Study On The Mechanism Of Bacillus Amyloliquefaciens SQR9-induced Systemic Resistance To Gibberella Stalk Rot Based On The Transcriptome

Gibberella stalk rot is one of the most devastating diseases of maize,yet there is no efficient method to completely control this disease.Recently,it has been shown that bio-control agents using beneficial microbes is becoming a powerful and eco-green approach to manage crop diseases.Bacillus amyloliquefaciens SQR9 is a bacterial strain isolated from the rhizosphere of cucumber.It has been proven to promote the growth of cucumber and tomato,and it is also a potential boil-control strain activating plant’s systemic resistance.However,there is no report about the role of SQR9 in maize resistance to Gibberella stalk rot（GSR）caused by Fusarium graminearum infection.To dissect whether and how SQR9 is involved in modulating the resistance of maize to stalk rot at genome-wide level,we performed a series of experiments,including microbiome analysis,transcriptomics,gene expression analysis,phenotypic assays,and histological staining.We found that SQR9 could induce maize systemic resistance to GSR by F.graminearum,and the related molecular mechanism was investigated.The main results are as follows:1.SQR9 reshaped the microbial community structure in rhizosphere soil during the colonization on maize.Moreover,the rhizosphere microbial community structure responded differentially to SQR9 and F.graminearum,resulting in the specific enrichment of different types of bacteria,respectively.2.Seedling assay showed that SQR9 could induce maize systemic resistance to GSR,via significantly reducing the disease severity of GSR on maize,with a reduction of disease incidence as much as by 28%.3.RNA-seq data showed that phenylpropanoid biosynthesis,flavonoid biosynthesis and plant-pathogen interaction pathways were enriched in the root upon colonization of SQR9,indicating the involvement of antibacterial compounds and pathogen responsive pathways in resistance triggered by SQR9 to GSR.SQR9 also triggered H₂O₂ production,callose deposition,and MAPK signaling pathway in leaves,which are typical hallmarks of PTI pathway.In addition,the activation of plant hormone signal transduction and the up-regulation of SA and JA signal pathway marker genes suggested that these phytohormone signaling pathways are likely involved in the ISR activation by SQR9.4.RNA-seq data also showed that several genes of calcium sensor protein family were up-regulated by SQR9 treatment.q RT-PCR analysis confirmed that calcium sensor genes were indeed activated,and the SQR9-activated ISR was weakened by the calcium signaling inhibitor La Cl₃,suggesting that the calcium signal pathway is involved in the ISR induced by SQR9.In summary,the beneficial microbe SQR9 could activate the maize ISR against GSR,likely through activation of a series of immune responsive pathways,such as calcium signaling pathways.The results will not only be helpful to understand the molecular mechanisms underlying the resistance triggered by beneficial microbes,but also provide gene resources and strategies to manage maize stalk rot via deoplying the beneficial microbes in the future.