The Rhizophagus irregularis permease RiFTR1 functions without a ferroxidase partner for reductive iron transport

Area of Science:

• Plant-microbe interactions
• Mycology
• Molecular biology

Background:

• Arbuscular mycorrhizal fungi (AM fungi) are crucial for plant nutrient acquisition, particularly iron (Fe).
• The AM fungus Rhizophagus irregularis uses a high-affinity reductive pathway for Fe uptake, mediated by the RiFTR1 transporter.

Purpose of the Study:

• To identify ferroxidase partners for the RiFTR1 iron transporter in R. irregularis using a genome-wide approach.
• To investigate the role of identified ferroxidases and RiFTR1 in iron uptake and arbuscular mycorrhizal symbiosis.

Main Methods:

• Genome-wide identification of multicopper oxidase (MCO) genes in R. irregularis.
• Yeast complementation assays to assess ferroxidase activity of MCOs.
• Expression analysis of RiFTR1 in arbuscules.
• Overexpression of RiFTR1 in Medicago truncatula roots.

Main Results:

• Nine putative MCO genes (RiMCO1-9) were identified in R. irregularis.
• RiMCO1 and RiMCO3 exhibited ferroxidase activity in yeast, suggesting a role in reductive Fe uptake.
• RiFTR1 demonstrated Fe transport in yeast independently of a ferroxidase.
• RiFTR1 expression increased in arbuscules, and its overexpression enhanced mycorrhizal colonization and arbuscule formation.

Conclusions:

• RiFTR1 plays a significant role in iron acquisition for arbuscular mycorrhizal symbiosis.
• The independent Fe transport capability of RiFTR1, similar to plant IRT1-like systems, is a key finding.
• Iron availability is critical for establishing and maintaining the AM symbiosis.