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New research from the Sainsbury Laboratory at the University of Cambridge has shed light on how plants precisely control their growth and development, revealing that seemingly similar molecular components fulfill surprisingly different jobs.

The study, in Science Advances, focuses on the SCAR/WAVE protein complex, a crucial molecular machine that helps shape plant cells by directing the formation of the internal cell scaffold known as the actin cytoskeleton. This is vital for processes like the growth of root hairs, which are essential for nutrient uptake, and the shape of leaf hairs, called trichomes.

Plants, much like other complex organisms, rely on the dynamic reorganization of their cytoskeleton to grow and respond to their environment. The SCAR/WAVE complex plays a pivotal role in this process by activating another group of proteins called the ARP2/3 complex, which initiates the branching of actin filaments. While SCAR/WAVE genes are found in many plant species, they often exist as small families of related genes, and scientists have been wondering whether these different versions perform unique roles.

The Cambridge team, led by Dr. Sebastian Schornack, investigated two closely related SCAR proteins, MtAPI and MtHAPI1, in the model legume plant Medicago truncatula. They discovered that these proteins, despite their similarity, are not interchangeable. MtAPI is crucial for normal root hair development, but MtHAPI1 cannot perform this role. Conversely, when tested in the model plant Arabidopsis thaliana, MtHAPI1, but not MtAPI, could rescue defects in trichome development caused by a faulty Arabidopsis SCAR protein. This clearly demonstrated that these two related SCAR proteins have distinct functions within the plant.

Digging deeper, the study identified a specific 42-amino acid sequence within an intrinsically disordered region of MtAPI that significantly impacts protein stability. The presence of this short segment leads to lower levels of the MtAPI protein in plant cells, suggesting a mechanism for finely tuning the protein's abundance. Intriguingly, this segment appears to function as a general destabilizing element, affecting other proteins it's attached to, even in different plant species.

"Our findings reveal a fascinating molecular basis for how plants can achieve functional diversity using closely related proteins," said Dr. Sabine Brumm, lead author of the study. "The intrinsically disordered regions, which were least understood, are actually critical for defining what these SCAR proteins do in the cell."

This research significantly advances our understanding of how the SCAR/WAVE complex is regulated and how it contributes to plant development.

"By uncovering how these SCAR proteins differ, we gain deeper insights into the intricate mechanisms that control cell shape and growth in plants," explained Dr. Schornack. "This knowledge could have implications for understanding plant-microbe interactions and potentially for developing strategies to improve plant growth and resilience."

The study highlights the complexity and elegance of molecular regulation in plants and opens new avenues for investigating how protein stability and function are controlled.