Fight for the iron throne: A plant–pathogen faceoff

Xing et al. find that Arabidopsis plants ward off pathogen infection by limiting iron availability. Plant Cell

By Yingying Xing, Deepak D. Bhandari, and Jun Liu

Background: Iron is an essential nutrient for most lifeforms. Organisms get their iron from their diet. Similarly, plant pathogens too get their iron from the plants they infect. Indeed, acquisition of nutrients from plant hosts is necessary for pathogen growth inside a plant. The bacterium Pseudomonas syringae (Pst) is used as a model pathogen to study plant bacterial diseases. Plant pathogens such as Pst, have an arsenal of weapons known as effectors that help them infect, acquire nutrients from, and grow inside plants. The functions of most pathogen effectors have yet to be fully elucidated.

Question: We wanted to know if and how Pst acquires iron from its host plant and if any of its effector(s) are involved in this process. We explored these questions by studying different Pst effectors. In addition, we wanted to know if plants could prevent pathogens from accessing iron.

Findings: We found that AvrRps4, an effector protein secreted by Pst, helps strip iron from plants. The Arabidopsis plant has a protein, BRUTUS, that acts as an iron sensor and fine-tunes iron uptake. AvrRps4 inactivates BRUTUS, resulting in increased iron accumulation in the plants, enabling iron uptake by Pst. This rich source of iron helps Pst thrive inside the plants. How AvrRps4 inactivates BRUTUS is not precisely known. This strategy only works for some plants that cannot identify AvrRps4. Plants that can identify AvrRps4 could initiate an immunity war that limits the iron availability for this pathogen. We show that iron availability for pathogens is linked to plant immunity, a phenomenon called “iron immunity” or “nutritional immunity”.

Next steps: Many Pst strains do not produce AvrRps4. It will be important to investigate how these bacteria acquire iron from plants during infection. How these bacteria acquire iron from plants in their quest for dominance is an exciting topic for future exploration.

Xing Y, Xu N, Bhandari DD, Lapin D, Sun X, Luo X, Wang Y, Cao J, Wang H, Coaker G, Parker JE, and Liu J. (2021). Bacterial effector targeting of a plant iron sensor facilitates iron acquisition and pathogen colonization. Plant Cell.