Conservation and Expression Patterns Divergence of Ascorbic Acid d-mannose/l-galactose Pathway Genes in Brassica rapa.

Weike Duan,Xilin Hou,Yan Li,Jun Ren,Tongkun Liu,Zhinan Huang,Ying Li,Zhongwen Chen,Xiaoming Song,Xiaoming Song

doi:10.3389/fpls.2016.00778

Abstract

Ascorbic acid (AsA) participates in diverse biological processes, is regulated by multiple factors and is a potent antioxidant and cellular reductant. The D-Mannose/L-Galactose pathway is a major plant AsA biosynthetic pathway that is highly connected within biosynthetic networks, and generally conserved across plants. Previous work has shown that, although most genes of this pathway are expressed under standard growth conditions in Brassica rapa, some paralogs of these genes are not. We hypothesize that regulatory evolution in duplicate AsA pathway genes has occurred as an adaptation to environmental stressors, and that gene retention has been influenced by polyploidation events in Brassicas. To test these hypotheses, we explored the conservation of these genes in Brassicas and their expression patterns divergence in B. rapa. Similar retention and a high degree of gene sequence similarity were identified in B. rapa (A genome), B. oleracea (C genome) and B. napus (AC genome). However, the number of genes that encode the same type of enzymes varied among the three plant species. With the exception of GMP, which has nine genes, there were one to four genes that encoded the other enzymes. Moreover, we found that expression patterns divergence widely exists among these genes. (i) VTC2 and VTC5 are paralogous genes, but only VTC5 is influenced by FLC. (ii) Under light treatment, PMI1 co-regulates the AsA pool size with other D-Man/L-Gal pathway genes, whereas PMI2 is regulated only by darkness. (iii) Under NaCl, Cu2+, MeJA and wounding stresses, most of the paralogs exhibit different expression patterns. Additionally, GME and GPP are the key regulatory enzymes that limit AsA biosynthesis in response to these treatments. In conclusion, our data support that the conservative and divergent expression patterns of D-Man/L-Gal pathway genes not only avoid AsA biosynthesis network instability but also allow B. rapa to better adapt to complex environments.

Highlights

L-Ascorbic acid (AsA, the reduced form of vitamin C (VTC)) is an essential metabolite in plants and animals
VTC5 is one of putative targets of FLOWERING LOCUS C (FLC), which plays a key role in the timing of the initiation of flowering and potentially regulates genes that function in many developmental pathways (Deng et al, 2011)
Syntenic alignments of the AsA D-Man/L-Gal pathway genes between the model crucifers A. thaliana, B. rapa, B. oleracea and B. napus confirmed that triplication and fractionation had occurred in these three Brassica species (Figure 1A)

Summary

Introduction

L-Ascorbic acid (AsA, the reduced form of VTC) is an essential metabolite in plants and animals. Conklin et al (2000) screened Arabidopsis mutants with defective AsA synthesis and identified 5 loci (VTC1, VTC2, VTC3, VTC4 and VTC5) related to the AsA D-Man/L-Gal pathway (Conklin et al, 2000). VTC1 encodes the AsA biosynthetic enzyme GMP (Conklin et al, 1999), VTC2 and VTC5 encode GGP (Dowdle et al, 2007; Linster et al, 2007), and VTC4 encodes GPP (Conklin et al, 2006; Laing et al, 2007) All of these genes are essential for AsA biosynthesis, as evidenced by experiments performed in vtc mutants. The AsA D-Man/LGal pathway is essential for plant growth, participates in stress resistance, and appears to be involved in flowering time control. The D-Man/L-Gal pathway genes are highly conserved and there are no significant differences in the gene numbers among selected representative plant species (Duan et al, 2015)

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Discussion

Conclusion