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

Stefan Petrasch,Christian J Silva,Barbara Blanco-Ulate,Karina Gallegos,Victor Papin,Francisco J Fernandez-Acero,Steven J Knapp,Casper Van Den Abeele,Saskia D Mesquida-Pesci

doi:10.3389/fpls.2019.00223

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.

Highlights

Fungi are important plant pathogens that cause large economic losses due to their ability to inflict diseases such as rot, rust, and wilt in various plant organs both preharvest and postharvest (Dean et al, 2012; Nabi et al, 2017)
At 3 dpi, RR fruit inoculated with B. cinerea and F. acuminatum showed water-soaked lesions of approximately 15 mm covered by dense mycelia, whereas RR fruit inoculated with R. stolonifer were almost decomposed and entirely covered by mycelia
To provide initial support that both F. acuminatum and R. stolonifer are capable of inducing disease responses in the host, like B. cinerea, and do not merely behave as saprotrophs, we evaluated the expression of the host gene SlWRKY33 (Solyc09g014990), which is well-known to be pathogen-responsive but is not induced by abiotic stresses (Zhou et al, 2014, 2015)

Summary

Introduction

Fungi are important plant pathogens that cause large economic losses due to their ability to inflict diseases such as rot, rust, and wilt in various plant organs both preharvest and postharvest (Dean et al, 2012; Nabi et al, 2017). Necrotrophic fungi feed on dead host cells and cause necrosis by secreting toxins and cell walldegrading enzymes (CWDEs), among other virulence factors (van Kan, 2006). Due to their ability to feed on dead host tissue, necrotrophic fungi are sometimes grouped into the less defined group of saprotrophic fungi, which includes many fungi that do not actively kill host cells (Dyakov, 2007). The brute force strategy of secreting toxins and CWDEs as well as the broad host range of many necrotrophic fungi is often interpreted as indiscriminate killing of host cells without the requirement for host-pathogen compatibility (van Kan, 2006). The relevance of host-pathogen compatibility in necrotrophic infections is highlighted by the fact that necrotrophic fungi can readily infect ripe fruit but fail to infect unripe fruit or remain quiescent until host and environmental conditions stimulate a successful infection (Prusky and Lichter, 2007; Alkan and Fortes, 2015; Blanco-Ulate et al, 2016a,b)

Methods

Results

Conclusion