Trends in Plant Science
Licensed to kill: the lifestyle of a necrotrophic plant pathogen
Section snippets
Necrotrophic plant pathogens
Biotrophic plant pathogenic microbes such as downy or powdery mildews and rusts are generally accepted to have an intricate biological interaction with their host plant [1], presumably as a result of co-evolution [2]. Many plant pathologists have long considered that necrotrophic plant pathogenic fungi do not have much of a ‘real’ interaction with their host. Necrotrophs kill host cells by means of toxic molecules and lytic enzymes and they subsequently decompose the plant tissue and consume it
Brief history of white and gray mould
The genus Botrytis was established as early as 1729. Fungi from the family of Sclerotiniaceae are among the earliest studied plant pathogens. In 1886, Anton De Bary [4] described the ability of S. sclerotiorum, a close relative of B. cinerea, to kill and macerate plant cells. He could microscopically distinguish both processes and could mimic these by administering cell-free extracts of fungal cultures to plant tissues. Similar studies were performed on B. cinerea in the first half of the 20th
Penetrating the host tissue
Pathogens landing on a leaf must penetrate the host surface, which is composed of cutin covered with wax. There is cytological [11] and molecular-genetic [12] evidence for B. cinerea developing appressoria (infection structures that differentiate on the surface and form a penetration peg that breaches the cuticle) (Figure 2b) [11]. Appressoria of the rice blast fungus Magnaporthe grisea penetrate by exerting an extreme physical pressure on the host tissue, resulting from high osmotic turgor in
Host cell death requires the active participation of the pathogen and the host
B. cinerea possesses multiple tools that facilitate host cell death.
Decomposition and consumption of plant biomass
The ultimate goal of a necrotrophic plant pathogen is not to kill its host plant per se but to decompose plant biomass and convert it into fungal mass. A common feature shared by all plant species that are colonized by B. cinerea (i.e. dicots and corolliferous monocots) is their relatively high content of pectin in the cell wall. Plant species with low pectin contents are considered poor hosts for B. cinerea. It was therefore postulated that the host preference of B. cinerea reflects its
Infection requires intricate sensing and signaling
The infection of a host plant is a highly regulated process in which the pathogen must decide whether or not to germinate, when and where to develop an infection structure (appressorium) or produce enzymes and metabolites. There must be continuous sensing of the physical and chemical environment to make the correct decisions (Figure 2a). Sensing and signaling therefore play an important role in all stages of infection [51]. For example, M. grisea can germinate on hydrophilic as well as on
Perspectives
Botrytis cinerea uses multiple strategies to subdue its host plant(s). So far, the ‘silver bullet’, which is crucial for an infection to succeed on all host tissues without affecting fungal growth in vitro, has not been identified and might not exist. Certain virulence factors can be important for one isolate on one particular host species, but they might be dispensable on another host species, or they might be dispensable for a different isolate 24, 47, 48. I prefer not to use the term
Acknowledgements
I am grateful to Bettina and Paul Tudzynski, Matthias Hahn, Günther Doehlemann, Peter van Baarlen and Jos Raaijmakers for critical reading of the manuscript.
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