A molecular conversation between the two tissues of a developing seed has been decoded, and turning up one side of it has already produced larger seeds in a crop. Scientists led by Cyril Zipfel's group at The Sainsbury Laboratory in Norwich, working with the Plant Reproduction and Development Laboratory at the École Normale Supérieure in Lyon, report in Science that the peptide PIPL7 is the signal recognised by the receptor HAIKU2, or IKU2.

IKU2 was identified more than 20 years ago as a regulator of seed size, but the peptide that switches it on had never been found. The new work shows that PIPL7 — PATHOGEN-INDUCED PEPTIDE-LIKE 7 — is produced by the embryo and detected by IKU2 in the endosperm, the nutrient-rich tissue that feeds the growing seed, so that the two tissues develop in step and the seed forms properly.

The discovery was accidental. Zipfel's group normally studies receptors that respond to pathogens and environmental stress, and was mapping where particular peptide–receptor pairs are expressed when it noticed one peptide unusually active in the silique, the pod that holds developing Arabidopsis seeds. "We kind of stumbled into this project," said Jack Rhodes, a team leader in the group. Single-cell and spatial expression analysis then showed PIPL7 strongly expressed in developing seeds; its relatives are recognised by the receptor RLK7, whose closest kin is IKU2.

In Lyon, Gwyneth Ingram's laboratory had meanwhile shown from iku2 mutants that the receptor controls endosperm polarity and development in response to an external peptide from the zygote or early embryo — and had written that the ligand still needed to be found. "We thought we had been able to identify the ligand. That is how our collaboration began," Zipfel said.

Combining seed genetics and bioimaging in Lyon with the Norwich group's fast transient-expression assays, the teams confirmed that IKU2 recognises PIPL7, and that plants lacking the peptide show the same seed phenotype as plants with a broken receptor. Embryo cells express PIPL7; endosperm cells express IKU2.

Most of the work was done in Arabidopsis, but early proof-of-concept experiments in a crop showed that increasing PIPL7 expression produced larger seeds, pointing to a possible target for engineering seed size. The researchers and PBL Technology have filed a patent on the discovery. For breeders of grain and oilseed crops, where seed weight is a direct yield component, a single tunable signal is a notable lead. The report is drawn from a press release by The Sainsbury Laboratory.

Source: Seed World Europe