Endophytic Bacillus amyloliquefaciens YTB1407 elicits resistance against two fungal pathogens in sweet potato (Ipomoea batatas (L.) Lam.).

The endophytic Bacillus amyloliquefaciens YTB1407 was previously reported to promote the growth of sweet potato (Ipomoea batatas cv. Yanshu 25). Here, we demonstrate in both in vitro and pot trial assays that pre-treatment with YTB1407 suspension could enhance resistance against root rot disease and black rot disease, caused by Fusarium solani Mart. Sacc. f. sp. batatas McClure and Ceratocystis fimbriata Ell. & Halst on sweet potato, respectively. When seedlings were infected with fungal pathogens at 10 days post irrigation, pre-treatment with YTB1407 suspension decreased these pathogens and YTB1407 bacterial biomass in sweet potato roots. The pre-treatment activated the expression of salicylic acid (SA)-responsive PR-1 gene, raised SA content, and reduced hydrogen peroxide (H2O2) in the host to resist F. solani, while it enhanced the expression levels of SA-responsive NPR1 and PR1 genes and increased SA content to resist C. fimbriata. The disease resistance control effect initiated by pre-treatment with YTB1407 for root rot pathogen (F. solani) was better than for black rot pathogen (C. fimbriata). The results indicated that Bacillus amyloliquefaciens YTB1407 played a pivotal role in enhancing resistance to two fungi pathogens in sweet potato, through production of some antifungal metabolites to decrease infection in the early stage as well as induction of SA-dependent systemic resistance.

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We isolated the endophytic Bacillus amyloliquefaciens YTB1407 from the root of Panax quinquefolium, which has both biological control and growth promoting effects. To investigate its potential applications, a pot experiment of sweet potato was tested to assess the capacity of endophytic colonization of YTB1407 and the selection of its optimum concentration by investigating the performance of root characteristics on three time points in the whole early growth phase after irrigating with different concentrations of bacterial suspensions with treatment of sterile water as control. The activities of endogenous hormone IAA, ZR, t-ZR and IAA oxidase (IAAO, PPO, POD) were analyzed. The results showed that YTB1407 promoted the specific colonization of root system, the elongation of adventitious root and branch roots, and root activity in the early growth stage of sweet potato. At later growth stage, it formed greater fresh mass of absorption root and lower aboveground/root system mass ratio. YTB1407 suspensions with OD600 of 0.50 (T0.50) had more significant effect, which induced the highest fresh tuber mass and the largest effective tuber numbers of per plant at top cover stage. YTB1407 promoted the differentiation of adventitious roots into tubers at initial point of tuberization by increasing IAA content and the ratio of (t-ZR+ZR)/IAA, decreasing IAAO activity and enhancing PPO activity. Moreover, it promoted the differentiated roots into tubers at tuberization stage by keeping the higher content of IAA, lower ratio of (t-ZR+ZR)/IAA, and decreasing IAAO and PPO activities.