Infection Strategies Deployed by Botrytis cinerea, Fusarium acuminatum, and Rhizopus stolonifer as a Function of Tomato Fruit Ripening Stage

Abstract

Worldwide, 20–25% of all harvested fruit and vegetables are lost annually in the field and throughout the postharvest supply chain due to rotting by fungal pathogens. Most postharvest pathogens exhibit necrotrophic or saprotrophic lifestyles, resulting in decomposition of the host tissues and loss of marketable commodities. Necrotrophic fungi can readily infect ripe fruit leading to the rapid establishment of disease symptoms. However, these pathogens generally fail to infect unripe fruit or remain quiescent until host conditions stimulate a successful infection. Previous research on infections of fruit has mainly been focused on the host’s genetic and physicochemical factors that inhibit or promote disease. Here, we investigated if fruit pathogens can modify their own infection strategies in response to the ripening stage of the host. To test this hypothesis, we profiled global gene expression of three fungal pathogens that display necrotrophic behavior— Botrytis cinerea , Fusarium acuminatum , and Rhizopus stolonifer —during interactions with unripe and ripe tomato fruit. We assembled and functionally annotated the transcriptomes of F. acuminatum and R. stolonifer as no genomic resources were available. Then, we conducted differential gene expression analysis to compare each pathogen during inoculations versus in vitro conditions. Through characterizing patterns of overrepresented pathogenicity and virulence functions (e.g., phytotoxin production, cell wall degradation, and proteolysis) among the differentially expressed genes, we were able to determine shared strategies among the three fungi during infections of compatible (ripe) and incompatible (unripe) fruit tissues. Though each pathogen’s strategy differed in the details, interactions with unripe fruit were commonly characterized by an emphasis on the degradation of cell wall components, particularly pectin, while colonization of ripe fruit featured more heavily redox processes, proteolysis, metabolism of simple sugars, and chitin biosynthesis. Furthermore, we determined that the three fungi were unable to infect fruit from the non-ripening ( nor ) tomato mutant, confirming that to cause disease, these pathogens require the host tissues to undergo specific ripening processes. By enabling a better understanding of fungal necrotrophic infection strategies, we move closer to generating accurate models of fruit diseases and the development of early detection tools and effective management strategies.

Physiological and Proteomic Approaches to Address the Active Role of Botrytis cinerea Inoculation in Tomato Postharvest Ripening

“…Differentially expressed genes revealed functions during tomato inoculations that are likely to be pathogenic, such as catabolisms of ROS (e.g., SOD, CAT, POD) and cell wall breakdown molecules, including cellobiose and lignin [56]. ROS are rapidly produced by the host after pathogen detection, activating downstream signaling of different defense responses [57].…”

“…In addition to ROS generation machinery, fungal pathogens must be shielded from the infection site's oxidative stress, with activation of enzymatic and non-enzymatic mechanisms [59]. SODs activation is the first step to convert O 2 − into H 2 O 2 , and then H 2 O 2 can be converted to water by either CATs or peroxidases (GPXs or PRXs), and B. cinerea revealed upregulation of mechanisms for the H 2 O 2 catabolism [56], evidence that was also obtained in the present study.…”

Grey mould of strawberry, a devastating disease caused by the ubiquitous necrotrophic fungal pathogen Botrytis cinerea

“…B. cinerea 's infection mechanisms have been studied in model organisms and further characterized thanks to the availability of high‐quality reference genome sequences (Amselem et al , ; Van Kan et al , ; Staats and van Kan, ). The fungus is known to actively promote plant susceptibility by employing a variety of virulence factors (Choquer et al , ; Nakajima and Akutsu, ; Petrasch et al , ). In early stages, B. cinerea deploys sRNAs and effector proteins to suppress premature host cell death and immune responses, which enables the fungus to establish inside the host and accumulate biomass prior to the necrotrophic phase (Veloso and van Kan, ).…”

Salmonella enterica Elicits and Is Restricted by Nitric Oxide and Reactive Oxygen Species on Tomato

“…Within this regulon, the flavohemoglobin HmpA is identified as the main protein responsible for NO detoxification activities in the presence of an oxygenated environment (Karlinsey et al, 2012). A transcriptomic study of S. Typhimurium on tomato leaves and roots identified multiple up-regulated NsrR regulon genes, ygbA, ytfE, yoaG STM1808 and yfhH (Han et al, unpublished), which in E. coli is known to offer an NsrR binding site (Partridge et al, 2009). Supporting this, we assayed two genes in the NsrR regulon, hmpA and yoaG, both of which were differentially up-regulated compared to all native fruit and leaf environments.…”

“…The transition from green to red fruit is marked by accruement of high ROS concentrations (Kumar et al, 2016). Petrasch et al (2019) investigated ROS detoxification by fungal pathogens in red ripe fruit. Thus, the ascorbic acid injected in our plant colonization assays could have been targeting ripening-related ROS, hence providing a more hospitable environment for colonizing S. enterica.…”