Why peas in a pod are like they are

The expression “like peas in a pod” usually refers to looking alike, but there’s actually more to the story.

Open up a pod, and the peas will be the same distance apart. The same is true of some other pulses and even non-pulses, irrespective of the size and number of seeds and the size of the pod size.

To biologists, that’s not as surprising as it might seem, but until now it has not been clear how it happens. A team led by Rüdiger Simon from Germany’s Heinrich-Heine University decided to find out.

Geneticists based in Germany, Australia, Japan, the US and Italy used the much-studied thale cress (Arabidopsis thaliana) to examine the genetic processes taking place behind the initiation of ovules – the primordia from which seeds emerge after fertilisation – and the growth of the pod.

There is considerable variation in fruit size and seed number between different wild varieties of A. thaliana, but the researchers say they also discovered a uniform genetic mechanism that controls seed position regardless of environmental factors such as temperature.

In a paper in the journal Current Biology, they describe how they established that seed formation is controlled by several signalling pathways at precisely defined positions. These signalling pathways are activated by small secreted proteins from the EPFL family.

One of the peptides, EPFL2, is formed between the developing ovules, where it adjusts the spacing between the seeds. Where this peptide is not present, the researchers found irregular spacing – meaning that adjacent seeds compete more for nutrients – or even ovule twinning, which generally results in neither ovule developing fully.

“The individual seeds compete with each other for nutrients,” says Simon. “To ensure that each seed gets an equal supply and can develop well, it is important that the seeds are spread as evenly as possible at equal distances in the pod.”

EPFL2 and a closely related peptide, EPFL9, also control fruit development. As a result, seed formation is closely linked to pod growth.

And there’s more. “The same signalling substances and receptors that we have identified as being responsible for relative pod size and seed spacing are also in charge of the spacing of leaf stomata and the microstructure of serrated leaves,” says colleague and first author Nozomi Kawamoto.