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Roles of ASYMMETRIC LEAVES2 (AS2) and Nucleolar Proteins in the Adaxial–Abaxial Polarity Specification at the Perinucleolar Region in Arabidopsis

1
Graduate School of Bioscience and Biotechnology, Chubu University, 1200, Matsumoto-cho, Kasugai, Aichi 487-8501, Japan
2
Graduate School of Medical Sciences, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
3
Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
*
Authors to whom correspondence should be addressed.
Int. J. Mol. Sci. 2020, 21(19), 7314; https://doi.org/10.3390/ijms21197314
Received: 21 August 2020 / Revised: 27 September 2020 / Accepted: 29 September 2020 / Published: 3 October 2020
(This article belongs to the Special Issue Molecular Mechanisms of Leaf Morphogenesis)
Leaves of Arabidopsis develop from a shoot apical meristem grow along three (proximal–distal, adaxial–abaxial, and medial–lateral) axes and form a flat symmetric architecture. ASYMMETRIC LEAVES2 (AS2), a key regulator for leaf adaxial–abaxial partitioning, encodes a plant-specific nuclear protein and directly represses the abaxial-determining gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3). How AS2 could act as a critical regulator, however, has yet to be demonstrated, although it might play an epigenetic role. Here, we summarize the current understandings of the genetic, molecular, and cellular functions of AS2. A characteristic genetic feature of AS2 is the presence of a number of (about 60) modifier genes, mutations of which enhance the leaf abnormalities of as2. Although genes for proteins that are involved in diverse cellular processes are known as modifiers, it has recently become clear that many modifier proteins, such as NUCLEOLIN1 (NUC1) and RNA HELICASE10 (RH10), are localized in the nucleolus. Some modifiers including ribosomal proteins are also members of the small subunit processome (SSUP). In addition, AS2 forms perinucleolar bodies partially colocalizing with chromocenters that include the condensed inactive 45S ribosomal RNA genes. AS2 participates in maintaining CpG methylation in specific exons of ETT/ARF3. NUC1 and RH10 genes are also involved in maintaining the CpG methylation levels and repressing ETT/ARF3 transcript levels. AS2 and nucleolus-localizing modifiers might cooperatively repress ETT/ARF3 to develop symmetric flat leaves. These results raise the possibility of a nucleolus-related epigenetic repression system operating for developmental genes unique to plants and predict that AS2 could be a molecule with novel functions that cannot be explained by the conventional concept of transcription factors. View Full-Text
Keywords: ASYMMETRIC LEAVES2; AS2/LOB domain; adaxial–abaxial polarity; ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3); AS2 body; nucleolus; gene body methylation; ribosomal DNA (rDNA) ASYMMETRIC LEAVES2; AS2/LOB domain; adaxial–abaxial polarity; ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3); AS2 body; nucleolus; gene body methylation; ribosomal DNA (rDNA)
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MDPI and ACS Style

Iwakawa, H.; Takahashi, H.; Machida, Y.; Machida, C. Roles of ASYMMETRIC LEAVES2 (AS2) and Nucleolar Proteins in the Adaxial–Abaxial Polarity Specification at the Perinucleolar Region in Arabidopsis. Int. J. Mol. Sci. 2020, 21, 7314. https://doi.org/10.3390/ijms21197314

AMA Style

Iwakawa H, Takahashi H, Machida Y, Machida C. Roles of ASYMMETRIC LEAVES2 (AS2) and Nucleolar Proteins in the Adaxial–Abaxial Polarity Specification at the Perinucleolar Region in Arabidopsis. International Journal of Molecular Sciences. 2020; 21(19):7314. https://doi.org/10.3390/ijms21197314

Chicago/Turabian Style

Iwakawa, Hidekazu; Takahashi, Hiro; Machida, Yasunori; Machida, Chiyoko. 2020. "Roles of ASYMMETRIC LEAVES2 (AS2) and Nucleolar Proteins in the Adaxial–Abaxial Polarity Specification at the Perinucleolar Region in Arabidopsis" Int. J. Mol. Sci. 21, no. 19: 7314. https://doi.org/10.3390/ijms21197314

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