Lights Out. The Chloroplast Under Attack During Phytoplasma Infection?

Phytoplasma infections lead to high economic losses in different crops worldwide. The array of symptoms induced by an infection depends on the causative phytoplasma species and the infected plant host. The symptom of yellowing or chlorosis is common and characteristic for all so far known phytoplasma diseases. Yellowing and chlorosis indicate that the chloroplast and its components are affected during phytoplasma infection and indeed, perturbations of the photosynthetic apparatus and carbohydrate metabolic pathways of the host plant are the most prominent alterations seen in diverse phytoplasma pathosystems. Even though the role of the chloroplast and its function as a target in effector-mediated plant pathologies becomes more evident, its importance during phytoplasma infection remains elusive. In this article, we provide an overview of the current knowledge of the chloroplast role in plant immunity, and why it is an interesting target for phytoplasma. A special focus will be on pathogen effectors that directly target the chloroplast. On this basis, we propose three hypotheses about the potential role of this organelle in phytoplasma infection. The aim of this article is to motivate research to experimentally test these hypotheses and provide important information that goes beyond the mere description of physiological changes seen during phytoplasmal infections.