| Soil salinization is a global environmental problem caused by human activities,which seriously limits the agricultural production.In our country,the development and utilization of saline alkali wasteland is less.So with the decrease of cultivated land area,cotton cultivation gradually to the saline alkali land,so these saline alkali land has huge development potential.Cotton is one of the crops that are more tolerant to salt and alkali,but from the present study,the salt concentration in the saline alkali land is still the main factor restricting the production of cotton.The improving of salt tolerance is the important target in modern cotton breeding.On the other hand,with the aggravation of global warming,extreme weather occurs frequently,extreme hot weather has caused substantial impact to many countries and regions of agriculture,whether it is temporary or sustained high temperatures will cause on cotton morphology,physiological andbiochemical of negative effects,resulting in vegetative growth and reproductive growth the deformity and sterility.Therefore,the situation of soil salinization,high temperature and other abiotic stress increasing,strengthen the understanding of the physiological mechanism of plant stress,screening of excellent germplasm with resistance quality and identification of stress-related genes are the urgent problem of agricultural science has become an urgent problem of agricultural science.This research is based on the above three aspects to carry on the design discussion:(1)For explorering the efficient methodology of salt tolerance evaluation in cotton,three-leaf cotton seedlings of two salt-tolerant varieties and two salt-sensitive varietieswere treated by water and 4%(40 g/L)NaCl solution,respectively.A total of 13 parameters related to salt tolerance including salt injury index(S?),shoot fresh weight(SRW),root fresh weight(RFW),leaf relative water content(RWC),chlorophyll fluorescence parameters(Fo,Fm,Fv/Fm),relative electric conductivity(REC),manlondialdehyde(MDA)content and the activity of antioxidant enzymes were monitored after 72h of treatment.Comprehensive assessment of salinity tolerance based on grey relation clustering method,principal component analysis and stepwise regression analysis indicated that the maximum quantum yield of PSII was the most significant correlated indices with salt tolerance in upland cotton,which could be used as a single parameter to assess salt tolerance based on the equation:y=1.943x-0.882(where y is the salt tolerance index,x is the related value of maximum efficiency of photosystem II).The other two salt-tolerant varieties and two salt-sensitive varieties were used to rate the classification of salt tolerance.Salt tolerance index(y)of 23 varieties with known salt tolerance were calculated to validate the accuracy of the equation,the result was consistent with field investigation.In this study,the maximum efficiency of photosystem II was used as the main index to validate the salt tolerance in cotton,together with the construction of salt tolerance index equation and salt tolerance rating,which could greatly improve the efficiency of salinity tolerance evaluation for massive germplasm in future.(2)The capacity for ion compartmentalization among different tissues and cells is the key mechanism regulating salt tolerance in plants.In this study,we investigated the ion compartmentalization capacity of two upland cotton genotypes with different salt tolerances under salt shock at the tissue,cell and molecular levels.We first found the glandular trichome in upland cotton can secrete salt ions,in addition,the leaf glandular trichome could secrete more salt ions in the salt-tolerant genotype than in the sensitive genotype,demonstrating the excretion of ions from tissue may be a new mechanism to respond to short-term salt shock.Furthermore,an investigation of the ion distribution demonstrated that the ion content was significantly lower in critical tissues and cells of the salt-tolerant genotype,indicating the salt-tolerant genotype had a greater capacity for ion compartmentalization in the shoot.By comparing the membrane H+-ATPase activity and the expression of ion transportation-related genes,we found that the H+-ATPase activity and Na+/H+ antiporter are the key factors determining the capacity for ion compartmentalization in leaves,which might further determine the salt tolerance of cotton.The novel function of the glandular trichome and the comparison of Na+compartmentalization between two cotton genotypes with contrasting salt tolerances provide a new understanding of the salt tolerance mechanism in cotton.(3)We investigated two distinct salt stress phases—dehydration(4 h)and ionic stress(osmotic restoration;24 h)—that were identified by physiological changes of 14-day-old seedlings of two cotton genotypes,one salt tolerant(Earlistaple 7)and one salt sensitive(Nan Dan Ba Di Da Hua),during a 72h NaCl exposure.A comparative transcriptomics approach was used to monitor gene and miRNA differential expression at two time points(4h and 24h)in leaves of the two cotton genotypes under salinity conditions.The expression patterns of differentially co-expressed unigenes were divided into six groups using STEM software.During a 24h salt exposure,819 transcription factor unigenes were differentially expressed in both genotypes,with 129 unigenes specifically expressed in the salt-tolerant genotype.Under salt stress,108 conserved miRNAs from known families were differentially expressed at two time points in the salt-tolerant genotype.We further analyzed the predicted target genes of these miRNAs along with the transcriptome for each time point.Important expressed genes encoding membrane receptors,transporters,and pathways involved in biosynthesis and signal transduction of calcium-dependent protein kinase,mitogen-activated protein kinase,and hormones(abscisic acid and ethylene)were up-regulated.We also analyzed the salt stress response of some key miRNAs and their target genes and found that the expressions of five of nine target genes exhibited significant inverse correlations with their corresponding miRNAs.On the basis of these results,we constructed molecular regulatory pathways and a potential regulatory network for these salt-responsive miRNAs.(4)In this study,four genotypes of upland cotton(Gossypium hirsutum L.)withknown field thermotolerance were evaluated under normal and HTs at the seedlings stage in a growth cabinet with 11 physiological,biochemical,and phenotypic assays.Consistent with previous field observations,the thermotolerance could be identified by genotype differences at the seedling stage under HT in a growth cabinet.Comparative transcriptome analysis was performed on seedlings of two contrasting cotton genotypes after 4 and 8 hours of HT exposure.Gene ontology analysis combined with BLAST annotations revealed a large number of HT-induced differentially expressed genes(4,698)that either exhibited higher expression levels in the heat-tolerant genotype(Nan Dan Ba Di Da Hua)compared with the heat-sensitive genotype(Earlistaple 7),or were differentially expressed only in Nan Dan Ba Di Da Hua.These genes encoded mainly protein kinases,transcription factors,and heat shock proteins,which were considered to play key roles in thermotolerance in upland cotton.Two heat shock transcription factor genes(homologs of AtHsfA3,AtHsfCl)and AP2/EREBP family genes(homologs of AtERF20,AtERF026,AtERF053,and AtERF113)were identified as possible key regulators of thermotolerance in cotton.Some of the differentially expressed genes were validated by quantitative real-time PCR analysis.Our findings provide candidate genes that could be used to improve thermotolerance in cotton cultivars. |
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