Research And Application Of Salt Tolerance Of Homologous Octoploid Switchgrass

Switchgrass（Panicum virgatum L.）is a perennial herbaceous C₄ plant.It is considered to be a good candidate for remediation of salinized soil and still has wide adaptability on saline-alkali marginal soil.Studies have shown that polyploid plants often show stronger resistance,and ploidy breeding can be used as an important means to cultivate more resistant switchgrass.In this study,we used colchicine to double tetraploid switchgrass materials to create a stable genetic octoploid switchgrass.However,the salt tolerance and salt tolerance mechanism of homologous octoploid switchgrass are still unclear.How to grow on saline-alkali land is still unclear.Therefore,in-depth exploration of the salt tolerance,physiological and biochemical,and molecular mechanisms of octoploid switchgrass can enrich the study of artificial induction of homologous polyploid salt tolerance,and at the same time demonstrate planting in coastal saline-alkali land,providing strong support for the application of homologous polyploid switchgrass under natural saline-alkali conditions,which has important theoretical and practical significance.In this study,octoploid and tetraploid switchgrass of different ecotypes（highland and lowland）were used as experimental materials to study the response to salt stress and salt tolerance,and the salt tolerance mechanism was discussed from physiological and molecular aspects,and then the growth adaptability of homologous polyploid switchgrass in coastal saline-alkali land was analyzed.The results showed that octoploid switchgrass could enhance salt tolerance through physiological and molecular response mechanisms under salt stress.The results provided a theoretical basis for the improvement of homologous octoploid switchgrass in saline-alkali land and the restoration of vegetation in saline-alkali land.The main conclusions of this study are as follows:（1）Salt treatment had a significant inhibitory effect on plant height,stem diameter,leaf length,leaf width and biomass of two ecotypes of switchgrass with different ploidy.The octoploid is more tolerant than tetraploid.The highland Ranlow diploid plants were significantly different at low salt concentration,while the lowland Alamo was significantly different at medium and high salt concentration.The salt tolerance index of biomass can be used as an index for salt tolerance identification of switchgrass seedlings.The salt tolerance thresholds of Ranlow and Alamo tetraploids and octoploids were 139.78 mmol/L,143.42mmol/L,146.66 mmol/L and 147.38 mmol/L,respectively.Alamo octoploid switchgrass has the strongest salt tolerance.（2）Under 150 mmol/L NaCl stress,compared with the octoploid Alamo,the tetraploid leaves were significantly yellowing and wilting,and the degree of salt toxicity was greater.There are two physiological reasons for the enhanced salt tolerance of octoploid switchgrass:on the one hand,it can better cope with oxidative stress damage by enhancing osmotic regulation ability and antioxidant enzyme activity;on the other hand,it can accumulate more K⁺and increase K⁺/Na⁺ratio.（3）Transcriptome analysis showed that there were 50 differentially expressed genes（DGEs）between octoploid and tetraploid Alamo under salt treatment,of which 31 were up-regulated and 19 were down-regulated.Based on GO enrichment analysis,it was found that DGEs were related to oxidoreductase.KEGG enrichment analysis showed that the up-regulated and down-regulated genes in response to salt stress were significantly enriched in the biosynthesis of secondary metabolites,metabolic pathways,plant MAPK signaling pathways,phenylpropanoid biosynthesis and plant hormone signaling pathways.In addition,the up-regulated genes were also related to arginine and proline metabolism,starch and sucrose metabolism.The analysis of plant hormone signal transduction pathway showed that octoploid activated auxin AUX1/IAA and GH3,abscisic acid PP2C,Sn RK2,ABF,cytokinin A-ARR and ethylene EIN3,which made octoploid switchgrass better adapt to salt stress.WGCNA analysis identified three modules with high correlation,and a total of 17 hub genes were screened.These genes were mainly enriched in metabolic pathways such as phenylpropanoid biosynthesis,plant hormone signal transduction,arginine and proline.（4）Under natural coastal saline-alkali conditions,the tiller number,leaf length and biomass of tetraploid Ranlow decreased faster than that of octoploid,and tetraploid was more damaged by salt stress.The stem diameter and inflorescence length of octoploid and tetraploid in saline-alkali land increased,but there was no significant difference in plant height and leaf width.At the physiological and biochemical level,the MDA content of octoploid switchgrass was lower than that of tetraploid in moderate saline-alkali land,the activities of SOD and CAT were significantly higher than those of tetraploid,and the contents of chlorophyll and soluble sugar were significantly different from those of tetraploid.The octoploid Ranlow has a strong growth advantage in moderate saline-alkali soil.