Regulatory Mechanism Of Root Secretion In Puccinellia Tenuiflora Under Alkali Stress

Alkaline soil is widely present and has become an important factor limiting the sustainable development of the world’s agriculture and ecological environment.For example,in Northeast China,alkalized grassland has reached 70%.Planting and enclosing highly alkali-resistant halophytic forage grass has become an important approach to manage alkalized land.Puccinellia tenuiflora is an extremely alkali-tolerant halophyte with a well-developed root system capable of completing life at p H 10-11,and is widely used in biological management of alkalized land in Northeast China.Therefore,studying the alkali tolerance mechanism of P.tenuiflora has vital theoretical value for the restoration and utilization of alkalized land in Northeast China.Previous studies have proved that root secretion plays a crucial role in plant alkali resistance,but little is known about the root secretion process and its regulatory mechanism under alkali stress.In this paper,P.tenuiflora were treated with salt stress（96 mmol/L Na Cl）and alkali stress（16 mmol/L Na₂CO₃,64mmol/L Na HCO₃）at 60 days of seedling age for 3 days.To reveal the regulatory mechanism of root secretion under alkali stress,liquid chromatography-mass spectrometry（LC-MS）was used to analyze the composition and content of root exudates and accumulated metabolites in roots,and the root transcriptome was analyzed.The main findings and conclusions are as follows:1.Alkali stress induces the root system of P.tenuiflora to secrete a large number of phenolic acids and fatty acids.A total of 622 root exudates were detected,including 148phenolic acids,92 fatty acids,37 organic acids,78 flavonoids,42 nucleotides and their derivatives,29 lignans/coumarins,40 alkaloids,27 carbohydrates,28 amino acids and their derivatives,14 vitamins,1 tannin,10 terpenes,53 lipids and 23 other metabolites.There were60 root secretions unique to the alkali stress condition,including 20 phenolic acids,12 fatty acids,5 organic acids,1 amino acid derivative,2 flavonoids,2 alkaloids,3 carbohydrates,3nucleotides and their derivatives,6 lignans/coumarins,1 terpenoid,1 lipid and 4 other metabolites,a total of 32 phenolic acids and fatty acids,accounting for more than half of the number of unique root exudates under alkali stress treatment.In addition,this paper focuses on the root secretion AISM induced by alkali stress（AISM:the relative content of metabolites under alkali stress is higher than that in control and salt stress conditions）,and a total of 166AISMs,including 56 phenolic acids and 43 fatty acids,were detected in the root exudates of P.tenuiflora.The above results show that alkali stress induces the secretion of a large number of phenolic acids and fatty acids in the root system of P.tenuiflora,and the secretion of these metabolites may play an important role in the regulation of rhizosphere p H of P.tenuiflora.2.Alkali stress induces the accumulation of large number of phenolic acids and fatty acids in the P.tenuiflora root system.A total of 933 metabolites were detected in P.tenuiflora root tissue,including 171 phenolic acids and 95 fatty acids,of which 165 phenolic acids and90 fatty acids were detected under alkali stress conditions.Similar to root secretion,this article also focuses on the metabolite AIAM accumulated in the root induced by alkali stress（AIAM:the relative content of metabolites accumulated in roots under alkali stress condition is higher than that in control and salt stress conditions）.A total of 58 AIAMs containing 23phenolic acids and 1 fatty acid were detected in the root tissue of P.tenuiflora,indicating that alkali stress induced the accumulation of large number of phenolic acids and fatty acids in the root system of P.tenuiflora.Phenolic acids and fatty acids induced by alkali stress can be used not only for osmoregulation,but also to support root secretion.3.Alkali stress in P.tenuiflora promotes root secretion by enhancing respiration.Transcriptome analysis showed that the expression levels of key genes of 19 glycolytic pathways in P.tenuiflora root were upregulated under alkali stress treatment,and the gene of ATP-dependent 6-phosphate fructokinase and glyceraldehyde-3-phosphate dehydrogenase encoding the rate-limiting enzyme of the glycolysis pathway were upregulated by 47-fold and56-fold,respectively,indicating that alkali stress enhanced the respiration of P.tenuiflora.In addition,under alkali stress,the expression of rate-limiting enzyme phenylalanine ammonia-lyase gene and cinnamic acid 4-monooxygenase gene synthesized by phenolic compounds in P.tenuiflora root were upregulated,thereby promoting the accumulation of phenolic acids in P.tenuiflora root.In addition,in this paper,the expression of key genes in the process of fatty acid synthesis was also upregulated,thereby promoting the accumulation of fatty acids in the root of P.tenuiflora.The above results showed that under alkali stress,P.tenuiflora promotes root secretion and regulated rhizosphere p H by strengthening root respiration and increasing the expression level of genes related to phenolic acid and fatty acid synthesis.In conclusion,the root system of P.tenuiflora promotes rhizosphere p H regulation by secreting a large number of phenolic acids and fatty acids under alkali stress,which may be an important physiological reason for its survival in high alkaline environment.Increasing the accumulation of phenolic acids and fatty acids,enhancing respiration and the expression of genes related to the synthesis of fatty acids and phenolic acid substances in the roots can promote the root secretion process of P.tenuiflora under alkali stress.This paper reveals the regulation mechanism of P.tenuiflora root secretion under alkali stress,which can not only enrich the theories of plant alkali tolerance,but also provide new ideas for the restoration and utilization of alkalized land.