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Abstract：
The methylerythritol 4-phosphate (MEP) pathway of isoprenoid biosynthesis produces chlorophyll side chains and compounds that function in resistance to abiotic stresses, including carotenoids, and isoprene. Thus we investigated the effects of moderate and severe drought on MEP pathway function in the conifer Picea glauca, a boreal species at risk under global warming trends. Although moderate drought treatment reduced the photosynthetic rate by over 70%, metabolic flux through the MEP pathway was reduced by only 37%. The activity of the putative rate-limiting step, 1-deoxy-D-xylulose-5-phosphate synthase (DXS), was also reduced by about 50%, supporting the key role of this enzyme in regulating pathway metabolic flux. However, under severe drought, as flux declined below detectable levels, DXS activity showed no significant decrease, indicating a much-reduced role in controlling flux under these conditions. Both MEP pathway intermediates and the MEP pathway product isoprene incorporate administered 13CO2 to high levels (75-85%) under well-watered control conditions indicating a close connection to photosynthesis. However, this incorporation declined precipitously under drought, demonstrating exploitation of alternative carbon sources. Despite the reductions in MEP pathway flux and intermediate pools, there was no detectable decline in most major MEP pathway products under drought (except for violaxanthin under moderate and severe stress and isoprene under severe stress) suggesting that the pathway is somehow buffered against this stress. The resilience of the MEP pathway under drought may be a consequence of the importance of the metabolites formed under these conditions.
摘要：
类异戊二烯生物合成的甲基赤藓糖醇4-磷酸（MEP）途径产生叶绿素侧链和在抵抗非生物胁迫中起作用的化合物，包括类胡萝卜素,和异戊二烯。因此，我们研究了中度和重度干旱对针叶树云杉MEP途径功能的影响，在全球变暖趋势下处于危险之中的北方物种。虽然适度的干旱处理使光合速率降低了70%以上,通过MEP途径的代谢通量仅减少了37%.推定的限速步骤的活动，1-脱氧-D-木酮糖-5-磷酸合酶（DXS），也减少了大约50%，支持该酶在调节途径代谢通量中的关键作用。然而,在严重干旱的情况下，随着通量下降到可检测水平以下，DXS活性无明显下降,表明在这些条件下控制通量的作用大大降低。MEP途径中间体和MEP途径产物异戊二烯均在充分浇水的控制条件下掺入施用的13CO2至高水平(75-85%),表明与光合作用密切相关。然而,这种合并在干旱下急剧下降，展示替代碳源的开发。尽管MEP途径通量和中间池减少，在干旱下，大多数主要的MEP途径产物没有可检测到的下降（中度和重度胁迫下的紫黄质和重度胁迫下的异戊二烯除外），表明该途径以某种方式缓冲了这种胁迫。MEP途径在干旱下的恢复力可能是在这些条件下形成的代谢物的重要性的结果。
