Response Of Root Morphology To Low Phosphorus Stress And QTL Analysis In Brassica Napus

Rapeseed is one of the major oilseed crops in China, and planting area and total yield are the first in the world. The middle and lower valley of the Yangtze River in South China is the largest cultivated region for rapeseed (Brassica napus), where the soil available P concentration is usually lower. B. napus has high P requirement for its optimal seed yield and quality, so low available P concentration in soils seriously limits the production of B. napus in this area. Exploiting or breeding P-efficient cultivars has become an attractive prospect for rapeseed production. Modification of root morphology and architecture is the key adaptive strategy of plant in response to P deficiency. In this study, a B. napus F10 recombinant inbred lines (was named as BE-RILs) population, which developed from a cross between P-efficient rapeseed cv. Eyou Changjia and P-inefficient cv. B104-2, was employed to investigate plant dry weight, P concentration, and root morphology traits at the seedling stage under high P (HP) and low P (LP) conditions in paper culture experiments. Based on a genetic linkage map constructed with SSR, AFLP, SRAP markers, quantitative trait loci (QTL) for these traits were identified, and the genetic basis of P-efficiency was dissected through QTL meta-analysis and comparative mapping method.The main results were as follows:1 Constructing a genetic linkage map for P efficiency in B. napus and comparative mapping with Arabidopsis Used genomes DNA of BE-RILs as template, the genotypes were analyzed with SSR, AFLP, SRAP markers. A genetic linkage map with 553 molecular markers was constructed by Jionmap4.0, including 202 SSR,62 AFLP,234 SRAP and 55 functional markers. The total genetic distance was 1592.7 centimorgans (cM) and an average distance was 2.9 cM between adjacent markers. Futhermore, the comparative genomics analysis between B. napus and Arabidopsis was mapped employing SSR markers with known sequence information as anchored markers. And 27 synteny blocks were aligned with some regions of ten linkage groups of BE-RILs.2 Investigating the phenotypic traits of two parents and BE-RILs under HP and LP conditions by three independent paper culture experiments The paper culture system which employed P-free blue germination paper as support was designed to investigate the phenotypic traits. After harvesting, total root length of plant, root surface area, root volume, and root average diameter were analyzed with WinRhizo root image analysis software. Plant dry weight and plant P concentration were also detected. As results, under low P condition, compared to the P-inefficient parent B104-2, the P-efficient parent Eyou Changjia not only developed bigger root system, but also produced higher biomass and acquired more P. However, Eyou Changjia had lower P use efficiency. This suggested that high P efficiency of Eyou Changjia was mainly attributed to developed root system. The phenotypic traits showed continuous distribution in BE-RILs. The coefficients of variation (CV) for these traits ranged from 20.5% to 40.6%. This indicated that each parent possesses alleles that have both positive and negative effects on the traits.3 Detecting and analyzing the QTL of P efficiency3.1 Detecting the significant-QTL for various traits under two P conditions QTL for twelve traits-SDW, RDW, TDW, RSR, RL, RSA, RD, TIP, PC, PU, and PUE under HP and LP conditions were detected in the three experiments using the composite interval method by Wincart2.5 software. As results, a total of 136 significant-QTL were identified, where 63 and 73 QTL were detected for HP and LP condition, respectively. These significant-QTL were distributed on eight linkage groups-A1, A3, A6, C1, C2, C3, C6 and C8. The contribution to phenotypic variations for the single QTL ranged from 7.95%-22.00%.3.2 Meta-analysis QTL of P efficiency The overlapping QTL for the traits in different experiments were integrated using QTL meta-analysis. First, the overlapping significant-QTL for the same traits in different experiments were integrated, and were got 94 consensus-QTL, which included 44 for HP and 50 for LP, respectively. Second, the overlapping consensus-QTL for different traits were integrated into 37 unique-QTL, which included 10 QTL specific for HP,15 QTL specific for LP and 12 constitutive QTL, respectively.3.3 Distributing of functional markers in the confidence intervals of unique-QTL 10 functional markers were distributed in the confidence intervals of unique-QTL. Five markers were located in the intervals of LP-specific QTL. One marker was located in the interval of HP-specific QTL. Four markers were located in the intervals of constitutive QTL. The five markers located in the intervals of LP-specific QTL were BnSQDl-Cl, BnGPT2-C1, BnPHO1-C1, BnIPS2-C3 and BnGPT1-C3, which correspond to LP-specific QTL uq.C1a, uq.C1b, uq.C1b, uq.C3a and uq.C3c. Some RI lines pyramiding favorable alleles were found using these functional makers in the intervals of LP-specific QTL3.4 Locating the orthologous genes of Arabidopsis on BE-RILs linkage map by in silico mapping On the basis of comparative genomics between B. napus and Arabidopsis,423 orthologous genes for P metabolism, P-responsive transcription factors, root development and auxin transporter in Arabidopsis were located on 850 loci of BE-RILs linkage map by in silico mapping, and 67 orthologous genes were corresponded to the intervals of QTL, which were speculated as the candidate genes of QTL in B. napus. These information of QTL and candidate genes provide a solid foundation for improving the adaptability of P deficiency, and paves the way for map-based cloning of these genes.