The Sugar Heist: A Sneaky Pathogen Siphons off Sugars with Plant’s Own Pipes
By Tegan Armarego-Marriott
We tend to think of the world’s little nasties — parasites and pathogens — as intrinsically lazy. Instead of having the common decency to either make their own carbons (big shout out to the plant world!), or to hunt and gather them, these little sneaks simply find a host, and suck out whatever goodness they can find. But in the natural world, it turns out that really there is no such thing as a free feed. For the pesky pathogen known as clubroot (Plasmodiophora brassicacae), getting the sugars it needs requires multi-level manipulation, both to override the host’s control of its own sugar metabolism, and to completely remodel its host plant’s inner pipes for sugar delivery.
Clubroot is a soil borne pathogen that infects members of the Brassicacae family, a.k.a the cabbages. As well as including many important crops, such as broccoli, cauliflower, turnips, radishes, and the oil plant canola (rapeseed), the Brassicaceae family also includes the favourite plant of many scientists — the ‘model rat’ of the plant world, Arabidopsis thaliana. Cue research.
Infection by clubroot can result in massive losses to crops. Unfortunately, although infection is easily recognizable by the formation of club-like galls on plant roots (get it: clubroot), it isn’t so easy to treat or cure. But previous research suggested that a critical point in the development of the disease might be the pathogen’s ability to steal sugars from its host. Walerowski and colleagues recently investigated this sugar link further, with their research providing vital clues that could help in the fight against clubroot.
In order to better understand how clubroot coaxes its sugary feed from its host, the authors deliberately infected Arabidopsis thaliana with the pathogen. They noticed that infection resulted in changes in specific plant enzymes involved in sugar metabolism (making and breaking sugars), and transport. Generally speaking, enzymes used for making and moving sugars were increased, while those involved in breaking them down were decreased. These manipulations seemed to work in favor of the pathogen: following infection, sugar accumulation was increased in the phloem of the plant (specialized pipes used for sugar transport).
Interestingly, when the authors tried infecting mutant plants, defective in some of those up-regulated sugar transporters (aptly named SWEET transporters), they found significant delays in the life cycle of the pathogen. So disrupting sugar supply may be a key to slowing down the disease.
As if that wasn’t enough, Walerowski and colleagues demonstrated that not only is there more sugar in the phloem of infected plants, but the phloem cells are completely remodeled, becoming more complex in infected plants. Again, these changes seem to benefit the pathogen — mutant plants whose pipes could not be remodeled hosted smaller pathogen galls. Amazingly, pathogens forced to infect these inferior hosts simply adapted! They sped up their life cycle and released spores earlier, presumably in order to send the next generation off to find a fitter host to colonize.
In fact, it was the mutant plants themselves who suffered the most. Plants unable to remodel their phloem died prematurely, whereas those which could lived long enough to produce seeds. So although the sugar pipe intervention may indeed benefit the pathogen, it might also helps out the host. It seems as though the complex architectural undertaking is something of a win-win.
All in all, this work reveals the amazing lengths to which pathogens will go to get what they need, while also providing some clues that might ultimately help us assist crops in overcoming the clubroot disease.
Piotr Walerowski, André Gündel, Nazariyah Yahaya, William Truman, Mirosław Sobczak, Marcin Olszak, Stephen Rolfe, Ljudmilla Borisjuk, Robert Malinowski. (2018). Clubroot Disease Stimulates Early Steps of Phloem Differentiation and Recruits SWEET Sucrose Transporters within Developing Galls. Plant Cell 30: 3058–3073; DOI: https://doi.org/10.1105/tpc.18.00283
Armarego-Marriott Tegan. (2018). Sugar Architect: The Brassicaceae Pathogen Clubroot Manipulates Plants on Multiple Levels to Secure Sucrose Supply. Plant Cell 30: 2896–2897; DOI: https://doi.org/10.1105/tpc.18.00886