Loss of Function of Arabidopsis C-Terminal Domain Phosphatase-Like1 Activates Iron Deficiency Responses at the Transcriptional Level

The expression of genes that control iron (Fe) uptake and distribution (i.e. Fe utilization-related genes) is tightly regulated. Fe deficiency strongly induces Fe utilization-related gene expression; however, little is known about the mechanisms that regulate this response in plants. Transcriptome analysis of an Arabidopsis (Arabidopsis thaliana) mutant defective in RNA polymerase II C-terminal domain-phosphatase-like1 (CPL1) revealed significant up-regulation of Fe utilization-related genes (e.g. IRON-REGULATED TRANSPORTER1), suggesting the importance of RNA metabolism in Fe signaling. An analysis using multiple cpl1 alleles established that cpl1 mutations enhanced specific transcriptional responses to low Fe availability. Changes in protein level were less prominent than those in transcript level, indicating that cpl1-2 mainly affects the Fe deficiency response at the transcriptional level. However, Fe content was significantly increased in the roots and decreased in the shoots of cpl1-2 plants, indicating that the cpl1 mutations do indeed affect Fe homeostasis. Furthermore, root growth of cpl1-2 showed improved tolerance to Fe deficiency and cadmium (Cd) toxicity. cpl1-2 plants accumulated more Cd in the shoots, suggesting that Cd toxicity in the roots of this mutant is averted by the transport of excess Cd to the shoots. Genetic data indicate that cpl1-2 likely activates Fe deficiency responses upstream of both FE-DEFICIENCY-INDUCED TRANSCRIPTION FACTOR-dependent and -independent signaling pathways. Interestingly, various osmotic stress/abscisic acid (ABA)-inducible genes were up-regulated in cpl1-2, and the expression of some ABA-inducible genes was controlled by Fe availability. We propose that the cpl1 mutations enhance Fe deficiency signaling and promote cross talk with a branch of the osmotic stress/ABA signaling pathway.