Swiping Leaf: Can plants choose their partners?

Written by Alejandro Izquierdo Lopez
Illustrated by Haely Oh

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This article is part of our Special’s Week series, The Science of Romance.

The life of a plant may look static, uneventful. Plants are photosynthetic: they can produce their own food using CO₂ and water, powered by sunlight. But behind their green façade, plants have a more complex, alien lifestyle than we usually give them credit for. 

You may be familiar with sunflowers, which follow the Sun’s position, or Mimosa’s leaves, which close after being touched¹, or even carnivorous plants, which close their highly modified leaves to trap insects, but these are not isolated cases. Many plants have the ability to respond to their environment, although usually in a much slower pace than animals. Plants, for example, can grow and move in response to the direction of light and gravity² and even their roots have sensory organs that allow them to detect and grow around obstacles¹. Plants communicate with each other through networks of subterraneous fungi (mycorrhizae), which they can use to share nutrients and compounds for multiple roles, even warning about the presence of predators³.

If plants can eat, move and communicate, what else can they do? In the animal kingdom, finding and choosing a partner is a big deal: the antlers of a deer or the plumage of the birds of paradise illustrate the idea that only the best are chosen as partners. If animals can choose, then, can plants choose their partners, as well?

First, we must understand how flowers work. Flowers are a complex reproductive organ with generally four organs: petals and sepals are the exterior “leafy” sections, with different shapes and colours that attract pollinators; the stamens and the pistil are the reproductive organs that produce gametes. Stamens produce pollen: a multicellular, but small structure with a protective capsule that contains gametes. Their role is to travel and fecundate the gametes enclosed in the pistil: the ovules. Most flowers are hermaphroditic⁴: they produce pollen and ovules, and thus, they can both send and receive gametes. Flowers usually receive more pollen than ovules are available⁴; not all pollen will be able to fecundate an ovule and form a seed. It is also known that plants highjack pollen coming from their own flowers⁵ to avoid inbreeding. Based on that, can plants also choose among the different pollen grains they receive? The answer is yes. 

From their creation, pollen embarks on a quest, with multiple trial that it has to surpass to finally form a seed. In their first trial, pollen goes on a journey to reach another plant. While many plants just release pollen into the wind, this does not ensure pollen will reach another flower. Attracting pollinators, for example, is usually a great strategy to ensure that this journey is successful. Pollinators can choose which flowers they visit based on their shape⁵ or colour⁶, and thus, only the most appealing flowers may be pollinated. This creates a situation in which plants compete to see who gets the highest number of pollinators. 

But further trials are still waiting. Now the pollen has reached the stigma, the top of the pistil. There, it needs to germinate and create a structure known as the pollen tube. But germination can be controlled by multiple factors that can be altered by the stigma itself⁷, including humidity⁷ or calcium levels⁸. For example, the stigma may choose to provide only a tiny bit of water, so that only pollen with fewer water requirements germinates⁹. After germination, the pollen tubes must grow through the style, the “neck” of the pistil. Different pollen tubes “race” through the style to be the first to reach the ovary, where the ovules reside. The pistil, then, becomes a racing track, and its shape and size can influence the final winner¹⁰. However, the flower itself can also cheat, and provide more nutrients to its favourite pollen tubes, giving them an edge⁸. 

Finally, a few selected pollen tubes reach the ovaries and fecundate them. You may think that the winners have already been decided, but this is not the case. Once the ovaries have been fecundated, they create small embryos. Embryos, for example, can highjack the growth of their neighbours¹¹, ensuring their growth against their competitors. But the mother flower can still have the final call. Seeds can still be aborted, if not desired; this process is not random, and depends on which plant was the pollen donor⁵. After the seeds have been formed, the flower transforms into a fruit. However, there is still a final trial, and some fruits may still be aborted, if considered of a lesser quality¹².

So, can plants choose their partners? In a sense, they can: pollen has to go through multiple screening processes until it finally creates a fruit. The life of a plant is not as peaceful as it seems: maybe your Ficus at home has a more exciting life that you think. 

Sources

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