Unveiling the phloem: a battleground for plant pathogens

Abstract

Phloem is the primary conduit for transporting photosynthates and signaling molecules in plants, facilitating communication between various plant organs. As an ancient vascular tissue, phloem transports sugars, proteins, and hormones from source tissues to sinks over long distances. However, this vital transport system also serves as a battlefield where plants and pathogens compete for survival. The phloem’s nutrient-rich environment offers pathogens a secure habitat, protecting them from external threats while providing ample metabolic resources. Phloem-feeding insects, bacteria, fungi, and viruses exploit this system to access nutrients, leading to widespread diseases and yield losses. These insects can also transmit pathogens, such as viruses, which can evade the plants’ defense systems, causing systemic damage throughout the transport network. This review describes the mechanisms by which pathogens invade and colonize the phloem, the plant’s defense strategies, and their dynamic interactions. Understanding the phloem’s structural intricacies, physiological functions, and defense mechanisms provides a foundation for comprehending phloem–pathogen interactions. Insights into these interactions at the molecular level are crucial for developing innovative and effective disease management strategies. Genomics, proteomics, and bioinformatics advances have elucidated the interactions between phloem defenses and pathogen offenses. Finally, this review discusses integrated disease management strategies to counteract these pathogens, paving the way for improving plant health and resilience.