Impact of rRNA Biogenesis Defect in Arabidopsis Embryogenesis

Regulatory mechanisms of ribosomal RNA (rRNA) gene transcription and processing remain elusive in plants, especially their connection to nucleolar organization. We identified a previously uncharacterized SAS10/C1D family protein Thallo (THAL) as a novel component important for both rRNA gene expression and processing, which further impacts nucleolar structure. THAL disruption caused enlarged nucleoli in arrested embryos, aberrant processing of precursor rRNAs at the 5' External Transcribed Spacer, and repression of the major rDNA variant (VAR1). THAL overexpression lines showed de-repression of VAR1 and overall reversed effects on rRNA processing sites. Strikingly, THAL overexpression also induced formation of multiple nucleoli per nucleus phenotypic of mutants of heterochromatin factors. THAL physically associated with histone chaperone Nucleolin 1 (NUC1), histone-binding NUC2, and histone demethylase Jumonji 14 (JMJ14), suggesting that it participates in chromatin regulation. THAL also interacted with putative Small Subunit processome components, including previously unreported Arabidopsis homologue of yeast M Phase Phosphoprotein 10 (MPP10). This study uncovering the dual role of THAL in transcription and processing events critical for proper rRNA biogenesis and nucleolar organization during reproduction is the first to define the function of SAS10/C1D family members in plants pollination.