Two bHLH Transcription Factors, bHLH34 and bHLH104, Regulate Iron Homeostasis in Arabidopsis thaliana

IRON MAN (IMA) peptides, a family of small peptides, control iron (Fe) transport in plants, but their roles in Fe signaling remain unclear. BRUTUS (BTS) is a potential Fe sensor that negatively regulates Fe homeostasis by promoting the ubiquitin-mediated degradation of bHLH105 and bHLH115, two positive regulators of the Fe deficiency response. Here, we show that IMA peptides interact with BTS. The C-terminal parts of IMA peptides contain a conserved BTS interaction domain (BID) that is responsible for their interaction with the C terminus of BTS. Arabidopsis thaliana plants constitutively expressing IMA genes phenocopy the bts-2 mutant. Moreover, IMA peptides are ubiquitinated and degraded by BTS. bHLH105 and bHLH115 also share a BID, which accounts for their interaction with BTS. IMA peptides compete with bHLH105/bHLH115 for interaction with BTS, thereby inhibiting the degradation of these transcription factors by BTS. Genetic analyses suggest that bHLH105/bHLH115 and IMA3 have additive roles and function downstream of BTS. Moreover, the transcription of both BTS and IMA3 is activated directly by bHLH105 and bHLH115 under Fe-deficient conditions. Our findings provide a conceptual framework for understanding the regulation of Fe homeostasis: IMA peptides protect bHLH105/bHLH115 from degradation by sequestering BTS, thereby activating the Fe deficiency response.

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Glutathione regulates subcellular iron homeostasis via transcriptional activation of iron responsive genes in Arabidopsis

10.1101/2021.05.09.443283 ◽

2021 ◽

Author(s):

Ranjana Shee ◽

Soumi Ghosh ◽

Pinki Khan ◽

Salman Sahid ◽

Chandan Roy ◽

...

Keyword(s):

Transcription Factors ◽

Transcriptional Activation ◽

Iron Homeostasis ◽

Fe Deficiency ◽

Fe Content ◽

Increased Sensitivity ◽

Fe Homeostasis ◽

Bhlh Transcription Factors ◽

Dependent Pathway

Glutathione (GSH) is a ubiquitous molecule known to regulate various physiological and developmental phenomena in plants. Recently, its involvement in regulating iron (Fe) deficiency response was established in Arabidopsis. However, the role of GSH in modulating subcellular Fe homeostasis remained elusive. In this study, we dissected the role of GSH in regulating Fe homeostasis in Arabidopsis shoots under Fe limited conditions. The two GSH depleted mutants, cad2-1 and pad2-1 displayed increased sensitivity to Fe deficiency with smaller rosette diameter and higher chlorosis level compared with the Col-0 plants. Interestingly, the expression of the vacuolar Fe exporters, AtNRAMP3 and AtNRAMP4, chloroplast Fe importer, AtPIC1, along with AtFer1 and AtIRT1 were significantly down-regulated in these mutants. The expression of these genes were up-regulated in response to exogenous GSH treatment while treatment with BSO, a GSH inhibitor, down-regulated their expression. Moreover, the mutants accumulated higher Fe content in the vacuole and lower in the chloroplast compared with Col-0 under Fe limited condition suggesting a role of GSH in modulating subcellular Fe homeostasis. This regulation was, further, found to involve a GSNO-dependent pathway. Promoter analysis revealed that GSH induced the transcription of these genes presumably via S-nitrosylation of different Fe responsive bHLH transcription factors.

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