Each of the world's 300,000 plant species is a target for attack from a range of nearly 400,000 species of plant-eating insects. Herbivory is among the Earth's most important interactions in terms of number of taxa, biomass and mass transfer, as well as evolutionary impact on plant traits, community structure and ecosystem function. The population biologists Paul Ehrlich and Peter Raven have even claimed that insect herbivory has generated much of terrestrial biodiversity. Yet we are now realizing that the interactions between plants and insects are far more dynamic than was previously thought, and involve much shared chemistry.

Insects are responsible for 15% of the world's crop losses; even in natural systems, they consume 10% of plant production each year. Herbivory is limited directly or indirectly by plant chemicals, which at first glance seem to have no role in normal plant metabol- ism but are highly active in animal tissues. Plant–herbivore interactions are often described as an ongoing biochemical warfare that occurs on an evolutionary timescale.

But our perspective of plant defences and plant–insect interactions is now changing. Plants are not static, chemically defended fortresses — they respond to attack with rapid, long-lasting, variable, and often specific biochemical, physiological and developmental changes. Plants respond differentially to many stimuli, including various insect species. Every class of constitutive chemical defence responds to a physical insect attack, or to insect regurgitant or saliva, over periods of minutes to days. The few 'stealthy' insect species that fail to elicit any response at all are now considered exceptional.

Several signalling pathways coordinate these responses. Fatty-acid signals (for example, oxylipins, which are synthesized from linolenic acid released from membranes by lipases) regulate expression of defence-related genes and are central to most wound-mediated plant responses. Peptides, phenolics, terpenoids and classical plant hormones (such as cytokinins and ethylene) also can help to coordinate plant responses.

As plants recognize pathogens by detecting specific molecules, it surely follows that feeding insects must likewise present identifying chemistry to plant receptors. But few such response elicitors have been isolated from insects. Salivary digestive enzymes have some activity, as do several fatty-acid derivatives. The most complete study so far indicates that phospholipid/amino acid conjugates found in insect guts — and hence presumably in 'spit' (or regurgitant) — elicit what may be herbivore-specific volatile emissions from the host plant. These emissions in turn attract parasitoids, which kill the insect pests.

Plant-response elicitors, and the fatty-acid-based oxylipins used by plants themselves to organize defence, are similar to signals used in animal defence systems. Eicosanoids in animals (including insects) match plant oxylipins in terms of biosynthesis, structure, function and oxidative modification. These fatty-acid signals, including prostaglandins, leukotrienes and thromboxanes, mediate many immune and inflammation responses in animals. Eicosanoid receptors and their genes, as well as their structure–function relationships, have been characterized in vertebrates. Much of their activity is attributed to structural features that are shared by plant oxylipins.

Other chemical signals that are shared across the plant and animal kingdoms include peptides and glycoproteins, nitric oxide, reactive oxygen species, neurotransmitters and plant growth hormones (auxins and cytokinins). Plant phenolics and steroids interact with animal steroid receptors, and many plants produce molecules that act as insect hormones. The signalling activities of many of these molecules are known only in one of the kingdoms, but this probably reflects an absence of evidence — or study — rather than evidence of absence.

Plant defence responses require differential gene expression in signalling, sensory and metabolic pathways. It now seems that 800–1,500 differentially regulated genes are involved in a plant's response to a single insect, a complexity that matches that of animal responses.

Searching plant and animal genomes for sequences that are likely to share functions has the potential to revise our view of plant defences completely— for example, a sequence that encodes a putative glutamate receptor has now been found in the genome of the mustard weed (Arabidopsis thaliana). Ecologists have long known that plants produce neurotransmitters, including glutamate, that are presumably involved in defence against animals. The discovery of such receptors in plants suggests that such 'secondary metabolites' are more than purely defensive, and that plants may be able to sense and respond to common animal signals.

There are important ecological and evolutionary implications of plants and animals sharing signalling systems. Theoretically, each could manipulate the other's detection and response mechanisms. Insects present familiar signals to plants when they attack, and plant food contains chemicals that interfere with signalling in animals. This latter observation is the basis of phytopharmacology, but is unappreciated in ecological and evolutionary contexts. Although the phylogenetic distance between plants and herbivores is great, the organisms share a common code-book, which explains how plants might identify insects, how both plants and insects might 'jam' each other's signalling, and how plants could wreak havoc in insects by interfering with immune responses, reproduction and behaviour.

This new view of herbivory, emphasizing rapid, dynamic behaviours, shifts our focus from the chemical composition of plant tissues to the factors that elicit change and the signals that coordinate it. Here there are as many similarities between plants and insects as there are differences, raising the possibility of much 'signal stealing' and 'biochemical espionage' between their two kingdoms. Perhaps, in terms of perception, signalling and biochemical behaviour, plants are really just very slow animals after all.


Ehrlich, P. R. & Raven, P. H. Evolution 18, 586–608 (1964). Karban, R. & Baldwin, I. T. Induced Responses to Herbivory (Univ. Chicago Press, Chicago, 1997). Shiu, S.-H. & Bleecker, A. B. Proc. Natl Acad. Sci. USA 98, 10763–10768 (2001). Chiu, J., DeSalle, R., Lam, H. M., Meisel, L. & Coruzzi, G. Mol. Biol. Evol. 16, 826–838 (1999).