Endophytic Bacteria as Modulators of Fragrance Biosynthesis in Plants: Mechanisms and Prospects

Plant fragrance is the blend of volatile terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and nitrogen- or sulfur-containing compounds released from flowers, leaves, and fruit. It drives pollination and defence and accounts for much of the economic value of aromatic, medicinal, and ornamental crops. However, it shifts with genotype, developmental stage, tissue type, and stress, and a large share of it still comes from wild-harvested plants. Endophytic bacteria are the microorganisms that colonize internal plant tissues without causing apparent disease symptoms, may modulate these specialized metabolic pathways. Only a limited number of studies have directly investigated these interactions. In contrast, a substantially larger body of research has demonstrated the roles of endophytic bacteria in promoting plant growth and enhancing plant tolerance to environmental stresses. However, direct evidence establishing links between bacterial endophytes, specific genes within volatile-biosynthetic pathways, and the volatile compounds actually released by plants remains limited. The current review summarizes the current evidence on the regulation of major plant volatile biosynthetic routes by endophytic bacteria, including the MVA and MEP pathways for terpenoids, the shikimate-derived phenylpropanoid/benzenoid pathway, as well as the lipoxygenase pathway leading to green-leaf volatile production. The suggested mechanisms are generally grouped into several inter-related categories, such as microbial elicitation and pattern recognition, phytohormonal reprogramming, nutrient and precursor acquisition, modulation of carbon flux through the MVA/MEP and phenylpropanoid pathways, redox regulation and stress priming, and bacterial volatile-mediated signalling. More speculative mechanisms are discussed separately, such as microbial biotransformation, direct microbial biosynthesis of plant volatiles, and horizontal gene transfer. Evidence from culinary and essential oil herbs, medicinal plants, ornamental flowers, and fruit crops is evaluated using an evidence hierarchy that distinguishes phenotypic associations from experimentally validated causal mechanisms. Establishing a robust gene-to-metabolite causal framework will require targeted approaches, including gnotobiotic inoculation, volatilomic profiling, and stable-isotope tracing.