Abramson Bradley W, Kachel Benjamin, Kramer David M, Ducat Daniel C
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI, USA.
Department of Cell and Molecular Biology, Michigan State University, East Lansing, MI, USA.
Plant Cell Physiol. 2016 Dec;57(12):2451-2460. doi: 10.1093/pcp/pcw169. Epub 2016 Oct 13.
In plants, a limited capacity to utilize or export the end-products of the Calvin-Benson cycle (CB) from photosynthetically active source cells to non-photosynthetic sink cells can result in reduced carbon capture and photosynthetic electron transport (PET), and lowered photochemical efficiency. The down-regulation of photosynthesis caused by reduced capacity to utilize photosynthate has been termed 'sink limitation'. Recently, several cyanobacterial and algal strains engineered to overproduce target metabolites have exhibited increased photochemistry, suggesting that possible source-sink regulatory mechanisms may be involved. We directly examined photochemical properties following induction of a heterologous sucrose 'sink' in the unicellular cyanobacterium Synechococcus elongatus PCC 7942. We show that total photochemistry increases proportionally to the experimentally controlled rate of sucrose export. Importantly, the quantum yield of PSII (ΦII) increases in response to sucrose export while the PET chain becomes more oxidized from less PSI acceptor-side limitation, suggesting increased CB activity and a decrease in sink limitation. Enhanced photosynthetic activity and linear electron flow are detectable within hours of induction of the heterologous sink and are independent of pigmentation alterations or the ionic/osmotic effects of the induction system. These observations provide direct evidence that secretion of heterologous carbon bioproducts can be used as an alternative approach to improve photosynthetic efficiency, presumably by by-passing sink limitation. Our results also suggest that engineered microalgal production strains are valuable alternative models for examining photosynthetic sink limitation because they enable greater control and monitoring of metabolite fluxes relative to plants.
在植物中，从光合活跃的源细胞向非光合的库细胞利用或输出卡尔文-本森循环（CB）终产物的能力有限，会导致碳捕获和光合电子传递（PET）减少，以及光化学效率降低。因光合产物利用能力降低导致的光合作用下调被称为“库限制”。最近，一些经过工程改造以过量生产目标代谢物的蓝藻和藻类菌株表现出光化学增强，这表明可能涉及源-库调节机制。我们直接检测了在单细胞蓝藻聚球藻PCC 7942中诱导异源蔗糖“库”后的光化学特性。我们发现总光化学与实验控制的蔗糖输出速率成比例增加。重要的是，PSII的量子产率（ΦII）随着蔗糖输出而增加，同时PET链因PSI受体侧限制减少而变得更氧化，这表明CB活性增加且库限制降低。在诱导异源库后的数小时内即可检测到光合活性和线性电子流增强，且与色素沉着改变或诱导系统的离子/渗透效应无关。这些观察结果提供了直接证据，即分泌异源碳生物产品可作为提高光合效率的替代方法，大概是通过绕过库限制来实现。我们的结果还表明，工程化微藻生产菌株是研究光合库限制的有价值的替代模型，因为相对于植物，它们能够对代谢物通量进行更好的控制和监测。
