Synthesis and physiological function of lignin in the reproductive structures of angiosperms

Lignin, a polymer of hydroxycinnamyl alcohols and related monomers, is found in great abundance in the plant vasculature, where it confers mechanical strength and facilitates water transport. Lignin is also found in many other plant organs, including roots, flowers, fruits, and abscission zones. While in these organs impregnation of lignin is usually limited to a thin layer of cells, or just a few cells, this thin lignin linin often fulfills important physiological functions related to reproduction and plant fitness. In this perspective review, we focused on the lignin of flowers and fruits. After a brief introduction on the anatomy and morphology of these organs, we provided an overview of the molecular mechanisms governing the spatial and temporal regulation of lignin synthesis. We further described the consequences that perturbations of these processes have on the biochemical structure of lignin polymers, plant reproduction, and fitness. Recent discoveries on the chemical composition of seed coat lignin and the formation of the petal abscission zone are also discussed. With this Perspective review, we hope to bring the topic of lignin in reproductive organs to a wider audience, while highlighting the impact that future research could have on breeding and agriculture.