by Piter Kehoma Boll
Parasites are almost as old as life itself and they can be a mild to moderate nuisance to their hosts most of the time. However, by domesticating many species, we humans helped many parasites to thrive and become a greater threat to their hosts and to ourselves.
One of these cases is today’s fellow, Puccinia graminis, a fungus that causes a cereal disease known as stem rust. Known since ancient times, as it is already mentioned in works such as those of Aristotle, this species has become a serious problem in more recent centuries after the expansion of agriculture.
The stem rust belongs to the order Pucciniales, a group of fungi known as rusts. This name refers to the appearance that they cause in plants after infecting them, as parts of the stem and leaves can look like if they have rusted. Although the stem rust can infect several species, its most important hosts, at least from a human perspective, are wheat and barley, especially wheat.
When the stem rust infects a wheat plant, which occurs in summer, it starts to grow as a mycelium inside the plant’s tissue and, after about 1 to 2 weeks, it begins to produce rust-red pustules that appear mostly at the leaf sheaths, although they can appear anywhere on the plant. These pustules are uredinia and contain many stalked spores called urediniospores.
The urediniospores are dikaryotic cells, i.e., they have two haploid nuclei inside them, and are easily spread by the wind. When they fall on a new wheat plant, they germinate to produce a new mycelium and infect the new host. This way they can spread asexually across a large area. Infected wheat plants are often smaller, produce fewer or smaller grains, and sometimes can even die if the infection is too severe.
By the end of the wheat’s life cycle, the fungus produces other structures, called telia, which produce a different form of dikaryotic spores known as teliospores. The telia have a black color and the disease is also known as black rust because of this. The teliospores can overwinter without a host. During this period, their two nuclei fuse and, as spring arrives, the teliospore undergoes meiosis and produces four spores known as basidiospores.
The basidiospores are carried by the wind until they reach the so-called alternate host, a plant in which the fungus reproduces sexually. This host is often a shrub of the genus Berberis (barberry). When the basidiospore falls on a barberry leaf, it germinates to produce a haploid mycelium. This mycelium originates structures known as pycnia, which produce both female hyphae known as receptive hyphae and male spores known as pycniospores. The pycniospores are covered by a sticky honeydew that attracts insects. As these insects move from one plant to another exploring this sweet gift, they carry the pycniospores with them.
When a pycniospore meets a receptive hypha from another fungus, they fuse and grow into a dikaryotic mycelium that will produce again a different structure, the aecium, that contains again another type of spore, the aeciospore. The aeciospores are carried by the wind from the barberry leaves to the wheat plants, where they germinate and restart the cycle.
This is a complex and amazing cycle, isn’t it? However, humans hate the stem rust since ancient times. Its effect on wheat is so important that the Romans even had a whole festival, the Robigalia, dedicated to preventing it. It was a horrid festival that included the sacrifice of a poor innocent dog to Robigus, the “rust god”.
In recent history, the fungus caused serious damage in wheat crops across the world several times, especially in Europe, Asia, and Africa. Many modern cultivars have been selected to be resistant to the stem rust, but the fungus also evolves, frequently originating new strains that are able to bypass the acquired resistance of the plants.
But if the fungus has to infect barberry plants to complete its life cycle, wouldn’t it be enough to avoid having barberry plants near wheat plantations? Kind of, but remember that when the fungus is infecting a wheat plant, it produces urediniospores, which can infect new wheat plants. If you plant different wheat varieties that grow across different times of the year, the fungus can keep infecting new wheat plants asexually, not requiring any barberry.
We, humans, are really very good at making diseases more virulent by trying to prevent them. Aren’t we?
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More rusts:
Friday Fellow: Pear Rust (on 27 April 2018)
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References:
Bhattacharya, S. (2017). Deadly new wheat disease threatens Europe’s crops. Nature News, 542(7640), 145. https://doi.org/10.1038/nature.2017.21424
Lewis, C. M., Persoons, A., Bebber, D. P., Kigathi, R. N., Maintz, J., Findlay, K., … & Saunders, D. G. (2018). Potential for re-emergence of wheat stem rust in the United Kingdom. Communications biology, 1(1), 1-9. https://doi.org/10.1038/s42003-018-0013-y
Wikipedia. Stem rust. Available at < https://en.wikipedia.org/wiki/Stem_rust >. Access on 16 December 2021.
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