Biological control of potato common scab and growth promotion of potato by Bacillus velezensis Y6

Abstract

Potato common scab, caused mainly by Streptomyces scabies, causes surface necrosis and reduces the economic value of potato tubers, but effective chemical control is still lacking. In this study, an attempt was made to control potato common scab by inoculating potatoes with Bacillus velezensis (B. velezensis) and to further investigate the mechanism of biological control. The results showed that B. velezensis Y6 could reduce the disease severity of potato common scab from 49.92 ± 25.74% [inoculated with Streptomyces scabies (S. scabies) only] to 5.56 ± 1.89% (inoculated with S. scabies and Y6 on the same day) and increase the potato yield by 37.32% compared with the control under pot experiment in this study. Moreover, in the field trial, it was found that Y6 could also significantly reduce disease severity from 13.20 ± 1.00% to 4.00 ± 0.70% and increase the potato yield from 2.07 ± 0.10 ton/mu to 2.87 ± 0.28 ton/mu (p < 0.01; Tukey's test). Furthermore, RNA-seq analysis indicated that 256 potato genes were upregulated and 183 potato genes were downregulated in response to B. velezensis Y6 inoculation. In addition, strain Y6 was found to induce the expression of plant growth-related genes in potato, including cell wall organization, biogenesis, brassinosteroid biosynthesis, and plant hormone transduction genes, by 1.01-4.29 times. As well as up-regulate hydroquinone metabolism-related genes and several transcription factors (bHLH, MYB, and NAC) by 1.13-4.21 times. In summary, our study will help to understand the molecular mechanism of biological control of potato common scab and improve potato yield.

Keywords: Bacillus velezensis; Streptomyces scabies; antagonism; lipopeptide; potato; transcriptome.

Figure 1

Antagonistic activity of Y6 against S. scabies and the analysis of lipopeptide crude extracts from Y6 by UPLC/Q-TOF. (A) Antagonistic activity of Y6 against S. scabies via a spot-on lawn assay. The white line indicated the size of the inhibition circle, measured after 24 h of incubation at 30°C (Y6, 32.0 ± 0.5 mm, mean ± SD, n = 3). The bar represents 10 mm. (B) Representative chromatograms of the LPs (iturin, fengycin, and surfactin) from strain Y6 using UPLC/Q-TOF analysis. After 24 h of incubation at 30°C, a 600 mg agar sample was collected from the clear area, which was minced and mixed with 2 mL of acetonitrile/water (1:1 v/v), shaken well, and sonicated twice for 30 s, then centrifuged. The supernatant was collected, filtered, and analyzed using UPLC/Q-TOF. As a control, LP was isolated in a similar way but without the addition of S. scabies in 0.7% YME agar.

Figure 2

Identification of LP compounds in Y6 extracts using UPLC/Q-TOF analysis. The figure shows the mass spectra [M + H]⁺ for LPs, iturin, fengycin, and surfactin.

Figure 3

The antagonistic activity of the strain Y6 and mutants (453 (srfAA:mls), 454 (ituA:mls), 459 (fenC:spc)). The white line indicated the size of the inhibition circle, measured after 24 h of incubation at 30°C (Y6: 32.50 ± 0.50 mm, 453: 27.05 ± 0.05 mm, 454: 30.00 ± 0.50 mm, 459: 32.75 ± 0.43 mm, mean ± SD with n = 3). The bar represents 10 mm. The clearance zone indicated by the white lines was measured after 24 h incubation at 30°C.

Figure 4

Biocontrol and yield-increasing efficacy of Y6 under greenhouse conditions. (A) Potato tubers harvested from three treatments: (a) inoculated with Y6 and S. scabies on the same day; (b) inoculated with S. scabies only; (c) control without Y6 or S. scabies. The bar represents 2 cm. (B) Potato yields from three treatments. (C) Disease severity of potato common scab. (D) Disease incidence of the potato common scab. p-values were calculated using Tukey’s test. Asterisks (**) indicate p < 0.01 as compared to the treatment of control.

Figure 5

Biocontrol and yield-increasing efficacy of Y6 under field conditions. (A) Potato tubers were harvested from two treatments: (a) inoculated with Y6, tuber pieces with a bud were mixed with the biofertilizer made of Y6 before planting in soil; (b) no treatment. The bar represents 5 cm. (B) Data about the disease severity and disease incidence of the potato common scab from two treatments. One hundred twenty randomly chosen tubers from each treatment were used to calculate the severity of the disease as a percentage. We collected 40 potato tubers randomly from the middle block of each plot to assess disease severity and incidence in each plot, and three plots of the same treatment were used to determine the mean ± standard error and compare with the other treatments. p-values were calculated using Tukey’s test. Asterisks (**) indicate p < 0.01 as compared to the treatment of control.

Figure 6

Volcano diagram of differentially expressed genes (DEGs) in the transcriptome of plants inoculated with strain Y6 and control. Red dots indicate gene up-regulation and blue dots indicate gene down-regulation.

Figure 7

GO enrichment analysis of differentially expressed genes (DEGs) in the transcriptome of plants inoculated with strain Y6 and control. The X-axis represents the enrichment factor. The Y-axis represents the GO term name.

Figure 8

KEGG enrichment analysis of differentially expressed genes (DEGs) in the transcriptome of plants inoculated with strain Y6 and control. The X-axis represents the enrichment factor. The Y-axis represents the pathway name.