Abstract
In the legume-rhizobia symbiosis, rhizobia enter into the root tissues via tubular plant-derived structures termed infection threads. Between two accessions of Lotus japonicus, MG-20 Miyakojima and B-129 Gifu, the number of infection threads is different. The stem color is also distinct by different accumulation of anthocyanins: red (B-129) or green (MG-20). Composite interval mapping using recombinant inbred lines between B-129 and MG-20 revealed two quantitative trait loci, on the southern area of chromosome 5 (QTL1) and near the stem color locus (VIC6) on chromosome 2 (QTL2). We also found significant correlations among infection thread number, pod width, seeds per pod, and stem color. A mutant of the stem color locus (vic6-1) with B-129 background showed the characteristics of the four traits similar to MG-20. Comparison of the response to a precursor and an inhibitor of ethylene biosynthesis between B-129 and MG-20 suggested that there is each optimal concentration of the precursor or the inhibitor for the number of infection threads and that MG-20 has lower endogenous ethylene level than B-129.
