Alga of the year 2025: newly discovered Draparnaldia erecta will help shed light on plant evolution

Why to study Draparnaldia?

There are two groups of green algae – chlorophytes and streptophytes. They both evolved multicellularity independently and they often co-exist in the same habitats. However, only Streptophyte algae have managed the transition from water to land (= plant terrestrialization) and diversified into the amazing variety of the land plants that we can see nowadays. This raises an important question: Why was the conquest of land accomplished solely by Streptophyte algae and not by their sister lineage, the Chlorophyte algae?

Multicellularity and terrestrialization in green plants are studied using a variety of morphologically complex model organisms. There are well established models in land plants, such as Marchantia, Physcomitrium and Arabidopsis. There are also being developed new models in Streptophyte algae - direct ancestor of land plants– for example Klebsormidium, Chara and Zygnema. However, there are no models and genomes of Chlorophyte algae that would morphologically resemble early land plants and at the same time has adaptations to live on land. Only the colonial Volvox and the folious Ulva are established as multicellular Chlorophyte models. Draparnaldia is closing this gap (Fig 1).

Figure 1. Position of Draparnaldia in the phylogeny of green plants. Adapted from Caisova et al. 2024.

Uniqueness of Draparnaldia

Draparnaldia is very special in a sense that it is the only Chlorophyte alga, which acquired a morphological complexity comparable to early land plants. It has entirely unique morphological adaptations to both, aquatic and terrestrial habitat. Crucially, these adaptations can be induced under controlled laboratory conditions. Draparnaldia, thus allows studying terrestrialization in green plants from a completely different point of view. At present, Draparnaldia is the only model that can address either of these topics: (i) alternative evolution of multicellularity and complexity and (ii) terrestrialization.

This alga also possesses features that are important for research: it is easy to culture and it is fast growing, completing its life cycle within 7-9 days. Draparnaldia reproduces via zoospores (unicellular stages), that can be induced simultaneously in large amounts. This is important to conduct experiments on multicellularity.

Establishment of Draparnaldia as a new model

Our aim is to establish Draparnaldia as a model for both the algal and plant community. To do so, we have developed several essential tools and resources. We have protocols to grow the alga in laboratory. We have a good knowledge about Draparnaldia’s morphology, development and its adaptations to water and to land. We have profiled Draparnaldia’s phytohormone repertoire. We have also sequenced the de novo genome and several transcriptomes of Draparnaldia. Moreover, we have established the transient transformation of this alga and we are currently developing protocols for stable transformation and CRISPR/Cas.

Figure 2. Morphology of Draparnaldia. The alga consists of two distinct parts: the Upright System (US) (left) and Prostrate System (PS) (right). Image taken from Caisova 2020.

Draparnaldia and Evolution of multicellularity

The origin of multicellularity in algae is very poorly understood, and so far it has been studied only in two multicellular Chlorophyte green algae - the colonial Volvox and the folious Ulva. Interestingly, a comparison of their genomes with a genome of the unicellular Chlorophyte alga Chlamydomonas did not show any expanded gene families directly associated with multicellularity. However, when we compared the genome of Draparnadia with the genome of Chlamydomonas, we found expanded gene families associated with multicellularity, development and abiotic stresses. This gives strong evidence that Draparnaldia provides and will provide key novel insights into the evolution of multicellularity.

Draparnaldia and Terrestrialization in green plants

Draparnaldia diverged from Streptophyte algae 1000 million years ago. Despite this independent evolution, Draparnaldia shows striking morphological, physiological and molecular analogies to the process of terrestrialization observed in Streptophyte algae and land plants. Similarly, despite Draparnaldia synthesizing most of the phytohormones used by land plants, it lacks many of their canonical signaling components. This suggests that Draparnaldia evolved an alternative mechanism of hormonal signal transduction. Since plant hormones and their signaling are key innovations in plant terrestrialization, our findings support the importance of Draparnaldia as an exceptionally valuable model to study alternative ways of terrestrialization in green plants.

In summary, Draparnaldia represents a major breakthrough in our understanding of plants’ acquisition of multicellularity and their conquest of land – a key event in Earth’s history. Moreover, the function of phytohormones in algae is not known. Draparnaldia has, thus, a huge potential to bring valuable insights into this topic.

Literature

Caisová, L., Crombez, E., Arellano, M.S.T., Gut, M., Alioto, T.S, Gómez-Garrido, J., Dabad, M., Esteve-Codina, A., Petřík, I., Pěnčík, A., Novák, O., Van de Peer, Y., Vicoso, B., and Friml, J. (2024). The Draparnaldia Genome: Alternative Mechanisms for Multicellularity and Terrestrialization in Green Plants. bioRxiv. https://doi.org/10.1101/2024.09.12.612648.

Caisová, L. (2020). Draparnaldia: A Chlorophyte Model for Comparative Analyses of Plant Terrestrialization. J Exp Bot 71, 3305–3313. https://doi.org/10.1093/JXB/ERAA102.

Caisová, L., and Jobe, T.O. (2019). Regeneration and Transient Gene Expression in Protoplasts of Draparnaldia (chlorophytes), an Emerging Model for Comparative Analyses with Basal Streptophytes. Plant Methods 15, 1–14. https://doi.org/10.1186/s13007-019-0460-6.