Sex change by fungus


White campion infected with anther smut fungus. Image by Martin C. Fischer (CC BY 4.0)

Fungi reproduce by releasing spores that lie dormant in the environment until conditions are right for them to grow. The spores of fungi that infect plants are often released from fungal structures that develop on the surface of their hosts’ leaves or stems. However, the anther smut fungus (Micobrotryum lychnidis-dioicae) employs a more unusual strategy. Its spores are displayed on its host’s flowers so that they can be carried to other plants by insect pollinators.

Anther smut fungus can infect a small flowering plant called white campion (Silene latifolia). White campion is dioecious, meaning that each plant can only produce either male or female flowers. In the flowers of the male plants, pollen is produced by structures called stamens. When the fungus infects a male plant, it manipulates the plant so that the stamens no longer produce pollen and display fungal spores instead.

The flowers of the female plants do not have stamens, so infecting a female would appear to be a dead end for the fungus. However, the fungus has another trick up its sleeve: it induces a partial sex change in female white campion plants so that they do produce stamens (albeit primitive ones).

Like us, white campion has sex chromosomes (X and Y) that determine whether a plant will be male or female. Some of the genes on the sex chromosomes regulate the activity (or expression) of genes on other chromosomes. Therefore, certain genes are more active in a male plant than a female plant, and vice versa. This sex-biased gene expression contributes to the physical differences between the males and females. To better understand how anther smut fungus causes the female plants to develop male characteristics, Niklaus Zemp and colleagues used a technique called RNA-seq to study gene expression in white campion (Zemp et al., 2015).

They found that the fungus causes different changes in gene activity in the male and female plants. The biggest differences were in genes that are more highly expressed in healthy male plants than healthy female plants (male-biased genes). Zemp et al. show that fungal infection decreases the expression of many of these genes in male plants, but has the opposite effect on these genes in female plants. The fungus also altered the expression of female-biased genes differently in males and females, but to a lesser extent.

The up-shot to these changes in gene expression is that the male plants become a bit more feminine when anther smut fungus infects, while the female plants become more masculine. How the fungus achieves this is still a mystery, but it is not the only microbe to cause sex changes in its host. Wolbachia bacteria can cause some male insects to become more female, and a parasite called Nosema granulosis also feminizes some crustaceans. So, in the natural world, gender is a more fluid concept than you might think.

Anther smut fungus is the Organism of the Month here at Plant Scientist.


Zemp N, Tavares R, Widmer A (2015) Fungal Infection Induces Sex-Specific Transcriptional Changes and Alters Sexual Dimorphism in the Dioecious Plant Silene latifolia. PLoS Genet 11(10): e1005536. doi:10.1371/journal.pgen.1005536

The helpful onion

Field of onions in Ismaning, Germany. Image by Rainer Haessner (CC BY SA 3.0 via Wikimedia Commons)

Field of onions in Ismaning, Germany. Image by Rainer Haessner (CC BY SA 3.0 via Wikimedia Commons)

This month it was all change in my vegetable patch as I harvested the last of the crops I planted in the spring and planted new things to grow over the winter. On a bit of whim I decided to plant some onion sets (mini bulbs) at one end of the patch, which should be ready to eat in early summer next year.

Onion is one of the oldest known cultivated plants and the earliest archaeological evidence of onions in human settlements dates back to around 5000 BC (Bronze age). It is grown all over the world where it features as a staple vegetable in a variety of dishes. It is not clear where they originated from, but there is some evidence that they may have come from southwestern Asia. Most cultivated onions are varieties of the common onion (Allium cepa L.) but some other onion species are cultivated too.

If you have ever cooked with onions you will know that when the bulbs are wounded they release a chemical that stings our eyes and can make us cry. This chemical – which has the catchy name syn-propanethial-S-oxide – doesn’t tend to put us off eating onions, but it does help them to defend themselves against herbivores and other pests. Charles Darwin hypothesized that tears triggered by cutting onions are the same as tears of sadness (2). However, he was later proved wrong because tears of sadness actually release extra “waste” proteins that aren’t found in onion tears.

Onions may also help to protect other plants from disease. Intercropping is a farming practice in which two or more crop species are grown in alternating rows. It has been used for a long time to increase crop productivity and to help control disease. Intercropping may help to protect plants against diseases by decreasing the number of attacks by the microbes that cause them, or by boosting the resistance of the host plant. Most studies so far have focused on investigating the first possibility, but little is known about whether plants release signals that can boost defense in their intercropping companion.

In Northeast China, a variety of A. cepa L called the potato onion is often the preferred companion plant to tomatoes. Tomatoes (but not onions) are susceptible to infection by a fungus called Verticillium dahlia, which causes a disease called tomato Verticillium wilt. A group of researchers recently investigated whether intercropping tomato with the potato onion is an effective way to control this disease (Fu et al. 2015).

Fu et al. found that when tomato and potato onion plants were grown together, molecules secreted from the tomato plants (but not the onion plants) inhibited the germination of fungus spores and also inhibited the growth of the fungus. This effect is due to the presence of the onion plants because molecules secreted from tomato and onion plants that were grown separately did not limit the growth of the fungus. Further experiments show that the onion plants trigger the expression of defense genes in the tomato plants.

These results indicate that intercropping tomatoes and potato onions may help to reduce the number and severity of outbreaks of Verticillium wilt. However, since these experiments were carried out under controlled conditions and the tomato plants were deliberately exposed to the fungus, large scale field trials would be needed to find out whether this effect is actually relevant in the field.

