Exploration Of Bacteria-root Interaction System And Functional Analysis Of Rhizosphere Bacteria MLR

The root microenvironment,which includes the roots,soil,microorganisms,and all organic and inorganic components,is referred to as the rhizosphere.The root system secretes a large amount of organic components such as sugars,amino acids,organic acids,and small peptides into the soil,and then recruits some microorganisms to colonize and grow in the rhizosphere,among which the rhizosphere bacteria are the main component.Plant growth-promoting rhizobacteria(PGPRs),as the name implies,are bacteria that promote plant growth or nutrient uptake through a variety of pathways,such as the metabolites of PGPRs,including bacteria volatile components(BVCs).The perennial problem of CO2 has been debated for decades:whether the CO2 produced by bacteria in confined systems is the main factor that promotes plant growth.Taking this as a breakthrough point,we first conducted in-depth optimization of the previous experimental system and carried out large-scale bacterial experiments to prevent the experimental results of a single strain from affecting the final conclusion.Preliminary results showed that most of the bacteria promoted plant growth in Paraflm(?) sealed medium(closed system),suggesting that this promotion may be a common feature of the most of bacteria.Later,rhizosphere bacteria S.marcescens 8C-3 and pathogenic bacteria Pst DC3000 were co-cultured with Arabidopsis seedlings and both promoted plant growth under the Paraflm(?) closed system.However,when we used the MTT(Microporous Pressures-Sensitive Tape,Aerable System)culture dish,the growth-promoting effect disappeared.In addition,we sealed the dishes with two different sealing materials(Paraflm(?) and MBT)in a bacteria-free culture.However,the Paraflm(?) system inhibited plant growth,while the MBT system did not.Therefore,we hypothesize that the inhibition of plant growth in a closed system is due to the obstruction of normal gas exchange.It is further speculated that the growth promotion effect of bacteria on plants in the closed system is because the CO2 produced by bacterial respiration complements the carbon source demand of plants due to photosynthesis.Therefore,the CO2 content in Paraflm(?) and MBT systems was determined by gas chromatography.The results showed that the CO2 content in Paraflm(?) system was significantly higher than that in MBT system.Moreover,in the Paraflm(?) system,the CO2 content in the inoculated group was significantly higher than that in the uninoculated group,which supported the hypothesis that CO2 was a direct growth-promoting factor.To further reinforce this conclusion,we applied the CO2 scavenger BA(OH)2 into culture system,and the results showed that the growth-promoting effect of bacteria was significantly reduced.Then we added some CO2 to the culture system,which achieved the same growth-promoting effect as bacterial co-culture.Together,we concluded that the main effect factor of bacterial growth promotion was CO2,but we could not rule out the existence of other effect factors.Since CO2 and other unknown volatile factors may interfere with the screening of "promote-growth bacteria",we used MBT system to screen for the true rhizosphere probiotics.In a large number of screening process,we found a strain can significantly promote the generation of lateral root(LR),and named it the MLR(More lateral root),because the increasing of the LR has important scientific research value,for example,the absorption of phosphorus(mainly distributed in the soil surface).Therefore,we take MLR as the research object and focus on exploring "the effect of MLR on LR development" and "how plant response to MLR".Interestingly,although MLR promoted LR formation,it significantly inhibited the growth of primary root and severely damaged the root tip structure.We deeply sequenced MLR,and the results showed that MLR was Paenibacillus xylanexedens.Therefore,it was speculated that the damage of MLR to root tip was due to the degradation of xylan in cell wall by the xylanase secreted by MLR.The destruction of the primary root disrupts the normal growth of the root system,forcing the plant to produce more LRs.Further experiments showed that the MLR-promoted LRs were intact and the LR tips were not damaged.In order to explore the reason of MLR-induced LR increase and the molecular mechanism of the increased disease resistance of LRs,we used auxin and salicylic acid(SA)related mutants to conduct inoculant experiments,respectively.The results showed that the increase of LR was not dependent on auxin signaling,but may be related to SA signaling.However,the enhancement of the tolerance of LR tips to MLR was dependent on the auxin signaling,so MLR may promote the increase of LRs through the SA signaling,and enhance the disease resistance of LRs through the auxin signaling.In addition,qRT-PCR results showed that MLR treatment could induce the expression of SA,JA,and PTI related genes,indicating that MLR could induce the immune response of plants.Studies have shown that there is a tug of war between plants and bacteria:bacteria produce xylanase to break down the cell wall of plants to colonize and obtain nutrients;at the same time,plants secrete xylanase inhibitors to inhibit the destruction of cell walls by bacteria.However,bacteria produce xylanase analogues that interact with xylanase inhibitors to competitively protect xylanase activity.Next,we will focus on exploring the game mechanism between plants and bacteria and the molecular mechanism behind it.