Sounding the alarm
Early detection of a pathogen attack can prevent the spread of an infection. Scientists know how plants detect pathogens and contain infections locally. But how this early detection is then translated into warning for the rest of the plant is less well understood. Part of the problem was the absence of a reporter that can capture the dynamics of this process. Now British researchers show in Nature Plants using a reporter system that the alarm passes through the plant much faster than first thought.
After a pathogen infects one leaf, the immune system sends a warning signal to the rest of the plant, telling it to prepare for an attack. As such if after that initial infection a second pathogen tries to infect the plant it is met by the plant’s immune system in full riot gear. No pathogen comes through. How fast and through which signalling molecules the alarm is activated is unknown, although scientist suggested that the plant hormone salicylic acid might be involved, as it is with other infection responses.
Unsuspected alarm raiser
Now to investigate this properly required a reporter for the alarm signal. This the researchers of the study found in a gene they now call JISS. By coupling the promoter region – the part that tells the gene when to be active – to luciferase – a gene that encodes for a protein that lights up – the researchers could see when and where JISS was activated. Using this reporter the team showed that within three to five hours after infection JISS was activated throughout the plant. This was before there where even visible signs that the plant was infected. That was surprisingly fast.
Next came trying to find out where in the process of sounding the alarm the activation of JISS fitted. So, the researchers checked if the alarm was still sounded in a bunch of known defence response mutants. In all of the mutants tested the warning signal was still able to travel through the plant. Suggesting that the alarm by which JISS is activated comes before the defence response is raised.
Subsequently the researchers wanted to find out the signalling molecule that sets off this early pathogen alarm. Surprisingly they found that this was not the plant hormone salicylic acid or a signalling molecule related to salicylic acid. Instead, the plant hormone jasmonic acid activated the alarm. They confirmed this using mutans that could not make or detect jasmonic acid. In these mutants the JISS-reporter signal did not travel, and the plant was not prepared for a second infection.
Calcium required for alarm
To find the alarm signal the researchers next checked if they could detect any electrical signal. By connecting electrodes to different leaves of the plant the researchers noticed that after infection, first there is a clear electrical signal in the infected leaf, followed by one in its neighbours and later a weak signal in a more distant leaf. This electrical signal was absent in mutans that could not make or detect jasmonic acid.
The researchers suspected that the source of this electric signal might be calcium. They tested this with blocking calcium signals and checking the JISS-reporter signal. After blocking calcium, no reporter signal was detected. Indicating that calcium is required for spreading the warning signal.
So, within hours after detecting an infection, before outward signs are even seen, the whole plant is protected via this jasmonic acid sounded and calcium facilitated alarm signal.
Literature
Gaikwad, T., Breen, S., Breeze, E. et al. Rapid local and systemic jasmonate signalling drives the initiation and establishment of plant systemic immunity. Nat. Plants 12, 152–163 (2026). https://doi.org/10.1038/s41477-025-02178-4
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