Study On The Growth Promoting Effect Of The Root Endophytes And Its Colonization Ability

Microorganisms can also biosynthesize plant auxin indole acetic acid?IAA?.Studies have shown that many bacteria promote plant growth by synthesizing IAA,to affect plant development,health and yield.Studying the mechanisms about the endophytic bacteria how to promote plant growth and colonizate in plant hosts will lay a solid scientific and theoretical foundation to understand the plant-bacteria interaction,improve the promotion effect of microbial fertilizers in agricultural production practice,and excavate new growth-promoting bacterial resources for a establishing of a new type of sustainable,modern ecological agricultural production system.On the basis of previous work,a total of 208 isolated bacterial strains were purified and identified.The 208 tomato endophytic strains belonged to 3 phylums,7 orders,11 genus and 25 species.Among them,Bacillus,Pseudomonas and Enterobacteriales from phyla of Firmicutes and Proteobacteria were the dominant bacterial species.In this study,wheat was used as a model plant to screen the plant growth promoting bacteria from the208 tomato endophytic strains.A total of 55 bacterial strains with the promoting effect on wheat germination?26.45%?were identified.Among these,mr₈9,cz23,cz30,cz34,cz35,cz36,cz67,B50,B51,B58,B60,B68,B70,B72 and B75 showed a significant promoting effect.In addition,20 bacterial strains,and 3 bacterial strains also demonstrated ability to promote the elongation of plant roots and the growth of plant whole plant height?from root to tip?,respectively.Combined with the previous molecular identification of the endophytes,most of the plant growth-promoting bacteria were Bacillus and Enterobacter.The result indicated that there was a broad and diverse range of plant growth-promoting bacterial resources in plant root microbiome.Further,the 47 strains were also demonstrated their plant-promoting roles effects on plant root development,plant growth and total plant weight in the pot experiment.To reveal the mechanism of the endophytes to promote plant growth and its ability to colonizate in roots,the production of IAA,and cellulase activity and xylanase activity of the isolated 208 bacterial strains were determined.The results showed that among the tested 208 endophytic bacteria,136 bacterial strains demonstrated their ability ro produceIAA in the medium containing tryptophan.Most of the identified plant hrowth-promoting endophytes also secreted auxin IAA,indicating that plant endophytes promote plant growth mainly through the production of IAA.In the detection of enzyme activity,177 endophytic bacteria,accounting for 85.1% of the tested bacteria,showed had obvious cellulose hydrolysis activities;163 strains,accounting for 78.37%,showed obvious xylan hydrolysis activities.The result indicates that most of the tested endophytes demonstrated a obvious lignocellulose-biodegrading ability.Based on the screened plant growth-promoting bacteria,the ability of endophytes to colonizate and built their population in plant tissue was studied by high-throutht sequencing.Microbial community composition was analyzed on the root microbiome of single-infected plants to judge whether the tested endophytes can be successfully colonized in plants to become a dominant population in the plant root.The results showed that among the tested 55 bacterial strains,there were 10 strains,Bac₉8,Ent₁30,Bac₆1,Bac₁17,Ent₉5,Bac₇9,Bac₇1,Bac₁33,Bac₉2,Ent₁89,mainly of Enterobacter and Bacillus bacteria successful colonizated and became the most dominant bacterial group in the cooresponding plant root microbiome,respectively.14 strains of the tested bacteria,Ste₂6,Ser₉9,Rhi₃3,Ent₁14,Pan₁20,Ent₂,Ent₃9,Ent₄,Ent₁29,Lys₁59,Ent₁5,Ent₁7,Bac₆4,Ach₄5,etc.,were detected in the plant root microbiome with a low abundance.The results showed that even for the low-abundance bacteria in plant microbiome,such as Enterobacter and Bacillus,could infect and colonizate in plant tissue.The studies have shown that cellulase play an important role in infection and colonization of the plant endophyte in plant.Enzyme screening also indicated that most of the endophytes tested had cellulase and xylanase activity.Total RNA was extracted for transcriptomic sequencing from 4 bacterial strains,which showed the ability to successfully colonize to become a dominant population in plant host in previous infect experiments.After transcriptome sequencing and KEGG metabolic pathway analysis,the expression of genes for cellulase??/?-glucosidase?and pectin esterase associated with degradation of plant cell walls was detected in the starch and sucrose metabolic pathways in all four bacterial strains.And also,expression of genes associated with thetryptophan-dependent IAA biosynthetic pathway was also detected in Bac₇3 and Ent₁89.The transcriptomic analysis of the plant-endophyte on colonization and growth promoting mechanism laid a theoretical foundation for us in understanding of the infection mechanism and the research of plant benefiting bacteria.