster assembly all take place in the chloroplast and all require iron. Reduction of iron via a membrane bound Fe(III) chelate reductase is required prior to iron transport across membranes in a variety of systems but to date there has been no definitive genetic proof that chloroplasts have such a reduction system. Here we report that one of the eight members of the Arabidopsis FRO (ferric reductase oxidase) family, FRO7, is targeted to the chloroplast. Chloroplasts prepared from fro7 loss of function mutants have 75% less Fe(III) chelate reductase activity and contain 33% less iron per µg chlorophyll than wild type chloroplasts. This decreased iron content is presumably responsible for the observed defects in photosynthetic electron transport. When germinated in alkaline soil, fro7 seedlings show severe chlorosis and die without setting seed unless watered with high levels of soluble iron. Overall, our results provide molecular evidence that FRO7 plays a role in chloroplast iron acquisition and is required for efficient photosynthesis in young seedlings and for survival under iron-limiting conditions.

e-mail address of presenting author: Jeeyon.Jeong@Dartmouth.EDU