Plant Signalling: How a Local Attack Triggers a Wider Response

When an insect damages a leaf, the plant's response can extend beyond the injured tissue. Cells detect chemical cues, signals travel through tissues and defence processes can change in parts of the plant that were never directly attacked.

Plant signalling connects the detection of a change with responses within and between tissues. Defence is one part of this wider network, which also regulates growth and development.

How a plant detects a threat

Plant cells have receptors that recognise molecules associated with microorganisms or damaged tissues. This recognition can trigger rapid changes in calcium concentrations within cells, the production of reactive oxygen species and the activation of signalling proteins.

These early events affect gene expression and metabolism. Depending on the stimulus, the plant may produce antimicrobial compounds, strengthen cell walls or activate other protective responses.

Plant hormones help coordinate these processes. Three are particularly important in defence: salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). They do not operate as independent switches; their activity is connected through a wider signalling network.

Three hormones with different roles

Salicylic acid (SA) is strongly involved in immunity against many pathogens that depend on living plant tissue. It influences defence-related gene expression through regulatory proteins including NPR1 and plays an important role in systemic acquired resistance.

Jasmonic acid (JA) is particularly associated with responses to insect feeding, wounding and pathogens that kill plant tissue. Jasmonate signalling can stimulate the production of defensive compounds and proteins that interfere with attackers.

Ethylene (ET) is best known for its role in fruit ripening, but it also participates in responses to pathogens, flooding and mechanical damage. In plant defence, it often interacts with jasmonate signalling and can alter the strength or nature of the response.

These associations are useful starting points, not fixed rules. The outcome depends on the plant, the attacker, the timing and the interaction with other signalling pathways (Zhang et al., 2025; Tian et al., 2025).

Why defence signals influence one another

The interaction between signalling pathways is called hormonal crosstalk. Salicylic acid and jasmonic acid can suppress each other's activity under some conditions, while supporting each other under others.

This helps explain why plant defence cannot be divided neatly into one pathway for fungal diseases and another for insects. Aphids and whiteflies, for example, interact with plant signalling in ways that do not always fit the familiar SA–JA distinction. These exceptions are explored in Plant Defence Doesn't Follow Simple Rules.

Defence signalling also interacts with growth. Producing protective compounds and maintaining immune activity requires resources, while some defence pathways directly influence growth regulation. The balance depends on the circumstances; stronger defence activity does not necessarily produce a healthier or more productive plant.

For the difference between a challenging condition, a protective response and actual damage, see Do Plants Get Stressed Out?.

How local interactions influence defence elsewhere

An infection in one leaf can increase resistance in tissues that were not directly infected. This is known as systemic acquired resistance (SAR).

Salicylic acid is important in this process, but the response involves more than SA moving away from the infection site. Other signals, including N-hydroxypipecolic acid (NHP), help coordinate defence in distant tissues.

A different starting point is found around the roots. Certain beneficial microorganisms can influence plant signalling and contribute to induced systemic resistance (ISR). In this case, an interaction in the root environment can affect how aboveground tissues respond to a later challenge.

ISR has traditionally been associated with jasmonate- and ethylene-related signalling, although salicylic acid and other pathways can also be involved. The response depends on the plant and microorganism involved.

This is one reason certain Bacillus strains are studied for their effects on plant immunity. The bacteria do not necessarily need to reach the leaves to influence their defence responses. See How Do Bacillus Bacteria Help Plants?.

Priming: preparing for a later challenge

A plant can respond to a signal by activating defence processes immediately. It can also enter a primed state, in which a later challenge triggers a faster or stronger defence response. Defence activity does not necessarily remain high between the first stimulus and the subsequent challenge.

Hönig et al. (2023) describe how certain chemical treatments can induce this effect. Priming may persist after the treatment itself and can influence tissues beyond the application site.

The distinction matters when evaluating a treatment. Increased expression of defence-related genes immediately after application demonstrates a signalling response, but not necessarily priming. To establish priming, researchers must examine what happens when the plant encounters a later challenge.

Priming can reduce the need for continuous defence activation, although it is not automatically free of physiological costs.

What plant signalling means for growers

Plant signalling explains why applying a substance associated with defence does not always produce the same result. Concentration, timing, plant development and environmental conditions all influence the response.

Salicylic acid is a good example. It participates in immunity, but also interacts with pathways involved in growth and responses to environmental stress (Janda et al., 2020). An application that changes SA signalling does not automatically improve resistance or crop performance.

When comparing treatment evidence, check whether researchers measured only an early signalling response or also tested disease severity, plant damage or growth after a challenge.

Salic Signal is based on willow bark extract containing naturally occurring salicylates. Its composition and application information should be considered separately from experimental findings using pure salicylic acid.

References

Zhang, P., Jackson, E., Li, X., & Zhang, Y. (2025). Salicylic acid and jasmonic acid in plant immunity. Horticulture Research, 12(7), uhaf082. DOI: 10.1093/hr/uhaf082.

Tian, H., Xu, L., Li, X., & Zhang, Y. (2025). Salicylic acid: The roles in plant immunity and crosstalk with other hormones. Journal of Integrative Plant Biology, 67(3), 773–785. DOI: 10.1111/jipb.13820.

Hönig, M., Roeber, V. M., Schmülling, T., & Cortleven, A. (2023). Chemical priming of plant defense responses to pathogen attacks. Frontiers in Plant Science, 14, 1146577. DOI: 10.3389/fpls.2023.1146577.

Janda, T., Szalai, G., & Pál, M. (2020). Salicylic Acid Signalling in Plants. International Journal of Molecular Sciences, 21(7), 2655. DOI: 10.3390/ijms21072655.