The Microtubule-Associated Protein IQ67 DOMAIN5 Modulates Microtubule Dynamics and Pavement Cell Shape

The dynamic arrangement of cortical microtubules (MTs) plays a pivotal role in controlling cell growth and shape formation in plants, but the mechanisms by which cortical MTs are organized to regulate these processes are not well characterized. In particular, the dynamic behavior of cortical MTs is critical for their spatial organization, yet the molecular mechanisms controlling MT dynamics remain poorly understood. In this study, we used the puzzle piece-shaped pavement cells of Arabidopsis (Arabidopsis thaliana) leaves as a model system in which to study cortical MT organization. We isolated an ethyl methanesulfonate mutant with reduced interdigitation of pavement cells in cotyledons. This line carried a mutation in IQ67 DOMAIN5 (IQD5), which encodes a member of the plant-specific IQ motif protein family. Live-cell imaging and biochemical analyses demonstrated that IQD5 binds to MTs and promotes MT assembly. MT-depolymerizing drug treatment and in vivo MT dynamics assays suggested that IQD5 functions to stabilize MTs. Hence, our findings provide genetic, cell biological, and biochemical evidence that IQD5 regulates MT dynamics that affect MT organization and subsequent cell shape formation.