Analysis On Pyramiding Effect Of Phosphorus-efficient QTLs In Brassica Napus

Brassica napus which is extremely sensitive to phosphorus(P) deficiency is one of the most important oil crops in China, and has high P requirement for its optimal seed yield and quality. The largest cultivated region for rapeseed in China is located in the middle and lower valley of the Yangtze River where the available P in soils is very low. Chemical P fertilizer is widely applied to maintain its optimal seed yield and quality in the Yangtze River region. In our previous study, we identified two B. napus cultivars with contrast P efficiency(97081, a P efficient cultivar and 97009, a P in-efficient cultivar) with an approach of two step selection. A recombinant inbred lines(RILs) population with 124 lines named as the BE-RIL population was constructed previously using the two cultivars. Then, phenotypes for each RI lines were investigated, including seed yield and yield-related traits at maturing stage, shoot and root dry weight, P concentration at seedling stage. Then, quantitative trait loci(QTLs) were detected by QTLcart2.5 using a genetic map containing a total of 840 loci. Based on our previous work, the advanced backcross lines of B. napus were obtained through backcross between P-efficient cultivar 97081/P-inefficient cultivar 97009 as the donor parents and two commercial varieties Zhongshuang No.9(ZS9)/Huashuang No.5(HS5) as the recurrent parents. Co-dominant SSR molecular markers under P-efficient QTL region were used for genotype analysis for the backcross population, and hydroponic culture and field trial were conducted to validate the effect of P-efficient QTLs pyramiding of B. napus. The main results are as follows: 1. Selection of low-P-specific QTLs and the corresponding molecular markers inBrassica napusIn this study, robust low-P specific QTLs and the correlated traits including shoot dry weight, root dry weight, root surface area, plant height, grain yield etc. were selected for the further analysis. Finally, 18 QTLs(9 QTLs from 97081, and the other from 97009) and 41 SSR molecular markers in the QTL intervals were selected as the target QTLs. These QTLs were distributed on 9 linkage groups, including A1, A2, A3, A6, A8, A10, C3, C5, C8. 2. Analysis on pyramiding effect of phosphorus-efficient QTLs at seedling stage ofBrassica napusForty-one SSR markers were used to analyze the genotype BC3F1, BC3F2 and BC4F2 plants. The results showed that 13 of 107 BC3F1 lines pyramided 1-6 P-efficient QTLs. Three P-efficient lines of No. 8-1 pyramided 5-7 QTLs, and 4 P-inefficient lines of No. 8-1 pyramided 1-4 QTLs,while P-efficient and inefficient lines of No.9-1 pyramided 4-5 and 2-3 QTLs, respectively. Moreover, 4 P-efficient lines of No. 16-2 pyramided 7-8 QTLs, but P-inefficient lines just pyramided 3-6 QTLs. The P-efficient lines pyramided more QTLs, and showed large biomass under low P condition. On the contrary, the P-inefficient lines pyramided fewer QTLs, and showed obvious P deficiency symptom and had small biomass. The above results indicated that more P-efficient QTLs were pyramided in the P-efficient materials compared to that in the P-inefficient materials, P-efficient QTLs may have additive effect for the tolerance of low P stress in B. napus. 3. Analysis on pyramiding effect of phosphorus-efficient QTLs at maturing stage inBrassica napusSeven BC4F2 population were grown under low P stress field, seed yield and yield-related traits were investigated. The genotype of P-efficient materials and P-inefficient materials among the seven BC4F2 populations were analyzed, and the results showed that more P-efficient QTLs were pyramided in the P-efficient materials than those in the P-inefficient materials. In addition, for the materials from the same family, more QTLs were pyramided in BC4F2 generation than that in BC3F1 and BC3F2 generations, indicating that the continuous backcross could effectively increase the number of QTLs pyramiding. Compared to the plants with fewer QTLs, lines with more P-efficient QTLs have higher plant height, larger pod number per plant, larger seed number per pod and higher seed yield. Compared to the recipient parents, 13 of 18 P-efficient QTLs pyramiding lines had 72.2% increase in pods number per plant, 14 of 18 P-efficient QTLs pyramiding lines had 77.8% increase in seed number, and 13 of 18 P-efficient QTLs pyramiding lines had 72.2% increase in seed yield. However, P-inefficient lines were generally lower than the recurrent parents for the above traits. Our results showed that P-efficient QTLs pyramided in plants has played an important role in the improvement of yield and other traits at maturing stage. More P-efficient QTLs pyramided in plants may lead to higher resistance to low P stress and finally lead to higher yield under low P environment.