Laccaria bicolor adapts to phosphate deficiency at the developmental, transcriptional and metabolic levels

The amount of soluble inorganic phosphate (Pi) in soils is typically low and limiting plant growth. Roots of trees in several forest ecosystems form association with ectomycorrhizal (ECM) fungi, where fungi forage and supply inorganic nutrients, such as Pi, in exchange for fixed carbon. While adaptations of model fungi, such as Saccharomyces cerevisiae, to Pi deficiency has been extensively studied, much less is known about how mycorrhizal fungi adapt to Pi deficiency. This study aimed to decipher how the free-living ECM Laccaria bicolor mycelium adapts to Pi deficiency. L. bicolor grown for 7 days in medium without Pi showed very low Pi and polyphosphate reserves and displayed less compact colonies with spreading hyphae. Pi deficiency resulted in approximately 1500-2000 genes being up- and down-regulated more than 2-fold compared to mycelium grown with abundant Pi, with most genes partially reverting their expression pattern in cultures spiked with Pi for 24 hours. Numerous genes involved in Pi mobilization from organic sources, such as phosphatases and ribonucleases, were induced by Pi deficiency, as well as genes involved in Pi transport, and such expression patterns correlated with increased enzymatic activities. Pi deficiency also induced the synthesis of the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS). Several genes induced by mycorrhization, such as those encoding protease inhibitors belonging to the mycocypin family and Mycorrhizae-Induced Small Secreted Peptides (MiSSP), were also induced by Pi deficiency. Altogether, this study shows that L. bicolor can robustly respond to Pi deficiency and identifies parallels between these adaptations and those involved in mycorrhization.