Abstract
The ASYMMETRIC LEAVES2 (AS2) gene in Arabidopsis thaliana is responsible for the development of flat, symmetric, and extended leaf laminae and their vein systems. AS2 protein is a member of the plant-specific AS2/LOB protein family, which includes 42 members comprising the conserved amino-terminal domain referred to as the AS2/LOB domain, and the variable carboxyl-terminal region. Among the members, AS2 has been most intensively investigated on both genetic and molecular levels. AS2 forms a complex with the myb protein AS1, and is involved in epigenetic repression of the abaxial genes ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3), ARF4, and class 1 KNOX homeobox genes. The repressed expression of these genes by AS2 is markedly enhanced by the cooperative action of various modifier genes, some of which encode nucleolar proteins. Further downstream, progression of the cell division cycle in the developing organs is stimulated; meristematic states are suppressed in determinate leaf primordia; and the extension of leaf primordia is induced. AS2 binds the specific sequence in exon 1 of ETT/ARF3 and maintains methylated CpGs in several exons of ETT/ARF3. AS2 forms bodies (designated as AS2 bodies) at nucleolar peripheries. AS2 bodies partially overlap chromocenters, including inactive 45S ribosomal DNA repeats, suggesting the presence of molecular and functional links among AS2, the 45S rDNAs, and the nucleolus to exert the repressive regulation of ETT/ARF3. The AS2/LOB domain is characterized by three subdomains, the zinc finger (ZF) motif, the internally conserved-glycine containing (ICG) region, and the leucine-zipper-like (LZL) region. Each of these subdomains is essential for the formation of AS2 bodies. ICG to LZL are required for nuclear localization, but ZF is not. LZL intrinsically has the potential to be exported to the cytoplasm. In addition to its nuclear function, it has been reported that AS2 plays a positive role in geminivirus infection: its protein BV1 stimulates the expression of AS2 and recruits AS2 to the cytoplasm, which enhances virus infectivity by suppression of cytoplasmic post transcriptional gene silencing.
Highlights
By using the estradiol-inducible expression system for AS2fused yellow fluorescence protein (AS2-YFP), we investigated the subcellular localization of the ASYMMETRIC LEAVES2 (AS2) protein in cells of a transgenic Arabidopsis plant and cultured cell lines of Arabidopsis and tobacco
We reported that AS2 forms granules at the periphery of the nucleolus in interphase cells in the epidermis of the adaxial domains of cotyledonary and leaf primordia (Figs. 7a, 8), some amounts of AS2-YFP are present in the nucleoplasm of these cells (Luo et al 2012; Ueno et al 2007)
The observation that AS2 bodies are overlapped with two nucleolar chromocenters in A. thaliana (Col-0) (Luo et al 2020) predicts that a role of AS2 bodies might be related to the repressive regulation of ETT/ARF3, such as the inactivation of the 45S rDNA genes in the chromocenters
Summary
Molecular and genetic analyses of the AS2 gene in Arabidopsis thaliana have shown that it is a key regulator for the development of flat symmetric leaves containing vascular bundles and fine networks of venation systems, the morphology of which seems to be evolved suitably for efficient photosynthesis (Fig. 1a) (Byrne et al 2000; Ikezaki et al 2010; Iwakawa et al 2020; Machida et al 2015; Ori et al 2000; Semiarti et al 2001; Serrano-Cartagena et al 1999). As described, it should be worth pointing out that such transcriptional repression of ETT/ARF3 and ARF4 is further achieved by cooperative action with various modifier genes, such as NUC1, RH10, and RID2, the proteins of which are localized to the nucleolus (Matsumura et al 2016).
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