Fusarium oxysporum f. sp. cubense, the causative agent of Panama disease, is classified into three races: Foc1, Foc2, and Foc4. However, the histological characteristics, the accumulation of fusaric acid (FA), and resistant gene expression in banana infected with different races remain unclear. In this study, we compared the infection processes, FA contents, and gene expression levels in a Cavendish banana cultivar (Musa AAA Brazilian) inoculated with Foc1 and Foc4. Results showed that Foc4 can rapidly extend from the roots to the leaves, whereas Foc1 expands slowly from the roots to the rhizomes but cannot expand further upward. In addition, the colonization of plants by Foc4 was significantly higher compared with Foc1, as was the content of FA in those infected plant tissues. We observed that a large amount of starch granules was produced in the rhizomes and the number of starch granules was significantly higher after infection with Foc1 than after infection with Foc4. We further found that starch has an important inhibitory effect on the phytotoxicity induced by FA, thus leading to more resistance to the pathogens in the plants with high amounts of starch accumulation than in those with a low amount of starch accumulation. Moreover, the expression levels of 10 defense-related genes were analyzed and the results showed that the induction levels of those genes were higher after infection with Foc1 than after infection with Foc4. These results suggest that the observed differences in the invasion of host tissues and FA accumulation, and the number of starch granules and expression of defense-related genes, may contribute to a difference in virulence between the two races and the resulting difference in host resistance response, respectively.

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Plant cell walls, which are mainly composed of pectin, play important roles in plant defence responses to pathogens. Pectin is synthesised in a highly esterified form and then de-esterified by pectin methylesterases (PMEs). Because of this, PMEs are directly involved in plant defence. However, the molecular mechanisms of their interactions with pectins remain unclear. In this study, we compared the expression level and enzyme activities of PMEs in a banana Cavendish cultivar (Musa AAA ‘Brazilian’) inoculated with Fusarium oxysporum f. sp. cubense pathogenic races 1 (Foc1) and 4 (Foc4). We further examined the spatial distribution of PMEs and five individual homogalacturonans (HGs) with different degree of pectin methylesterification (DM). Results suggested that the banana roots infected with Foc1 showed lower PME activity than those infected with Foc4, which was consisted with observed higher level of pectin DM. The level of HGs crosslinked with Ca2+ was significantly higher in roots infected with Foc1 compared with those infected with Foc4. Therefore, banana exhibited significantly different responses to Foc1 and Foc4 infection, and these results suggest differences in PME activities, DM of pectin and Ca2+-bridged HG production. These differences could have resulted in observed differences in virulence between Foc1 and Foc4.

Background Banana Fusarium wilt is a devastating disease of bananas caused by Fusarium oxysporum f. sp. cubense (Foc) and is a serious threat to the global banana industry. Knowledge of the pathogenic molecular mechanism and interaction between the host and Foc is limited. Results In this study, we confirmed the changes of gene expression and pathways in the Cavendish banana variety ‘Brazilian’ during early infection with Foc1 and Foc4 by comparative transcriptomics analysis. 1862 and 226 differentially expressed genes (DEGs) were identified in ‘Brazilian’ roots at 48 h after inoculation with Foc1 and Foc4, respectively. After Foc1 infection, lignin and flavonoid synthesis pathways were enriched. Glucosinolates, alkaloid-like compounds and terpenoids were accumulated. Numerous hormonal- and receptor-like kinase (RLK) related genes were differentially expressed. However, after Foc4 infection, the changes in these pathways and gene expression were almost unaffected or weakly affected. Furthermore, the DEGs involved in biological stress-related pathways also significantly differed after infection within two Foc races. The DEGs participating in phenylpropanoid metabolism and cell wall modification were also differentially expressed. By measuring the expression patterns of genes associated with disease defense, we found that five genes that can cause hypersensitive cell death were up-regulated after Foc1 infection. Therefore, the immune responses of the plant may occur at this stage of infection. Conclusion Results of this study contribute to the elucidation of the interaction between banana plants and Foc and to the development of measures to prevent banana Fusarium wilt.

The pathogenic bacterium Ralstonia solanacearum caused tomato bacterial wilt (TBW), a destructive soil-borne disease worldwide. There is an urgent need to develop effective control methods. Myxobacteria are microbial predators and are widely distributed in the soil. Compared with other biocontrol bacteria that produce antibacterial substances, the myxobacteria have great potential for biocontrol. This study reports a strain of Myxococcus xanthus R31 that exhibits high antagonistic activity to R. solanacearum. Plate test indicated that the strain R31 efficiently predated R. solanacearum. Pot experiments showed that the biocontrol efficacy of strain R31 against TBW was 81.9%. Further study found that the secreted protein precipitated by ammonium sulfate had significant lytic activity against R. solanacearum cells, whereas the ethyl acetate extract of strain R31 had no inhibitory activity against R. solanacearum. Substrate spectroscopy assay and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of secreted proteins showed that some peptidases, lipases, and glycoside hydrolases might play important roles and could be potential biocontrol factors involved in predation. The present study reveals for the first time that the use of strain M. xanthus R31 as a potential biocontrol agent could efficiently control TBW by predation and secreting extracellular lyase proteins.