Ascorbic acid – more commonly known as vitamin C – is a widespread antioxidant in living organisms and plays essential roles in the growth and development of animals and plants as well as in the response to abiotic stress. Ascorbic acid is present in a wide range of plant tissues, and is a multifunctional metabolite linked to many diverse physiological processes, including photosynthesis, cell wall biosynthesis, seed germination, flowering time and fruit softening to name a few. Ascorbic acid in produced via one of four biosynthetic pathways in plant cells; the L-galactose pathway, the L-glucose pathway, the D-galacturonic acid and the myo-inositol pathway. Of these, the L-galactose pathway is the best established and is considered to be the only predominant pathway for ascorbic acid accumulation in most plant species, including vascular plants, mosses and green algae. GDP-L-galactose phosphorylase (GGP) is a key regulatory gene of this pathway. The function and regulation mechanisms of GGP are well understood; however, the molecular evolutionary patterns of the gene remain unclear.
In their new study published in AoBP, Tao et al. explore the molecular evolutionary patterns of plant GGP genes. They found that most GGPs in their 71 studied species had similar gene structure and motif patterns, indicating plant GGPs have evolutionary conserved functions. Molecular evolutionary analyses showed that GGPs were mainly constrained by purifying selection, indicating that the gene is functionally conserved due to its vital importance in ascorbic acid biosynthesis. A few branches of the GGP evolutionary tree were identified to be under positive selection, indicating that episodic diversifying selection played a role during the evolution of plant GGPs. Whole-genome duplication was identified in seed plants, which may explain the increased ascorbic acid content observed in angiosperms compared to other plants. This has allowed angiosperms to adapt more easily to changing environments.