SEN1 gene from Lotus japonicus MG20 improves nitrogen fixation and plant growth

In Lotus japonicus, quantitative trait loci (QTLs) for nitrogen fixation activity (acetylene reduction activity, ARA) and seed weight are located in the same region on chromosome 4 as the gene responsible for the Fix(-) mutant sen1. The SEN1 gene of L. japonicus Gifu B129 has a deletion of three nucleotides in comparison with that of L. japonicus Miyakojima MG20, which results in a deletion of one amino acid. Here, we examined whether SEN1 is responsible for these QTLs. Because the MG20 allele of the QTL for ARA and seed weight has a positive effect, we generated a near-isogenic line (NIL) in which the SEN1 gene region of B129 was replaced with that of MG20. Growth parameters of this NIL (MG20-type) 21 days after inoculation with Mesorhizobium loti were significantly higher than those of B129-type, and ARA was also significantly higher. Nodules formed on hairy roots in which the sen1 mutant was complemented with the MG20 SEN1 gene had higher ARA than those of B129. The seeds of the MG20-type NIL were significantly heavier than those of B129-type. These results indicate that the SEN1 gene is a very likely candidate for the QTL for ARA and seed weight on chromosome 4.