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Research Project: Mining Collections of Wild Germplasm and Novel Defense Regulators for Enhanced Plant Defenses

Location: Plant Gene Expression Center

Title: The phloem as an arena for plant pathogens

item Lewis, Jennifer
item KNOBLAUCH, MICHAEL - Washington State University
item TURGEON, ROBERT - Cornell University - New York

Submitted to: Annual Review of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2022
Publication Date: 3/29/2022
Citation: Lewis, J.D., Knoblauch, M., Turgeon, R. 2022. The phloem as an arena for plant pathogens. Annual Review of Phytopathology. 60:4.1-4.20.

Interpretive Summary: The phloem is the key distribution system of sugars to the plant and thus is targeted by pathogens, particularly those that are transmitted by insects. The phloem is difficult to access for experimental studies, which has made the study of phloem restricted pathogens challenging. We discuss aspects of phloem biology that are particularly relevant to pathogen infection and focus on selected phloem-limited pathogens, including Candidatus Liberibacter asiaticus (which causes citrus greening), phytoplasmas such as Candidatus Phytoplasma solani and Candidatus Phytoplasma asteris, and viruses such as Citrus tristeza virus. We further discuss how the phloem may be sampled, to determine how the contents of the phloem change during infection. This work will help to better understand phloem biology and pathogen infection, in order to help protect plants from these devastating pathogens.

Technical Abstract: Although the phloem is a highly specialized tissue, certain pathogens, including phytoplasmas, spiroplasmas and viruses have evolved to access and live in this sequestered and protected environment, causing substantial economic harm. In particular, Candidatus Liberibacter spp. are devastating citrus in many parts of the world. Given that most phloem pathogens are vectored, they are not exposed to applied chemicals and are therefore difficult to control. Furthermore, pathogens use the phloem network to escape mounted defenses. Our review summarizes current knowledge of phloem anatomy, physiology and biochemistry relevant to phloem/pathogen interactions. We focus on aspects of anatomy specific to pathogen movement, including vascular patterns, sieve plate structure and phloem-specific proteins. Phloem sampling techniques are discussed. Finally, pathogens that cause particular harm to the phloem of crop species are considered in detail.