Yue Liu,Lida Wang,Ying Lan,Qingchao Li,Xiumei Zhao,Ying Yang,Chuanyu Zhou

Qiqihar Branch,Heilongjiang Academy of Agricultural Sciences,Qiqihar 161006,China

Abstract In order to promote the development and application of environmental-friendly,efficient and safe beneficial Bacillus sp.preparations,the paper summarizes and systematically elaborates the colonization of Bacillus sp.in host plants and the mechanism of synergistic effect on disease prevention of host plants,further reviews the application of rhizospheric Bacillus sp.in promoting the growth of agricultural and forestry crops and controlling plant diseases,and prospects the scientific issues and application of plant rhizospheric Bacillus sp.in the future.

Keywords Rhizospheric Bacillus sp.;Growth promotion;Disease prevention;Colonization;Mechanism

In recent years,chemical fertilizers have been widely used to improve the yield of agricultural and forestry crops.However,unreasonable use of chemical fertilizers not only fails to achieve the desired effect,but also has negative impact on the ecological environment[1].Additionally,in the process of ensuring efficient production of agricultural and forestry crops,long-term uncontrolled use of chemical fertilizers and pesticides will lead to the loss of the balance of nutrients such as nitrogen and phosphorus in the soil,and further cause great harms to plant growth.Excessive application of chemical agents has seriously affected the sustainable development of agriculture and forestry in China[2],so it is urgent to study beneficial green preparations with good environmental compatibility and high efficiency.

Plant growth promoting rhizobacteria(PGPR)refers to a group of beneficial bacteria that inhabit inside or on the surface of plant root tissue and in rhizosphere soil,and can promote plant growth and control plant diseases[3].There are a variety of PGPR,among which Bacillus sp.has been widely used in agricultural production and afforestation because of strong tolerance and obvious growth promoting and disease preventing effect.Bacillus cereus[4],B.subtilis[5]and B.thuringiensis[6]are all important Bacillus sp.isolates in agriculture.B.velezensis FZB42 has also been commercially produced and utilized because it can effectively colonize the roots of plants,resist various plant pathogens and promote plant growth[7].In this paper,the application status of rhizospheric Bacillus sp.in promoting plant growth and controlling plant diseases,and the potential mechanism of interaction between Bacillus sp.and plants are reviewed,and the future research direction,development and utilization of rhizospheric Bacillus sp.are put forward.

1 Overview of Bacillus sp.

Bacillus sp.is a kind of gram-positive bacteria that can form endospores,which has the biological characteristics of fast proliferation and metabolism,large volume,strong tolerance and vitality.Bacillus sp.is rod-shaped,with thick cell wall,and the bacteria contain DNA,RNA and synthetic proteins.The spore produced by Bacillus sp.is a hypopus with multilayered structure,which has strong resistance to external adversity and other physical and chemical factors[8].The germination of the spores goes through several stages of differentiation and is affected by factors such as the age of the spores and the status of RNA[9].Liu et al.[10]found that the number of spores produced by B.cereus SQL0164 and their state changed with the incubation time.

Bacillus sp.can produce abundant metabolically active substances,and some Bacillus sp.are considered as ideal cell factories for producing enzymes and polysaccharides in food processing industry.Peng et al.[11]realized the efficient production of mannan with engineered strains of B.subtilis by further optimizing the synthetic pathway and dynamic regulation of competitive pathway.In addition,Bacillus sp.has strong decomposition ability of organic matter.Wani et al.[12]showed that under different concentrations of chromium,Bacillus sp.PSB10 significantly improved the growth of chickpea.Meantime,because of obvious spatial advantage,Bacillus sp.is widely distributed and easy to be separated from the soil and other environments,being a widely used microbial resource in industrial and agricultural production.

