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Ran is an evolutionarily conserved eukaryotic GTPase. We previously identified a cDNA of TaRAN1, a novel Ran GTPase homologous gene in wheat (Triticum aestivum) and demonstrated that TaRAN1 is associated with regulation of genome integrity and cell division in yeast (Saccharomyces cerevisiae) systems. However, much less is known about the function of RAN in plant development. To analyze the possible biological roles of Ran GTPase, we overexpressed TaRAN1 in transgenic Arabidopsis (Ambidopsis thaliana) and rice (Oryza sativa). TaRAN1 overexpression increased the proportion of cells in the G2 phase of the cell cycle, which resulted in an elevated mitotic index and prolonged life cycle. Furthermore, it led to increased primordial tissue, reduced number of lateral roots, and stimulated hypersensitivity to exogenous auxin. The results suggest that Ran protein was involved in the regulation of mitotic progress, either in the shoot apical meristem or the root meristem zone in plants, where auxin signaling is involved. This article determines the function of RAN in plant development mediated by the cell cycle and its novel role in meristem initiation mediated by auxin signaling.
Ran is one of the important small G-protein families in organisms. In animals and yeast (Saccharomyces cerevisiae), it functions in many aspects, including nuclear transport, cell cycle control, postmitotic nuclear assembly, and spindle assembly (for review, see Dasso, 2001). The core biochemistry of Ran is similar to that of many Ras-related GTPases (for review, see Gorlich and Kutay, 1999; Sazer and Dasso, 2000). Ran's intrinsic rates of nucleotide exchange and hydrolysis are slow. In vivo, these reactions require a nucleotide exchange factor (RCC1) and a GTPase-activating protein (RanGAP1) to achieve physiological rates. The Ran binding protein 1 (RanBP1) binds to RanGTP with high affinity and acts as an essential accessory factor to increase RanGAP1-mediated nucleotide hydrolysis (Sazer and Dasso, 2000). During interphase, RCC1 is a chromatinassociated nuclear protein, while RanBP1 and RanGAP1 are largely cytosolic. The asymmetric distribution of nucleotide exchange and hydrolysis enzymes across the nuclear envelope suggests that RanGTP should be largely nuclear and RanGDP largely cytosolic. This distribution plays a key role in determining the directionality of nuclear transport (Dasso, 2001). The requirement for Ran in nuclear transport has been extensively studied in animals (for review, see Gorlich and Kutay, 1999)....