Abstract
Plant diseases caused by necrotrophic fungal pathogens result in large economic losses in field crop production worldwide. Effectors are important players of plant-pathogen interaction and deployed by pathogens to facilitate plant colonization and nutrient acquisition. Compared to biotrophic and hemibiotrophic fungal pathogens, effector biology is poorly understood for necrotrophic fungal pathogens. Recent bioinformatics advances have accelerated the prediction and discovery of effectors from necrotrophic fungi, and their functional context is currently being clarified. In this review we examine effectors utilized by necrotrophic fungi and hemibiotrophic fungi in the latter stages of disease development, including plant cell death manipulation. We define “effectors” as secreted proteins and other molecules that affect plant physiology in ways that contribute to disease establishment and progression. Studying and understanding the mechanisms of necrotrophic effectors is critical for identifying avenues of genetic intervention that could lead to improved resistance to these pathogens in plants.
Highlights
Plant pathogens are often categorized based on their relationship with host plants
CtNUDIX-overexpressing strains of C. truncatum and M. oryzae fail to induce disease symptoms on lentil and barley, respectively, but light-brown discoloration is observed, which is suspected to be hypersensitive response (HR)-like cell death on infected plants (Bhadauria et al, 2013). These results suggest that the timely secretion of CtNUDIX is key to normal disease progression, and its premature secretion likely triggered cell death in the biotrophic phase, preventing these two hemibiotrophs from establishing infection
Recent studies have revealed that non-coding small RNAs derived from necrotrophic fungi can be delivered into host cells, hijack plant RNAi machinery, and silence plant genes that are involved in immunity (Weiberg et al, 2013; Wang et al, 2016, 2017)
Summary
Plant pathogens are often categorized based on their relationship with host plants. These categories include biotrophs, which infect living plants with the objective of suppressing the plant immune system and acquiring nutrients from living cells; necrotrophs, which infect living plants with the objective of killing the plant upon or shortly after infection and acquiring nutrients from dead or dying tissues; and hemibiotrophs, which infect living plant tissues akin to biotrophs to first establish infection before “switching” to necrotrophy and killing the plant (Giraldo and Valent, 2013). We broadly discuss the participation of effectors from necrotrophic and late stage hemibiotrophic fungi in disease development, plant cell death modulation, and both the suppression and the hijacking of plant immune responses. Many examples of effectors from necrotrophic fungi have been reported for their cell death-inducing activities.
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