Onion is the (long-awaited!) Organism of October (apologies for the delay).


  1. Wikipedia: Onion  (retrieved 21/10/15)
  2. Law, B (2010) Fifty plants that changed the course of history. David and Charles.
  3. Fu X, Wu X, Zhou X, Liu S, Shen Y and Wu F (2015) Companion cropping with potato onion enhances the disease resisitance of tomato against Verticillium dahlia. Frontiers in Plant Science 6:726 doi: 10.3389/fpls.2015.00726

How to live with a legume

Peas, beans and other members of the legume family of plants can form friendly relationships (symbioses) with nitrogen fixing bacteria from the soil. Guided by the plant, the bacteria infect into the root and colonise plant organs called nodules, which supply sugars to the bacteria. Nodules also provide conditions that enable the bacteria to efficiently convert nitrogen gas (N2) into a form of nitrogen that plants can use to grow.

Endophytic bacteria (red) in surrounded by nitrogen-fixing bacteria in a nodule from the legume Lotus japonicas. Scale bar = 50 um. Image from Figure 1 by Zgadzaj et al (2015) (CC BY 4.0)

Endophytic bacteria (red) in surrounded by nitrogen-fixing bacteria (green) in a nodule from the legume Lotus japonicas. Scale bar = 50 um. Image from Figure 1 by Zgadzaj et al (2015) (Licenced under CC BY 4.0)

To set up a symbiosis, the plant and bacteria exchange signals to enable them to identify each other. The plants release molecules called flavonoids into the soil and, in return, the nitrogen-fixing bacteria produce molecules called Nod factors. These Nod factors activate signalling pathways that trigger many responses in the plant and allow the bacteria to enter. The bacteria need to produce the correct Nod factors to gain admittance and so most legumes are only able to team up with a few species of bacteria. Other bacterial molecules such as exopolysaccharides also play important roles in establishing the symbioses. Continue reading

Producing the perfect tomato

Image by regan76 (CC BY 2.0)

Image by regan76 via Flickr (CC BY 2.0)

This summer, I’ve been growing some vegetables in my garden. As a novice gardener, I selected some plants to grow on the basis of what is “easy to grow” rather than any other concern. Fortunately for me, one of my favourite foods happens to be the tomato, and tomato plants (Solanum lycopersicum) are very easy to grow even in small gardens like mine.

For culinary purposes, we generally treat the tomato as a vegetable, but it is in fact a fruit. The tomato plant comes from the Andes in South America where it grows as a vine (1). It is not certain when the plant was first cultivated but it was already being grown in Southern Mexico by 500 BC. At this time, the fruits were about the same size as cherry tomatoes and likely to be yellow in colour. Continue reading

The (un)natural history of maize

Image by Spiritia licensed under CC BY-SA 3.0 via Wikimedia Commons

Image by Spiritia licensed under CC BY-SA 3.0 via Wikimedia Commons

As a PhD student, I once scared some work colleagues by making popcorn during a tea break (in the kitchen, not the lab, I hasten to add). They did not expect to hear the microwave making a series of “popping” noises, and for a few moments, I think they were genuinely worried that the microwave might explode.

Popcorn is made from heating the grains (kernels) of the plant maize until they explode, or “pop”. Also known by its latin name Zea mays ssp. mays, maize is the second most important crop plant in the world behind rice and is widely used in many human foods, as well as for animal feed and to make biofuels. It has many characteristics that make it useful to humans. Maize produces large kernels with high starch content (except for the varieties that are grown to make sweetcorn). The case surrounding a kernel is firm, but soft enough to allow us to grind these kernels to make cornflour. Also, harvesting the crop is relatively easy because the kernels stay on the cob even when ripe. Continue reading

The precious pods of the vanilla orchid

By H. Zell licensed under CC BY-SA 3.0 via Wikimedia Commons.

By H. Zell licensed under CC BY-SA 3.0 via Wikimedia Commons.

At the mention of “vanilla”, the first things I think of are ice cream, cake and other tasty foods. Next, I think of the little bottles of vanilla extract or flavouring that I often use when baking. In turn, that makes me think of the brown, shrivelled vanilla pods I have occasionally used in particular recipes. Vanilla is a popular ingredient in many foods and perfumes, but where does it come from?

The answer lies in an orchid called Vanilla planifola (also known as flat-leaved vanilla), which originates from Mexico. The Totonac people of Mexico were the first to cultivate vanilla. In their mythology, this plant was born when Princess Xanat—whose father had forbidden her from marrying a mortal—fled to the forest with her human lover. The lovers were captured and killed, and it is said that the vine of the first vanilla plant grew from the ground where their blood landed (2). Continue reading

Negative but not useless: the results of a GM wheat field trial

Image by by Bluemoose (CC-BY-SA-3.0) via Wikimedia Commons

Image by by Bluemoose (CC-BY-SA-3.0) via Wikimedia Commons

Last week it was reported that a GM wheat variety that was found to deter aphids in laboratory tests failed to do the same in field trials (1). Some opponents of GM technology have called the trial a “waste of over £1 million of public money” and said that the trial “confirms the simple fact that when GM tries to outwit nature, nature adapts in response” (2). Are these fair criticisms?

The GM wheat variety—which was developed by researchers at Rothamstead Research in the UK—can make an insect pheromone called (E)-β-farnesene. This pheromone is normally produced by aphids when they are under threat to warn other aphids so they disperse. Some of the natural predators of aphids (e.g. parasitic wasps) are also able to detect this pheromone and are attracted by it. Continue reading