Hunsperger Heather M, Ford Christopher J, Miller James S, Cattolico Rose Ann
Department of Biology, University of Washington, Seattle, Washington, United States of America.
PLoS One. 2016 Jul 1;11(7):e0158614. doi: 10.1371/journal.pone.0158614. eCollection 2016.
Diatoms (Bacilliariophyceae) encode two light-dependent protochlorophyllide oxidoreductases (POR1 and POR2) that catalyze the penultimate step of chlorophyll biosynthesis in the light. Algae live in dynamic environments whose changing light levels induce photoacclimative metabolic shifts, including altered cellular chlorophyll levels. We hypothesized that the two POR proteins may be differentially adaptive under varying light conditions. Using the diatom Phaeodactylum tricornutum as a test system, differences in POR protein abundance and por gene expression were examined when this organism was grown on an alternating light:dark cycles at different irradiances; exposed to continuous light; and challenged by a significant decrease in light availability.
For cultures maintained on a 12h light: 12h dark photoperiod at 200μE m-2 s-1 (200L/D), both por genes were up-regulated during the light and down-regulated in the dark, though por1 transcript abundance rose and fell earlier than that of por2. Little concordance occurred between por1 mRNA and POR1 protein abundance. In contrast, por2 mRNA and POR2 protein abundances followed similar diurnal patterns. When 200L/D P. tricornutum cultures were transferred to continuous light (200L/L), the diurnal regulatory pattern of por1 mRNA abundance but not of por2 was disrupted, and POR1 but not POR2 protein abundance dropped steeply. Under 1200μE m-2 s-1 (1200L/D), both por1 mRNA and POR1 protein abundance displayed diurnal oscillations. A compromised diel por2 mRNA response under 1200L/D did not impact the oscillation in POR2 abundance. When cells grown at 1200L/D were then shifted to 50μE m-2 s-1 (50L/D), por1 and por2 mRNA levels decreased swiftly but briefly upon light reduction. Thereafter, POR1 but not POR2 protein levels rose significantly in response to this light stepdown.
Given the sensitivity of diatom por1/POR1 to real-time light cues and adherence of por2/POR2 regulation to the diurnal cycle, we suggest that POR1 supports photoacclimation, whereas POR2 is the workhorse for daily chlorophyll synthesis.
硅藻（硅藻纲）编码两种光依赖性原叶绿素酸酯氧化还原酶（POR1和POR2），它们在光照条件下催化叶绿素生物合成的倒数第二步。藻类生活在动态环境中，光照水平的变化会诱导光适应代谢转变，包括细胞叶绿素水平的改变。我们推测，在不同光照条件下，这两种POR蛋白可能具有不同的适应性。以三角褐指藻作为测试系统，研究了该生物在不同辐照度的明暗交替循环中生长、暴露于连续光照以及受到光照可用性显著降低挑战时，POR蛋白丰度和por基因表达的差异。
对于在200μE m-2 s-1（200L/D）的12小时光照：12小时黑暗光周期下培养的藻株，两个por基因在光照期间均上调，在黑暗中下调，尽管por1转录本丰度比por2上升和下降得更早。por1 mRNA与POR1蛋白丰度之间几乎没有一致性。相比之下，por2 mRNA和POR2蛋白丰度呈现相似的昼夜模式。当200L/D的三角褐指藻培养物转移到连续光照（200L/L）时，por1 mRNA丰度的昼夜调节模式被破坏，而por2的调节模式未受影响，并且POR1蛋白丰度急剧下降，而POR2蛋白丰度未下降。在1200μE m-2 s-1（1200L/D）下，por1 mRNA和POR1蛋白丰度均呈现昼夜振荡。在1200L/D下，受损的昼夜por2 mRNA反应并未影响POR2丰度的振荡。当在1200L/D下生长的细胞转移到50μE m-2 s-1（50L/D）时，光照减少后，por1和por2 mRNA水平迅速但短暂下降。此后，POR1蛋白水平因光照降低而显著上升，而POR2蛋白水平未上升。
鉴于硅藻por1/POR1对实时光照线索敏感，而por2/POR2调节遵循昼夜循环，我们认为POR1支持光适应，而POR2是日常叶绿素合成的主要酶。
