Domatia are hollow structures in plants occupied by ant colonies, in turn ants provide protection against herbivores. In plants, competition for resources has driven sex-related changes in the patterns of resource allocation to life-history traits and defence traits. The resource-competition hypothesis (RCH) proposes that female plants due to their higher investment in reproduction will allocate fewer resources to defence production, showing greater herbivore damage than other sexual forms. We hypothesise the existence of sex-related differences in defensive traits of domatia-bearing plants, being female plants less defended due to differences in domatia traits, such as size, number of domatia and their position, exhibiting more herbivore damage than hermaphrodite plants of Myriocarpa longipes, a facultative neotropical myrmecophyte. We found eight species of ants inhabiting domatia; some species co-inhabited the same plant, even the same branch. Our results are consistent with the predictions of RCH, as female plants had ant-inhabited domatia restricted to the middle position of their branches and exhibited greater herbivore damage in leaves than hermaphrodites. However, we did not find differences in domatia size and leaf area between sexual forms. Our study provides evidence for intersexual differences in domatia position and herbivory in a facultative ant–plant mutualism in M. longipes. We highlight the importance of considering the plant sex in ant–plant interactions. Differences in resource allocation related to sexual reproduction could influence the outcome of ant–plant interactions.

Thousands of plant species throughout tropical and temperate zones secrete extrafloral nectar (EFN) (see www.biosci.unl.edu/emeriti/keeler/extrafloral/worldlistfamilies.htm) to attract ants, whose presence leads to an indirect defence against herbivores (Chamberlain & Holland 2009, Heil 2008, Heil & McKey 2003, Rico-Gray & Oliveira 2007). Although termed ‘extrafloral’ because the nectar is not involved in pollination, EFN can also be secreted within the inflorescences (Bentley 1977, Holland et al. 2010, Martins 2009). Because ants tend to defend reliable food sources against all types of putative competitors, it has been hypothesized that the presence of extrafloral nectaries close to flowers may lead to competition among ants and pollinators, or even to direct ant–pollinator conflicts. Such antagonistic interactions would reduce the access of pollinators to flowers and, thereby, may cause significant ‘ecological costs’ of indirect, ant-mediated defences (Heil 2002).

Aphids are phloem-feeding insects that damage many important crops throughout the world yet, compared to plant–pathogen interactions, little is known about the mechanisms by which plants become resistant to aphids. Medicago truncatula (barrel medic) is widely considered as the pre-eminent model legume for genetic and biological research and in Australia is an important pasture species. Six cultivars of M. truncatula with varying levels of resistance to two pests of pasture and forage legumes, the bluegreen aphid Acyrthosiphon kondoi Shinji and the spotted alfalfa aphid Therioaphis trifolii f. maculata. (Buckton) are investigated. Two resistance phenotypes against T. trifolii f. maculata are described, one of which is particularly effective, killing most aphids within 24 h of infestation. Each resistance phenotype provided a similar but somewhat less effective degree of resistance to the closely-related spotted clover aphid Therioaphis trifolii (Monell). In the case of A. kondoi only one resistance phenotype was observed, which did not vary among different genetic backgrounds. None of the observed resistance against A. kondoiorT. trifolii f. maculata significantly affected the performance of green peach aphid Myzus persicae (Sulzer) or cowpea aphid Aphis craccivora Koch. The existence of multiple aphid resistance mechanisms in similar genetic backgrounds of this model plant provides a unique opportunity to characterize the fundamental basis of plant defence to these serious agricultural pests.

Plant roots enter symbiotic as well as pathogenic interactions with fungi in the rhizosphere. We studied the response of bean (Phaseolus vulgaris L. cv. Saxa) roots to infection by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and the pathogen Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burkholder) Snyder & Hansen. In a time-course study of the symbiotic interaction between bean roots and G. mosseae, covering all stages of mycorrhiza development, we detected little change in the expression of the defence-related genes chitinase, β-1,3-glucanase and phenylalanine ammonia-lyase compared with non-mycorrhizal control roots. The only difference observed was a transient increase in chalcone synthase transcripts at later stages of mycorrhizal root colonization. In interactions with the pathogen, a marked induction of chitinase and phenylalanine ammonia-lyase expression was observed at the level of both the transcripts and enzyme activities. Class I β-1,3-glucanase levels strongly increased at the transcript level, but there was little change in the overall β-1,3-glucanase enzyme activity. In the non-host interaction between common bean and Fusarium solani (Mart.) Sacc. f. sp. pisi (Linford) Snyder & Hansen defence responses increased only slightly and transiently, if at all.