Credit: HOLT

Plants can't run away from trouble and have developed sophisticated chemical defences instead. Maize, for example, releases a cocktail of volatile indole and terpenoid compounds when attacked by the beet armyworm caterpillar (Spodoptera exigua, pictured). These compounds attract a parasitic wasp, which deposits its eggs in the caterpillar; the wasp larvae then devour the caterpillar.

Writing in Proceedings of the National Academy of Sciences USA (online early edition, 5 December), two groups describe their investigations of how maize produces the substances. Monika Frey and colleagues have identified a gene, Igl, that is involved in the synthesis of indole. And Binzhang Shen and co-workers show that another gene, stc1, is required for maize to make a sesquiterpene compound (a terpenoid).

Maize releases the compounds only when under attack, so it seemed likely that the genes are activated only temporarily. Using techniques such as treating maize plants with volicitin, an 'elicitor' substance in the caterpillar's saliva, both groups show that each gene is indeed switched on only in response to damage. Finally, Shen et al. look at maize plants in which the stc1 gene is mutated, and discover that they do not produce a major volatile compound seen in the normal plants.