Zhou Youping, Stuart-Williams Hilary, Grice Kliti, Kayler Zachary E, Zavadlav Saša, Vogts Angela, Rommerskirchen Florian, Farquhar Graham D, Gessler Arthur
Institute for Landscape Biogeochemistry, ZALF, Germany; Department of Chemistry, Curtin University, Perth, Australia; SKLLQG, Chinese Academy of Sciences, Xi'an, China; Leibniz Institute for Freshwater Ecology & Inland Fisheries, Germany.
RSB, Australian National University, Canberra, Australia.
Phytochemistry. 2015 Mar;111:14-20. doi: 10.1016/j.phytochem.2014.12.005. Epub 2015 Jan 6.
It has long been theorized that carbon allocation, in addition to the carbon source and to kinetic isotopic effects associated with a particular lipid biosynthetic pathway, plays an important role in shaping the carbon isotopic composition ((13)C/(12)C) of lipids (Park and Epstein, 1961). If the latter two factors are properly constrained, valuable information about carbon allocation during lipid biosynthesis can be obtained from carbon isotope measurements. Published work of Chikaraishi et al. (2004) showed that leaf lipids isotopic shifts from bulk leaf tissue Δδ(13)C(bk-lp) (defined as δ(13)C(bulkleaftissue)-δ(13)C(lipid)) are pathway dependent: the acetogenic (ACT) pathway synthesizing fatty lipids has the largest isotopic shift, the mevalonic acid (MVA) pathway synthesizing sterols the lowest and the phytol synthesizing 1-deoxy-D-xylulose 5-phosphate (DXP) pathway gives intermediate values. The differences in Δδ(13)C(bk-lp) between C3 and C4 plants Δδ(13)C(bk-lp,C4-C3) are also pathway-dependent: Δδ(13)C(ACT)(bk-lp,C4-C3) > Δδ(13)C(DXP(bk-lp,C4-C3) > Δδ(13)C(MVA)(bk-lp,C4-C3). These pathway-dependent differences have been interpreted as resulting from kinetic isotopic effect differences of key but unspecified biochemical reactions involved in lipids biosynthesis between C3 and C4 plants. After quantitatively considering isotopic shifts caused by (dark) respiration, export-of-carbon (to sink tissues) and photorespiration, we propose that the pathway-specific differences Δδ(13)C(bk-lp,C4-C3) can be successfully explained by C4-C3 carbon allocation (flux) differences with greatest flux into the ACT pathway and lowest into the MVA pathways (when flux is higher, isotopic shift relative to source is smaller). Highest carbon allocation to the ACT pathway appears to be tied to the most stringent role of water-loss-minimization by leaf waxes (composed mainly of fatty lipids) while the lowest carbon allocation to the MVA pathway can be largely explained by the fact that sterols act as regulatory hormones and membrane fluidity modulators in rather low concentrations.
长期以来，人们一直认为，除了碳源以及与特定脂质生物合成途径相关的动力学同位素效应外，碳分配在塑造脂质的碳同位素组成（(13)C/(12)C）方面起着重要作用（Park和Epstein，1961年）。如果后两个因素得到适当限制，那么从碳同位素测量中可以获得有关脂质生物合成过程中碳分配的有价值信息。Chikaraishi等人（2004年）发表的研究表明，叶片脂质相对于叶片组织整体的同位素偏移量Δδ(13)C(bk-lp)（定义为δ(13)C(叶片组织整体)-δ(13)C(脂质)）取决于途径：合成脂肪脂质的产乙酸（ACT）途径具有最大的同位素偏移，合成甾醇的甲羟戊酸（MVA）途径具有最小的同位素偏移，而合成植醇的1-脱氧-D-木酮糖5-磷酸（DXP）途径给出中间值。C3和C4植物之间的Δδ(13)C(bk-lp)差异Δδ(13)C(bk-lp,C4-C3)也取决于途径：Δδ(13)C(ACT)(bk-lp,C4-C3)>Δδ(13)C(DXP)(bk-lp,C4-C3)>Δδ(13)C(MVA)(bk-lp,C4-C3)。这些途径依赖性差异被解释为是由于C3和C4植物脂质生物合成中关键但未明确的生化反应的动力学同位素效应差异所致。在定量考虑了（黑暗）呼吸、碳输出（到库组织）和光呼吸引起的同位素偏移后，我们提出，途径特异性差异Δδ(13)C(bk-lp,C4-C3)可以通过C4 - C3碳分配（通量）差异成功解释，其中进入ACT途径的通量最大，进入MVA途径的通量最小（当通量较高时，相对于源的同位素偏移较小）。向ACT途径的最高碳分配似乎与叶蜡（主要由脂肪脂质组成）在最小化水分损失方面最严格的作用相关，而向MVA途径的最低碳分配在很大程度上可以由甾醇作为调节激素和膜流动性调节剂以相当低的浓度起作用这一事实来解释。
