Much of our understanding of plant defense response is built upon the responses that occur in leaves. Many pathogens colonize the phloem system, which is both nutrient-rich and provides an easy conduit for spreading through systemically through the plant body. These phloem-inhabiting prokaryotic pathogens are introduced and spread through phloem-feeding insects. Jiang et al. describe the challenges in studying these pathogens, which can cause serious diseases including citrus greening disease (aka “huanglongbin” or HLB). Phloem-resident prokaryotic pathogens include phytoplasmas and spiroplasmas, which are notable both for having a reduced genome and being difficult (or impossible) to culture in vitro, contributing to the challenges of studying them. (Summary by Mary Williams) Proc. Natl. Acad. Sci. USA 10.1073/pnas.1915396116. (This PNAS perspective is contributed by Sheng Yang He, who is also featured in a QnA https://doi.org/10.1073/pnas.1918514116).
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