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Writer's pictureAndré Geremia Parise

The unknown photosynthesis of dodders

Around these days, another article came out resulting from my master's thesis, the last publication of this research with the mysterious parasitic plant Cuscuta racemosa. This dissertation "spin-off" was a small experiment that we did alongside the main work, which was published last year in Frontiers in Plant Science. As I explained in a previous post, dodders are parasitic plants. Better saying, they are holoparasites, because they depend entirely on their host plants to survive. This way of life made some species of the genus lose completely, throughout their evolution, the ability to carry out photosynthesis. In other species, this ability remained very limited.


The Cuscuta racemosa, main protagonist of my master's research | personal archive

The event that gave rise to my master's research was when a friend and I were walking home and found a dodder growing on crown of thorns plant (Euphorbia milii) in a flower bed in the centre of Pelotas, Brazil. We plucked a twig to observe it better and took it to my friend's house, where we left it on an aloe plant. When I left, we forgot the dodder twig there and, a few days later, my friend called me in surprise saying that the dodder twig had turned completely green! The natural colour of this species is golden yellow and, as far as we know, there were no reports of the presence of chlorophylls in C. racemosa.


Because of that, we started asking some questions and formulating hypotheses. First, did the dodder turn green because it wasn't finding a good host, so it needed to photosynthesise on its own to survive? Second, would it have turned green if it had detected a suitable host nearby? And if there are different hosts around, does it notice and change the amount of chlorophyll depending on how much it will need to survive? These questions gave rise to the master's main research, and the answers were yes, they perceive hosts at a distance, they produce more chlorophylls when no one is around, and they change the amount of pigments depending on the host species that is close to them.


Furthermore, we noticed that when the dodder turns green, it does not change colour uniformly, but becomes particularly greener in the region of the node of the branches; the region where the shoots grow. This also made us question whether there was a "localised photosynthesis" taking place there, perhaps to support the growth of the new shoot, which is the part of the plant that will look for a new host. To test this hypothesis, a group of friends and I developed a very simple experiment: we wrapped strips of aluminium foil around the node of the dodder branches and observed what happens after a while. According to our hypothesis, if there would be no incidence of light on the nodes, the new shoots would not develop well.


The node of C. racemosa was noticeably greener than the rest of the branch when it was separated from the host | personal archive

As a control, in another group of plants, we wrapped equal strips of aluminium foil not at the node, but immediately below. Thus, both groups (treatment and control) would receive the same amount of light and would have the influence of aluminium foil. Hence, differences in the results would be due to the blockage of light to the node, and not another factor. The result of this work was that, in fact, the new shoot grew less when there was no light to the node. However, the effect was only on length, which was about a centimetre smaller in average, and not on other parameters such as shoot weight. Another interesting result was that the dodders that had the node of their branches shaded presented higher amounts of chlorophyll than the control group, and the proportion of two different types of chlorophyll (called a and b) changed as well. The chlorophylls proportion was similar to that of green plants when they are in shaded environments. That is, apparently, when the nodes, and only the nodes of C. racemosa, are not illuminated, the twigs behave in a similar way to what green, autotrophic plants do.


This work adds another brick to the knowledge we have about the biology of dodders—in particular, their photosynthesis, which is poorly known and mysterious. Some studies have already demonstrated that certain species produce rather high amounts of chlorophylls in certain situations. It has been suggested that they do so when they are detached from their hosts or in hosts of poor quality. In this way, they would prolong their survival until they found a more nutritious host to parasitise. The interesting thing is that, as suggested by Hibberd and colleagues, due to the fact that dodders have few stomata (pores with adjustable openings through which the plant exchanges gases), most of the carbon used in photosynthesis would come from the respiration of the plant itself! It would be recycling its own carbon to stay alive longer. Unfortunately, for our work, it was not possible to directly measure photosynthesis, but this is a goal for the future!


Photo of the test treatment with the dodder nodes covered in aluminum foil. A rather simple setup! | personal archive

Anyway, the results are already quite exciting. Another noteworthy observation is that this work was done with a minimum of resources and materials, showing that, in science, the most important thing is to have good questions and creativity to test them. In addition, we published the research in Acta Botanica Brasilica, an open access journal, so this knowledge is freely available to anyone who wants to access it!


Finally, the authors of the article, in addition to myself, are Gabriela Niemeyer Reissig, Luís Felipe Basso and Ricardo Padilha de Oliveira. The work was part of my master's degree in the Graduate Program in Plant Physiology at the Federal University of Pelotas (Brazil), carried out with the supervision of Prof. Gustavo Maia Souza at the Laboratory of Plant Cognition and Electrophysiology. And, of course, with grants from CNPq and CAPES.


This post was published in Portuguese on the 14th of February, 2022.

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