2 Mechanism of Bacillus sp.for Growth Promotion and Disease Prevention

2.1 Colonization of Bacillus sp. The colonization ability of Bacillus sp.on host plants is an important prerequisite for its growth promotion and biocontrol functions.Bacillus sp.has strong adaptability to the microenvironment around plant roots,and it can colonize the roots of plants more effectively by competing with other microorganisms[13-14].Fan et al.[15]showed that B.velezensis FZB42 could effectively colonize the roots of corn,tomato and other plants to promote its growth.Strain FZB42 effectively inhibits pathogenic bacteria in the plant rhizosphere by producing antibacterial substances,but when the colonization-related genes of strain FZB42 are mutated,its antibacterial ability will also be lost[16].Meantime,there are many factors affecting bacterial colonization,including chemotaxis of bacteria to plant root exudates,formation and motility of bacterial biofilms.

2.1.1 Chemotaxis.In nature,Bacillus sp.faces constantly changing environmental conditions that require rapid and continuous adaptation by bacteria to ensure optimal growth and proliferation.Chemotaxis of bacteria means that bacteria accumulate to high concentration of attractants or stay away from repellent molecules,so that organisms can reach ecological niche with high nutrient concentration and avoid toxins at the same time[17-18].Plant root exudates act as chem-oattractants to Bacillus sp.Allard et al.[19]showed that root exudates of Arabidopsis thaliana attracted B.subtilis in vitro,and chemotaxis receptors of B.subtilis played an important role in effective colonization of plant roots and interactions with plants.Malic acid isolated from tomato root exudates significantly induced the chemotactic reaction of B.amyloliquefaciens T-5,and promoted the effective colonization of T-5 strain in plant roots through directed motion mediated by chemotaxis[20].B.cereus YL6 showed strong positive chemotaxis to the root exudates of cabbage,and adding organic acids secreted by roots to soil could promote the colonization of YL6 in the root of cabbage[21].In addition, the complete chemotaxis mechanism of bacteria is essential for early colonization in plant roots.Liu et al.[22]showed that the chem-oattractant of B.velezensis SQR9 was involved in the early establishment of plant root population colonized by the strain;B.velezensis SQR9 induced d-galactose secretion from cucumber roots,and d-galac-tose,as a signaling molecule,induced more effective colonization of strain SQR9.In conclusion,the chemotaxis of Bacillus sp.to plant secretions is necessary for plant-microbe interaction.

2.1.2 Motility.The motility of Bacillus sp.is critical for its early colonization in plant roots.Bacteria can move under the chemotaxis of plant root exudates.The motility of bacteria can be divided into swimming,swarmming and twitching[23].Zhao et al.[24]suggested that the motilityrelated genes of B.amyloliquefaciens FZB42 are the key genes in plant root colonization.In addition,Bacillus sp.strains can move to find suitable ecological sites to colonize host plants more efficiently.The colonization of B.cereus YL6 in Chinese cabbage is a dynamic process in which it moves from root surface to root tissue and then to stems and leaves and produces growth hormone to promote plant growth more effectively[21].In summary,the motility of Bacillus sp.is one of the key factors affecting its colonization efficiency in host plants.

2.1.3 Formation of Bacillus sp.biofilm.Bacillus sp.can move to plant roots through chemotaxis and colonize early in plant roots,but the formation of Bacillus sp.biofilm in plant roots is a necessary condition for long-term colonization.Bacterial biofilms refer to organized bacterial aggregates wrapped by extracellular macromolecules of bacteria.The formation of bacterial biofilms provides good protection and enables them to effectively colonize the roots of host plants[25].Beauregard et al.[26]found that many genes of B.subtilis involved in biofilm formation also involved in effective colonization of A.thaliana roots.Other studies have shown that B.pumilus FAB10 could effectively colonize the roots of wheat by forming biofilm and help alleviate salt stress in wheat[27].Besides,the formation of Bacillus sp.biofilms is also important for the control of plant diseases.Chen et al.[28]isolated a B.subtilis strain resistant to tomato bacterial wilt from the natural environment,and found that the biofilm-forming ability of this strain was closely related to its protective function on plants.Overall,the formation of Bacillus sp.biofilm is important for bacterial colonization on plant root surface and its function of growth promotion and disease prevention.

