Quantitative trait locus analysis of root ferric reductase activity and leaf chlorosis in the model legume, Lotus japonicus

Ferric reductase activity is a rate-limiting step in the accumulation of iron by Strategy I plants. Preliminary work with Lotus japonicus accessions Miyakojima MG-20 and Gifu B-129 identified differences in shoot chlorosis and ferric reductase activity. This study assessed the genetic basis for these differences. Lines of a recombinant inbred population, derived from Miyakojima and Gifu, were tested for whole-root ferric reductase activity and shoot chlorosis following iron-limited growth. A ferric reductase gene (LjFRO1) was cloned from both parents. Protein sequence analysis, transcript abundance, and yeast complementation studies were conducted with the two parental alleles. A single quantitative trait locus (QTL) was identified for both ferric reductase activity and shoot chlorosis, with each QTL explaining similar to 30% of the variation and both overlapping across the same region of chromosome 3. LjFRO1 mapped to chromosome 3, but to a region adjacent to the reductase and chlorosis loci. Nucleotide variation in LjFRO1 parental alleles was identified, as were minor functional differences between the two proteins. The results indicate that both allelic variation (providing potential functional differences) and unidentified molecular components (derived from non-LjFRO1 genetic loci) can contribute to the regulation of ferric reductase activity and chlorosis susceptibility.