Fungal and Bacterial Biotrophy and Necrotrophy

Abstract

Plant pathogens have been divided into two classes, namely, biotrophs and necrotrophs. These pathogens lead to significant economic losses by infecting various crops. Biotrophs complete their life cycle by using the living host cell machinery, while necrotrophs feed on the host cell after killing them. Hemibiotrophs, a third group, show both the forms for obtaining nutrition i.e., early biotrophic stage to later necrotrophic phase. After infecting the plants, both the groups of plant pathogens can trigger and suppress plant immune responses by synthesizing and secreting effector proteins. In case of biotrophic pathogens, effector proteins were found to be Avr proteins (identified by resistance proteins), hrp genes, and cell wall-degrading enzymes, while necrotrophic pathogen has additional effectors called as host-selective toxins. Significant differences have been observed between these two groups in the disease symptoms they cause, their host range, morphogenesis of the infection, production of secondary metabolites and hormones, and nature of plant resistance. Biotrophs possess a sophisticated way of infection, i.e., it enters the host cell using the haustoria, colonizes the intercellular space, and overpowers the host defenses. Necrotrophs have been further grouped into host-specific and broad host range necrotrophs depending on the toxins they secrete. In case of necrotrophic infections, host cell death has been shown to trigger production of hormones like ethylene, abscisic acid, salicylic acid, and jasmonic acid. Both bacterial and fungal plant pathogens belonging to the above mentioned category have been identified. In this chapter we are going to discuss the current state of knowledge about bacterial and fungal biotrophs and necrotrophs.