Difference Of Low Phosphorus Tolerance Between Tibetan Wild Barley And Cultivated Barley And Related QTL Location

Phosphorus(P)is one of the most important nutrient for plant growth,participating in various physiological and biochemical processes and being the key element for ensuring normal growth and development,yield formation and stress resistance of crops.Because of the low concentration of available phosphorus in soil and the limited phosphate rock resources,phosphorus has gradually become a limiting factor for the sustainable development of agricultural production.On the other hand,the application of phosphorus fertilizer causes serious water pollution.Therefore,improving crop phosphorus use efficiency or low phosphorus tolerance is an important way to solve phosphorus deficiency and reduce the use of phosphorus fertilizer.Understanding the molecular mechanism of phosphorus-efficient genotypes adapting to low phosphorus stress is helpful for breeding the crop cultivars with high P efficiency.In the present study,low-phosphorus-tolerant wild barley XZ26 and low-phosphorus-sensitive cultivate barley ZU9 were used as parents to construct the DH population.We compared the responses of different DH lines under different phosphorus levels and carried out the QTL mapping of phosphorus efficiency-related traits.Then,we identified LP-tolerant line L138 and LP-sensitive line L73 and used these two genotypes for carrying out transcriptome profiling.The main results were summarized as follows:1.Identification of phenotypic differences between low phosphorus sensitive and tolerant genotypesTibet wild barley and cultivated barley were grown under low phosphorus(LP)and normal phosphorus(CK)conditions in hydroponics to analyse phenotypic traits,including shoot dry weight,root dry weight,root length and element content.LP-tolerant wild barley XZ26 and LP-sensitive cultivated barley ZU9 were identified and used as parents to construct the DH population.LP-tolerant line L138 and-sensitive line L73 were identified.The results showed that,compared with ZU9,the low phosphorus tolerance of XZ26 was mainly reflected in the significant increase of root biomass and the distribution of more phosphorus in roots under low phosphorus conditions,thus ensuring root growth and improving nutrient acquisition efficiency.Compared with L73,L138 responded earlier to low phosphorus stress,and had the same tolerance mechanism as XZ26.Root biomass and length increased significantly,and amount of phosphorus allocated to roots increased.At the same time,L138 maintained better shoot growth by increasing P use efficiency under long-term stress.Therefore,it can be suggested that the low phosphorus tolerance was mainly due to the plasticity of root system and more re-distribution of phosphorus within plants.2.Identifation of QTLs for the traits related to phosphorus efficiencyThe genetic linkage map of DH population was constructed by using SNP markers.The shoot dry weight,root dry weight and length of the population under two phosphorus levels(LP and CK)were analyzed.The results showed that there were significant differences in low phosphorus tolerance and superparental segregation among the DH population.Map QTL 5 was used to analyze the phenotypic data and genetic linkage map.Only one QTL was detected,which is located near 5.7 cM of 6H chromosome(LOD>2),explaining 9.0%of phenotypic variation.The representative candidate genes in this region included HORVU6Hr1G002400(cytochrome P450),HORVU6Hr1G002410(isoflavone reductase),HORVU6Hr16002600(BURP protein),HORVU6Hr1G003300(nitrate reductase 1).Two key candidate genes HORVU6Hr1G002410(isoflavone reductase IFR)and HORVU6Hr16003300(nitrate reductase NLA1)were associated with relative root length,which were up-regulated and down-regulated by low phosphorus,respectively.3.Transcription profiling analysis of the two genotypes with different low P toleranceTotal RNA was extracted from roots of L138 and L73 for RNA-sequencing.It was found that L138 and L73 had different transcriptional levels in response to low phosphorus stress.In terms of root system changes,the auxin regulatory genes associated with lateral root and root hairs formation in L138 were significantly up-regulated under low phosphorus condition,while the expression of the relevant genes in L73was inhibited at the early stage of stress.In terms of phosphorus uptake and transport,L138 initiated phosphorus transport systems(PHO1,SPX-MFS)earlier than L73 in the early stress treatment,and transported phosphorus was stored in vacuoles for plant growth.In the late stress treatment,L138 initiated the PHT1 family to adapt to low phosphorus stress by improving phosphorus acquisition efficiency.In addition,the expression of sugar transporters in L138 was up-regulated by low phosphorus stress,which improved phosphorus uptake efficiency and promoted phosphorus transport and distribution in plants.Therefore,it can be concluded that the molecular mechanisms of L138 adapting to low phosphorus stress included the growth of lateral roots regulated by auxin and sugar,the enhancement of Pi uptake capacity mediated by PHT1,and the redistribution of Pi and Pi homeostasis regulated by PHO1 and SPX-MFS.