3 unexpected things we owe to the plant pathogenic bacterium Xanthomonas

KamounLab
6 min readJan 30, 2023

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I have a special relationship with the bacterium Xanthomonas. As a grad student at UC Davis, I did my thesis research on Xanthomonas campestris pathovar campestris, a species of this genus of bacterial pathogens that causes disease on the vegetable and seed crops known as brassicas, which include plants such as cabbage, broccoli, radish, and mustard.

Colonies of the bacterium Xanthomonas on an agar plate. Source: Jim Deacon, University of Edinburgh.

I had a lot of fun during my Ph.D. both in the lab and outside. I relished being asked at social events what I work on. I would answer with a straight face and a deadpan tone: “Black rot of cabbage.” The looks on people’s faces were priceless. Some were confused, some were disgusted, and some were just plain amused. But for me, it was all worth it just to see their reactions.

I won’t go as far as saying that this was my favorite pickup line, but I ended up finding love with someone who appreciates my cabbage-related humor and other nerdish pleasures. At least, I like to believe that.

While it may not have made me sound like a rocket scientist compared to some of my fellow students who worked on more high-profile topics, my humorous introduction did serve its purpose: it caught people’s attention and gave me an opportunity to explain the importance of studying plants and plant diseases.

Animation video of Xanthomonas bacteria infecting a tomato plant. Must watch. Credit: Duke Pkan in collaboration with Ulla Bonas Lab.

You see, plant diseases can have serious consequences for agriculture and food security. Understanding how these diseases spread and how to control them is crucial for maintaining healthy crops and feeding the growing global population.

So while studying black rot of cabbage may not seem like the most glamorous or groundbreaking research, it’s actually a vital part of the larger puzzle. And who knows, maybe one day research on cabbage rot will lead to breakthroughs that change the world.

And that’s pretty much what happened. The bacterium Xanthomonas has given us scientific breakthroughs that go way beyond the world of plant pathology. Here are three products or technologies that we owe to Xanthomonas:

1. Xanthan gum

Xanthan gum is a high-molecular-weight carbohydrate polymer produced by Xanthomonas. It is a ubiquitous ingredient in the processed food industry, being used as a thickener, stabilizer and emulsifier. It can improve the texture, suspension, and stability of food products.

The food industry loves xanthan gum and it must have been beneficial to improving a number of products. However, I have to confess here that I’m not a fan of processed food, particularly the ultra processed type that includes multiple industrially produced ingredients. Xanthan gum, for example, is touted as a gluten-free baking replacement for gluten. But as I and many others have discovered — I’m gluten-intolerant and have been avoid gluten products for about 10 years — xanthan gum can cause digestive discomfort, and can contain gluten when the bacterium is fermented on a culture medium based on wheat.

I once had dinner from a bottled Tomato, Pepper & Cucumber Gazpacho Soup, which appeared nutritious and appealing when I saw it in the store. It was a mistake as I experienced bloating and had a rough night as a consequence. Upon closer examination, I found that the soup was mostly xanthan gum (possibly over 60% according to the ingredients list). I was essentially eating a soup of fermented Xanthomonas extracts, an example of how the food industry tricks us into purchasing processed foods disguised as healthy options.

A xanthan gum soup?

Xanthan gum is utilized in industries beyond food, such as cosmetics, pharmaceuticals, and even oil and gas drilling as an additive. The global xanthan gum market is expanding at a rate of over 5% annually and is projected to reach a value of $1 billion in the near future.

2. TALEN gene editing technology

TALENs (Transcription Activator-Like Effector Nucleases) were developed in the mid-2000s as a new tool for gene editing (GE). The TALEN technology originated from the discovery that virulence effectors proteins, so-called TAL Effectors (TALEs), secreted by Xanthomonas proteins form a totally novel type of DNA binding proteins. TALEs are programmable DNA-binding proteins because they follow a specific code where one particular protein repeat sequence binds a specific base of DNA. You can almost hear me gasp in this tweet when I realized the technology’s potential upon first hearing about it at a plant pathology conference in 2009.

It didn’t take long after that for scientists to fuse TALEs with a DNA-cutting domain derived from the FokI endonuclease to design programmable enzymes that can target specific sequences in an organism’s genome. This combination of DNA-binding and cutting capabilities makes TALENs a powerful tool for gene editing with applications in medicine and agriculture. In one gene therapy trial, TALENs were used to correct the mutation and “cure the incurable cancer” of a one-year old girl named Layla Richards. Thank you Xanthomonas.

TALENs have lost some popularity after the discovery of CRISPR-based gene editing, but the applications of TAL effectors have expanded beyond the realm of plant-pathogen interactions.

A Xanthomonas TAL effector protein wrapped up around a DNA double helix

3. The antibiotic albicidin

Albicidin is an antibiotic produced by Xanthomonas albilineans, a pathogen of sugarcane. Just like TALEs, albicidin has evolved as a virulence factor that favors the pathogen’s infection of host plants. In this case, albicidin is a phytotoxin that inhibits DNA gyrase — a critical DNA replication enzyme in plant chloroplasts — killing the plant cell as a result. Given that chloroplasts are evolutionarily related to bacteria, it turned out that albicidin can also kill various bacteria, therefore qualifying as an antibiotic with therapeutic potential.

Albicidin and leaf scald symptoms on sugarcane. Source: Pieretti et al. 2015.

Albicidin was back in the news very recently. My colleague Dmitry Ghilarov who recently joined our campus at the John Innes Centre resolved the structure of albicidin in complex with DNA gyrase and a DNA fragment, demonstrating that the molecule obstructs the gyrase enzyme, keeping it away from acting on the DNA.

Molecular mechanism of topoisomerase poisoning by the Xanthomonas peptide albicidin. Source: Michalczyk et al. 2023.

This work opens up opportunities for improving the antibiotic and clinical properties of albicidin. It might also help delay the emergence of albicidin-resistant bacteria, which have been shown to arise through an increase in the copy number of the gyrase gene.

Basic research — You don’t know what you will cure

Studying topics as odd as black rot of cabbage can lead to unexpected discoveries in science. As Nobelist Barry Marshall said when talking about scientific research: “You don’t know where you will end up. You don’t know what you will cure.” In the meantime, let’s just keep making people laugh (and maybe a little grossed out) to communicate the importance of basic research and the transformative impact of science.

The world owes to Xanthmonas…

Acknowledgements

This post was written with assistance from ChatGPT and Quillbot.

This article is available on a CC-BY license via Zenodo. Cite as: Kamoun, S. (2023). 3 unexpected things we owe to the plant pathogenic bacterium Xanthomonas. Zenodo https://doi.org/10.5281/zenodo.7586037

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KamounLab

Biologist; passionate about science, plant pathogens, genomics, and evolution; open science advocate; loves travel, food, and sports; nomad and hunter-gatherer.