Role of plant autophagy in biotic stress response

Autophagy is an essential and evolutionarily conserved cellular process for the degradation or recycling of damaged or unwanted intracellular materials under stress conditions or during specific developmental processes. Cytoplasmic components, including mis-formed, unwanted, dysfunctional intracellular components and organelles, are engulfed into a double-membraned vesicle called autophagosome, and transported to lysosomes (animals) or vacuoles (yeast and plants) for degradation through autophagy, and completed the recycling of intracellular contents. There are three major types of autophagy in eukaryotic cells: macroautophagy(hereafter referred to as autophagy, which is mediated by autophagosome), microautophagy, and chaperone-mediated autophagy (CMA). Genes involved in autophagy processes are termed autophagy-related (ATG) genes. Among these, the essential genes for autophagosome formation are referred to as the core machinery genes. One can distinguish the following key steps in the life cycle of an autophagosome: initiation, elongation, cargo uptake, closure/maturation, and fusion with vesicles (including the lysosomes or vacuoles) followed by destruction of the cargos. As a mechanism for maintaining intracellular homeostasis, autophagy plays critical roles in various cellular processes such as cell metabolism, differentiation, development, and adaptive and innate immunity against various pathogens. In metazoans, autophagy is highly engaged during the immune responses through interfacing either directly with intracellular pathogens or indirectly with immune signaling molecules. Plants evolved several layers of defense mechanisms to combat various pathogens, including pathogen-triggered immunity, effector-triggered immunity, RNA silencing, and ubiquitination. Autophagy may link plant immunity in different ways, with autophagy playing a role in degrading pathogen effectors or defense-related plant proteins, or pathogen effectors interfering with autophagy. During incompatible plant-pathogen interactions, autophagy prevents cells from death beyond pathogen infection sites. During the compatible plant-virus interactions, autophagy acts as a defense mechanism and mediates the degradation of pathogenic components or particles, while plant pathogens have evolved mechanisms to counteract or hijack autophagic processes to promote their own infection or virulence. In addition, autophagy may prevent senescence and tissue death of infected plants to promote plant fitness to the benefit of pathogen production and transmission. Recent studies have demonstrated that autophagy plays a dual role, akin to a 'double-edged sword', during plant- pathogen interactions. On the one hand, autophagy acts as a defense mechanism against invading pathogens by recognizing and interacting with their virulence factors directly, further transporting these factors to vacuoles for degradation. In turn, certain pathogens possess the ability to suppress or manipulate autophagy by directly interacting with ATG proteins or hijacking autophagy to remove host immune-related proteins, such as AGO1, SGS3, thus evading the host immune response and promoting pathogen infection. In this review, we will summarize the latest progress and discuss the significant roles of autophagy in plant responses to biotic stresses, including viral, bacterial, and fungal infections, and its role in virus-insect vector interactions. This will provide a reference and idea for plant resistance against pathogens including viruses, bacteria, fungi, and insects.