Evidences for a Nutritional Role of Iodine in Plants

Abstract

Little is known about the role of iodine in plant physiology. We evaluated the impact of low concentrations of iodine on the phenotype, transcriptome and proteome of Arabidopsis thaliana. Our experiments showed that removal of iodine from the nutrition solution compromises plant growth, and restoring it in micromolar concentrations is beneficial for biomass accumulation and leads to early flowering. In addition, iodine treatments specifically regulate the expression of several genes, mostly involved in the plant defence response, suggesting that iodine may protect against both biotic and abiotic stress. Finally, we demonstrated iodine organification in proteins. Our bioinformatic analysis of proteomic data revealed that iodinated proteins identified in the shoots are mainly associated with the chloroplast and are functionally involved in photosynthetic processes, whereas those in the roots mostly belong and/or are related to the action of various peroxidases. These results suggest the functional involvement of iodine in plant nutrition.

Keywords: Arabidopsis; iodine; plant growth; plant nutrition; plant phenotype; proteomics; transcriptomics.

FIGURE 1

Impact of iodine on plant growth and development (exp. 1-phenotype). (A) Lateral view of plants after 4 or 6 weeks from the onset of KIO₃ treatment. (B) Flowering time curve; the percentage of bloomed plants/tray was calculated every 3 days after the opening of the first flower (day 0). (C) Morphological data on plant FW, DW, dry matter content, rosette diameter, inflorescence length and number of produced siliques/plant, determined 1 month after the onset of KIO₃ treatments. Values indicated by different letters significantly differ from each other (according with one-way ANOVA, LSD post hoc test, P ≤ 0.05). In particular, the statistical analysis of flowering (B) was performed by comparing the percentage of bloomed plants of each tray (considered as biological replicates) within each sampling point. When data followed a Normal distribution and there was homogeneity of variances, they were subjected to one-way ANOVA and values indicated by different letters significantly differ from each other (LSD post hoc test, P ≤ 0.05). When one of this two prerequisite was violated, a Kruskal-Wallis test was performed. Error bars (±SE) are shown in graphs.

FIGURE 2

Impact of iodine on plant growth and development (exp. 2-phenotype). (A) Flowering curve; the percentage of bloomed plants/tray was calculated every 2 days after the opening of the first flower (day 0). (B) Representative control, and KI-, NaI- or KBr-treated plants (10 and 30 μM) after 15 days from the onset of the treatments. Pictures were taken after 4 days from the opening of the first flower on the main stem. (C) Morphological data on plant FW, DW, dry matter content determined 15 days after the onset of the treatments. Values indicated by different letters significantly differ from each other (according with one-way ANOVA, LSD post hoc test, P ≤ 0.05). In particular, the statistical analysis of flowering (A) was performed by comparing the percentage of bloomed plants of each tray (considered as biological replicates) within each sampling point. When data followed a Normal distribution and there was homogeneity of variances, they were subjected to one-way ANOVA and values indicated by different letters significantly differ from each other (LSD post hoc test, P ≤ 0.05). When one of this two prerequisite was violated, a Kruskal-Wallis test was performed. Error bars (±SE) are shown in graphs.

FIGURE 3

Transcriptional regulation of gene expression induced by iodine. Venn diagram showing the number of genes differentially regulated in shoot (A) or root (B) tissues of KBr-, NaI-, and KI-treated plants (10 μM—48 h), when compared with the control. Heatmap showing the pattern of expression of the genes analysed in the shoot (C) or root (D) tissues in response to NaI, KI or KBr treatments, when compared with the control. qPCR validation of selected genes up- or down-regulated by iodine treatments (commonly regulated by NaI and KI, but not KBr) in shoot (E) or root (F) tissues. qPCR data are mean ± SE of four biological replicates, each composed of a pool of three different rosettes. Values indicated by different letters significantly differ from each other (according with one-way ANOVA, LSD post hoc test, P ≤ 0.05).

FIGURE 4

Overview of the main biological processes affected by iodine based on the GO terms enrichment analysis carried out in root tissues. Only genes regulated in NaI- and KI-treated plants, and not in KBr-treated plants, when compared with the control, were analysed. The figure was extracted from GOrilla (http://cbl-gorilla.cs.technion.ac.il) and reproduced. In this analysis, DEGs with log₂FC ≥ 2.5 or log₂FC ≤ –2.5 were used.

FIGURE 5

Autoradiographies of the SDS-PAGE gels. Comparison between the position and relative intensities of ¹²⁵I radiolabelled bands of representative shoot (A) and root (B) protein extracts from ¹²⁵I treated Arabidopsis (exp. 1-radioactive), and maize, tomato, wheat and lettuce plants (exp. 2-radioactive). Sampling was performed after 48 h of ¹²⁵I incubation. In both the experiments, autoradiographies were acquired after 72 h of gel exposition to the multipurpose phosphor storage screen. Representative pictures of total stained protein extracts (SDS-PAGE) and of the autoradiographies of control samples after 15 days of exposition are also shown. Controls consisted in protein extracts obtained from plants untreated with ¹²⁵I during their growth, to which the radioactive solution containing ¹²⁵I was added during the extraction process.

FIGURE 6

Iodination in A. thaliana proteins identified by database searching of nano-LC-ESI-MS/MS raw data from a public repository (PRIDE). (A) Unambiguous assignment of iodination sites by MS/MS analysis in two peptides from chloroplast (light harvesting complex of photosystem II 5, AT4G10340.1), upper panel, and roots (peroxidase superfamily protein, AT4G30170.1), lower panel. The peptides are identified by both y, and b ions. Red labels in the spectra evidence the mass shift corresponding to the iodinated tyrosine (i-Y). (B) Venn Diagram showing the iodinated peptide sequences identified in the datasets of chloroplasts (Chlor), cauline (Cau), rosette (Ros), and roots (Root).