A trio of Arabidopsis proteins acts as the executor complex in the pollen aperture formation program.

Abstract

Pollen apertures are specialized sites on the pollen surface that have little to no pollen wall exine and their numbers, positions, and morphology are species specific. Apertures have become a useful model for studying how cells develop distinct plasma membrane (PM) domains. During pollen development, these sites recruit specific factors and become protected from exine deposition. However, the molecular mechanisms underlying this process and the identities of the associated aperture factors remain poorly understood. Previous studies identified 2 Arabidopsis (Arabidopsis thaliana) proteins, INAPERTURATE POLLEN 1 (INP1) and the non-paralogous INP2, as essential for aperture formation and showed that they form a species-specific protein complex proposed to localize to the extracellular surface of the aperture PM domains. In this study, we identified STRUBBELIG RECEPTOR FAMILY 2 (SRF2), a receptor-like pseudokinase that spans the PM, as another factor required for pollen aperture formation in Arabidopsis. SRF2 shares key features with INP1 and INP2, including mutant phenotype, expression pattern, localization to aperture PM domains, genetic interactions, and evolutionary trends. We present evidence that the extracellular domain of SRF2 interacts with INP2, and that these 3 proteins form a complex essential for executing the aperture formation program, with properties that differ among species.