2.2 Interactions between Bacillus sp.and plants By chemotactic migration of plant root exudates,Bacillus sp.can gather in plant roots to form biofilms and colonize plants for a long time.The mechanisms of Bacillus sp.in promoting plant growth and preventing disease mainly include the following four aspects.First,plant growth regulating substances are produced to promote plant development.For example,B.subtilis B9 isolated from sugarcane roots by Liu et al.[29]produced indole-3-acetic acid(IAA)and promoted the growth of corn seeds and seedlings.Second,biological nitrogen fixation,phosphorus solubilization,and production of siderophore promote the absorption of nutrients by host plants.The endophytic Bacillus sp.isolated from cucumber by Mahmood et al.[30]significantly promoted the growth of host plants by generating siderophore and solubilizing phosphate.Third,antibacterial compounds are produced to inhibit the growth of plant pathogens,induce the resistance of plants to pathogens,and improve the health level of host plants.Studies showed that B.vele-zensis FKM10 destroyed Fusarium cell wall by secreting glucoside enzyme,thus effectively inhibiting the growth of the pathogen[31].Fourth,it induces the host to develop resistance to biotic and abiotic stresses.Stavropoulou et al.[32]enhanced tomato tolerance to salt by inoculating with plant growth-promoting B.subtilis.In summary,after effective colonization in host plants,Bacillus sp.can play its synergistic and disease-prevention effects through the above several aspects,but the specific mechanism of each aspect needs to be further studied.

3 Application of Bacillus sp.in Promoting the Growth of Agri-cultural and Forestry Crops

Bacillus sp.increases crop yields by producing plant nutrients to maintain soil fertility.Franco-Sierra et al.[33]isolated B.subtilis EACB0575 from the rhizosphere of banana plants,which produced surfactant and gengycin,solubilized phosphate and fixed nitrogen,and produced indoles and siderophore compounds.The EACB0575 strain increased the total dry weight(TDW)of tomato plants by 34.60%.Ker et al.[34]found that B.polymyxa played a biological nitrogen fixation role in the roots of willow branches and stimulated the growth of willow branches by producing auxin.

In addition,Bacillus sp.triggers the inductive system tolerance(IST)of plants by producing volatile organic compounds(VOC),and enhances the tolerance of plants under stress.Saoussen et al.[35]found that B.velezensis GJ11 that promoted plant growth produced volatile compounds such as 2,3-butanediol and acetosyringone,which could close plant stomata and trigger plant defense response to external stress.The stress tolerance of Linum plants can be enhanced by inoculating with Bacillus sp.Strain1(B2),and the relative water content,plant height,water-soluble carbohydrate and proline contents and antioxidant enzyme activities of the plants inoculated with the strain were enhanced[36].In addition,salt stress is one of the environmental stresses that have adverse effects on plant growth.High salt will cause plant growth retardation and affect crop yield.Bacillus sp.help plants resist salt stress by producing indole-acetic acid,ethylene and other compounds.These salt-tolerant Bacillus sp.can be used as effective defense agents for plants.Moreover,Bacillus sp.can promote plant growth and reduce the effects of salt stress without damaging the environment by regulating the physiological and molecular responses of plants,which greatly helps to solve the problem of plant yield due to high salt content[37].Rhizospheric Bacillus sp.,which produces ACC deaminase,can reduce salt stress and promote corn growth by regulating ethylene metabolic pathways[38].B.firmus SW5 plays a crucial role in alleviating the adverse effects of high salinity on soybean growth[39].The development of Bacillus sp.strains has been widely used in agricultural production,and has achieved good economic and ecological benefits.

