Effector biology during biotrophic invasion of plant cells

Chaudhari, Prateek; Ahmed, Bulbul; Joly, David L; Germain, Hugo

Fungal spores grow on plant surfaces upon germination. It has been shown that the rust fungus Uromyces appendiculatus uses topographical cues for orientation and the formation of infection structures. 51 Once U. appendiculatus detects a 0.5-μm ridge, which it interprets as the presence of the stomatal lip (its entry point into tissue), it starts producing its infection structure. 51 When the pathogen has forced its way into plant tissue, nutrient acquisition and defense suppression occur primarily through haustoria, although effectors are also released from growing hyphae. Support for such a mechanism is lent by deep sequencing of the biotrophic growth phase of Colletotrichum higginsianum during A. thaliana infection. 52 In this pathosystem, effector genes are expressed in consecutive waves associated with pathogenic transition, and some are expressed before host invasion at the appressorial stage. 52 In fact, multi-stage transcriptome analysis of Melampsora larici-populina, the causative agent of the poplar leaf rust (obligate biotroph), revealed that a number of small-secreted proteins were even expressed in resting urediniospores. 53 Therefore, we can infer that suppression of plant immunity starts prior to the formation of haustorial structures in host tissue. While our understanding of molecular partners at play is progressing, we have made few inroads into the establishment of plant-haustoria interactions and post-invasion events. Dynamic interplay could be mainly driven by the invader, and as we progress in this review, we will examine some important phenomena that may hold clues to these questions.