The Arabidopsis Hypoxia Inducible AtR8 Long Non-Coding RNA also Contributes to Plant Defense and Root Elongation Coordinating with WRKY Genes under Low Levels of Salicylic Acid

AtR8 lncRNA was previously identified in the flowering plant Arabidopsis thaliana as an abundant Pol III-transcribed long non-coding RNA (lncRNA) of approximately 260 nt. AtR8 lncRNA accumulation is responsive to hypoxic stress and salicylic acid (SA) treatment in roots, but its function has not yet been identified. In this study, microarray analysis of an atr8 mutant and wild-type Arabidopsis indicated a strong association of AtR8 lncRNA with the defense response. AtR8 accumulation exhibited an inverse correlation with an accumulation of two WRKY genes (WRKY53/WRKY70) when plants were exposed to exogenous low SA concentrations (20 µM), infected with Pseudomonas syringae, or in the early stage of development. The highest AtR8 accumulation was observed 5 days after germination, at which time no WRKY53 or WRKY70 mRNA was detectable. The presence of low levels of SA resulted in a significant reduction of root length in atr8 seedlings, whereas wrky53 and wrky70 mutants exhibited the opposite phenotype. Taken together, AtR8 lncRNA participates in Pathogenesis-Related Proteins 1 (PR-1)-independent defense and root elongation, which are related to the SA response. The mutual regulation of AtR8 lncRNA and WRKY53/WRKY70 is mediated by Nonexpressor of Pathogenesis-Related Gene 1 (NPR1).