Above: Main parts of as mature flower, showing the plant’s reproductive organs (Wikimedia Commons/Mariana Ruiz)
Did you know? Just like animals, plants have an immune system. When a disease-causing pathogen is detected, plants can mount their defences to try and fend off the attacker!Sexually transmitted infections (STIs), also commonly called sexually transmitted diseases (STDs), are no joke. Sexual contact between two people can spread dangerous infections with serious consequences. The idea of a plant catching an STI may sound silly, but diseases that occur on a plant’s reproductive organs or as a direct result of spreading pollen from one plant to another could be considered the plant equivalent of an STI.
Generally speaking, it is possible for plants to catch many different kinds of diseases. For instance, you might have noticed that maple leaves often have big, scabby black spots on them that are about the size of a quarter. These spots are a symptom of tar spot, a fungal disease. Another example of plant disease would be when bacteria clog up vascular tissues, which are the “veins” that transport water and sugars throughout a plant. In fact, there are thousands of microorganisms and viruses that can attack every part of a plant, so it shouldn’t be such a surprise that a plant’s sexual organs can also host an infection.
Of course, plant sex is a little different from animal sex. Flowers contain the sexual organs of the plants that produce them. Make sure you remind your date of this next Valentine’s Day! Pollen is transferred by the wind or by pollinators onto the stigma, the female part of a flower. Sperm cells carried within the pollen grain can then be transferred into the flower’s ovary, where fertilization occurs.
Some plant STIs affect the development of a flower’s reproductive parts. For example, when spores of the fungus Ustilago violacea land on their wildflower hosts, they germinate and grow through the plants' tissues until they reach a developing flower bud. From there, the fungus infects the developing anther.
Did you know? The fungus Ustilago maydis attacks corn and causes the kernels to become hugely swollen and turn black. This diseased corn is eaten as a delicacy called huitlacoche!Instead of producing pollen, the anther of an infected plant swells up and turns black before releasing millions of fungal spores. In plants that make separate female and male flowers, if the fungus finds itself in a female flower (which wouldn’t normally form an anther at all), it can alter the plant’s development and force the female flower to produce a strange, rudimentary stamen so that the fungus can proceed with its normal infection process!
If an STI that can force a female flower to change its sex isn’t strange enough, how about an STI that can jump the 1.5 billion year evolutionary gap between the plant kingdom and the animal kingdom? Despite its name, tobacco ringspot virus (TRSV) doesn’t just affect tobacco. It also causes big problems for many other plants, including cucumbers, soybeans, and other important food crops. These plants can become infected when the virus is spread by a well-meaning visitor: bees! If bees unwittingly pick up pollen from a plant that is infected with TRSV, the virus can hitch a ride on the pollen and spread to a healthy plant.
That’s bad news not only for plants, but also for the bees. Researchers have found evidence that TRSV can actually reproduce inside bees, and bee colonies with higher levels of TRSV also have higher levels of other bee viruses and are more likely to suffer a population collapse when compared to bee colonies without the virus. This doesn’t mean that TRSV directly causes bee colonies to collapse, but it probably makes a bad situation even worse for colonies that are already struggling.
Did you know? The fungus Claviceps purpurea attacks plants’ anthers and produces powerful psychoactive compounds, including lysergic acid (LSD).This research suggests that the virus might have found a way to hop hosts from a plant to an animal, which is a pretty impressive evolutionary feat! Although it’s unusual, there are a few other groups of viruses known to infect both plants and insects. For example, flock house virus can actually multiply in plants, insects, or fungi!
Around the world, bee numbers have been dropping dramatically, and although many possible causes have been proposed there isn’t yet a clear cause or solution. The newly-discovered link between TRSV and honeybees probably isn’t the cause of this mystery, but the unexpected relationship between bees and a plant STI demonstrates just how complicated ecological relationships can be.
What is Plant Pathology? (2014)
Canadian Phytopathological Society
Introduction to phytopathology, the study of disease in plants.