Evaluation Of The Adaptability Of Different Ramie (Boehmeria Nivea L.) Germplasms To Poor Soil Condition And Exploration Of The Adaptation Strategies

Ramie（Boehmeria nivea L.）is a crop which requires a large amount fertilizer application for high yield and good fiber quality.However,the current agricultural production in China goes into a new era of sustainable development.One of the most important tasks for sustainable agriculture is to reduce the fertilizer application.The breeding for varieties with tolerant of stressful conditions is critical for sustainable agriculture,which is also true for ramie.However,lack of breeding materials and the related information on how the ramie plant tolerate nutrient-deficient conditions has hindered efforts to breed varieties,which are suitable for stressful conditions.Thus,two long-term（8 years after planting）poor soil field trials and 2018 wild germplasms collection（from the mining area）were conducted with the first objective to select the germplasms with highly adaptability to poor soil conditions.Based on the above materials,exploration of the adaptability strategies of ramie plants to poor soil condition was also conducted in terms of nutrient translocation,soil microbe contribution and transcriptome.Results are shown as following:（1）The adaptability of ramie to nutrient-deficient conditions（based on the field phenotype）was not consistent in the establishment year and year after a long-term cultivation,which is also true between different growing seasons（divided by mid-season harvest）within one year In order to accurately and comprehensively evaluate the poor soil tolerance adaptability of ramie,the plant phenotype should be measured from the third year after cultivation and throughout the entire growth period（divided by the mid-season harvest）within a year should be considered.Based on this criterion,the field phenotypic indicators(measured in the 8^th year after planting)of the six tested ramie genotypes were normalized to build a comprehensive plant field phenotype index（NFPI index）,which was used to compare the poor soil adaptabilities of different germplasms.Finally,the germplasms of Duobeiti 1 and Xiangzhu XB with better field phenotype were screened to have a stronger adaptability to poor soil condition..（2）The 20 wild ramie germplasms collected from 5 mining areas in Linxiang,Cili,Huayuan,Taojiang and Shaodong have rich phenotypic variation（the highest CV value of65.10%）and genetic diversity（the highest Shannon index value of 2.02）.Based on the grey correlation analysis results,the wild germplasm J3-4（from the Huayuan mining area）has a similar phenotypic performance with Xiangzhu XB（the screened germplasm with strong poor soil adaptability）as indicated by the grey relational grade of 0.739.Moreover,J3-4 also has a strong ability to accumulate Pb and Cd（their transfer coefficients are both greater than1）,and the per square meter harvested aboveground biomass can absorb and enrich 40.556mg Pb and 4.371 mg Cd,respectively.Results indicate that germplasm of J3-4 has strong adaptability to low fertility and heavy metal polluted soils.（3）This study confirmed the existence of N,P,and K retranslocation in ramie plants at the end of the GP2（the second growth period within a growing season divided by mid-season harvest,defined as August-December）,but not at GP1（the first growth period,defined as March-July）.The correlation between the changing rate of N and P in the underground root system at the end of GP2 and the NFPI index were 0.959（p=0.025）and0.843（p=0.035）,respectively,suggesting that nutrients（mainly N and P）retranslocation to the root at the end of GP2 helped ramie plants adapting to poor soil conditions.In addition,this study also found that the changing rate of above-ground plant contented nutrients at GP1was relatively low by the germplasm with strong poor soil adaptability.In particular,the negative correlation（-0.819）between the change rate of N in the stem and the NFPI index at the end of GP1 reached a significant difference level（p=0.047）.These results indicated that reducing the amount of nutrient（especially N）taken away from the soil by the mid-term harvest may be also a strategy for ramie to copes with poor soil.In terms of nutrient absorption and transformation between different plant parts,it was found that the roots’nutrients（especially N）absorption and stem’s nutrient transformation ability significantly correlated with the ramie plant’s NFPI index.These results indicate that the ramie plants can also reduce the impact of soil nutrient deficiency on plant growth by improving the root’s ability to extract nutrients from the soil and accelerating the migration cycle of elements between new and old tissues.（4）Different ramie genotypes adapt to the poor soil condition by altering the microbial community composition rather than its diversity.On the one hand,the ramie genotypes with strong adaptability to poor soil can accumulate more beneficial bacteria in its root system,which can promote plant nutrient absorption and utilization.From the perspective of bacterial functional guilds,nitrogen-fixing bacterial guilds and cellulose degrading bacterial guilds showed significant and positive correlation with NFPI index.On the other hand,ramie genotypes with strong adaptability to poor soils can reduce the abundance of pathogenic bacteria,endophytic fungi and other harmful fungi represented by Aspergillus.Through reducing the negative impact of diseases on the growth of ramie plants,the ramie’s adaptability to poor soil conditions can be improved.The results of fungal function prediction analysis showed that the significant（p<0.001）and negative correlation was exist between lichen parasitic flora,endophytic fungi and wood-based saprophytic flora and NFPI.（5）Results of this experiment confirmed the positive contribution of nitrogen-fixing bacteria to the adaptation of ramie plants to poor soil environments.The number of nitrogen-fixing bacteria and the nitrogenase activity in the rhizosphere soil of ramie plants with strong poor soil adaptability were higher than those with weak adaptability.In addition,this study also determined the positive contribution of microbial phosphorus solubilization to the adaptation of ramie plants to poor soil environments.The phosphate solubilizing ability and acid phosphatase activity of microorganisms in the rhizosphere soil of ramie plants with strong poor soil adaptability are higher than those of plants with weak adaptability.（6）Transcriptome sequencing was performed on the two ramie materials with the best and the worst adaptability to poor soil obtained in present study using high-throughput sequencing technology.A total of 52,759 unigenes were obtained by assembling.Among them,the functions of unigene with large differences in expression are related to ABC transporter,triangular pentapeptide repeat protein,aging-specific cysteine protease,purple acid phosphatase activity,and papain family cysteine protease.Through the functional enrichment of differentially expressed genes,the most abundant genes are related to involved in carbohydrate metabolism and environmental adaptation functions.Among the differentially expressed genes involved in environmental adaptation,the genes that are only negatively related to ramie adaptability to poor soil conditions are related to disease resistance proteins,death-related protein kinases,pathogenicity-related proteins.The top four up-expressed unigenes（i.e.,positively correlated with the poor soil adaptability of ramie）in Xiangzhu XB include DN721＿c0＿g1（controlling formation of the RNA1polyprotein）,DN9098＿c0＿g1（integral component of membrane）,DN7103＿c0＿g1（plant transposase related）and DN11078＿c0＿g1（unknown function）,while top four down-expressed（i.e.,negatively correlated）unigenes include DN41995＿c0＿g1（unknown function）,DN5624＿c0＿g2（transmembrane transport and defense related）,DN8330＿c0＿g5（controlling the plant senescence-related enzyme）and DN7998＿c1＿g1(Ca²⁺signal transduction related).The selected ramie genotypes with poor soil condition adaptability can served as materials for the subsequent breeding of new barren-tolerant varieties.On the other hand,the present study discusses the strategies of poor soil tolerance in ramie,and the related results have improved the agronomic and biological characteristics of barren tolerance in ramie,which can provide theoretical support for the subsequent breeding of new varieties of barren tolerance.