Abstract
The flavonoid pathway is one of the best characterized specialized metabolite pathways of plants. In angiosperms, the flavonoids have varied roles in assisting with tolerance to abiotic stress and are also key for signaling to pollinators and seed dispersal agents. The pathway is thought to be specific to land plants and to have arisen during the period of land colonization around 550–470 million years ago. In this review we consider current knowledge of the flavonoid pathway in the bryophytes, consisting of the liverworts, hornworts, and mosses. The pathway is less characterized for bryophytes than angiosperms, and the first genetic and molecular studies on bryophytes are finding both commonalities and significant differences in flavonoid biosynthesis and pathway regulation between angiosperms and bryophytes. This includes biosynthetic pathway branches specific to each plant group and the apparent complete absence of flavonoids from the hornworts.
Highlights
The flavonoid pathway is one of the best characterized specialized metabolite pathways of plants
Regardless of whether the ancestral genes for flavonoid biosynthesis were present in algal ancestors, the flavonoid pathway we see in extant land plants is hypothesized to have arisen when the land plant ancestors were first colonizing the land about 550–470 million years ago (MYA) (Markham, 1988; Stafford, 1991; Jorgensen, 1994; Koes et al, 1994; Kenrick and Crane, 1997; Rozema et al, 2002)
The origin of the flavonoid pathway for providing tolerance to UV-B light (UVB) has been supported by recent studies on the liverwort species Marchantia. (“Marchantia” is used in this article to refer to M. polymorpha subsp. ruderalis, which is the model experimental species.) Marchantia is an excellent research model: it is small; has a rapid growth rate; can asexually reproduce in large numbers through single-cell-derived clonal gemmae; and, has a small genome which, larger than that of Arabidopsis, contains significantly fewer genes (Ishizaki et al, 2015; Bowman et al, 2016; Shimamura, 2016; Bowman et al, 2017)
Summary
The flavonoid pathway is a core component enabling land plants to interact with their environment. Flavonoid pathway diversity has increased greatly, with more than 8,000 different structures reported from the relatively small number of plant species studied to date (Andersen and Markham, 2006). This multiplicity of structures and functions is thought to have assisted land plants to colonize the wide range of environments they occupy. The over 8,000 compounds are grouped into relatively few classes of flavonoids, based on the core structure and/or biosynthetic origin. There are notable groups of related non-flavonoid compounds produced with the same starting precursors as used for chalcone formation, such as the stilbenes and bibenzyls.
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