Dynamic Control of the High-Affinity Iron Uptake Complex in Root Epidermal Cells

Area of Science:

• Plant Biology
• Molecular Plant Physiology
• Biochemistry

Background:

• Iron is essential for plant growth, and its uptake is tightly regulated.
• The IRT1 transporter in Arabidopsis roots mediates iron uptake but also transports non-iron metals like Zn, Mn, and Co.
• IRT1's endocytosis and degradation are regulated by its non-iron metal substrates via ubiquitination.

Purpose of the Study:

• To identify proteins interacting with IRT1 in Arabidopsis roots.
• To elucidate the molecular mechanisms regulating iron uptake.
• To understand the composition and dynamics of the iron uptake machinery.

Main Methods:

• Mass spectrometry to identify IRT1-interacting proteins.
• Co-immunoprecipitation and split-ubiquitin assays to confirm protein interactions.
• Analysis of protein ubiquitination and endocytosis dynamics.

Main Results:

• Identified a tripartite complex of IRT1, FRO2 reductase, and AHA2 proton pump involved in iron uptake.
• Confirmed direct, tripartite interactions between IRT1, FRO2, and AHA2.
• Demonstrated that FRO2 and AHA2 ubiquitination is independent of non-iron metal substrates and they are not significantly endocytosed in response to metal excess.
• Provided evidence that IRT1 phosphorylation upon high non-iron metal levels triggers complex dissociation.

Conclusions:

• A dedicated iron-acquisition protein complex, comprising IRT1, FRO2, and AHA2, exists at the Arabidopsis root epidermal cell surface.
• This complex optimizes iron uptake and its regulation.
• Phosphorylation of IRT1 plays a key role in dissociating the complex under high non-iron metal conditions, preventing metal toxicity.