Opportunistic Pathogens of Terrestrial Plants

Abstract

Pathogens of terrestrial plants are categorized as biotrophic, hemibiotrophic, or necrotrophic based on similarities in how they manipulate plant cells and based on when or if in the plant-microbe interaction plant cell death occurs. Biotrophic pathogens require live plant cells for infection, hemi-biotrophs require live plant cells to initiate colonization, but the plant cell eventually dies, and necrotrophs kill plant cells early in their interactions with their hosts. Pathogens in each of these categories that have arisen in multiple kingdoms and species within single genera may fall into multiple pathogen categories. The borders between these classifications are blurred and can be affected by environment and plant host species. Broad host range necrotrophic plant pathogens are sometimes referred to as opportunistic because some infect mainly damaged plants or only cause disease on harvested and stored fruits and vegetables. However, true opportunistic pathogens are rarely studied in plant pathology because they do not cause significant economic losses in agriculture. The broad host range necrotrophs may be the closest to true opportunistic pathogens. Recent mechanistic and genomic data shows that these pathogens are adept at manipulating plant defenses and subverting them to the benefit of their necrotrophic lifestyle. This subversion of conserved plant defenses likely contributes to the broad host range of these pathogens and the constant challenge of identifying resistant germplasm. Plants can also resist necrotrophic pathogens by producing antimicrobial compounds or plant cell wall modifications, but the genes encoding for these resistance mechanisms are little studied compared to canonical gene-for-gene resistance that act against biotrophic and hemibiotrophic pathogens.