Molecular Evolution of GDP-D-Mannose Epimerase (GME), a Key Gene in Plant Ascorbic Acid Biosynthesis

Abstract

The widespread ascorbic acid (AsA) plays a vital role in plant development and abiotic stress tolerance, but AsA concentration varies greatly among different plants. GDP-D-mannose epimerase (GME), which catalyzes GDP-D-mannose to GDP-L-galactose or GDP-L-gulose, is a key enzyme in plant AsA biosynthesis pathway. Functions and expression patterns of GME have been well studied in previous works, however, little information is known about the evolutionary patterns of the gene. In this study, GME gene structure, corresponding conserved protein motifs and evolutionary relationships were systematically analyzed. A total of 111 GME gene sequences were retrieved from 59 plant genomes, which representing almost all the major lineages of Viridiplantae: dicotyledons, monocotyledons, gymnosperms, pteridophytes, bryophytes, and chlorophytes. Results showed that homologs of GME were widely present in Viridiplantae. GME gene structures were conservative in higher plants, while varied greatly in the basal subgroups of the phylogeny including lycophytes, bryophytes, and chlorophytes, suggesting GME gene structure might have undergone severe differentiation at lower plant and then gradually fixed as plant evolution. The basic motifs of GME were strongly conserved throughout Viridiplantae, suggesting the conserved function of the protein. Molecular evolution analysis showed that strong purifying selection was the predominant force in the evolution of GME. A few branches and sites under episodic diversifying selection were identified and most of the branches located in the subgroup of chlorphytes, indicating episodic diversifying selection at a few branches and sites may play a role in the evolution of GME and diversifying selection may have occurred at the early stage of Viridiplantae. Our results provide novel insights into functional conservation and the evolution of GME.

Molecular evolution of GDP-L-galactose phosphorylase (GGP), a key regulatory gene in plant ascorbate biosynthesis

“…These results were also consistent with the results in the GME, where most of the positively selected branches detected in the GME species were located in the green algae lineage [27], and also suggesting that the evolutionary innovation of GGP genes also mainly occurred in the early stages of plant evolution and played an important role in helping plants adapt to new and challenging environments.…”

“…The molecular evolutionary results of GGP were similar to that of GME, which is the upstream gene of GGP in L-galactose pathway and is also considered as a key gene in plant AsA biosynthesis, and also had undergone strong purifying selection during evolution (ω 0 = 0.0287) [27]. Moreover, a total of 22 branches were identified under positive selection.…”

“…AsA is widely found in almost all plants, while the concentration of AsA varies considerably across different plant species. For example, AsA concentrations in higher plants range approximately from 2 to 135 μmol g -1 FW (fresh weight), however, green algae species of Ulva compressa and bryophyte species of Hypnum plumaeforme exhibit AsA concentrations of about 0.5 μmol g -1 FW and 0.1-0.6 μmol g -1 FW, respectively[26,27]. Plant AsA level is closely related to external environmental factors such as light intensity, UV-B and temperature, and is controlled by internal metabolism mechanisms including biosynthesis, recycling and degradation.…”

Molecular evolution of GDP-L-galactose phosphorylase, a key regulatory gene in plant ascorbate biosynthesis

“…In general, the concentration of AsA in higher plants is usually much higher than that in bryophytes and green algae ( Gest et al 2013 ; Vidal-Meireles et al 2017 ). For example, AsA concentrations in higher plants range approximately from 2 to 135 μmol g −1 FW (fresh weight); however, green algae species of Ulva compressa and bryophyte species of Hypnum plumaeforme exhibit AsA concentrations of ~0.5 μmol g −1 FW and 0.1–0.6 μmol g −1 FW, respectively ( Gest et al 2013 ; Tao et al 2018 ). The relationship between the increased copy number of GGP gene and the higher AsA content in angiosperms remains to be further studied.…”

Root Proteomic Analysis of Two Grapevine Rootstock Genotypes Showing Different Susceptibility to Salt Stress

“…Our proteomic analyses revealed the presence of the same glutathione transferase (GST) isoforms in both genotypes, all decreased under salt stress conditions (#30, #139 and #134 in 101.14, #36, #86 and #107 in M4). GSTs are a large group of multifunctional enzymes that show different responses to salt stress [70] and that in some cases resulted in the improvement of salt tolerance [71]. Further work is necessary to define the biochemical/physiological role(s) of the GSTs whose levels have been observed to change in the present study.…”