Effect Of Plant Growth-Promoting Rhizobacteria And Arbuscular Mycorrhizal Fungi On Growth And Development Of Trifoliate Orange

China is a significant citrus-producing country,the development of citrus sector is extremely important,in which seedling quality plays a critical role.At present,Poncirus trifoliata(L.)Raf is the most extensively used citrus rootstock variety,and it is well-liked for its advantages of cold resistance,early fruiting,and disease resistance.In China’s citrus producing areas in the middle and upper reaches of the Yangtze River,such as Chongqing,it is mainly in medium to slightly alkaline soil.Slow growth in the early stages,alkali resistant soil,and iron deficiency yellowing are common concerns in citrus seedlings producing trifoliate rootstocks,all of which have a negative impact on plant growth and development,as well as citrus yield and quality.As a result,maximizing benefits of trifoliate orange rootstocks while minimizing negative impacts is critical for citrus industry’s high-quality and efficient growth.Arbuscular mycorrhizal fungi(AMF)are vital microorganisms that can create a reciprocal symbiosis with plant roots,allowing them to infect trifoliate orange roots,boost plant stress resistance,and exchange mineral nutrition.Plant growth-promoting rhizobacteria(PGPR)is a type of beneficial soil fungus that can help plants grow and develop by improving soil environment.PGPR is a type of biological fertilizer with a wide range of applications.Many studies have shown that there is an interaction between AMF and PGPR,which can improve AMF inoculation and form a good ecological cycle with rhizosphere soil.However,there have been few studies on trifoliate orange,and mechanism of their interaction as well as combination of dominant strains is unclear.Therefore,it is necessary to conduct in-depth research and discussion.Arbuscular mycorrhizal fungi(AMF)and plant growth-promoting rhizobacteria(PGPR)are two soil microorganisms that have received a lot of attention,they have a great relationship.The inoculation impact of AMF is considerably enhanced by PGPR.There are few studies on their symbiotic relationship,and mechanism of their interaction and combination of dominant strains is unknown.As a result,much research and discussion are required.The test plant was the trifoliate orange,and two dominant AMF strains with high symbiotic effect on trifoliate orange were chosen: Glomus mosseae(Fm)and Glomus etunicatum(Ge).Bacillus subtilis(Bs)and Bacillus mucilaginosus(Bm)were used as test agents,in addition to two typical PGPR strains.Inoculation with AMF alone,PGPR alone,and AMF plus PGPR at the same time were the three therapies considered.There were a total of nine treatments,each of which was repeated three times.Trifoliate orange seedlings were evaluated in terms of growth and development,mineral nutrition,rhizosphere soil environment,and symbiotic associated gene expression.Here are the key findings:1.Both Fm and Ge have potential to infect trifoliate Orange seedling roots.The mycorrhizal infection rate was dramatically raised after PGPR inoculation,and mycorrhizal infection rates of double inoculation treatments increased by 13.89%,8.06%,11.57%,and 14.57%,respectively.At the same time,the dependence of seedlings on mycorrhizal fungi was higher under double inoculation conditions.2.Inoculation of AMF and PGPR at the same time could significantly promote root development and increase plant biomass.The effect of double inoculation of AMF on root volume,number of first-order new roots,longest lateral root length,root activity,plant height,stem diameter,root-shoot ratio,and leaf photosynthetic capacity of trifoliate orange seedlings was better than that of single inoculation of AMF.The overall effect of Fm and Bs immunization at the same time is superior.In terms of mineral nutrition,single inoculation raised element content in trifoliate orange seedlings to a degree,while double inoculation had a greater effect,and there were variances across strains.3.Single inoculation and double inoculation improved trifoliate orange physiological metabolism and stress response ability.Free proline,soluble sugar,and soluble protein accumulation in leaves and roots of trifoliate orange seedlings increased,malondialdehyde and hydrogen peroxide production was inhibited,and catalase(CAT),peroxidase(POD),superoxide dismutase(SOD),and ascorbic acid peroxidase(APX)activities were increased.When compared to single AMF or single PGPR inoculation,effect of double inoculation on antioxidant capacity and physiological metabolism was more pronounced,implying that AMF and PGPR promote each other.4.When compared to single AMF or single PGPR inoculation,contents of various elements in the rhizosphere soil of trifoliate orange seedlings in double inoculation treatment were significantly higher(especially available nutrients beneficial to plant absorption),activities of soil enzymes such as urease,phosphatase,and polyphenol oxidase were increased,and the content of glomalin-related soil protein(GRSP)was significantly higher.It reveals that AMF and PGPR can help trifoliate orange growth by improving soil nutrition and microclimate.5.In single AMF inoculation and double inoculation treatments,transcription of DMI1,DMI2,and DMI3 genes associated to symbiotic-related was dramatically elevated.Among them,expression of genes related to Fm was higher than Ge,expression of Fm Bs was better than other double inoculation treatments.The inoculation strains were connected to expression levels of different strains.The results showed that double inoculation treatments enhanced infection rate of mycelium and intensity of mycorrhizal root colonization on trifoliate orange seedlings,as well as proliferation of AMF mycelium.6.The concomitant inoculation of Fm and Bs exhibited the best growth promotion effect on trifoliate orange seedlings and the strongest antioxidant capacity of plants,according to growth and development,element regulation,and physiological metabolism indexes of trifoliate orange seedlings.Fm and Bs double inoculation is more favourable to improving rhizosphere soil environment of trifoliate orange seedlings,improving enzyme activity,and is of major importance to stabilize soil.Upregulation of expression levels of symbiotic-related genes DMI1,DMI2 and DMI3 revealed that Bs might better promote establishment of symbionts between Fm and P.trifoliata roots.Therefore,symbiotic combination of Glomus mosseae and Bacillus subtilis(Fm Bs)is a new technology of Citrus mycorrhizal seedling breeding.