MICROBIAL REGULATION OF FLORAL SECONDARY METABOLISM IN ORNAMENTAL PLANTS: MOLECULAR MECHANISMS UNDERLYING FLOWER COLOR, FRAGRANCE AND QUALITY

Abstract

Flower-associated microorganisms have been receiving growing scientific interest in recent years as important constituents of the floral ecosystem. However, possible participation of microorganisms in the specialized metabolism of ornamentals still requires more investigation. Various microbial populations living in nectar, petals, reproductive organs, and other parts of the flower may interact with the hosts by means of microbial metabolites, volatile organic compounds, elicitors, nutrient-dependent effects, and signaling. The interactions might affect the synthesis, accumulation, and composition of floral secondary metabolites connected with traits of flower color, fragrance, and nectar chemical characteristics, among others. The present review considers the current knowledge about the molecular mechanisms of floral specialized metabolism and possible interactions of floral microorganisms with the hosts affecting flower metabolic processes. Special attention is paid to the microbial signaling, calcium and reactive oxygen species signaling, mitogen-activated protein kinase pathways, phytohormone response, transcriptional regulation, and metabolic pathways involved in floral pigmentation and fragrance. Information from ornamental plants, such as chrysanthemum, poinsettia, Hibiscus mutabilis, and Osmanthus fragrans, is provided. Recent development in various omics technologies, including microbial community profiling together with host transcriptomics, metabolomics, and volatilomics, allows the identification of microbial functions and host pathways responsible for variations in flower metabolic processes. Direct causal links between specific microorganisms or their microbial signaling molecules and host floral biosynthetic genes have not yet been established. Future research will need to include microbial inoculation, multi-omics analysis at the tissue level, synthetic microbial communities, and validation in order to differentiate between microbial synthesis or transformation of floral metabolites and the true modulation of host metabolic pathways.