Abstract
Nucleolin is a major nucleolar protein implicated in many aspects of ribosomal biogenesis, including early events such as processing of the large 35S preribosomal RNA. We found that the Arabidopsis (Arabidopsis thaliana) parallel1 (parl1) mutant, originally identified by its aberrant leaf venation, corresponds to the Arabidopsis nucleolin gene. parl1 mutants display parallel leaf venation, aberrant localization of the provascular marker Athb8:beta-glucuronidase, the auxin-sensitive reporter DR5:beta-glucuronidase, and auxin-dependent growth defects. PARL1 is highly similar to the yeast (Saccharomyces cerevisiae) nucleolin NUCLEAR SIGNAL RECOGNITION 1 (NSR1) multifunctional protein; the Arabidopsis PARL1 gene can rescue growth defects of yeast nsr1 null mutants. This suggests that PARL1 protein may have roles similar to those of the yeast nucleolin in nuclear signal recognition, ribosomal processing, and ribosomal subunit accumulation. Based on the range of auxin-related defects in parl1 mutants, we propose that auxin-dependent organ growth and patterning is highly sensitive to the efficiency of nucleolin-dependent ribosomal processing.
Highlights
Nucleolin is a major nucleolar protein implicated in many aspects of ribosomal biogenesis, including early events such as processing of the large 35S preribosomal RNA
These defects suggest Arabidopsis PARL1 is involved in plant growth and patterning
Ribosomes are responsible for protein translation in living eukaryotic cells, and their biogenesis is a complex process involving the coordination of rDNA, rRNA, RNA polymerase I, and a plethora of nonribosomal/ribosomal proteins
Summary
Nucleolin is a major nucleolar protein implicated in many aspects of ribosomal biogenesis, including early events such as processing of the large 35S preribosomal RNA. Based on the range of auxin-related defects in parl mutants, we propose that auxin-dependent organ growth and patterning is highly sensitive to the efficiency of nucleolin-dependent ribosomal processing. Venation phenotypes are associated with defects in other signals, including xylogen, brassinosteroids, cytokinin, and small peptides (Casson et al, 2002; Fukuda, 2004; Motose et al, 2004) It is currently unknown whether these diverse factors have direct or indirect roles in vein patterning, but it is remarkable that of the many plant morphological features, venation pattern is especially sensitive to perturbations in basic cellular machinery. Since most parl effects appear to be auxin related, we suggest that auxin-dependent organ growth and patterning is sensitive to nucleolin deficiency, possibly because auxin regulation depends on protein turnover and ribosome biogenesis in areas of growth
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