Adaptations to biotic and abiotic stress: Macaranga-ant plants optimize investment in biotic defence

Obligate ant plants (myrmecophytes) in the genus Macaranga produce energy- and nutrient-rich food bodies (FBs) to nourish mutualistic ants which live inside the plants. These defend their host against biotic stress caused by herbivores and pathogens. Facultative, 'myrmecophilic' interactions are bas ed on the provision of FBs and/or extrafloral nectar (EFN) to defending ins ects that are attracted from the vicinity. FB production by the myrmecophyt e, M. triloba, was limited by soil nutrient content under field conditions and was regulated according to the presence or absence of an ant colony. Ho wever, increased FB production promoted growth of the ant colonies living i n the plants. Ant colony size is an important defensive trait and is negati vely correlated to a plant's leaf damage. Similar regulatory patterns occur red in the EFN production of the myrmecophilic M. tanarius. Nectar accumula tion resulting from the absence of consumers strongly decreased nectar flow , which increased again when consumers had access to the plant. EFN flow co uld be induced via the octadecanoid pathway. Leaf damage increased levels o f endogenous jasmonic acid (JA), and both leaf damage and exogenous JA appl ication increased EFN flow. Higher numbers of nectary visiting insects and lower numbers of herbivores were present on JA-treated plants. In the long run, this decreased leaf damage significantly. Ant food production is contr olled by different regulatory mechanisms which ensure that costs are only i ncurred when counterbalanced by defensive effects of mutualistic insects.