| P availability is considered a major limitation for crops development and production. Brassica napus is one of the most important oilseed crops in the China, which has high P requeirment and is extremely sensitive to soil P deficiency. Developing B. napus varieties with improved P efficiency is regarded as a sustainable strategy for coordinating the relation of plant P requirement and soil P deficiency. Hence, understanding the P efficiency mechanism will be beneficial to develop Brassica napus cultivars with enhanced P-efficiency. In this study, two ways are employed to study the mechanism of P efficiency in B. napus. One is to screen and identify P efficient mutants from an ethylmethanesulfonate (EMS) mutant population of B. napus varieties Ningyou7; Another is to study the difference of QTLs for seed yield and its related traits in TN DH population uner low P (LP) and normal P (NP) conditions with three field trials. The main results are as follow:1Brassica napus root mutants insensitive to exogenous cytokinin show phosphorus efficiencyA concentration of0.2μM6-BA was set for screening the root mutants based on the data from the experiment with6-BA concentration gradient from0to0.2μM. Six types of root mutants were obtained out of4200M2EMS mutagenesis plants, prll (a mutant with an elongated primary root) and lrn1(a mutant with more lateral roots) were used to evaluated P efficiency in hydroponics and pot trial, alongside wild type (WT) plants. The results showed that root dry weight of both mutants was higher than that of WT at low P (LP), with a greater increase in lrn1than in prll. lrn1not only acquired more P from the culture medium but utilized it effectively to produce higher shoot dry weight and greater seed yield than WT plants at LP.Endogenous cytokinin concentrations and P starvarion induced genes expression were tested in the shoots and roots of lrn1and prll in hydroponics. The results showed that trans-zeatin riboside (tZR) and isopentenyl adenosine (iPA) concentration in lrn1roots were both significantly lower than that in WT roots at both P conditions. Root iPA concentration in prll was lower than that in WT under both P conditions, however, root tZR concentration was greater in prll than WT under LP condition. Although the transcription of P starvation induced genes was up-regulated both in the shoots and roots of mutants, significant differences in the increment of gene expression levels existed between Irn1and prl1.In conclusion, the decrease in endogenous cytokinin concentrations in lrn1roots impaired the cytokinin-induced repression of root growth and P starvarion induced genes expression, and thereby enhanced the P uptake and utilization efficiency simultaneously. Genetic variation in P efficiency between lrn1and prll relulted form the differences in root growth and P starvarion induced genes expression between them.2Response of seed yield and its related traits to phosphorus defficiency and QTL mapping in TH DH population of B. napusA DH (double haploid) population derived from a cross between a P-efficient cv. Ningyou7and a P-inefficient cv. Tapidor was used to evalutated for seed yield (SY), first-effective branch height (FBH), plant height (PH), relative branch height (RBH), branche number (BN), seed number (SN), pod number (PN) and thousand seed weight (SW) under low P (LP) and normal P (NP) conditions. P deficiency significantly decreased PH, BN, SN, PN and SY, increased FBH and RBH, and had little effect on SW in TN DH population. Of the seven seed yield-related traits, PN, BN and RBH were most severely influenced by P-deficiency.53gene-based markers (GBM) designed from46genes involved P hometasis in Arabidopsis were added on the framework of the existing well-defined G-map. The new genetic linkage map had a total of798molecular markers and the average distance between two adjacent markers was2.57cM. QTL for FBH, PH, RBH, BN, SN, PN, SW and SY under LP and NP conditions of the three field trials were detected by composite interval mapping using WinQTL Cartographer2.5software. A total of155significant QTL were identified. Intervals of some QTL were found overlapped with each other. As a result, the155QTL were clustered to be81regions on15chromosomes. These QTL regions were mainly distributed on linkage groups A2, A3, A5, A9, C6and C9.29and29of the81QTL regions were specially detected under LP and NP condition, respectively, and23QTL regions were detected under both P conditions. Three LP-specific regions were consistently detected for the same traits across two of the three field trials. Six of23constitutive QTL regions were reproducible for the same trait under LP condition at least in two trials. These repeatedly detected loci in response to P-deficiency provided useful information for breeding B. napus with improved P-efficiency.Of the53GBM located in TN DH linkage map,30GBM designed from19P homeostasis genes in Arabidopsis were distributed in or near the QTL confidence intervals. These homologous genes in B. napus could be the candidate genes for the corresponding QTL. Of the19candidated genes,12were refracted to the corresponding genetic position on B.rapa or B.olerecea genome. LRP1, GPT1, SIZ1, MGD2and PYK10were located in the QTL regions repeatedly detected under LP conditions. The genomic sequence information of the homologous genes in B.rapa and B.olerecea will facilitate fine-mapping of these QTL...
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