Iron (Fe) deficiency is a major constraint for plant growth and affects the quality of edible plant parts. To investigate the mechanisms underlying Fe homeostasis in plants, Fe deficiency-induced changes in the protein profile of Arabidopsis (Arabidopsis thaliana) roots were comprehensively analyzed using iTRAQ (Isobaric Tag for Relative and Absolute Quantification) differential liquid chromatography-tandem mass spectrometry on a LTQ-Orbitrap with high-energy collision dissociation. A total of 4,454 proteins were identified with a false discovery rate of less than 1.1%, and 2,882 were reliably quantified.

ORCID IDs: 0000-0002-7967-5136 (J.R.-C.); 0000-0001-5470-5901 (J.A.); 0000-0002-7850-6832 (W.S.).The generally low bioavailability of iron in aerobic soil systems forced plants to evolve sophisticated genetic strategies to improve the acquisition of iron from sparingly soluble and immobile iron pools. To distinguish between conserved and species-dependent components of such strategies, we analyzed iron deficiency-induced changes in the transcriptome of two model species, Arabidopsis (Arabidopsis thaliana) and Medicago truncatula. Transcriptional profiling by RNA sequencing revealed a massive upregulation of genes coding for enzymes involved in riboflavin biosynthesis in M. truncatula and phenylpropanoid synthesis in Arabidopsis upon iron deficiency. Coexpression and promoter analysis indicated that the synthesis of flavins and phenylpropanoids is tightly linked to and putatively coregulated with other genes encoding proteins involved in iron uptake. We further provide evidence that the production and secretion of phenolic compounds is critical for the uptake of iron from sources with low bioavailability but dispensable under conditions where iron is readily available. In Arabidopsis, homozygous mutations in the Fe(II)-and 2-oxoglutarate-dependent dioxygenase family gene F69H1 and defects in the expression of PLEIOTROPIC DRUG RESISTANCE9, encoding a putative efflux transporter for products from the phenylpropanoid pathway, compromised iron uptake from an iron source of low bioavailability. Both mutants were partially rescued when grown alongside wild-type Arabidopsis or M. truncatula seedlings, presumably by secreted phenolics and flavins. We concluded that production and secretion of compounds that facilitate the uptake of iron is an essential but poorly understood aspect of the reduction-based iron acquisition strategy, which is likely to contribute substantially to the efficiency of iron uptake in natural conditions.

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BackgroundQuantitative information on gene activity at single cell-type resolution is essential for the understanding of how cells work and interact. Root hairs, or trichoblasts, tubular-shaped outgrowths of specialized cells in the epidermis, represent an ideal model for cell fate acquisition and differentiation in plants.ResultsHere, we provide an atlas of gene and protein expression in Arabidopsis root hair cells, generated by paired-end RNA sequencing and LC/MS-MS analysis of protoplasts from plants containing a pEXP7-GFP reporter construct. In total, transcripts of 23,034 genes were detected in root hairs. High-resolution proteome analysis led to the reliable identification of 2,447 proteins, 129 of which were differentially expressed between root hairs and non-root hair tissue. Dissection of pre-mRNA splicing patterns showed that all types of alternative splicing were cell type-dependent, and less complex in EXP7-expressing cells when compared to non-root hair cells. Intron retention was repressed in several transcripts functionally related to root hair morphogenesis, indicative of a cell type-specific control of gene expression by alternative splicing of pre-mRNA. Concordance between mRNA and protein expression was generally high, but in many cases mRNA expression was not predictive for protein abundance.ConclusionsThe integrated analysis shows that gene activity in root hairs is dictated by orchestrated, multilayered regulatory mechanisms that allow for a cell type-specific composition of functional components.