Conservation and Adaptability of Ascorbic Acid D-Mannose/L-Galactose Pathway Genes in Brassica rapa

Ascorbic acid (AsA) participates in diverse biological processes, is regulated by multiple of factors and is a potent antioxidant and cellular reductant. The D-mannose/L-galactose pathway, the major plant AsA biosynthetic pathway, is highly connected within biosynthetic networks; thus, genes of this pathway exhibit preferential retention and conservation. However, in response to fluctuating environmental conditions, plants change their AsA levels. In our study, we used Brassicas to further explore the conservation of these genes. Similar retention and a high degree of gene sequence similarity were identified in AA and CC Brassicas genomes. Additionally, their divergence was analyzed in Brassica rapa. We found two major divergence types: the divergence of multigene families, i.e., GMPs; and divergence of duplicated genes, i.e., VTC2 and VTC5, which were paralogous genes, although VTC5 retained less than VTC2 and might be regulated by FLC. For the complex environments, under light/darkness treatment, we found that the mechanism of the AsA pool size is light regulated as is the co-regulation of D-Man/L-Gal pathway genes except for PMI2; under NaCl, Cu²⁺, MeJA and wounding stresses, GME and GPP are the key regulatory enzymes that limit AsA biosynthesis. More importantly, the genes that did not express under normal growth conditions would be regulated in response to multiple stresses. The retention, gene structure and associated expression data under stress treatments suggests that D-Man/L-Gal pathway genes are conservative and adaptable that allows Brassica to adapt better to complex environments. In addition, GME and GPP play key roles in the regulation and limitation of AsA biosynthesis in Brassica.