Arabidopsis bHLH100 and bHLH101 Control Iron Homeostasis via a FIT-Independent Pathway

Abstract

Iron deficiency induces a complex set of responses in plants, including developmental and physiological changes, to increase iron uptake from soil. In Arabidopsis, many transporters involved in the absorption and distribution of iron have been identified over the past decade. However, little is known about the signaling pathways and networks driving the various responses to low iron. Only the basic helix–loop–helix (bHLH) transcription factor FIT has been shown to control the expression of the root iron uptake machinery genes FRO2 and IRT1. Here, we characterize the biological role of two other iron-regulated transcription factors, bHLH100 and bHLH101, in iron homeostasis. First direct transcriptional targets of FIT were determined in vivo. We show that bHLH100 and bHLH101 do not regulate FIT target genes, suggesting that they play a non-redundant role with the two closely related bHLH factors bHLH038 and bHLH039 that have been suggested to act in concert with FIT. bHLH100 and bHLH101 play a crucial role in iron-deficiency responses, as attested by their severe growth defects and iron homeostasis related phenotypes on low-iron media. To gain further insight into the biological role of bHLH100 and bHLH101, we performed microarray analysis using the corresponding double mutant and showed that bHLH100 and bHLH101 likely regulate genes involved in the distribution of iron within the plant. Altogether, this work establishes bHLH100 and bHLH101 as key regulators of iron-deficiency responses independent of the master regulator FIT and sheds light on new regulatory networks important for proper growth and development under low iron conditions.

SORTING NEXIN1 Is Required for Modulating the Trafficking and Stability of the Arabidopsis IRON-REGULATED TRANSPORTER1

“…Surprisingly, IRT1 amounts in CHX-treated Col-0 were slightly elevated in comparison to the corresponding control, suggesting that factors with a high turnover rate might negatively influence IRT1 stability. Similar effects were recently reported for the transcriptional regulation of the IRT1 gene (Sivitz et al, 2012). In the snx1-2 and snx triple mutants, a strong reduction in IRT1 levels was observed after CHX treatment.…”

A Program for Iron Economy during Deficiency Targets Specific Fe Proteins

“…Abundance under control conditions increases to the right in the plots. As expected, after 2 d of Fe deficiency, FER1 expression was decreased, while PYE, BHLH38, and BHLH100, markers for plant Fe deficiency (Long et al, 2010;Sivitz et al, 2012), were up-regulated. Generally, a good correlation was observed between protein and mRNA abundance.…”

Iron-Binding E3 Ligase Mediates Iron Response in Plants by Targeting Basic Helix-Loop-Helix Transcription Factors

“…1A), further validating the up-regulation of BTS under iron deficiency. Microarray analysis has revealed that apart from roots, BTS is also expressed in shoots in Arabidopsis (Sivitz et al, 2012). To characterize BTS expression in the shoot and during reproductive growth, we performed GUS staining on young seedlings and mature plants.…”

“…FIT acts in concert with two other bHLH proteins (Yuan et al, 2008), ETHYLENE INSENSITIVE3 and ETHYLENE INSENSITIVE3-LIKE1 , and is posttranslationally regulated by the presence of nitric oxide via 26S proteasomedependent degradation (Meiser et al, 2011;Sivitz et al, 2011). A FIT-independent iron homeostasis pathway has also been identified involving bHLH100 and bHLH101 (Sivitz et al, 2012). Recently, it has been shown that root exudates including phenolics and riboflavin derivatives function as iron-binding compounds in Arabidopsis and Medicago truncatula, similar to those in the chelation-based Strategy II response.…”