| Drought is one of the vital abiotic stresses that fatally affect plant growth and agricultural production globally.For the moment,the drought stress recurrently occurs in both natural and agricultural environment.Thus,a development in drought tolerance in crops is a requirement for reaching economic growth.The most and best efficient approach in this regard is to develop drought tolerant plant species.It is essential to identify tolerant genetic resources and understand the mechanisms of drought tolerance in socially and economically important plants such as cotton.Cotton is not only industrial fiber crops,but also plays a role in the feed and oil industries with its seed,protein(20-40%)and rich in oil(18-24%).Cotton is widely known as "white gold",it is most exclusive gift of nature to the mankind.While cotton is greater in terms of drought resistance to the most other plants,its growth and prduction is also greatly affected by drought stress.Consequently,the current research was carried out to illuminate the physiological and molecular mechanism in the drought tolerance by using three cotton species(Gossypium hirsutum,Gossypium arboretum and Gossypium barbadense),and to identify specific genes as well as key pathways for the tolerance to the drought stress through transcriptomic analysis.The significant results were precised as follows:1.Molecular and Physio-Biochemical Characterization of Cotton Species for Assessing Drought Stress ToleranceDrought stress significantly limits cotton growth and production due to the necessity of water at every stage of crop growth.Hence,it is essential to identify tolerant genetic resources and understand the mechanisms of drought tolerance in economically and socially important plants such as cotton.In this study,molecular and physio-biochemical investigations were conducted by analyzing different parameters by following standard protocols in three different cotton species,namely TM-1(Gossypium hirsutum),Zhongmian-16(Gossypium arboreum),and Pima4-S(Gossypium barbadense).Drought stress significantly decreased plant growth,chlorophyll content,net photosynthetic rate(Pn),stomatal conductance(Gs),maximum photochemical efficiency of PSII(Fv/Fm),and relative water content.TM-1 resulted in more tolerance than the other two species.The accumulation of proline,soluble proteins,soluble sugars,hydrogen peroxide(H2O2),and superoxide radicals(O2·-)increased significantly in TM-1.In addition,TM-1 maintained the integrity of the chloroplast structure under drought conditions.The relative expression level of drought-responsive genes including coding for transcription factors and other regulatory proteins or enzymes controlling genes(ERF,ERFB,DREB,WRKY6,ZFP1,FeSOD,CuZnSOD,MAPKKK17,P5CR,and PRP5)were higher in TM-1 under drought,conferring a more tolerant status than in Zhongmian-16 and Pima4-S.The findings of this research could be utilized for predicting a tolerant cotton genotype as well as evaluating prospective cotton species in the variety development program.2.Comparative Transcriptomic Analysis of Biological Process and Key Pathway in Three Cotton(Gossypiumspp)Species under Drought StressDrought is one of the most important abiotic stresses that seriously affect cotton growth and production worldwide.However,the molecular mechanism and responsible genes for drought tolerance of cotton have not been stated clearly.In this research,high-throughput next generation sequencing technique was utilized to investigate gene expression profiles of three cotton species(TM-1,Zhongmian-16 and Pima4-S)under drought stress.A total of 6968 differentially expressed genes(DEGs)were identified,where 2053,742 and 4173 genes for TM-1,Zhongmian-16 and Pima4-S were tested as statistically significant.Total DEGs were annotated and classified into functional groups under gene ontology analysis.The biological process was present only in tolerant species(TM-1)indicating drought tolerance condition.The kyoto encyclopedia of genes and genomes showed the involvement of plant hormone signal transduction and metabolic pathways enrichment under drought stress.Several transcription factors associated with ethylene responsive genes(ICE1,MYB44,FAMAA,etc.)were identified playing key roles in acclimatizing to drought stress.Drought also caused significant changes in the expression of certain functional genes linked to abscisic acid(ABA)responses(NCED,PYL,PP2C and SRK2E),ROS related small heat shock protein and 18.1 kDa,YLS3,ODORANTl.These results will provide deeper insights into the molecular mechanisms of drought stress adaptation in cotton.3.Effect of Drought During Reproductive Stage on Morphological,Physiological,Biochemical Parameters,Cotton Yield,and Fiber Quality of Three Cotton(Gossypium spp)SpeciesDrought is the deleterious abiotic stress that inhibits cotton growth and development,while its effect on vegetative stages,reproductive stages,leaf physiology,molecular response and metabolism have been studied on some cotton species.However,assessment of tolerant species based on response in reproductive phase has not been experimented extensively,hence the aim of this investigation was to evaluate the effects of drought on reproductive stage of three cotton species[TM-1(Gossypium hirsutum L),Zhongmian-16(Gossypium arboretum L),and Pima4-S(Gossypium barbadense L)].It was hypothesized that the drought stress would effect on these cotton species during reproductive stages but the tolerant species would ameliorate the response more efficiently and could produce good cotton yield.To investigate this hypothesis,pot experiment was conducted by imposing drought at reproductive stage for 10 days,at the experimental net house of the Zhejiang University,Zijingang Campus,Hangzhou,China in June 2016.Drought stress significantly(P<0.05)influenced plant height,total dry weights,growth parameters and physiological parameters but the intensity was different for the species.The proline content and soluble sugar both were increased in TM-1 genotypes compare to other two genotypes Zhongmian-16 and Pima4-S under drought stressed condition,which justified tolerability of cotton plant during critical situation through the osmotic adjustment.All the enzymes SOD,POD,CAT and APX increased significantly(P<0.05)but lower accumulation of H2O2,O2·-and MDA were observed in TM-1 genotype conferring higher tolerant status compared to the others.In addition,TM-1 produced higher seed cotton yield under drought stress which also affects micronaire value(fiber fineness),elongation,and strength and lint percentage.This experiment showed that the cotton plant affected more severely when drought imposed at the reproductive stages,but the tolerant variety always overcome the stress related limitation more efficiently then the sensitive species. |
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