Parasitic plants represent around 1.5% of all flowering species, adding up to ca. 4,750 species. The diversity of these plants, as well as aspects of their biology and evolution have drawn the attention of several naturalists and researchers — myself included! A point that is central to the discussion of how parasitic plants evolved from their non-parasitic ancestors is the development and functioning of the haustorium. This is the organ that promotes attachment, penetration and connection between parasitic plants and their hosts: “the very idea of plant parasitism.”
The prevalent interpretation is that haustoria would be modified roots since they carry out two of the same basic functions: attachment to a substrate and solute uptake. Haustoria and roots are also similar in terms of their morphological origin, emerging either from other roots or from the root-pole of a seedling. However, this is not always the case. In some parasitic species, such as dodders (Cuscuta), haustoria originate not from the roots, but from twining stems. In this case they are generally interpreted as modified and reduced adventitious roots. The parallels between haustoria and roots seems quite straightforward until we start digging deeper.
Transcriptome analyses suggest that, during the course of evolution, haustoria might have co-opted genes normally expressed in roots, but also in floral tissue. These data already indicate a more complex situation, even when the reports come from few species in the broomrape family (Orobanchaceae). But RNA molecules are not the only clue to solving this identity crisis. Studies of haustorium development and structure could also be especially useful to elucidate the nature of haustoria. With that in mind, I analyzed and compared haustorium morphology, ontogeny, and anatomy across all 12 different clades that include parasitic plants.
With my analysis, I found there is a high degree of convergence in the topology of haustorium tissues. Despite their broad morphological diversity, haustoria of different parasitic plants are more similar to each other than they are similar to other plant organs in terms of tissue organization and developmental processes. This observation suggests that a shared body plan can be identified in the mature haustorium of all parasitic plants.
Furthermore, considering various lines of evidence showing that haustoria are not fully homologous to neither roots, nor stems, I propose that this parasitic plant organ is better interpreted as a “root-shoot mosaic”. This proposal considers key features of both organs that result in a novel structure. Overlap between the identities of different plant organs is also found in other, non-parasitic plants of the riverweed (Podostemaceae) and bladderwort (Lentibulariaceae) families, which are collectively known as “morphological misfits”. Research dealing with these plants has reshaped and renewed the way we think about plant form, function, development and evolution.
Therefore, beyond offering a solution to the conflict of haustorium homology and organ identity, this new interpretation opens up new research pathways for the comparison between parasitic plants and other morphological misfits, especially in terms of their evolutionary development. It also provides a more comprehensive framework for analyzing the haustorium across the multiple angiosperm lineages, taking us one step further into the deciphering how parasitism evolved among plants.
Luiza Teixeira-Costa is a botanist based in Cambridge, Massachusetts. She is interested in the form, function, development and evolution of parasitic flowering plants. Luiza is also interested in urban trees and the history of landscaping. She is currently a postdoctoral fellow at the Harvard University Herbaria. You can find her on Twitter: @l_teixeiracosta.