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.

Alun Salt. ORCID 0000-0002-1261-4283

When he's not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there's such a thing as scientific heritage and if there is how would you recognise it?