Abstract
Nucleolin is involved in various steps of ribosome synthesis such as transcription and processing of pre-rRNA and assembly with ribosomal proteins. Arabidopsis thaliana contains two genes for nucleolin; AtNuc-L1 expressed in young tissues and AtNuc-L2 expressed only weakly in floral buds under normal conditions. Expression of AtNuc-L1 is inducible by sugars and linked with cell proliferation, and its knockout mutant (atnuc1-1) shows delayed growth and various developmental defects. Although knockout of AtNuc-L2 does not affect growth, expression of AtNuc-L2 is high in atnuc1-1 and double knockout of both genes is embryonic lethal. When the wild type plants were transferred from 22oC to 15oC, AtNuc-L1 mRNA was induced within 1 day and the level of AtNuc-L1 protein also increased. The atnuc1-1 plants stopped growth at 15oC. Expression of AtNuc-L1 under the RPS5A promoter in atnuc1-1 caused 20 to 70% recovery of AtNuc-L1 mRNA, and resulted in partial recovery from various developmental defects and growth arrest at 15oC. The reduced levels of 25S and 18S rRNAs also showed partial recovery. Thus, AtNuc-L1 is necessary for growth under low temperature through ribosome synthesis.
