To screen strains with good growth-promoting and drought-tolerant effects on C. oleifera, clarify their systematic classification status, and preliminarily explore their mechanisms, which provides a new research direction for improving the drought resistance of C. oleifera.

Drought-tolerant bacteria were isolated and screened from the rhizosphere soil of C. oleifera growing well in a drought environment by using polyethylene glycol (PEG) 6000 to simulate drought stress. Their nitrogen fixation, phosphate solubilization, potassium solubilization, secretion of indole-3-acetic acid (IAA), siderophore and 1-aminocyclopropane-1-carboxylate (ACC) deaminase were determined. Combined with morphological, physiological and biochemical tests and 16S rDNA sequence analysis, the phylogenetic status was clarified. The growth-promoting and drought-resistance were verified by seed germination experiments, and their effects on the activity of antioxidant enzymes and the expression of related resistance genes in the roots of C. oleifera were studied.

A total of 23 strains of bacteria that tolerate 300 g/L PEG6000 drought stress were screened, including 21 strains of nitrogen-fixing bacteria, 16 strains of phosphate-solubilizing bacteria, 5 strains of IAA-producing bacteria, 11 strains of siderophore-secreting bacteria and 6 strains of ACC deaminase-secreting bacteria. Combined with drought tolerance and growth-promoting effects, the optimal strain 1J81 is screened and identified as Enterobacter sichuanensis by morphological characteristics, physiological and biochemical tests and 16S rDNA sequencing results. Under both drought and non-drought conditions, strain 1J81 significantly promotes the activities of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) and other enzymes in the roots of C. oleifera and the expression levels of related resistance genes POD, SOD1 and PR1B1.

As a microbial fertilizer strain resource, E. sichuanensis 1J81 derived from the rhizosphere soil of C. oleifera has the potential to improve the drought resistance of C. oleifera.

Eucalyptus is one of the three fast-growing tree species in China, and as one of the four pillars of Guangxi’s forestry economy, it has important economic, social and ecological values. With the expansion of planting area, the change of ecological environment and poor management, the occurrence of Eucalyptus leaf diseases is becoming more and more serious, which affects the growth of Eucalyptus and restricts the healthy development of Eucalyptus industry. Eucalyptus grows rapidly, leaf diseases are difficult to detect in the early stage, and after the disaster, the leaves are lost in a large area, the plant growth cycle is shortened, and the growth is slow, causing significant economic losses. In order to identify the diseases pathogenic fungi, enriching the biological control resources of Eucalyptus leaf diseases and formulate a reasonable disease control plan.

Based on the Koch rule to determine the pathogenicity, identify the pathogenic fungi, use morphological characteristics and rDNA-ITS sequence analysis to identify the pathogenic fungi, and use the plate confrontation method to screen the pathogenic antagonistic bacteria from the surface of Eucalyptus leaves, and identify them, and the evaluation of the control efficacy of biocontrol bacteria was also carried out, so as to provide a basis for the biological control of the Eucalyptus leaf diseases.

The results showed that four pathogenic fungi were isolated from the leaves of eucalyptus trees in Da Gui Shan Stateowned forest farm, namely F202310093, F202310094, F202310095 and F202310096, and the leaf disease symptoms caused by them were consistent with the onset symptoms of forest disease. Combined with morphological characteristics and rDNA-ITS sequence analysis, the four fungi were Colletotrichum fructicola, C. cliviicola, C. siemese and Coniella quercicola. The strains B2311001 with obvious antagonism to the above four pathogenic fungi were screened and identified as Bacillus subtillis by plate confrontation method which identified as Bacillus subtilis by morphological and 16s rDNA sequence analysis. The control effect of antagonistic bacteria was better than that of azoxystrobin, and the potted control effect against C. fructicola was 84.01%, followed by C. siemese with 83.67%, and the lowest control effect was Coniella quercicola, with an interforest control effect of 81.33%. C. cliviicola and C. siamense was first reported as a pathogenic fungi of Eucalyptus leaves disease.

In this study, the antagonistic bacteria of eucalyptus leaf pathogens were screened, and the biocontrol resources of Eucalyptus leaf diseases were enriched, and a theoretical basis was provided for the biological control of Eucalyptus leaf diseases.

A strain of highly efficient phosphorus-solubilizing bacteria was isolated from the rhizosphere soil of Camellia oleifera, and its phosphorus-solubilization characteristics and effects on the growth of C. oleifera were investigated.

Inorganic phosphorus selection medium was used for the screening of the phosphorus-solubilizing bacteria. The morphological characteristics were observed, and the taxonomic status was determined. The growth curve was determined to identify the optimal growth time of the strain. Functional selection medium was used to determine the strain's ability of nitrogen fixation, potassium solubilization, protease solubilization, cellulase solubilization, amylase solubilization, and iron-producing carriers, as well as to determine the content of its secretion of indole-3-acetic acid (IAA). Antagonistic experiments against the pathogen of Camellia anthracnose were conducted to evaluate its disease resistance. The optimal salt concentration, suitable nitrogen and carbon sources and optimal calcium phosphate addition were determined. The fermented broth of the phosphate-solubilizing bacteria was applied to potted C. oleifera through root irrigation and foliar spraying, and the activities of phenylalanine ammonialyase (PAL), polyphenol oxidase (PPO), peroxidase (POD) in leaves, as well as urease and acid phosphatase activities in the soil and soil content of effective phosphorus, were measured.

The efficient phosphate-solubilizing strain CSUFT-GXP-084 was isolated, with a D/d value of 3.67. The dissolved phosphorus content was 60.35 mg/L. Molecular identification confirmed it as Burkholderia territorii. The strain exhibited nitrogen fixation, protease, and iron carrier production ability, with an IAA secretion content of 4.42 mg/L. It inhibited the growth of five kinds of pathogens causing anthracnose in C. oleifera. Additionally, the salt tolerance range of strain CSUFT-GXP-084 was 0 %~5 %. The most suitable nitrogen and carbon sources for the phosphorus solubilization ability of the strain CSUFT-GXP-084 were ammonium sulfate, and the optimal amount of calcium phosphate was 2 g/L. Foliar spraying of the CSUFT-GXP-084 bacterial solution had greater effects on the PAL, PPO, and POD activities of the oil tea leaves, while root irrigation had greater effects on urease and acid phosphatase activities in the soil. The effective phosphorus content in the soil increased by 1.58 times and 1.70 times compared with CK after the root irrigation and leaf spraying treatments, respectively.

The strain CSUFT-GXP-084 demonstrates a high phosphorus solubilization ability, enhancing enzyme activity of C. oleifera leaves, soil enzyme activity, and effective phosphorus content in the soil; thus, it holds potential applications in microbial fertilizers and biocontrol agents.