Evidence mapping of molecular pathways regulating citral biosynthesis and validation status in Cymbopogon spp.

Abstract

Citral is the principal monoterpene aldehyde responsible for the characteristic aroma, quality, and industrial value of Cymbopogon species. Despite rapid advances in transcriptomic, metabolomic, and biochemical research, the extent of experimental support for different stages of citral biosynthesis and their relevance to crop improvement remain unclear. This structured scoping review followed the PRISMA-ScR framework to characterize current evidence on citral biosynthetic pathways, functional validation of pathway components, regulatory mechanisms, environmental regulation, and implications for breeding and biotechnology. Eighty-four eligible publications were evaluated using a Pathway–Validation–Environment Interaction Matrix and an Evidence Maturity Index, which assessed the strength of experimental evidence across pathway components. The evidence synthesis showed that precursor supply pathways, particularly the methylerythritol phosphate pathway and geranyl diphosphate-associated steps, have received extensive research attention, although many conclusions remain based on transcript abundance rather than direct functional validation. Geraniol formation represents one of the most experimentally supported stages, with evidence from biochemical assays, localization studies, and functional analyses. Recent studies have improved understanding of terminal oxidation reactions through enzyme characterization and transient functional approaches. However, regulatory networks, metabolic flux control, chemotype stability, and genotype-to-phenotype relationships remain insufficiently characterized. Environmental conditions, genotype effects, harvest practices, and post-harvest handling influence citral accumulation through interactions between metabolic regulation and plant physiology. Future research should prioritize functional genomics, enzyme kinetics, metabolic flux analysis, regulatory network validation, and integrated multi-omics approaches. This evidence framework supports improved understanding of citral biosynthesis and informs breeding strategies, metabolic engineering, and quality standardization of Cymbopogon essential oils.