Involved in pre-rRNA processing and ribosome assembly (PubMed:17108323, PubMed:17286797, PubMed:17369435, PubMed:21124873). Is associated with intranucleolar chromatin and pre-ribosomal particles and plays a role in controlling activation and repression of a specific subset of rRNA genes located in distinctive nucleolar organizer regions. Binds specifically rDNA chromatin and may be required to maintain rDNA chromatin structure, but is probably not required for the overall histone methylation status of 45S rRNA genes (PubMed:17108323, PubMed:17286797, PubMed:21124873). Involved in leaf polarity establishment by functioning cooperatively with AS1 to repress abaxial genes ARF3, ARF4, KAN1, KAN2, YAB1 and YAB5, and the knox homeobox genes KNAT1, KNAT2, KNAT6, and STM to promote adaxial development in leaf primordia at shoot apical meristems at high temperatures (PubMed:27334696). Interacts with THAL in the nucleus. Found in the dense fibrillar component region of the nucleolus. Expressed in roots, leaves, shoots and flowers. By glucose and sucrose (at protein level). Reduced growth, bushy plants with stunted leaves, abnormal vascular patterns and many stems (PubMed:17286797, PubMed:17369435, PubMed:21124873). Plants exhibit a pointed narrow shape of leaves, but no filamentous leaves. Plants with double mutations in this protein and in AS2 or AS1 protein show severe defects in leaf shape and filamentous leaves are efficiently formed, although a few normally shaped leaves and markedly narrow leaves are also generated (PubMed:27334696). Altered nucleolus ultrastructure (PubMed:17108323). Disruption of NUCL1 induces NUCL2 expression.