Mapping Of QTL For Ferrous And Zinc Toxicity Tolerance At Seedling Stage Using Two Sets Of Reciprocal Introgression Lines In Rice

Ferrous and zinc are essential microelements for rice growth, but they are also important sources of heavy metal pollution in soil. Rice in lowland or acid soil is easily subjected to ferrous and zinc toxicities which harm plant growth and decrease biomass and yield production. The aim of this study was to identify quantitative trait loci (QTL) underlying ferrous toxicity tolerance (FTT) and zinc toxicity tolerance (ZTT) in rice and analyze the effects of genetic background on QTL expressions by using two sets of reciprocal advanced backcross introgression lines which were derived from a cross between Lemont (japonica) and Teqing (indica). These introgression lines were genotyped with308evenly distributed single nucleotide polymorphism (SNP) markers developed from the two parents. The genetic structure of the reciprocal introgression lines was also analyzed. We measured physiological traits in the two parents and extreme lines which were selected on the basis of relative ratio of shoot dry weight (SDW) of plants under stress to control conditions. The main results are summarized as follows:1. Genetic structure analysis of the reciprocal introgression linesThe genetic structure of the reciprocal introgression lines was analyzed by using genotype data from308SNP markers. A high frequency of segregation distortion markers were both found in the two introgression populations. Asymmetric over-introgression of Teqing alleles on chromosome9and specific regions on other chromosomes were observed in Lemont genetic background. This indicated that it was easy for Teqing alleles to introgress into Lemont genetic background and that there is a high genetic load in the japonica genome. According to the genetic structure of different populations and the loci of target genes, we can improve introgression frequency of the target genes using an appropriate breeding strategy.2. QTL for ferrous and zinc toxicity tolerance at the seedling stage and their genetic background effectsA total of42putative QTL affecting shoot height (SH), shoot dry weight (SDW), and root dry weight (RDW) under control and stress conditions and the relative ratio of the traits of plants under the stress to the control were identified. Most of the alleles that improved the tolerance to ferrous and zinc toxicity were from Lemont. Among them, only four (9.52%) QTL were detected in the two backgrounds, indicating that the expressions of most of these QTL are genetic background-specific. Nine QTL were detected from the same genetic background affecting the tolerance of both FTT and ZTT, of which QSdw5was only one expressed under the two backgrounds with the same direction and similar quantity of gene additive effect, suggesting that there is genetic overlap of FTT and ZTT at seedling stage in rice. It is possible, therefore, to improve both FTT and ZTT of Teqing by introgressing and pyramiding Lemont alleles at the overlapping QTL underlying FTT and ZTT to via marker-assisted selection (MAS).3. Physiological analysis of FTT and ZTT using the two extreme linesPhysiological traits including chlorophyll content index, shoot and root iron concentration, and shoot and root zinc concentration were tested on the two parents and extreme lines selected on the basis of relative ratio of SDW under stress and control conditions. Plant biomass and chlorophyll content index both decreased under stress condition compared with control, but there was no significant difference in chlorophyll content index between the extreme lines. Ferrous was more easily gathered in root than zinc. Whether under control or treated conditions, the concentration of ferrous or zinc in root were significantly higher than that in the shoot. The susceptible lines absorbed more ferrous and zinc than the tolerant lines under treated condition. Relative shoot dry weight can be used as an important reference index to evaluate the tolerance to ferrous and zinc toxicity in rice.