Identification of QTL and candidate genes involved in early seedling growth in rice via high-density genetic mapping and RNA-seq

Early seedling vigor (ESV) is a major breeding target in rice, especially under direct seeding. To identify quantitative trait locus (QTL) affecting ESV, a recombinant inbred line population derived from a cross between 02428 and YZX, two cultivars differing in vigor during early seedling growth, was used for QTL analysis. Nine traits associated with ESV were examined using a high-density map. Of 16 additive loci identified, three were detected in two generations and thus considered stable. Four epistatic interactions were detected, one of which was repeated in two generations. Further analysis of the pyramiding effect of the three stable QTL showed that the phenotypic value could be effectively improved with an increasing number of QTL. These results were combined with results from our previous QTL analysis of the germination index. The lines G58 and G182 combined all the favourable alleles of all three stable QTL for ESV and three QTL for germination speed. These two lines showed rapid germination and strong ESV. A total of 37 candidate differentially expressed genes were obtained from the regions of the three stable QTL by analysis of the dynamic transcriptomic expression profile during the seedling growth period of the two parents. The QTL are targets for ESV breeding and the candidate genes await functional validation. This study provides a theoretical basis and a genetic resource for the breeding of direct-seeded rice.