Genetic diversity, plant growth promotion potential, and antimicrobial activity of culturable endophytic actinobacteria isolated from Aconitum carmichaelii Debeaux.

AIM: This study evaluated the phylogenetic diversity, plant growth promotion capacity, antifungal activity, and biocontrol potential of culturable actinobacterial endophytes isolated from the medicinal plant Aconitum carmichaelii Debeaux. METHODS AND RESULTS: Isolation of actinobacteria from healthy A. carmichaelii plants was carried out on six different media. Full-length 16S rRNA gene was amplified by PCR from the genomic DNA of each strain. Indole-3-acetic acid and siderophore production were quantitatively assessed by the Salkowski and Chrome Azurol S methods, respectively. Rice seeds germination and seedling growth were employed to evaluate plant growth promotion capacities of candidate strains. Dual-culture assay and pot experiments were performed to investigate the antifungal and biocontrol potential of isolates. We obtained 129 actinobacterial isolates from A. carmichaelii, and they belonged to 49 species in 7 genera. These strains exhibited diverse plant growth promotion ability, among which one strain significantly enhanced rice seeds germination, while 31 strains significantly facilitated rice seedling growth. SWUST-123 showed strong antifungal activity against four pathogens in vitro and was most compatible with Qingchuan cultivar. SWUST-123 reduced around 40% of southern blight disease occurrence compared to blank control treatment. . CONCLUSION: Aconitum carmichaelii harbored genetically diverse actinobacterial endophytes exhibiting diverse plant growth promotion and antifungal potential, some of which can be served as good candidates for biofertilizers and biocontrol agents.

Culturable bacterial endophytes of Aconitum carmichaelii Debx. were diverse in phylogeny, plant growth promotion, and antifungal potential.

Medicinal plants harbor tremendously diverse bacterial endophytes that maintain plant growth and health. In the present study, a total of 124 culturable bacterial endophytes were isolated from healthy Aconitum carmichaelii Debx. plants. These strains were clustered into 10 genera based on full-length 16S rDNA sequences, among which Bacillus and Pseudomonas were the dominant genera. In addition, A. carmichaelii may capture 10 potential new bacterial species based on multi-locus sequence analysis of three housekeeping genes (gyrA, rpoB, and atpD). The majority of these bacterial endophytes exhibited plant growth-promoting ability through diverse actions including the production of either indole acetic acid and siderophore or hydrolytic enzymes (glucanase, cellulose, and protease) and solubilization of phosphate or potassium. A total of 20 strains inhibited hyphal growth of fungal pathogens Sclerotium rolfsii and Fusarium oxysporum in vitro on root slices of A. carmichaelii by the dual-culture method, among which Pseudomonas sp. SWUSTb-19 showed the best antagonistic activity. Field experiment confirmed that Pseudomonas sp. SWUSTb-19 significantly reduced the occurrence of southern blight and promoted plant biomass compared with non-inoculation treatment. The possible mode of actions for Pseudomonas sp. SWUSTb-19 to antagonize against S. rolfsii involved the production of glucanase, siderophore, lipopeptides, and antimicrobial volatile compounds. Altogether, this study revealed that A. carmichaelii harbored diverse plant growth-promoting bacterial endophytes, and Pseudomonas sp. SWUSTb-19 could be served as a potential biocontrol agent against southern blight.

Biocontrol and plant growth promotion potential of endophytic Bacillus subtilis JY-7-2L on Aconitum carmichaelii Debx.

Aconitum carmichaelii Debx. is a famous medicinal plant rich in alkaloids and widely used to treat various human diseases in Asian countries. However, southern blight caused by Sclerotium rolfsii severely hampered the yield of A. carmichaelii. Beneficial microbe-based biological control is becoming a promising alternative and an environmentally friendly approach for the management of plant diseases. In this study, we evaluated the biocontrol potential of an endophytic bacterial strain JY-7-2L, which was isolated from the leaves of A. carmichaelii, against southern blight in vitro and by a series of field experiments. JY-7-2L was identified as Bacillus subtilis based on multi-locus sequence analysis. JY-7-2L showed strong antagonistic activity against S. rolfsii in vitro and on A. carmichaelii root slices by dual-culture assay. Cell-free culture filtrate of JY-7-2L significantly inhibited the hyphal growth, sclerotia formation, and germination of S. rolfsii. In addition, volatile compounds produced by JY-7-2L completely and directly inhibited the growth of S. rolfsii. Furthermore, JY-7-2L was proved to produce hydrolytic enzymes including glucanase, cellulase, protease, indole acetic acid, and siderophore. The presence of bacA, fenA, fenB, fenD, srfAA, and baeA genes by PCR amplification indicated that JY-7-2L was able to produce antifungal lipopeptides and polyketides. Field trials indicated that application of the JY-7-2L fermentation culture significantly reduced southern blight disease severity by up to 30% with a long-acting duration of up to 62 days. Meanwhile, JY-7-2L significantly promoted the fresh and dry weights of the stem, main root, and lateral roots of A. carmichaelii compared to non-inoculation and/or commercial B. subtilis product treatments. Taken together, JY-7-2L can be used as a promising biocontrol agent for the control of southern blight in A. carmichaelii.