4 Application of Bacillus sp.in Preventing and Controlling Plant Diseases

With a highly resistant structure that can produce endospores,Bacillus sp.can survive in harsh environmental conditions and inhibit the growth of plant pathogens by secreting lipopeptide,antibiotics,enzymes and other substances,and has been widely used in the prevention and control of plant diseases caused by pathogenic fungi[40-41].B.subtilis SL-44 not only activates the systemic resistance of pepper seedlings to bacterial wilt,but also produces antimicrobial peptides such as surfactin,iturin and fengycin to inhibit or even destroy the mycelial growth of Rhizoctenia solanacearum[42].Fusarium sp.is one of the most destructive soil-borne diseases,and Bacillus sp.is also widely used to control Fusarium wilt,a plant disease caused by Fusarium sp.Chowdhury et al.[43]found that Bacillus sp.LBF-01 in tomato soil could control tomato Fusarium wilt by enhancing the activities of defense related enzymes in roots and leaves of the plants.Difficidin and oxdifficidin isolated from B.methylotrophicus DR-08 had good biological control effect on tomato Fusarium wilt[44].Meanwhile,B.subtilis V26 has the biological potential to control potato Fusarium wilt and tuber dry rot caused by Fusarium sp.[45].Besides,Bacillus sp.is also widely used in agricultural pest control.Ye et al.[46]found that the synergistic effect of B.subtilis,B.thuringiensis and other biocontrol bacteria had significant control effect on agricultural lepidopteran insect pests.It is found that B.thuringiensis Cry2Ab2 is a sustainable defense microbial agent of Helicoverpa zea[47].Overall,Bacillus sp.strains can be used as biological fertilizers and biological control agents for crops to perform green and sustainable control against plant diseases and insect pests.

5 Summary and Prospect

Plant rhizospheric Bacillus sp.can effectively promote plant growth and control plant diseases,and has been widely used in agricultural production and green prevention and control,providing a scientific basis for further promoting green,efficient and sustainable development of agriculture.This paper reviews the application of plant rhizospheric Bacillus sp.in promoting the growth of agricultural and forestry crops and controlling plant diseases,and summarizes and expounds the possibility that Bacillus sp.moves to plant roots through chemotaxis of plant root exudates,and differentiates from motor cells into biofilm to produce cells and colonize host plants for a long time after a short period of contact with plant roots.Meantime,Bacillus sp.produces plant hormones,antibacterial compounds and other substances to enhance the tolerance of host plants,so as to promote the growth of host plants and prevent diseases.The above research results and theoretical knowledge will provide a scientific basis for the research,development and application of plant growth-promoting Bacillus sp.

Many characteristics of Bacillus sp.are closely related to plants.The following three points should be noted in future research on the growth promotion and disease prevention of plant rhizospheric Bacillus sp.First,the colonization mechanism of Bacillus sp.in plant rhizosphere is deeply explored.The colonization ability of Bacillus sp.in host plants is an important prerequisite for its growth promotion and biocontrol functions.There-fore,special attention should be paid to the study of biological characters and molecular mechanisms related to the colonization of Bacillus sp.in plant roots.In addition,when Bacillus sp.colonizes and interacts with plants,plants can respond to external stress through the outbreak of reactive oxygen species,so how Bacillus sp.activates different cellular defense mechanisms to respond to oxidative stress and affects the colonization ability of strains also needs to be further studied[48].Second,based on the related research results on the mechanism of plant growth promotion and disease prevention by Bacillus sp.,the synergistic mechanism generated by the interaction between strains can be further studied.Niu et al.[49]simplified a representative bacterial community from corn roots by exploring interspecific interactions of bacteria.This model community significantly inhibits plant pathogenic fungi and is essential for the growth and health of host plants.Third,there are extensive studies on the mechanism of Bacillus sp.on the synergistic effect against crop disease prevention,but the effects of Bacillus sp.on forest growth promotion and disease prevention are not clear,and there are few reports on related studies[50].Therefore,the research and application of Bacillus sp.to promote the sustainable development of forestry is also a topic that needs to focus on in the future.