A living bridge between two enemies: haustorium structure and evolution across parasitic flowering plants

Parasitic flowering plants are characterized by the development of an organ known as haustorium, which has evolved in multiple independent angiosperms clades. The haustorium has also been deemed “the most plastic of organs” due to its ability to accommodate physiological and anatomical differences between the parasite itself and its host plants. This is achieved through the development of vascular connections, which involve the differentiation of various specialized cell types by the parasite. The development, structure, and evolution of the haustorium and the connections it fosters are reviewed here considering all 12 parasitic plant lineages. A multi-level comparison between “model” parasitic plants, such as Orobanchaceae and Cuscuta species, with members of often neglected groups, such as Lennoaceae, Mitrastemonaceae, and Santalales yields the idea of a shared general body plan of the mature haustorium. This proposed haustorium bauplan is composed of an upper part, including structures associated with mechanical attachment to the host body, and a lower part, including all parasitic tissues and cell types within the host body. The analysis of multi-level convergence is also applied here to the comparison between haustoria and other plant organs. Considering the structure, molecular development, and functionality of this organ under the framework of continuum and process plant morphology, I propose the interpretation of haustoria as morphological misfits.