Two AMP-Binding Domain Proteins from Rhizophagus irregularis Involved in Import of Exogenous Fatty Acids

Area of Science:

• Plant-Fungal Interactions
• Molecular Biology
• Biochemistry

Background:

• Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with plant roots, exchanging nutrients for host-derived carbon compounds.
• AMF lack key enzymes for fatty acid synthesis, necessitating the import of these essential lipids from the host plant.
• The precise mechanisms by which AMF acquire host fatty acids remain largely uncharacterized.

Purpose of the Study:

• To identify and characterize genes in *Rhizophagus irregularis* involved in the uptake of host-derived fatty acids.
• To investigate the function of two novel AMP-binding domain protein genes, RiFAT1 and RiFAT2, in fatty acid transport.
• To elucidate the role of these proteins in the symbiotic interaction within colonized plant roots.

Main Methods:

• Gene cloning and expression analysis of *RiFAT1* and *RiFAT2* in *Saccharomyces cerevisiae* Δ*fat1* mutant cells.
• Fatty acid uptake assays using ¹³C-labeled fatty acids and ²H-labeled monoacylglycerols.
• Analysis of gene expression in colonized roots versus extraradical mycelium.
• Tracing of ¹³C-labeled acetate and fatty acids in fungal lipids within colonized roots.

Main Results:

• Expression of *RiFAT1* or *RiFAT2* in yeast cells significantly enhanced the uptake of ¹³C-myristic acid (14:0) and ¹³C-palmitic acid (16:0).
• Uptake of fatty acids from labeled monoacylglycerols was increased upon RiFAT1/RiFAT2 expression, but intact monoacylglycerols were not detected, suggesting hydrolysis.
• *RiFAT1* and *RiFAT2* expression was upregulated in colonized roots compared to extraradical mycelium.
• De novo synthesized fatty acids from ¹³C₂-acetate were detected in AMF lipids in colonized roots, indicating rapid uptake of host-derived fatty acids.

Conclusions:

• RiFAT1 and RiFAT2 are functional fatty acid transporters involved in the uptake of myristic and palmitic acids by *R. irregularis*.
• Fatty acids are imported into AMF either directly or after hydrolysis of host lipids like monoacylglycerols.
• These transporters likely play a critical role in facilitating fatty acid import into fungal arbuscules within plant roots, supporting the symbiotic relationship.