Time course analysis of gene expression over 24 hours in Fe-deficient barley roots

Typical for a graminaceous plant, barley secretes mugineic acid-family phytosiderophores (MAs) to acquire iron (Fe). Under Fe-deficient conditions, MAs secretion from barley roots increases markedly. Secretion shows a diurnal pattern, with a clear peak 2-3 h after sunrise and cessation within a few hours. Microarray analyses were performed to profile the Fe deficiency-inducible genes in barley roots and diurnal changes in the expression of these genes. Genes encoding enzymes involved in MAs biosynthesis, the methionine cycle, and methionine biosynthesis were highly induced by Fe deficiency. The expression of sulfate transporters was also upregulated by Fe deficiency. Therefore, all of the genes participating in the MAs pathway from sulfur uptake and assimilation to the biosynthesis of MAs were upregulated in Fe-deficient barley roots. In contrast to MAs secretion, the transcript levels of these genes did not show diurnal changes. The amount of endogenous MAs gradually increased during the day after MAs secretion ceased, and was highest before secretion began. These results show that MAs biosynthesis, including the supply of the substrate methionine, occurs throughout the day, and biosynthesized MAs likely accumulate in barley roots until their secretion into the rhizosphere. In contrast, the levels of transcripts encoding an Fe(III)-MAs complex transporter, two putative metal-MAs complex transporters, and HvYS1 were also increased in Fe-deficient barley roots, and the levels of two of these transcripts showed diurnal rhythms. The Fe(III)-MAs complex transporters may absorb Fe(III)-MAs diurnally, synchronous with the diurnal secretion of MAs.