Mapping of quantitative trait loci related to primary rice root growth as a response to inoculation with <i>Azospirillum</i> sp. strain B510

<p><i>Azospirillum</i> sp. strain B510 has been known as the plant growth-promoting endophyte; however, the growth-promotion effect is dependent on the plant genotype. Here, we aimed to identify quantitative trait loci (QTL) related to primary root length in rice at the seedling stage as a response to inoculation with B510. The primary root length of “Nipponbare” was significantly reduced by inoculation with B510, whereas that of “Kasalath” was not affected. Thus, we examined 98 backcrossed inbred lines and four chromosome segment substitution lines (CSSL) derived from a cross between Nipponbare and Kasalath. The primary root length was measured as a response to inoculation with B510, and the relative root length (RRL) was calculated based on the response to non-inoculation. Three QTL alleles, <i>qRLI-6</i> and <i>qRLC-6</i> on Chromosome (Chr.) 6 and <i>qRRL-7</i> on Chr. 7 derived from Kasalath increased primary root length with inoculation (RLI), without inoculation, (RLC) and RRL and explained 20.2%, 21.3%, and 11.9% of the phenotypic variation, respectively. CSSL33, in which substitution occurred in the vicinity region of <i>qRRL-7</i>, showed a completely different response to inoculation with B510 compared with Nipponbare. Therefore, we suggest that <i>qRRL-7</i> might strongly control root growth in response to inoculation with <i>Azospirillum</i> sp. strain B510.