A deeper insight into plant-pathogen warfare
When a pathogen invades a plant, defences are raised and a battle ensues at the cellular level. Communication between cells allows resources and information to be shared around an organism when needed. In plants, this is done through the symplast, a unique system of thin connection lines called plasmodesmata (PD) that flow between cells. PD are thought to be a key battleground between plant and pathogen. During immune responses, plants regulate PD to close the links between cells. Pathogens try to suppress this regulation to keep pathways open. But the exact reasons behind these actions are unknown. In the INTERCELLAR project, which was funded by the European Research Council(opens in new window), researchers used a range of experimental and computational techniques to investigate how exactly the symplast contributes to strategies of infection and defence. “Plasmodesmal responses to pathogens and pathogens that target and suppress these plasmodesmal responses are common to a range of species and disease-causing microbes,” says Christine Faulkner(opens in new window), project leader at the John Innes Centre. “I expect our findings will be broadly relevant and translatable to other species.”
Uncovering the role of the plasmodesmata
The team hypothesised that plants close PD to build up defence hormones in cells to respond to a pathogenic threat. And the pathogen was thought to fight this response so their effector proteins(opens in new window) – the tools they use to infect – could move into new cells and start manipulating them, along with keeping an open supply line of sugars to drive the infection. To put these hypotheses to the test, the INTERCELLAR team looked for genetic changes in the immune system and molecule production in mutant plants that can’t close PD. They also surveyed a fungal pathogen to find effector proteins that can move between cells. The researchers also took more targeted approaches. In host plants, they checked how immune responses changed when PD were perturbed, and used models to see if the responses were consistent with the spread of chemical signals through PD.
Unexpected and significant results
The project discovered many effector proteins from a fungal pathogen that can move through PD to uninfected cells, and even one that targets PD themselves and opens them up. This shows how the manipulation can help the pathogen. Curiously, closed PD enhanced resistance to certain pathogens but increased susceptibility to others. One extremely important result was unexpected. The researchers created plants in which they could change whether PD were open or closed. As part of this, they examined calcium responses – a key element of immune signalling. They found that stress-triggered calcium waves don’t rely on PD as previously thought, but on a diffusion of amino acids through the cell wall. This suggests most current models for calcium wave transmission in plants are likely incorrect, and instead support a hypothesis proposed in 1926 by Ricca(opens in new window) for cell-to-cell transmission of stress signals. “The unexpected calcium wave findings challenge existing models and dogma, and open many new questions about signal transmission in plants,” adds Faulkner.
Rethinking plant cell communication
Faulkner’s lab will continue in the vein of this research, with a shifted emphasis in light of the project findings. “Some of these are already taking us in directions that are not solely focused on plasmodesmata,” remarks Faulkner. “It’s taking me into uncharted waters, which is exciting!”
