Changes in Fatty Acid Profiles in Seeds of <i>Camellia oleifera</i> Treated by Mycorrhizal Fungi and Glomalin

<i>Camellia oleifera</i> is an important oilseed forest tree, but it is unknown whether and how inoculation with arbuscular mycorrhizal fungi, as well as spraying easily extractable glomalin-related soil protein (EG), regulates the fatty acid profile in seeds of this species. This study explored how inoculation with <i>Rhizophagus intraradices</i> (800 g inoculum/tree) and spraying EG (2.5 L/tree, four times in total, once a week) modulated the fatty acid profile for potential nutritional qualities in the seeds of 20-year-old <i>C. oleifera</i>. Spraying exogenous EG significantly increased fruit transverse diameter, longitudinal diameter, fruit weight, number of seeds, and seed weight but had no significant effect on the root mycorrhizal colonization rate. Inoculation with <i>R. intraradices</i> had no significant effect on these fruit traits but significantly boosted the root mycorrhizal colonization rate. A total of 11 saturated fatty acids and 12 unsaturated fatty acids were detected from the seeds, with the unsaturated fatty acids consisting primarily of C18:1N-12, C18:1N-9C, and C18:2-N6. Spraying exogenous EG significantly increased the levels of major unsaturated fatty acid components such as C18:1N-12, C18:1N-9C, C18:1N-7, and C18:2N-6 by 140.6%, 59.7%, 97.6%, and 60.6%, respectively, while decreasing the level of C16:0. Inoculation with <i>R. intraradices</i> only decreased the levels of C16:0 and C18:0, while increased the level of C18:2N-6. Both treatments increased the percentage of unsaturated fatty acids in total fatty acids, resulting in an increase in the unsaturation index of fatty acids. In addition, inoculation with <i>R. intraradices</i> significantly up-regulated the expression of <i>CoFAD2</i>, spraying exogenous EG significantly increased the expression of <i>CoSAD</i>, <i>CoFAD2</i>, and <i>CoFAD3</i>, and both treatments also significantly suppressed the expression of <i>CoFAE</i>. These findings suggested that exogenous EG as a biostimulant, is more suitable to regulate the nutritional values of fatty acids in seeds of 20-year-old <i>C. oleifera</i>.