Fatty acid signatures distinguish autotrophy, partial mycoheterotrophy, and full mycoheterotrophy in angiosperms

Detecting partial mycoheterotrophy in green plants remains challenging, particularly in associations with arbuscular mycorrhizal (AM) or rhizoctonia fungi, because natural-abundance stable isotope patterns often provide limited diagnostic resolution in these systems. We tested whether fatty acid composition, widely used to infer trophic relationships in animal and soil ecology, provides complementary evidence of fungal carbon acquisition by plants. We characterized fatty acid profiles of 21 autotrophic reference plants, six partially mycoheterotrophic plants, and 17 fully mycoheterotrophic plants associated with AM, ectomycorrhizal, or saprotrophic fungi. In expanded analyses, we additionally examined three individuals each of the rhizoctonia-associated orchid Goodyera biflora and the presumed autotrophic AM plant Trillium camschatcense with marked ¹³C enrichment, together with six co-occurring autotrophic reference plants. Fatty acid composition differed significantly among the three nutritional groups. Increasing mycoheterotrophic dependence was associated with decreasing relative abundance of 18:3n-3 and increasing relative abundance of 18:2n-6, which is widely used as a fungal biomarker in soil ecology. Partially mycoheterotrophic plants occupied an intermediate position in ordination space, and all pairwise group comparisons were significant. Differentiation remained significant after exclusion of 18:3n-3 and 16:3n-3 and renormalization of the remaining components, indicating that chloroplast-associated fatty acids alone did not drive the pattern. Goodyera biflora resembled partially mycoheterotrophic orchids, whereas T. camschatcense resembled autotrophic references. Fatty acid profiling may therefore complement stable isotope analysis in assessments of partial mycoheterotrophy. The profiles likely reflect metabolic remodeling, incorporation of fungal-derived carbon into newly synthesized plant lipids, or both, although some retention of intact fungal lipids cannot be excluded.