Effective control of southern blight and root rot of Aconitum carmichaelii Debeaux by endophytic Bacillus velezensis YN-2-6S

Southern blight and root rot were two main soil-borne diseases of Aconitum carmichaelii Debeaux, a famous medicinal plant rich in alkaloids. Beneficial microbe based biological control has been a promising and sustainable alternative for disease control. In this study, an endophytic bacterium YN-2-6S was isolated from the stem of A. carmichaelii. YN-2-6S was identified as Bacillus velezensis by phylogenetic analysis based on concatenated sequence of gyrA, rpoB, and recA genes. YN-2-6S showed strong antagonistic capacity against Sclerotium rolfsii and Fusarium oxysporum both in vitro and on root slices of A. carmichaelii by dual-culture assay. Field experiment revealed that YN-2-6S culture significantly reduced southern blight and root rot disease occurrences on A. carmichaelii, with 91.00 % and 85.71 % biocontrol efficiency respectively. The biocontrol activity persisted at least for 54 days. In addition, YN-2-6S inoculation significantly increased fresh and dry weight of stems, main roots and lateral roots of A. carmichaelii compared with non-inoculation and commercial B. subtilis treatments. YN-2-6S inoculation significantly promoted the accumulation of total alkaloids content of A. carmichaelii lateral roots to 17.36 mg/g. Cell-free culture filtrate of YN-2-6S showed strong inhibitory effect against S. rolfsii and F. oxysporum in vitro. The presence of ituC, fenA, fenB, fenD, srfAA, bmyB, bacA, baeA, mnlA, and dfnA genes by PCR amplification indicated that YN-2-6S was able to produce antifungal lipopeptides and polyketides. Furthermore, volatile compounds released by YN-2-6S also significantly inhibited the growth of S. rolfsii and F. oxysporum in vitro. In addition, YN-2-6S was able to produce glucanase, cellulase, protease, indole-3-acetic acid with growth-promoting ability. Increase of peroxidase (POD) and catalase (CAT) enzyme activity of A. carmichaelii leaves was detected upon inoculation of YN-2-6S, which indicated that YN-2-6S may induce systemic resistance on A. carmichaelii. Taken together, this study proved that YN-2-6S was a promising biocontrol agent against southern blight and root rot of A. carmichaelii and the underline mechanism may involve the production of antifungal lipopeptides, polyketides, volatile compounds, hydrolysis and induction of host systemic resistance.