Botanists explore how mint makes a home for ants and mites

Domatia are hollow plant structures that house small organisms such as ants and mites, and are relatively common in tropical species. More than 600 plant species produce domatia in association with ants, and some require the presence of ants for the structures to be formed. In some cases, these plants, called myrmecophytes, also provide nutrients to their occupants via extrafloral nectaries. In exchange, the plant is protected from competitors sprouting nearby and from leaf-eating herbivores. In leaf-borne domatia, the pouch-like compartment may arise either through the curling-under of the leaf blade or by cell proliferation that creates a hollow cavity within the leaf.

In a recent study published in Annals of Botany, Emma Sarath and colleagues studied domatium development and structure in a Bornean member of the mint family, Callicarpa saccata, to determine how they formed. The researchers used scanning electron microscopy, sectioned leaf tissue, and microcomputed tomography to examine domatia throughout their growth and compare them to a closely-related, non-domatium-forming relative, Callicarpa subaequalis.

Imaging and tissue examination revealed that there is excess cell proliferation at the intersection of the leaf blade and its petiole, which leads to a warping of the leaf surfaces and the opening of a hollow cavity on each side of the midvein, accounting for about 10% of the leaf’s total length. This process occurred even in the absence of ants, and in all but the youngest leaves, indicating that ants are not required to induce the transformation. The inner surfaces of the domatia formed glands that exuded a sucrose solution, which also occurred independent of the presence of ants. The absence of a well-differentiated palisade layer within the part of the leaf blade forming the domatium confirms, however, that the structures are in support of the ant-plant symbiosis rather than for photosynthesis.

The authors write that similar cell proliferation can produce such pouch-like structures in several remotely related taxa. “Thus, organogenesis of tubular or dome-like shapes mediated by cell proliferation might be a common strategy in angiosperm leaves.” They also note that “the development of similar sac-like foliar domatia in these distantly related families also indicates that these structures represent an interesting case of parallel evolution.” Future work by the group will compare domatium formation in several taxa from different families, though the authors also note that work on gene expression in developing domatia “would allow us to understand the molecular mechanisms that alter leaf morphology and support mutualistic relationships with ants.”