ROP GTPases Act with the Receptor-Like Protein PAN1 to Polarize Asymmetric Cell Division in Maize

John A Humphries,Zuzana Vejlupkova,Robert B Meeley,Anding Luo,Laurie G Smith,Anne W Sylvester,John E Fowler

doi:10.1105/tpc.111.085597

Abstract

Plant Rho family GTPases (ROPs) have been investigated primarily for their functions in polarized cell growth. We previously showed that the maize (Zea mays) Leu-rich repeat receptor-like protein PANGLOSS1 (PAN1) promotes the polarization of asymmetric subsidiary mother cell (SMC) divisions during stomatal development. Here, we show that maize Type I ROPs 2 and 9 function together with PAN1 in this process. Partial loss of ROP2/9 function causes a weak SMC division polarity phenotype and strongly enhances this phenotype in pan1 mutants. Like PAN1, ROPs accumulate in an asymmetric manner in SMCs. Overexpression of yellow fluorescent protein-ROP2 is associated with its delocalization in SMCs and with aberrantly oriented SMC divisions. Polarized localization of ROPs depends on PAN1, but PAN1 localization is insensitive to depletion and depolarization of ROP. Membrane-associated Type I ROPs display increased nonionic detergent solubility in pan1 mutants, suggesting a role for PAN1 in membrane partitioning of ROPs. Finally, endogenous PAN1 and ROP proteins are physically associated with each other in maize tissue extracts, as demonstrated by reciprocal coimmunoprecipitation experiments. This study demonstrates that ROPs play a key role in polarization of plant cell division and cell growth and reveals a role for a receptor-like protein in spatial localization of ROPs.

Highlights

The generation of cell polarity is critical for a wide variety of cellular processes, including polarized cell growth, cell migration, asymmetric cell division, and targeted secretion
The Arabidopsis Rho family GTPases (ROPs) (e.g., ROP1, ROP2, and ROP4), known to play a role in polarized cell growth, are in a group known as the Type I ROPs, based on sequence and apparent plasma membrane–targeting mechanisms
Mutations in either rop2 or rop9 enhance the effect of the pan1 mutation on subsidiary mother cell (SMC) divisions, with rop2 again having a stronger effect (Figures 1B and 1C)

Summary

Introduction

The generation of cell polarity is critical for a wide variety of cellular processes, including polarized cell growth, cell migration, asymmetric cell division, and targeted secretion. Members of the Rho family of small GTPases (e.g., Rho, Rac, and Cdc42) are found in all eukaryotes and play important roles in establishing and maintaining cell polarity (Schmitz et al, 2000; Jaffe and Hall, 2005; Ridley, 2006). A variety of downstream effectors bind to the active form of these GTPases to mediate their effects on cellular processes important for cell polarity. Plants lack direct orthologs of Rho, Rac, and Cdc, they have a distinct group of GTPases in the Rho family, termed ROPs (Rho of plants) or RACs (Zheng and Yang, 2000; Christensen et al, 2003). Whereas ROPs are involved in a variety of processes, they have been investigated primarily for their roles in polarized cell growth (reviewed in Nibau et al, 2006; Yang and Fu, 2007).

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