The Ras-related nuclear GTPase RAN1 ensures pollen size and tube growth by maintaining the actin cytoskeleton

Controlling organ size in plants is a complex biological process influenced by various factors, including gene expression, genome ploidy and environmental conditions. Despite its importance for plant growth and development, the mechanisms underlying organ size regulation remain unknown. Here, we investigated the role of RAN1, a member of the Ras-related nuclear GTPase family, in regulating pollen size. A RAN1 knockdown mutant (ran1-1) exhibited a significant reduction in pollen size, accompanied by impaired germination and reduced pollen tube growth. RAN1 mutation caused disruptions in actin filament organization, such as aberrant structure of actin collar due to the dysregulation of expression of actin-binding proteins. Furthermore, we identified that mutation of the transcription activator SHORT HYPOCOTYL UNDER BLUE1 (SHB1) showed similar but milder phenotypes in pollen compared to ran1-1. Genetic evidence suggested that SHB1 acts downstream of RAN1. Transient expression assays in leaves showed that SHB1 was largely retained in the cytoplasm of the ran1-1 mutant, potentially affecting the expression of actin-binding proteins. These findings highlight the pivotal role of RAN1 in modulating pollen size and development, providing valuable insights into cell size regulation.