Physiological Regulation Mechanisms Of Antioxidant System And Endogenous Hormones And Molecular Basis In Cassava Under Drought Stress

Cassava(Manihot esculenta Crantz)is one of the most important potato crops.Its tubers are often used as raw materials for starch and alcohol.Cassava is a drought-tolerant crop,but its growth and root yield are vulnerable to drought stress.Therefore,it is of great practical significance to study the drought-tolerant mechanism of cassava under water stress.In this study,drought-resistant cassava cultivar Xinxuan 048 and drought-sensitive cassava cultivar KU50 were used as experimental materials to study the physiological changes of antioxidant system and endogenous hormone metabolism.Treatments were composed of three drought stress levels,viz.,keeping the soil moisture content as 75-80%,55-60 %,35-40 % and 15-20 % of field capacity,respectively.RNA-Seq was used to analyze the changes of transcriptional level of cassava seedlings under normal water supply and severe drought stress.Ten differentially expressed genes(DEGs)related to drought resistance were screened from them.The results were as follows:(1)The growth,development,and root yield of cassava were inhibited by drought stress,and the root yield of Xinxuan048 cultivar was higher than that of KU50 cultivar.Under drought stress,the relative electrical conductivity,malondialdehyde in cassava leaves increased at all growth stages.The cell membrane permeability and malondialdehyde content in KU50 were significantly higher than those in Xinxuan048,indicating that drought-sensitive cultivars exhibited stronger oxidative response than drought-tolerant cultivars under water stress.(2)Different cassava cultivars showed different response mechanisms to drought stress.Xinxuan048 mainly eliminates oxidative damage caused by drought stress by accumulating As A content and increasing POD and CAT activities.In respect of hormone level,the growth of Xinxuan048 was inhibited by increasing ABA content and restraining the synthesis of IAA for regulating environmental adaptability to water deficiency.While KU50 reduced lipid peroxidation by increasing APX activity and inhibited the synthesis of GA to delay growth and maintain water balance in response to drought stress.(3)The transcriptome sequences of different cassava cultivars were analyzed under drought stress.In the comparison of drought treatment and control samples in Xinxuan048(Cx vs Sx),the up-and down-regulated genes were 1789 and 227,respectively.In the comparison of drought treatment and control samples in KU50(Ck vs Sk),up-and down regulated genes were 1123 and 657,respectively.The number of differentially expressed genes in Xinxuan048 cultivar was more than that in KU50,especially for up-regulated genes.(4)GO Term enriched significantly in the sample Cx vs Sx is different from Ck vs Sk.The DEGs in Xinxuan048 under drought stress are mainly concentrated to the biological process of GO ontology,that is,drought-induced genes are mainly involved in regulating the metabolic pathways at the molecular level,while the DEGs of KU50 under drought stress are mainly concentrated to molecular function of GO ontology,that is to say,the drought-induced genes mainly play their own roles at the molecular level.(5)In Cx vs Sx and Ck vs Sk samples,the most significant pathways were“Histidine metabolism” and “Biosynthesis of secondary metabolites”,respectively.Their common significant pathways were “Limonene and pinene degradation”,“beta-Alanine metabolism” and “Galactose metabolism”,which indicated that KEGG items significantly enriched by DEGs were varied from different cassava cultivars under drought stress.(6)The number of alternative splicing events in Xinxuan048 and KU50 cultivars under drought stress was 17469 and 2308 more than that of their control,respectively.The number of alternative splicing events in Xinxuan048 under drought stress was significantly more than that in KU50,indicating that the transcripts of Xinxuan048 were more abundant under water stress,and the corresponding response mechanism was more complex.(7)In this study,10 drought-induced genes related to antioxidant system and endogenous hormone regulation mechanisms were screened from transcriptome sequencing results.Fluorescence quantitative PCR analysis showed that these genes were significantly induced by drought in the Xinxuan048 cassava cultivar,and the overall expression levels increased with the increase of drought stress and were higher than KU50.(8)The full-length sequences of GSTU7 and SIRD genes from cassava were1106 and 958 bp,respectively.The length of coding region in GSTU7 gene is669 bp,encoding 222 amino acids.The length of coding region in SIRD gene is819 bp,encoding 272 amino acids.Both proteins mainly consist of spiral and crimp elements.Gene GSTU7 showed the closest relationship with Hevea brasiliensis,while gene SIRD showed the closest relationship with Ricinus communis.In addition,6 and 18 phosphorylation sites were predicted for genes GSTU7 and SIRD,respectively.(9)Both genes GSUT7 and SIRD showed weak preference to codon,and the selection of bases tends to A or T,and the codon prefers to end with A or T.Yeast is more suitable as a heterologous receptor for the expression of GSTU7 and SIRD genes,while model plant tobacco is more suitable as a receptor for the functional verification of genetic transformation of these two genes.In this study,the response to drought stress in Xinxuan048 and KU50 cassava cultivars was revealed in respect of antioxidant system and endogenous hormone level.The drought-resistant genes were screened.These studies provide a theoretical basis for studying the drought-resistant mechanism of cassava and cultivating varieties with drought-resistant and high-yield traits.