Anatomy and ultrastructure floral osmophores of Catasetum fimbriatum (Orchidaceae)

Fine-tuned terpene synthase gene expression, functional promiscuity, and subcellular localization: implications for the evolution of complex floral volatile bouquet in Caladenia orchids

Chemically mediated floral volatile signals are crucial for pollinator attraction across angiosperms. However, beyond model plant systems, the molecular mechanisms underpinning their tissue-specific biosynthesis, regulation, and emission are still poorly understood. In this study of a food-deceptive insect-pollinated orchid (Caladenia denticulata), we elucidated the molecular basis of α-pinene biosynthesis-the major floral volatile emitted by this species and diverse lower abundance monoterpenes and sesquiterpenes. To achieve this, we combined comparative transcriptomics between active glandular trichome-rich sepal tips and labellum and non-active remaining flower tissues, floral volatile headspace profiling, phylogenetic analysis of a multigene family, and protein functional assays. We found (i) multiple branch points of the terpene synthase (TPS) biosynthetic pathway were highly expressed and coordinately upregulated in the active floral tissues compared to non-active ones, (ii) the monoterpene synthase CdTPS-b3 underpinning α-pinene biosynthesis and a bona fide promiscuous TPS CdTPS-b4 that may contribute to the diverse array of low-abundance mono- and sesquiterpenes found in its flowers, and (iii) dual localization (plastid and cytosol) of CdTPS-b3 and CdTPS-b4. Our findings highlight metabolic pathway specialization at multiple TPS pathway branch points supporting the biosynthesis and emission of α-pinene in C. denticulata flowers that are implicated in its generalist pollinator attraction. Furthermore, the complexity of diverse floral terpenes in Caladenia is likely mediated by finely tuned TPS gene expression, functional promiscuity, and subcellular localization. We predict that the combination of these three mechanisms underpin the evolution of multiple deceptive pollination strategies in Caladenia.

Structure and release function of fragrance glands

Volatile compounds serve physiological, signaling, and defensive purposes in plants and have beneficial effects on the growth, reproduction, resistance, and yield of horticultural plants. They are released through fragrance glands and become gasses by passing through the plasma membrane, cell walls that contain water, and cuticle. Transporter proteins facilitate their transport and reduce the resistance of these barriers. They also regulate the rate of release and concentration of volatiles inside and outside of the membrane. However, there has been no summary of the structure and function of the fragrance glands of horticultural plants, as well as an introduction to the latest research progress on the mechanism of the transport of volatiles. This review focuses on the structure and function of the release of aromas in horticultural plants and explores the mechanism of the release of volatiles through a transporter model. Additionally, it considers the factors that affect their release and ecological functions and suggests directions for future research.