Have you ever wondered how a flower ensures that it's pollen-catching "landing pad" (the stigma) stays neatly separated from the "pollen tube highway" beneath it (the style)? This clear boundary is crucial for plant reproduction, but for decades, biologists didn't know how it was drawn.

Scientists from the Institute of Botany of the Chinese Academy of Sciences (CAS), in collaboration with the University of Oxford, Tsinghua University, and the CAS Institute of Genetics and Developmental Biology, has cracked the mystery. Their study reveals that a plant hormone called auxin acts like a pair of molecular scissors, which deliberately cuts down a key protein exactly where the boundary should form, creating a sharp dividing line between the two tissues. The study was recently published in PNAS.

Using the cruciferous plant Capsella rubella as a model, the team discovered that auxin concentration is highest at the gynoecium tip (where the stigma forms), and this high auxin level triggers the degradation of CrETT through a cellular "recycling system" known as the 26S proteasome. The more auxin present, the more CrETT gets broken down. When disabled this degradation by mutating a tiny molecular switch-a specific motif called PxS within the protein—the boundary disappeared, and style-like tissues invaded the stigma territory, causing severe developmental defects.

The story gets even more fascinating when viewed through an evolutionary lens. The team compared similar proteins across the plant kingdom and found that in ancient plant groups like gymnosperms (such as pine trees and ginkgo), ferns, and mosses, these proteins are completely unresponsive to auxin. But in all flowering plants (angiosperms) tested, the proteins were degraded to varying degrees. This tells us that flowering plants evolved this "auxin-triggered demolition" strategy as a new trick, which may have been a key innovation that allowed them to develop the complex female reproductive organs we see today.

"We uncovered a completely new mechanism that plants use auxin, not just to turn genes on or off, but to actively remove a protein to define a tissue boundary," said Prof. DONG Yang, the corresponding author of the study from the Institute of Botany, CAS. "This discovery not only answers a fundamental question in developmental biology, but also offers a glimpse into how flowering plants evolved their remarkable reproductive success."

Schematic model for auxin-dependent CrETT degradation in stigma-style boundary establishment (Image by DONG Yang)