Love and Flowers: When analogies break down

I’ve learned a lot from a new article in AoB PLANTS, the Open Access sister Journal to Annals of Botany. It’s Green love talks; Cell-cell communication during double-fertilization in flowering plants by Tomokazu Kawashima and Frederic Berger and it shows how things get really interesting when simple analogies break down. The paper is a review of recent research on cell signalling and how it works to ensure successful fertilisation of flowers by pollen. Borrowing from the title, it’d be easy to try and conjure up a start like: “Experts have got it right, the key to a successful relationship is communication.” But Kawashima and Berger show that there are times when anthropomorphising plants can effectively hide what is so fascinating about them.

Couple holding hands
This post would have been a lot easier to write if plant communication was simply about holding hands. Photo by Rachel Davies.

The issue is how does signalling work to get the male material into the female reproductive cells to start the seed making process? I’ve had the talk, so I know that it’s a matter of delivering sperm to the egg, so it’s just a case of making sure that the female organs signal they’re receptive, yes? In the case of angiosperms, flowering plants, it’s more complex. You need two male gametes to fertilise two parts of the female. There’s a central cell and within that there’s an egg cell.

This has been source of a puzzle for plant scientists. There are two female cells, so presumably there are two male cells, yet they’re coming from the same source i.e. a pollen grain that’s landed on the stigma and germinated there. How does the plant stop the wrong male cell getting to the wrong female one?

The answer found from studies of flowers, including Arabidopsis thaliana, is a surprise. They’re not two types of male cell. As the authors report, advances in high resolution imaging mean that they can identify that the two male cells are identical. All the hard work seems to be done by the female parts of the plant. Cells surrounding the egg cell produce proteins to attract the pollen tubes, and signalling between the female cells makes sure everything is delivered to the right place.

To be honest, some of the language in the paper is daunting. Being a human I’m used to the idea that plants are passive. No doubt, Kawashima and Berger would emphatically disagree. There’s a lot going on in a pollinated flower and the uses of various terms and proteins can be dizzying. What they show is that the signalling is complex. When you think about the scale of the operations it’s hard to see how so many proteins can be shuffled, ordered and directed to make everything happen. Far from silence, it looks like there’s a well-orchestrated chemical symphony being played in the cells that makes the double-fertilization possible. Keeping on top of all the detail means that the paper is not light-reading. It can however be rewarding reading.

The best papers don’t simply answer questions, they also open up new avenues of research. Here Kawashima and Berger are extremely helpful. If you’re looking for something to research in signalling then the authors have erected big signposts in the conclusion with big arrows marked ‘mysteries this way’. If you’re looking for a departure point into cell-cell signalling, then this looks like a helpful guide to where the interesting puzzles in mechanisms are.

Photo: Holding Hands by Rachel Davies. Licenced under a Creative Commons BY licence.

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