Plant Defence Doesn’t Follow Simple Rules

Aphids feed on plant sap without chewing large holes in leaves. Yet their feeding can activate both salicylic acid and jasmonic acid signalling. Whiteflies can produce an even more surprising result: in some plant–insect interactions, the defence response they trigger can favour the insect rather than restrict it.

This challenges a familiar explanation of plant defence. Salicylic acid (SA) is often associated with biotrophic pathogens and phloem-feeding insects, while jasmonic acid (JA) is associated with chewing insects, tissue damage and necrotrophic pathogens.

These associations are useful, but they do not predict every response. What matters is how the signalling pathways interact and whether the resulting defence actually restricts the attacker.

Aphids can activate both defence pathways

Aphids insert needle-like mouthparts, called stylets, into plant tissues to reach the phloem. They cause relatively little visible tissue damage compared with caterpillars, but the plant still recognises their activity.

Rubil et al. (2022) studied cabbage aphids feeding on Arabidopsis. They observed both SA- and JA-related signalling around the points where aphids inserted their stylets. These responses spread along leaf veins over the following hours.

Earlier work also found that aphid feeding changes the expression of genes associated with both pathways. Kuśnierczyk et al. (2008, 2011) showed that the response develops over time and that jasmonate signalling can contribute to resistance against cabbage aphids.

The important distinction is that phloem feeding does not mean JA is irrelevant. Even insects that cause little visible damage can activate several interacting defence mechanisms.

Whiteflies can turn signalling to their advantage

Some insects appear to benefit from the way plant defence pathways influence one another. An increase in SA signalling can suppress parts of the JA response, a form of interaction known as hormonal crosstalk.

Zarate et al. (2007) investigated silverleaf whiteflies feeding on Arabidopsis. They found that whitefly feeding activated SA-related responses while several JA-related responses were reduced. In their experiments, whitefly development was faster on plants with enhanced SA defence or impaired JA defence.

Zhang et al. (2013) showed an important detail: whitefly feeding could activate genes involved in JA production while suppressing a downstream JA-regulated defence response.

This means that detecting JA-related activity is not enough to conclude that an effective JA defence is operating. Different parts of the same pathway can respond differently.

Why feeding style is only part of the story

Chewing insects generally cause substantial tissue damage and frequently activate JA-related responses. Phloem feeders often induce SA-related responses, while thrips damage surface cells before feeding and can trigger responses associated with tissue injury.

But a plant does not identify an attacker simply as a chewing or sap-feeding insect. Its response is shaped by the damage, insect-derived compounds, plant species, affected tissues and timing.

The same signalling pathway can also affect different attackers in different ways. A response that helps resist one insect may have little effect on another.

For the wider role of SA, JA and ethylene in plant physiology, see Plant Signalling. Beneficial root-associated bacteria can influence these pathways too, as explained in How Do Bacillus Bacteria Help Plants?.

Defence involves more than hormones

Plant protection also depends on physical structures and chemical compounds. Silicon, for example, has been studied for its contribution to tissue properties and defence-related responses against certain pests and pathogens (Debona et al., 2017).

These mechanisms can interact with hormonal signalling, but silicon accumulation and a change in defence-related gene expression are different observations. Neither automatically establishes improved resistance under growing conditions.

For more about silicon uptake and its role in plants, see Silicon in Plants: Food, Biostimulant or Structural Tool?.

What this means for growers

An insect's feeding behaviour offers clues about how a plant might respond, but it does not tell you which defence pathway should be stimulated.

When evaluating research or a treatment, look beyond hormone concentrations or changes in gene expression. Did the treatment actually reduce insect survival, reproduction or plant damage?

Salic Signal is based on willow bark extract containing salicylates; Ortho Origin supplies silicon as orthosilicic acid. Their composition and application guidance are described on the product pages. Evidence for salicylic acid or silicon in a particular experiment cannot automatically be transferred to a finished product.

References

1. Rubil, N., Kalachova, T., Hauser, T. P., & Burketová, L. (2022). Specialist Aphid Feeding Causes Local Activation of Salicylic and Jasmonic Acid Signaling in Arabidopsis Veins. Molecular Plant-Microbe Interactions, 35(2), 119–124. DOI: 10.1094/MPMI-08-21-0203-SC.
2. Kuśnierczyk, A. et al. (2011). Testing the importance of jasmonate signalling in induction of plant defences upon cabbage aphid (Brevicoryne brassicae) attack. BMC Genomics, 12, 423. DOI: 10.1186/1471-2164-12-423.
3. Kuśnierczyk, A. et al. (2008). Towards global understanding of plant defence against aphids – timing and dynamics of early Arabidopsis defence responses to cabbage aphid attack. Plant, Cell & Environment, 31, 1097–1115. DOI: 10.1111/j.1365-3040.2008.01823.x.
4. Zarate, S. I., Kempema, L. A., & Walling, L. L. (2007). Silverleaf Whitefly Induces Salicylic Acid Defenses and Suppresses Effectual Jasmonic Acid Defenses. Plant Physiology, 143, 866–875. DOI: 10.1104/pp.106.090035.
5. Zhang, P. J. et al. (2013). Feeding by whiteflies suppresses downstream jasmonic acid signaling by eliciting salicylic acid signaling. Journal of Chemical Ecology, 39, 612–619. DOI: 10.1007/s10886-013-0283-2.
6. Debona, D., Rodrigues, F. A., & Datnoff, L. E. (2017). Silicon's Role in Abiotic and Biotic Plant Stresses. Annual Review of Phytopathology, 55, 85–107. DOI: 10.1146/annurev-phyto-080516-035312.