Laboratory of Molecular Plant Physiology, College of Life Sciences, South China Agricultural University, Guangzhou 510642, China.
J Exp Bot. 2010 Jun;61(6):1625-34. doi: 10.1093/jxb/erq028. Epub 2010 Mar 1.
Oxalate is widely distributed in the plant kingdom. While excess oxalate in food crops is detrimental to animal and human health, it may play various functional roles in plants, particularly for coping with environmental stresses. Understanding its biosynthetic mechanism in plants, therefore, becomes increasingly important both theoretically and practically. However, it is still a matter of debate as to what precursor and pathway are ultimately used for oxalate biosynthesis in plants. In this study, both physiological and molecular approaches were applied to address these questions. First, it was observed that when glycolate or glyoxylate was fed into detached leaves, both organic acids were equally effective in stimulating oxalate accumulation. In addition, the stimulation could be completely inhibited by cysteine, a glyoxylate scavenger that forms cysteine-glyoxylate adducts. To verify the role of glyoxylate further, various transgenic plants were generated, in which several genes involved in glyoxylate metabolism [i.e. SGAT (serine-glyoxylate aminotransferase), GGAT (glutamate-glyoxylate aminotransferase), HPR (hydroxypyruvate reductase), ICL (isocitrate lyase)], were transcriptionally regulated through RNAi or over-expression. Analyses on these transgenic plants consistently revealed that glyoxylate acted as an efficient precursor for oxalate biosynthesis in rice. Unexpectedly, it was found that oxalate accumulation was not correlated with photorespiration, even though this pathway is known to be a major source of glyoxylate. Further, when GLDH (L-galactono-1,4-lactone dehydrogenase), a key enzyme gene for ascorbate biosynthesis, was down-regulated, the oxalate abundance remained constant, despite ascorbate having been largely reduced as expected in these transgenic plants. Taken together, our results strongly suggest that glyoxylate rather than ascorbate is an efficient precursor for oxalate biosynthesis, and that oxalate accumulation and regulation do not necessarily depend on photorespiration, possibly due to the occurrence of the anaplerotic reaction that may compensate for glyoxylate formation in rice.
草酸盐广泛分布于植物界。虽然食物作物中过量的草酸盐对动物和人类健康有害，但它可能在植物中发挥各种功能作用，特别是应对环境胁迫。因此，了解其在植物中的生物合成机制在理论和实践上都变得越来越重要。然而，关于植物中草酸盐生物合成最终使用的前体和途径是什么，这仍然是一个有争议的问题。在这项研究中，应用了生理和分子方法来解决这些问题。首先，观察到当将甘醇酸或乙醛酸喂入离体叶片时，这两种有机酸都能有效地刺激草酸盐的积累。此外，这种刺激可以被半胱氨酸完全抑制，半胱氨酸是一种乙醛酸清除剂，它形成半胱氨酸-乙醛酸加合物。为了进一步验证乙醛酸的作用，生成了各种转基因植物，其中涉及乙醛酸代谢的几种基因[即 SGAT（丝氨酸-乙醛酸氨基转移酶）、GGAT（谷氨酸-乙醛酸氨基转移酶）、HPR（羟丙酮酸还原酶）、ICL（异柠檬酸裂解酶）]通过 RNAi 或过表达进行转录调控。对这些转基因植物的分析一致表明，乙醛酸是水稻中草酸盐生物合成的有效前体。出乎意料的是，尽管该途径是乙醛酸的主要来源，但发现草酸盐的积累与光呼吸无关。此外，当作为抗坏血酸生物合成关键酶基因的 GLDH（L-半乳糖酸-1,4-内酯脱氢酶）下调时，尽管这些转基因植物中的抗坏血酸大量减少，草酸盐的丰度仍然保持不变。总之，我们的结果强烈表明，乙醛酸而不是抗坏血酸是草酸盐生物合成的有效前体，草酸盐的积累和调节不一定依赖于光呼吸，可能是由于发生了氨酰反应，该反应可能补偿了水稻中乙醛酸的形成。
