Mitigation Effect And Mechanism Of Plant Growth Promoting Bacteria Bacillus Mojavensis On Salt Stress In Oats

Oat（Avena sativa）is an important grain and feed crop with a huge demand.Among the imported forages in China,it is second only to alfalfa.Improving the salt tolerance of oats and increasing the planting area of oats on saline land can not only increase forage yield,but also promote the development and utilization of saline land（an important reserve land resource in China）.Salt stress is one of the main abiotic factors limiting plant growth.Inoculating PGPB（Plant growth promoting bacteria）to improve plant salt tolerance is a feasible and promising method.The growth promoting effect and adaptability of strains in different habitats vary,so it is necessary to screen effective salt tolerant PGPB resources for saline soil.The existing salt tolerant crops have solved the growth problem in saline environments,but the yield is relatively low and the bottleneck of increasing yield needs to be overcome.This study used the medium salt tolerant oat variety“Baiyan 7”as the experimental material to inoculate the growth promoting bacteria LrM1 and LrM2 isolated from the rhizosphere of Lycium barbarum in saline alkali land,and the growth promoting bacteria Acinetobacter calcoaceticus P19 in the rhizosphere of Trifolium pratense L.in non-saline alkali land into the oat root system.The most significant growth promoting strains were screened based on oat morphological indicators,and their effects on salt stress oat growth and physiology were studied,And non targeted metabolomics was used to analyze the changes in differential metabolites of the strain before and after NaCl treatment,exploring the possible mechanism of promoting growth bacteria to improve salt tolerance of oat seedlings from the perspectives of plant morphology,physiological level,and strain metabolism level.The main research findings are as follows:（1）The growth promoting bacteria LrM1 and LrM2 were identified as Paenibacillus peoriae and Bacillus mojavensis,respectively;LrM1 can tolerate a NaCl concentration of9%,while LrM2 and P19 can tolerate a NaCl concentration of 11%;100 mmol/L NaCl treatment significantly reduced the ACC deaminase activity and IAA secretion of LrM1,increased the ACC deaminase of LrM2,decreased IAA secretion,decreased P19phosphorus solubility and IAA secretion,and increased nitrogenase activity.（2）Under 100 mmol/L NaCl stress,inoculation with various strains can promote the increase of oat root morphology and biomass,with LrM2 being the most significant,followed by P19,and LrM1 being the weakest;Under 100 mmol/L NaCl stress,after inoculation with LrM2,the root length,root surface area,root volume,root tip number,and branch number of oats increased by 36.4%,32.2%,27.9%,25.6%,and 18.1%,respectively,while plant height,fresh leaf weight,leaf dry weight,and root dry weight increased by 9.0%,39.4%,41.0%,and 47.6%,respectively.At the same time,root vitality significantly increased;Each strain lost its promoting effect on oat growth under 150 mmol/L NaCl stress.（3）Under salt stress,inoculation with LrM2 significantly increased soluble sugar content,decreased proline content,and regulated osmotic stress;Inhibit the absorption of Na⁺by roots,promote K⁺absorption,and thereby reduce the amount of Na⁺transported to leaves;Reducing ion toxicity;It increased the activity of antioxidant enzymes（CAT,APX,GR,MDHAR）and the content of antioxidants（As A,GSH,As A/DHA）,reduced the accumulation of active oxygen species(O2^·-,H2O2),and significantly decreased the content of malondialdehyde（MDA）,alleviating oxidative stress;It increased the activities of glyoxase I（Gly I）and glyoxase II（Gly II）in leaves,decreased the content of methylglyoxal（MG）,and alleviated carbonyl stress;Increased chlorophyll a（Chl a）and carotenoid（Car）content in leaves,increased stomatal conductance（Gs）,transpiration rate（Tr）,and net photosynthetic rate（Pn）,decreased quantum yield of regulatory energy dissipation(Y（NPQ）,increased electron transfer rate（ETR）,maximum photochemical efficiency of PSII（F_v/F_m）,photochemical quenching coefficient（q P）,and actual photochemical efficiency Y（II）Relative water content（RWC）and inherent water use efficiency(WUE_intr)enhance photosynthesis.（4）Before and after 100 mmol/L NaCl treatment,there were 89 differentially expressed metabolites in LrM2,24 down-regulated and 65 up-regulated in positive ion mode,and 50 down-regulated and 20 up-regulated in negative ion mode.The most significantly upregulated metabolites are amino acids and their derivatives,as well as a small number of organic acids and lipids.These metabolites are mainly involved in osmoregulation of oats,alleviating ion toxicity and oxidative stress,and significantly downregulated C.I.Basic Orange 2,nicotine,and dihydrocoumarin,which are harmful to plants,microorganisms,and the environment,Upregulation of beneficial metabolites and downregulation of harmful metabolites may be one of the main reasons for promoting plant growth under salt stress;Some metabolites that can improve salt tolerance of plants are down-regulated,such as D-glucosamine,ADP-ribose,trehalose 6-phosphate,and syringic acid,leading to an up-regulation of 3-phenylpropionic acid,which can inhibit continuous cropping and reduce the number of beneficial microorganisms in the soil,Downregulation of some beneficial metabolites and upregulation of harmful metabolites of the strain may be one of the reasons for the reduced growth promoting effect of LrM2 under salt stress compared to salt free stress.（5）In the positive ion mode,12 differential metabolites were annotated into 18metabolic pathways,while in the negative ion mode,15 differential metabolites were annotated into 22 metabolic pathways.Treatment with 100 mmol/L NaCl significantly affected the degradation of aromatic compounds and the pentose phosphate phosphate pathway Several metabolic pathways such as biosynthesis of antibiotics,D-alanine metabolism,amino sugar and nucleotide sugar metabolism,phosphotransferase system（PTS）,and biosynthesis of unsaturated fatty acids have been implicated.（6）Correlation cluster marker thermograph analysis showed that amino acids and their derivatives,arachidonic acid,docosahexaenoic acid,tyrosol,hydroquinone,and 4-methylcatechol were significantly positively correlated with APX,GSH/GSSG,leaf K⁺content,Chl a,Y（II）,water use efficiency,root activity,fresh leaf weight,leaf dry weight,and root dry weight,and were significantly correlated with leaf Na⁺,root Na⁺,proline content,H₂O₂ content,root Ca²⁺The significant negative correlation of Ca²⁺content in leaves further indicates that the metabolites of the strain may be used to alleviate the effects of salt stress on oat growth by regulating osmotic stress,reducing ion toxicity,alleviating oxidative stress,and enhancing photosynthesis.