| The colonization of plant growth promoting rhizobacteria（PGPR）along plant roots to promote plant growth through various strategies.PGPR is mainly isolated from the rhizosphere of plants such as grains and cash crops in China.The exploitation of PGPR in extreme habitats will be beneficial to enrich the resources of rhizosphere growth-promoting bacteria,while the resources of xerophytic plants in desert areas of northwest China need to be explored.Haloxylon ammdendron is one of the important sand-fixing trees in the desert areas of northwest China.It has a well-developed root system,drought resistance and salt tolerance,and its rhizosphere may contain rich microbial resources.In our previous work,we analyzed the diversity of rhizosphere soil bacteria which grow with different enclosure years in the Minqin of the Gansu province and Alxa youqi of the Inner Mongolia,and identified multiple PGPR strains,among them,a bacterium strain numbered as WM13-24 could be attributed to Bacillus subtilis by whole-genome sequencing.Further studies showed that B.subtilis WM13-24 could promote the growth of ryegrass,alfalfa and other plants.However,the molecular mechanism underlying WM13-24-stimulated plant growth is still unknown.Therefore,in this study,a series of Arabidopsis mutants defective in specific plant hormone signaling pathways were used as materials to preliminarily study the effect of volatile compounds（VCs）released by strain WM13-24 on plant root development through genetic and pharmacological methods;the volatile compounds of strain WM13-24 were identified by solid-phase microextraction gas chromatography-mass spectrometry（SPME-GC-MS）and their effects on plant growth were analyzed;the biological process that strain WM13-24 might participate in the process of promoting plant growth was analysed by RNA-Seq.The main results are as follows:1.Strain WM13-24 had the activity of 1-aminocyclopropane-1-carboxylic acid deaminase and nitrogenase,and produced indolic compounds and exopolysaccharide.Both petri dish system,pot experiments showed that WM13-24 promoted the growth of Arabidopsis thaliana,especially the root development,and increased the accumulation of macroelements N,P and Mg and trace elements Fe and Zn.Inoculation of WM13-24 also promoted the growth of its host plant Haloxylon ammdendron and can colonize the root system.2.Volatile compounds released from strain WM13-24 could not complement the lateral root deletion phenotype of the auxin signaling-deficient mutant arf7arf19,indicating that WM13-24 promoted lateral root growth related to the auxin pathway;The volatiles compounds of WM13-24 enhanced the auxin response in DR5::GUS and DR5::GFP roots,increased the lateral root primordia,inhibited the expression of PIN4:GFP and induced the expression of PIN7:GFP,the results indicated that strain WM13-24 VCs stimulated lateral root formation by increasing auxin accumulation in root via enhancement of auxin biosynthesis and PIN4 or PIN7-mediated polar auxin transport.3.A total of 20 volatile compounds released by strain WM13-24 were identified,including 2 inorganic compounds,3 esters,5 alcohols and 10 other compounds.Among them,2,3-butanediol and benzyl alcohol were the main volatile compounds for the growth promotion of strain WM13-24,and either single or the mixture of both affected plant growth in a dose-dependent.4.RNA-seq analysis of Arabidopsis thaliana seedlings treated by strain WM13-24 revealed transcriptional changes in genes related to biological processes such as starch and sucrose metabolism enzymes,glucosinolate biosynthesis,nitrate uptake,plant hormones and defense,it is speculated that WM13-24 regulated plant growth through the above process.5.The inoculation with strain WM13-24 increased the contents of fructose,sucrose,glucose and starch in shoots and roots,and significantly induced the expression levels of SUC2,SUC4,SWEET11 and SWEET12 in shoots,SUC4 and SWEET11 in roots.In addition,strain WM13-24 enhanced the SPS enzymes activity in roots and significantly induced the expression levels of SPS1 and SPS2,suggesting that strain WM13-24 regulated the process of sugar synthesis and transport in plants.6.The inoculation with strain WM13-24 increased the S and cysteine content of plant seedling,GSH and total GSL content in roots.Under the condition of sulfur deficiency,strain WM13-24 significantly induced the expression levels of myrosinases genes BGLU28 and BGLU30 and the expression levels of sulfur deficiency-responsive genes CYP79F1,APR2,APR3,GGCT2.1,SULTR4;2,GTR1 and SDI1,and reduced the expression levels of GSLs synthesis-related genes MAM1,BCAT4 and CYP79F2,thus enhancing the tolerance of plants to sulfur deficiency.WM13-24 significantly induced the expression levels of immune-related genes PDF1.2A,PDF1.2B,PDF1.2C and PDF1.3,and increased the accumulation of glucosinolates and plant defense-related hormones jasmonic acid.7.The inoculation with strain WM13-24 significantly induced the expression levels of nitrate transporter genes NRT1.1,NRT2.1,NRT2.2 and NRT2.4 in roots under the treatment of different nitrate concentrations,and increased the nitrate nitrogen content in roots treated with 0.5 m M KNO3,indicating that WM13-24 plays a role in regulating nitrate uptake in plants.The above results showed that B.subtilis WM13-24 promoted both model plant Arabidopsis and the host plant Haloxylon ammodendron growth.Volatile compounds released from strain WM13-24 stimulated lateral root formation dependent on the auxin pathway.The inoculation with strain WM13-24 modulates plant growth through regulating changes in genes related to sugar transport,N and S uptake,phytohormones,defense and cell wall.This study provided a certain theoretical basis and practical experience for the in-depth study of the growth-promoting mechanism of PGPR,and lay a working foundation for the study of the functional characteristics of rhizosphere growth promoting bacteria of xerophytes in the desert area of northwest China. |
