Shugo Maekawa, Tetsuya Ishida, Shuichi Yanagisawa*


Ribosome biogenesis is one of the most energy-consuming events in the cell and must therefore be coordinated with changes in cellular energy status. Here, we show that the sugar-inducible gene ARABIDOPSIS PUMILIO PROTEIN 24 (APUM24) encodes a Pumilio homology domain-containing protein involved in pre-ribosomal RNA (pre-rRNA) processing in Arabidopsis thaliana. Null mutation of APUM24 resulted in aborted embryos due to abnormal gametogenesis and embryogenesis, whereas reduced expression of APUM24 caused several phenotypes characteristic of ribosome biogenesis or function-related mutants. APUM24 interacted with other pre-rRNA processing factors and a putative endonuclease for the removal of the internal transcribed spacer2 (ITS2) of pre-rRNA in the nucleolus. The APUM24-containing complex also interacted with ITS2, and reduced APUM24 expression caused the over-accumulation of processing intermediates containing ITS2. Thus, APUM24 likely functions as an ITS2 removal-associated factor. Most importantly, the apum24 knockdown mutant was hypersensitive to highly concentrated sugar, and the mutant showed sugar-dependent over-accumulation of processing intermediates and nucleolar stress (changes in nucleolar size). Furthermore, reduced APUM24 expression diminished sugar-induced promotion of leaf and root growth. Hence, a breakdown in the coordinated expression of ribosome biogenesis-related genes with energy status may induce nucleolar stress and disturb proper sugar responses in Arabidopsis.

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