Endophytic Bacterium Serratia plymuthica From Chinese Leek Suppressed Apple Ring Rot on Postharvest Apple Fruit

Abstract

Apple ring rot caused by Botryosphaeria dothidea is an economically significant plant disease that spreads across the apple production areas in China. The pathogen infects apple fruits during the growing season and results in postharvest fruits rot during storage, which brings about a huge loss to plant growers. The study demonstrated that an endophytic bacterium Serratia plymuthica isolated from Chinese leek (Allium tuberosum) significantly suppressed the mycelial growth, severely damaging the typical morphology of B. dothidea, and exerted a high inhibition of 84.64% against apple ring rot on postharvest apple fruit. Furthermore, S. plymuthica significantly reduced the titratable acidity (TA) content, enhanced the soluble sugar (SS) content, vitamin C content, and SS/TA ratio, and maintained the firmness of the fruits. Furthermore, comparing the transcriptomes of the control and the S. plymuthica treated mycelia revealed that S. plymuthica significantly altered the expressions of genes related to membrane (GO:0016020), catalytic activity (GO:0003824), oxidation-reduction process (GO:0055114), and metabolism pathways, including tyrosine metabolism (ko00280), glycolysis/gluconeogenesis (ko00010), and glycerolipid metabolism (ko00561). The present study provided a possible way to control apple ring rot on postharvest fruit and a solid foundation for further exploring the underlying molecular mechanism.

Keywords: Botryosphaeria dothidea; RNA-seq; Serratia plymuthica; biocontrol; fruit quality.

FIGURE 1

Serratia plymuthica significantly inhibits the mycelial growth of Botryosphaeria dothidea (A). The inhibition increases with the increase of the treatment time (B). Compared to the untreated control (C), Serratia plymuthica damages the mycelial morphology of Botryosphaeria dothidea (D,E). Different lowercase letters indicate significant differences between treatments or different times (P < 0.05).

FIGURE 2

The disease symptoms on the apple fruit inoculated with Botryosphaeria dothidea. The disease develops quickly on control fruits, and apple fruit rots 5 days later (A). But the Serratia plymuthica-treated apple only shows slight disease symptoms (B). Serratia plymuthica significantly reduces the disease spot diameter (C) and inhibits apple ring rot on postharvest fruits (D) caused by Botryosphaeria dothidea. Different lowercase letters indicate significant differences between treatments or different times (P < 0.05).

FIGURE 3

The dynamic changes of fruit quality indexes including titratable acidity (A), total soluble solid (B), TSS/TA (C), SS (D), SS/TA (E), vitamin C (F), and firmness (G) in the different treatment fruits.

FIGURE 4

The area-under-curve (AUC) of fruit quality indexes including titratable acidity (A), total soluble solid (B), TSS/TA (C), SS (D), SS/TA (E), vitamin C (F), and firmness (G) in the different treatment fruits. “*” means P < 0.05, “^**” means P < 0.01, “ns” means P < 0.05.

FIGURE 5

Differentially expressed genes screening in Botryosphaeria dothidea treated by Serratia plymuthica.

FIGURE 6

GO enriched analysis of differentially expressed genes. MF, Molecular function; CC, cellular component; BP, biological process.

FIGURE 7

Analysis of main KEGG pathways of differentially expressed genes.

FIGURE 8

DEGs involved in GO term membrane.

FIGURE 9

DEGs involved in glycerolipid metabolism (A), glycolysis/gluconeogenesis (B), and tyrosine metabolism (C) by KEGG analysis.

FIGURE 10

qRT-PCR validation of 15 randomly selected differentially expressed genes identified by Illumina high throughput RNA-sequencing.