Expressional and regulatory characterization of Arabidopsis RNA-dependent RNA polymerase 1.

RNA-dependent RNA polymerase 1 (RDR1), a component of gene silencing, participates in plant pathogen defense. However, there are few reports on its expression pattern or regulatory mechanism. To clarify how the Arabidopsis RDR1 gene is regulated at the transcriptional level in response to various stresses, its native 1,303 bp promoter sequence upstream of the translational start site and five truncated regions were inserted upstream of a fused reporter gene (β-glucuronidase-green fluorescent protein) in Arabidopsis. Histochemical staining and fluorescent signal detection revealed that AtRDR1 was expressed primarily in the plant vascular tissue system and its expression was specifically localized in phloem cell layers in roots. Stress experiments showed that the AtRDR1 promoter has a broad-spectrum response to various stresses and is sensitive to 1-naphthaleneacetic acid, abscisic acid, and salicylic acid. Analysis of promoter derivatives revealed that the -1,088 to -690 region was involved in auxin and dehydration responsiveness, that -690 to -434 was responsive to cold treatment, and the intron in the 5'-untranslated region (5'-UTR) responded to jasmonic acid molecules. The 5'-UTR intron was functional in transcript accumulation. Together, our findings suggest that AtRDR1-associated pathogen defense is conducted mainly in the plant vascular tissue system and is under complex regulation.