The Mechanism Of Arbuscular Mycorrhizal Fungi Enhancing Saline-alkali Tolerance Of Populus × Xiaohei

Soil salinization is increasing year by year.Plants growing in saline-alkali soil are subjected to ionic,osmotic and p H stresses,resulting in nutrient imbalance,organelle damage,photosynthesis and respiration are inhibited,affecting the normal growth of plants.Arbuscular mycorrhiza（AM）fungi can form mycorrhiza symbiotically with Populus×xiaohei and enhance its saline-alkali tolerance,but the mechanism of action is still unclear.In this study,the AM fungus Funneliformis mosseae was inoculated with Populus×xiaohei under saline-alkali stress to explore the effects of AM fungi on the growth status,photosynthesis,ion distribution,osmotic adjustment and antioxidant capacity of Populus×xiaohei,analysis of gene expression patterns in mycorrhizal Populus×xiaohei,and study the regulation and expression of AM fungi on the Na⁺/H⁺antiporter gene in Populus×xiaohei.Thus,the mechanism of AM fungi enhancing Populus×xiaohei saline-alkali tolerance from the physiological,biochemical and molecular levels was revealed.The main conclusions were as follows:1.Effects of AM fungi on the growth and physiology of Populus×xiaohei under saline-alkali stressAM fungi promoted the rapid recovery of Populus×xiaohei after long-time（8 weeks）saline-alkali stress（p H 9.5,100 m M Na⁺）.After being inoculated with AM fungi,the p H of Populus×xiaohei rhizosphere soil decreased by 2.93%,the Na⁺concentration in the aboveground and underground parts increased by 444.92%and 106.28%,which reduced the Na⁺content in the rhizosphere soil;promoted the absorption of water,K⁺and Ca²⁺in Populus×xiaohei,and the tissue water content（TWC）and relative water content of the aboveground part increased by 23.74%and 11.70%,the concentration of K⁺in the aboveground and underground parts increased by 63.14%and 8.53%,and the concentration of Ca²⁺increased by 62.31%and 0.93%,which was conducive to alleviating physiological drought and maintaining Na⁺homeostasis;significantly enhanced the photosynthesis of Populus×xiaohei,the net photosynthetic rate,stomatal conductance and transpiration rate increased by 184.76%,54.61%and 72.43%,the maximum photosystem II quantum yield,photosystem II actual photochemical quantum efficiency and photosynthetic electron transfer rate increased by3.10%,11.08%and 10.64%;promoted the growth of seedlings,and increased the plant height,fresh weight of aboveground and underground parts by 300.51%,368.74%and 5.31%.These results indicated that under saline-alkali stress,AM fungi could promote the growth of Populus×xiaohei and enhance its saline-alkali tolerance by reducing soil p H,regulating ion and water absorption,and improving photosynthesis.2.Effects of AM fungi on Populus×xiaohei physiology under short-time saline-alkali stressAfter the formation of mycorrhiza,the ability of Populus×xiaohei to cope with short-time（0 d～7 d）saline-alkali stress（p H 9.5,100 m M Na+）was enhanced,which delayed the impact of saline-alkali stress on Populus×xiaohei.Saline-alkali stress reduced the concentrations of TWC,K⁺and Ca²⁺in non-mycorrhized Populus×xiaohei and inhibited their life activities;at the same time,the concentration of Na⁺,content of proline（Pro）,content of malondialdehyde（MDA）,activities of peroxidase and superoxide dismutase（SOD）in Populus×xiaohei were increased with the prolongation of stress time.Inoculation with AM fungi could protect Populus×xiaohei from saline-alkali stress at the initial stage of stress.Compared with non-mycorrhizal poplar,AM fungi increased the TWC,Pro content,SOD activity and Na⁺,K⁺and Ca²⁺concentration of the aboveground part of Populus×xiaohei from 0 d to 7 d,and decreased the rhizosphere soil p H and MDA content.These