| Droughts have become more frequent and widespread due to global climate change,making drought one of the most prominent abiotic stresses limiting plant growth and production.As the constructive species of zonal grassland vegetation,S.klemenzii and S.grandis are widely distributed along the water gradient and have strong adaptability to their respective water conditions.To understand the tolerance of the two species of Stipa to drought stress and reveal the mechanism will help to further understand the vegetation-environment relationship in the eastern end of the Eurasian steppe,and reveal the nature of the alternative distribution of the Stipa steppe.In this study,two species of Stipa were used as the objects,and two kinds of Stipa seedlings were treated with 30% PEG-6000 to simulate drought stress for 10 h,20 h and 30 h.The changes in leaf and root phenotype of the two were observed and recorded,the phenotypic changes were tested and analyzed for the response of its physiological and biochemical indicators,and the molecular mechanism of the two Stipa seedlings responses to drought stress was explored by transcriptome sequencing technology(RNA-Seq).The main findings are as follows:1.The response of two Stipa phenotypes and physiological and biochemical characteristics to drought stress.(1)In terms of phenotype,S.grandis is more susceptible to drought stress than S.klemenzii.(2)In terms of photosynthetic physiology and endogenous hormone content,with the increase of drought stress,the photosynthetic gas parameters and total chlorophyll content of the two Stipa leaves were significantly decreased,and the abscisic acid content was significantly increased.(3)In terms of cell membrane permeability,drought stress led to a significant increase in the electrical conductivity(REC)of the leaves of the two types of Stipa,and the REC of the roots of S.klemenzii and S.grandis decreased significantly at the early and later stages of stress,respectively.The content of malondialdehyde(MDA)in leaves and roots of the two types of Stipa increased significantly with the increase of drought stress,and the content of MDA in leaves was higher than that in roots.(4)In terms of osmotic regulators,the contents of soluble protein(BCA)and proline(Pro)in S.klemenzii increased significantly with the increase of drought stress.The BCA content of S.grandis leaves was significantly decreased in the early stage of drought stress,and the root system was significantly increased in the middle stage of drought stress.With the increase of drought stress,the Pro content of S.grandis leaves increased significantly,and the roots showed a trend of first decreasing and then increasing.(5)Catalase(CAT)activity decreased significantly in two types of Stipa leaves and roots of S.grandis with increasing drought stress,and increased significantly in the later stage of drought stress in S.grandis roots.Phenylalanine ammonia lyase(PAL)activity in S.klemenzii decreased significantly with the increase of drought stress,in S.grandis leaves showed a trend of first increase and then decreased with the increase of drought stress,and in root system with the increase of drought stress intensity,the activity decreased significantly.2.Transcriptome analysis of two species of Stipa under drought stress.(1)905 and 108 differentially co-expressed genes and 18,734 and 7,188 coexpressed differentially genes were screened out from S.klemenzii and S.grandis respectively.(2)The co-expression network of differentially expressed genes(DEGs),the key module of WGCNA,was constructed,and 7 and 9 S.klemenzii and S.grandis Hub genes were obtained,respectively.The significantly enriched transcription factors identified were MYB,NAC,WRKY,AP2-EREBP and b HLH Wait.(3)Functional annotation was performed on the differentially expressed genes identified by the two Stipa species,and the pathways significantly enriched by the two at the same time included photosynthesis,phytohormone signal transduction and secondary metabolite synthesis.After mining,a large number of functional genes and their encoded enzymes were identified.For example,ALD,FBP,CAT,GOX,and SHMT related to Calvin cycle and photorespiration,NCED,PYR/PYL,ABF,PP2 C,and Sn RK2 related to abscisic acid synthesis and signal transduction,and PAL,PAL,flavonoid biosynthesis,etc.CHI,FLS,F3 H and DFR.3.Molecular mechanism of two species of Stipa in response to drought stress.(1)After drought stress,in terms of photosynthesis,both Stipa species can reduce water loss by closing stomata and reducing transpiration rate.S.klemenzii also upregulates the expression of enzymes related to photosynthesis,regulates carbon flux through photorespiration,realizes the compensation effect of photosynthesis,and then increases photosynthetic activity and promotes growth.(2)In terms of phytohormone signal transduction after drought stress,S.klemenzii accumulated a large amount of ABA to activate downstream signaling molecules,thereby inducing the expression of dehydration tolerance protective proteins and the synthesis of metabolites such as MDA and Pro.Simultaneously,ABA accumulation caused leaf stomatal closure and reduced water loss,thereby increasing the resistance of Stipa to drought stress.The ABA content in Stipa leaves was much higher than that in roots,indicating that phytohormone signal transduction in leaves plays a major role in response to drought stress.In addition,no matter the ABA content or the expression of ABA-related signal transduction molecules,the performance of S.klemenzii is better than that of S.grandis,indicating that S.klemenzii can improve drought tolerance through stronger phytohormone signal transduction.(3)After being subjected to drought stress,in terms of the synthesis of secondary metabolites,S.klemenzii and S.grandis mainly accumulated flavonoids in the roots and leaves respectively to improve their antioxidant capacity.It is worth noting that most functional genes related to flavonoid synthesis were up-regulated in S.grandis,implying that flavonoid synthesis mainly played a role in drought resistance of S.grandis.S.klemenzii is one of the constructive species of desert steppe,and the annual average precipitation in its distribution area is between 150-250mm;S.grandis is one of the constructive species of typical grassland,and the annual average rainfall in its distribution area is 250-350 mm area in between.It can be seen that compared with the large Stipa,the small Stipa is more able to adapt to the arid environment.This study shows that S.klemenzii can improve its photosynthetic activity through the compensation effect of photosynthesis when water is deficient,and its drought resistance performance is better than that of S.grandis in terms of transduction of plant hormones,and synthesis of flavonoid secondary metabolites plays a major role in drought resistance of S.grandis.The above conclusions clarified the water adaptation mechanism of S.klemenzii and S.grandis,and enriched the understanding of the difference in drought tolerance between the two Stipa.At the same time,it also has important reference value for mining plant drought resistance gene resources and understanding vegetation evolution in the eastern end of the Eurasian steppe. |
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