The Biosynthesis and Functions of Flavonoids: Recent Advances From Studies Across Land Plant Diversity

Over the last decade there have been significant advances in genome sequencing and model species development for ferns, lycophytes, and the bryophyte lineages-mosses, liverworts, and hornworts. This has facilitated research on the biosynthesis and function of flavonoids in these non-seed land-plant lineages. Most studies have been on the liverwort model species Marchantia polymorpha (Marchantia). There has been extensive characterisation of biosynthetic and regulatory genes of the Marchantia flavonoid pathway, including generation of loss-of-function mutant lines to examine flavonoid contribution to tolerance of abiotic stresses and pathogen infection. Notably, the red pigments of liverworts were shown to be a new class of flavonoids, named 'auronidins'. There are relatively few studies on mosses, lycophytes, or ferns. Yet these lineages also contain distinct red pigment structures not found in seed plants. They also contain novel enzymatic activities, unique horizontal gene transfer events, and expanded gene families for proteins such as the polyphenol oxidases. Additionally, the hornworts have been shown to have lost the flavonoid pathway during lineage-specific evolution. Indeed, evidence suggests that aspects of flavonoid biosynthesis may have been lost and regained on multiple occasions, in different lineages, during land plant evolution. In this review, we summarise recent advances in understanding of flavonoid biosynthesis in non-seed plants and examine how this informs theories of the evolution of the flavonoid pathway across the land-plant lineages.