Evaluation, Inheritance And QTL Analysis Of Aluminum Tolerance In Soybean [Glycine Max (L.) Merr.]

Soybean (Glycine max (L) Merr.) is one of the most important legume crops containing about 40% of protein and 20% oil. It supplies all eight amino acids for human health. Increasing soybean productivity on the declining arable land sizes is limited by the abiotic stresses of which aluminum (Al) toxicity is the major cause of low productivity in acid soils worldwide. Breeding and selection of A1-tolerant cultivars is the most strategic, environmentally responsive, and economical and universally accepted approach to aluminum toxicity management. The main objectives of this study were on:(1) evaluation procedure and indicator of Al tolerance, (2) inheritance and QTL analysis of tolerance to aluminum toxicity at young seedling stage, and (3) association analysis of aluminum toxicity tolerance in Chinese released soybean cultivar population. The main results are summarized below.For the experiment on the evaluation procedure, seventeen accessions selected as tolerant from a previous test program using average membership index (FAi) as indicator, plus one tolerant (PI.416937) and one sensitive (NN1138-2) check, were assayed in sand culture pot experiments, four experiments at V3, V5, V7 and V9 leaf stage respectively. The relative values of shoot dry weight (RSDW), root dry weight (RRDW), total plant dry weight (RTDW), total root length (RTRL) and total root surface area (RRSA) as the tolerance indicators as well as FAi were compared. All the indicators showed significant variation in Al tolerance among genotypes across the leaf-stages, but Genotype×Stage interactions were significant only for RTRL and RRSA, indicating they were less stable among stages than RTDW, RSDW and RRDW. Among the latter three, RTDW was chosen as the major indicator of A1 tolerance due to its relatively better stability, higher correlation with other indicators and easier measuring procedure than the others. The seedling age applicable for screening was not definitive, but V5 appeared to compromise between time spent resulting from screening the relatively older seedlings at later stages and low variation among genotypes at younger stage. Superior Al tolerant accessions identified using RTDW were PI.509080 (South Korea), N23533 and N24282 (Northeast China) and PI.159322 (USA), comparable to the putative tolerant check PI.416937 (Japan) at all vegetative stages.To reveal inheritance and the genetic architecture of Al tolerance, segregation analysis and quantitative trait loci (QTL) mapping were performed with a recombinant inbred line (RIL) population NJRIKY with 184 F2:7-derived lines from a cross between Kefeng No.1 (KF No.1) and Nannong 1138-2 (NN1138-2) parents and using the RTDW, RSDW and RRDW indicators in sand culture and relative root elongation (RRE) tolerance index in hydroponics. Significant difference among RILs was observed and frequency distributions showed characteristics of mixed distributions, which suggested that inheritance of Al tolerance conformed to major gene plus polygene mixed inheritance model. The genetic model typeâ… (four additive major genes plus additive polygenes) was the most fitting one for all the traits, indicating some common genetic mechanism lying among the traits. Averagely, the major genes contributed more than the collective polygenes to the phenotypic variation, however, both were important in breeding for Al tolerance. Four additive QTLs, four epistatic QTL pairs and collective minor QTL were detected for RTDW, with respective contributions of 22.30%,14.86% and 40.64%, in a total genetic contribution of 77.80% to phenotypic variation (PV) while QTL×Environment contribution was relatively negligible. Similar results were for RSDW and RRDW indicators. A QTL linked to marker GMKF046 on linkage group (LG) B1 was shared with RTDW, RSDW and RRDW across two environments, and explained respectively 9.23%,7.05% and 8.85% of phenotypic variation. Similarly in hydroponics, four additive QTLs, four epistatic QTL pairs and collective minor QTLs were detected, with respective contributions of 29.39%,18.75% and 43.07% in a total genetic contribution of 91.20% of PV. Among the four major QTLs, QTLs on LGs A2 and B1 contributed respectively 8.36% and 8.92% of PV and 9.17% and 9.78% of genotypic variance (GV) respectively, and both had positive additive effects alleles from the tolerant parent. Overall, the three QTL types additive, epistatic QTLs and collective minor QTLs are all relevant and should therefore, be comprehensively utilized for the improvement of Al-tolerance in soybean. The QTLs on A2 and B1 are potential ones in breeding for A1 tolerance in soybean, while for simplifying the evaluation procedure, using RTDW in sand culture is to be studied further.Association analysis was performed on 188 cultivars evaluated for A1 tolerance in hydroponics and using 197 SSR markers for genome-wide scan and 186 markers for extend of linkage disequilibrium (LD) and population structure. LD between syntenic markers generally decayed to a basal value of r2< 0.1 within the～25cM and extended upto 150cM. Six markers significantly (p<0.01) associated with A1 tolerance and accounted 57.9% of total PV. Five of these were in regions where Al tolerance QTLs have been mapped in previous published linkageship analysis but the genomic region with marker Satt005 on D1b1 has not been reported and appears to be new in this study. Five SSR markers, Satt209 (A2), Satt005 (D1b1), Satt186 (D2), Sat₂40 (F) and Satt284 (L) jointly explained 49.7% of PV and the elite alleles Satt209-A210, Satt209-A219, Satt005-A159, Satt005-A186, Satt186-A267, Satt186-A258, Sat₂40-A214 and Satt284-A285 contributed effect of greater than 20%. The allele carrier materials ranged 4-30 cultivars. These are potential donor materials in breeding for Al tolerance. The elite alleles traced in five pedigrees of the released cultivars indicated each pedigree progeny had its own unique or multiple elite alleles not necessarily descendent from the pedigree ancestor parents. The elite cultivars most tolerant to Al toxicity, N25388, N25019, N09125.100, N23774.00 (from Shandong province) and N02951.00 (from Jiangxi province) were carriers of the five main markers. Several Al resistant cultivars shared elite alleles while others had specific alleles, indicating certain cultivars shared common ancestry and similar breeding objectives. It is suggested that our association mapping identified markers Satt209 (A2), Sattl86 (D2) and Satt005 (Dl1b1) as strong candidates valuable for MAS; however, confirmation studies are necessary for the implementation of MAS in plant breeding.