Chapter Ten - Like Cures Like: Caffeine Immunizes Plants Against Biotic Stresses

Caffeine (1,3,7-trimethylxanthine) is a member of purine alkaloids and produced in over 80 plant species. It is one of the oldest and widely used secondary metabolites by mankind as stimulant and ingredient in drugs. Its physiological function in nature has not completely been elucidated but is thought to participate in the chemical defence against biotic attackers. To substantiate this idea, transgenic tobacco and chrysanthemum were constructed by expressing three distinct N-methyltranferases involved in the caffeine biosynthesis pathway. Resulting plants produced a low amount of caffeine (0.4–5 μg/g tissue) yet exhibited strong tolerance against herbivores and pathogens. Their self-defence system was autonomously activated without perceiving external stresses. This can be regarded as the priming of defence response, by which host plants become on standby to cope with a broad range of biotic stresses. The feature resembles mammalian immunization or vaccination, and it was proposed that plants can also be immunized by expressing a mildly toxic ‘antigenic’ chemical such as caffeine in planta. The caffeine signal was predicted to be successively transduced through phosphodiesterase, cyclic AMP, calcium flux and salicylic acid.