results indicated that mycorrhizal Populus×xiaohei were in the stage of osmotic stress,while non-mycorrhizal Populus×xiaohei were in the stage of transition from osmotic stress to oxidative stress during the period of 0 d to 7 d.AM fungi played a buffering role in the early stage of stress,increased the Na⁺capacity of Populus×xiaohei,promoted the synthesis and accumulation of resistant substances,and endowed root protection,thereby improving the adaptability of plants to saline-alkali stress.3.Transcriptome analysis of mycorrhized poplar under saline-alkali stressThe AM fungi affected the expression of ion transport pathway genes in Populus×xiaohei.AM fungi induced ammonium transporter gene 1（Px AMT1）,ABC transporter C family member gene 15 and G family member gene 1（Px ABCC15 and Px ABCG1）in the aboveground part of Populus×xiaohei and sodium potassium root deficient protein gene 2（Px NAKR2）,Fe²⁺transporter gene 1（Px IRT1）,natural resistance-associated macrophage protein gene 3（Px Nramps3）and 5（Px Nramps5）,ABC transporter C family members gene 10（Px ABCC10）and 15（Px ABCC15）,heavy metal-associated isoprenylated plant protein gene7,9 and 26（Px HIPP7,Px HIPP9 and Px HIPP26）,sulfate transporter gene 3.1（Px Sultr3;1）and zinc transporter gene 1（Px ZNT1）up-regulated expression,and down-regulated expression of multidrug and toxic compound extrusion gene 5（Px MATE5）,promoted poplar’s absorption and distribution of Na⁺,K⁺,Fe²⁺,Zn²⁺,sulfate and ammonium salt ions,and maintain intracellular Na⁺homeostasis.At the same time,it induced down-regulation of plasma membrane intrinsic protein gene 2-8（Px PIP2-8）expression in the aboveground part and up-regulation expression of Px PIP2-7 in the underground part,and regulates water absorption and distribution of Populus×xiaohei.In addition,AM fungi affected the distribution and secretion of malic acid in Populus×xiaohei by up-regulating the expression of aluminum-activated malate transporter gene 10（Px ALMT10）in the aboveground part and aluminum-activated malate transporters gene 7（Px ALMT7）and 14（Px ALMT14）in the underground part,maintained the acid-base balance of poplar cells and rhizosphere soil.The results showed that AM fungi enhanced the tolerance of Populus×xiaohei to saline-alkali stress by regulating the expression of genes involved in ion transport,water transport,and organic acid transport.4.Effect of AM fungi on the expression of Populus×xiaohei Na⁺/H⁺antiporter gene under saline-alkali stressAM fungi changed the expression pattern of Na⁺/H⁺antiporter gene（NHX）in Populus×xiaohei under saline-alkali stress.During the period of 0 d to 7 d under saline-alkali stress,AM fungi induced the expression of Px NHX2-1,Px NHX2-2,Px NHX2-3,Px NHX2-4,Px NHX2-5,Px NHX6,Px NHX7-1 and Px NHX7-2 in the aboveground part and Px NHX2-1,Px NHX2-2,Px NHX2-3.Px NHX2-5,Px NHX6 and Px NHX7-2 in the underground part continued to increase to the occurrence of saline-alkali stress.Saline-alkali stress for 8 weeks,AM fungi maintained the induction of Px NHX2-1,Px NHX2-2,Px NHX2-4,Px NHX6,Px NHX7-1 and Px NHX7-2 in the aboveground part of Populus×xiaohei,which expression levels increased by 27.34%,182.95%,57.24%,32.00%,20.53%and 143.56%respectively;and the induction of Px NHX2-1,Px NHX2-2,Px NHX2-4,Px NHX2-5,Px NHX6,Px NHX7-1and Px NHX7-2 in the underground part,which expression levels increased by 257.89%,1268.16%,379.65%,963.93%,298.09%,648.84%and 1996.46%,respectively.The sustained expression of these genes induced by AM fungi plays an important role in maintaining Na+homeostasis in Populus×xiaohei.The continuous expression of these genes directly/indirectly affected the fresh weight,plant height,rhizosphere soil p H and photosynthesis of Populus×xiaohei,thus enhancing the saline-alkali tolerance.