PATHOGEN - Mr. Kitty
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PATHOGEN - Mr. Kitty
Potato (Solanum tuberosum L.) is the primary vegetable crop consumed worldwide and is largely affected by bacterial pathogens that can cause soft rot and blackleg disease. Recently, resistance to these diseases has been identified in the wild potato S. chacoense, and the mechanism of resistance is unknown. Here, it was hypothesized that S. chacoense stems or tubers have unique chemistry that confers resistance to the pathogen Pectobacterium brasiliense through bactericidal, bacteriostatic, or antivirulence activity. Stem and tuber metabolite extracts were collected from S. chacoense and tested for effects on Pectobacterium bacterial multiplication rates, and activity and expression of known exoenzymes and virulence genes using S. tuberosum extracts as a comparative control. Comparatively, the S. chacoense extracts did not affect bacterial multiplication rate; however, they did reduce pectinase, cellulase, and protease activities. The chemical extracts were profiled using a bioassay-guided fractionation, and a nontargeted metabolomics comparison of S. chacoense and S. tuberosum stems and tubers was performed. The data showed that selected alkaloids, phenolic amines, phenols, amines, and peptides are integrative chemical sources of resistance against the bacteria.
Genus Pectobacterium is a broad-host-range necrotrophic pathogen that infects many crops in addition to potato (Ma et al. 2007) and, because of the large losses caused by this pathogen and the threat it imposes to agriculture, it was listed among the top 10 most relevant plant-pathogenic bacteria (Mansfield et al. 2012). Disease control measures in potato and other crops include limited generation production schemes, seed indexing, copper sprays, and sanitation. Other methods such as antibiotics, inorganic and organic salts, chlorine-based compounds, heat, and biological control have been proposed but they do not provide economical and effective disease management (Czajkowski et al. 2011; Motyka et al. 2017).
Solanaceous species also produce phytoanticipins, which are preformed metabolites (small molecules, generally less than 1,500 Da) that are toxic to many pathogens and pests. Potato plants produce these protective metabolites in both tubers and stems, including steroidal alkaloids (glycoalkaloids), steroidal glycosides (saponins), phenolics, amino glycosides, polyamine alkaloids (kukoamines), and tropane alkaloids (calystegines) (Chaparro et al. 2018; Friedman 2006). Phytoanticipins act as bioactive metabolites and provide protection by either acting as toxins or interrupting virulence pathways. For example, glycoalkaloids and saponins are directly toxic toward pests and herbivores by inhibiting proteinase activity and interrupting the integrity of animal gut membranes (Hussain et al. 2019). In contrast, phenolics and their derivatives are generally nonlethal but, instead, interfere with virulence signaling pathways of species of Phytophthora, Verticillium, Pectobacterium, Burkholderia, Aliivibrio, Agrobacterium, Chromobacterium, Pectobacterium, and Dickeya (Joshi et al. 2015, 2016; Muñoz-Cazares et al. 2017). In these pathogens, potato phenolics can specifically affect microbial pigment, biofilm, motility, exoenzyme, and QS pathways (Compean and Ynalvez 2014; Joshi et al. 2015; Othman et al. 2019; Silva et al. 2016). Likewise, alkaloids and terpenoids were shown to have antimicrobial and antivirulence activity against many gram-negative and gram-positive bacteria (Cushnie et al. 2014; Guimarães et al. 2019; Silva et al. 2016).
Several accessions of wild potato species have been demonstrated to be resistant or tolerant to Pectobacterium diseases. Here, we hypothesized that tubers and stems of domestic and wild potato differ in their profile of antimicrobial metabolites, and that metabolites contribute to resistance to soft rot disease by acting as toxins (e.g., bactericidal or bacteriostatic), or affect the virulence of these pathogenic