Functional divergence of plant SCAR/WAVE proteins is determined by intrinsically disordered regions

Dynamic actin cytoskeleton reorganization enables plant developmental processes requiring polarized transport such as root hair and leaf trichome formation. The SCAR/WAVE complex plays a crucial role in regulating these dynamics through ARP2/3-mediated actin branching. SCAR/WAVE genes occur as small families across a wide range of plant species, but whether and how they fulfill different functions remains unclear. We use a systematic chimera approach to define the differential functionality of two closely related Medicago truncatula SCAR proteins in plant development. We show that SCAR/WAVE contribution to M. truncatula root hair or Arabidopsis thaliana trichome formation is dependent on two central intrinsically disordered regions (IDRs). Differential functionalities of M. truncatula SCAR proteins were furthermore associated with the presence/absence of a 42-amino acid sequence within the IDR that affected protein stability. Through uncovering a molecular basis for functional differences, we advance our understanding of plant SCAR/WAVE complexes.