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

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.

抗坏血酸（AsA）参与多种生物过程，受多种因素调控，是一种有效的抗氧化剂和细胞还原剂。D - 甘露糖/L - 半乳糖途径是植物中主要的AsA生物合成途径，在生物合成网络中高度关联，且在植物中普遍保守。先前的研究表明，尽管在芸薹属植物的标准生长条件下该途径的大多数基因都有表达，但其中一些基因的旁系同源基因却没有表达。我们假设重复的AsA途径基因发生了调控进化以适应环境胁迫，并且基因保留受到了芸薹属植物多倍体化事件的影响。为了验证这些假设，我们探究了这些基因在芸薹属植物中的保守性以及它们在芸薹中的表达模式差异。在芸薹（A基因组）、甘蓝（C基因组）和欧洲油菜（AC基因组）中发现了相似的基因保留情况以及高度的基因序列相似性。然而，编码相同类型酶的基因数量在这三种植物中有所不同。除了GMP有9个基因外，编码其他酶的基因有1到4个。此外，我们发现这些基因之间广泛存在表达模式差异。（i）VTC2和VTC5是旁系同源基因，但只有VTC5受FLC影响。（ii）在光照处理下，PMI1与其他D - 甘露糖/L - 半乳糖途径基因共同调节AsA库的大小，而PMI2仅受黑暗调控。（iii）在NaCl、Cu²⁺、茉莉酸甲酯和创伤胁迫下，大多数旁系同源基因表现出不同的表达模式。此外，GME和GPP是响应这些处理限制AsA生物合成的关键调控酶。总之，我们的数据支持D - 甘露糖/L - 半乳糖途径基因保守和差异的表达模式不仅避免了AsA生物合成网络的不稳定，还使芸薹能够更好地适应复杂环境。