Comparative Transcriptome Reveals ART1-Dependent Regulatory Pathways for Fe Toxicity Response in Rice Roots.

Iron (Fe) is an essential element for plants, but an excess supply can have detrimental effects. Fe toxicity induces complex physiological and genetic responses, and due to this complexity, the knowledge of transcriptional regulatory mechanisms under Fe toxicity is very limited. Previous studies suggested that plant responses to excess Fe involve oxidative stress caused by reactive oxygen species (ROS), which itself causes transcriptional changes. We hypothesized that dissecting these complex responses could lead to the identification of a novel factor and conducted a comparative transcriptome analysis using roots of rice plants exposed to nutrient solutions containing 1 or 5 mM of hydrogen peroxide (a major form of ROS) or 300 mg L^-1 of Fe (as FeSO₄). Genes induced by hydrogen peroxide overlapped with 62%, 49%, and 30% of Fe toxicity-upregulated genes at 3 h, 1 day, and 3 days following treatment initiation. Subsequent gene co-expression analyses classified genes into 21 groups with varying responsiveness to ROS and Fe toxicity. Genes in group 15 were specifically upregulated by Fe toxicity and overlapped significantly with aluminum (Al)-inducible genes and target genes of the Zn-finger transcription factor, ART1, which regulates Al response in rice roots. Additional experiments using the art1 knock-out mutant demonstrated that ART1 is crucial for upregulating genes such as STAR2 and FRDL4 in response to Fe toxicity. This study reveals the contribution of ART1-dependent regulatory pathways in rice roots under Fe toxicity.