Endoparasites spend their entire life cycles within the tissues of other plants except for emerging briefly to flower and set seed. They exist for most of their lives as a hidden endophyte – a series of filaments or masses of cells from which sinkers (analogous to roots) tap into the conducting tissues within their host plants’ roots and stems.
Our recent work, carried out in collaboration with scientists at the Harvard University Herbaria and University of São Paulo, reviews the extent to which these extraordinary ‘plants that live inside plants’ show similarities to fungi – organisms that belong to a different kingdom altogether. The parallels are particularly clear in biotrophic fungi – those that derive nutrients from living tissues. Both groups produce organs for penetrating their hosts and extracting nutrients from them, and both show filamentous growth within their hosts’ tissues.
In the transition this extreme life history, endoparasitic plants and fungi have both lost genes. For example, the genes needed for photosynthesis are missing in many parasitic plants, meanwhile those used to encode certain enzymes in some fungi are also redundant. Besides gene loss, the intimate associations between plants and fungi and their hosts have also led to the accumulation of new genes: so-called horizontal gene transfers (HGTs) – the rare exchange of genetic material between non-mating organisms, which has been shown to be unusually frequent in parasitic plants. Plants in the Rafflesiaceae (a family containing the world’s largest flowers) contain genes from the cucumber and carrot relatives, presumably reflecting past associations with hosts from these families. Thus these HGTs can be thought of as ‘DNA fossils’ – clues that scientists can use to unravel the complex puzzle of extinct relationships among plants that spans evolutionary time. The connection between a parasitic plant and its host can also facilitate the movement of viruses, proteins, and RNA (which acts as a messenger, carrying the instructions of the DNA). Astonishingly, some parasitic plants and fungi have evolved strategies to infect their hosts through RNA-based communication and ‘molecular camouflage’ – stealthily invading their hosts, unnoticed. For example, some fungi have been shown to use molecular messaging to disarm their hosts’ immune responses.
Staff at Oxford Botanic Garden are propagating some of these unusual plants, which are rare in cultivation, for research and conservation purposes.