Department of Life Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea.
Department of Biological Sciences, Chungnam National University, Daejeon 34134 Korea.
Plant Cell. 2022 Feb 3;34(2):910-926. doi: 10.1093/plcell/koab293.
Photosynthetic organisms are exposed to various environmental sources of oxidative stress. Land plants have diverse mechanisms to withstand oxidative stress, but how microalgae do so remains unclear. Here, we characterized the Chlamydomonas reinhardtii basic leucine zipper (bZIP) transcription factor BLZ8, which is highly induced by oxidative stress. Oxidative stress tolerance increased with increasing BLZ8 expression levels. BLZ8 regulated the expression of genes likely involved in the carbon-concentrating mechanism (CCM): HIGH-LIGHT ACTIVATED 3 (HLA3), CARBONIC ANHYDRASE 7 (CAH7), and CARBONIC ANHYDRASE 8 (CAH8). BLZ8 expression increased the photosynthetic affinity for inorganic carbon under alkaline stress conditions, suggesting that BLZ8 induces the CCM. BLZ8 expression also increased the photosynthetic linear electron transfer rate, reducing the excitation pressure of the photosynthetic electron transport chain and in turn suppressing reactive oxygen species (ROS) production under oxidative stress conditions. A carbonic anhydrase inhibitor, ethoxzolamide, abolished the enhanced tolerance to alkaline stress conferred by BLZ8 overexpression. BLZ8 directly regulated the expression of the three target genes and required bZIP2 as a dimerization partner in activating CAH8 and HLA3. Our results suggest that a CCM-mediated increase in the CO2 supply for photosynthesis is critical to minimize oxidative damage in microalgae, since slow gas diffusion in aqueous environments limits CO2 availability for photosynthesis, which can trigger ROS formation.
光合生物会暴露在各种环境来源的氧化应激下。陆生植物有多种机制来抵御氧化应激，但微藻如何做到这一点尚不清楚。在这里，我们对莱茵衣藻的碱性亮氨酸拉链（bZIP）转录因子 BLZ8 进行了特征描述，该因子对氧化应激高度诱导。氧化应激耐受性随着 BLZ8 表达水平的增加而增加。BLZ8 调节可能参与碳浓缩机制（CCM）的基因的表达：高光激活 3（HLA3）、碳酸酐酶 7（CAH7）和碳酸酐酶 8（CAH8）。BLZ8 的表达增加了碱性胁迫条件下无机碳的光合作用亲和力，表明 BLZ8 诱导了 CCM。BLZ8 的表达还增加了光合作用线性电子传递速率，降低了光合电子传递链的激发压力，从而在氧化应激条件下抑制活性氧（ROS）的产生。碳酸酐酶抑制剂乙氧唑胺消除了 BLZ8 过表达赋予的碱性胁迫下增强的耐受性。BLZ8 直接调节三个靶基因的表达，并在激活 CAH8 和 HLA3 时需要 bZIP2 作为二聚化伴侣。我们的结果表明，CCM 介导的光合作用 CO2 供应增加对于最大限度地减少微藻中的氧化损伤至关重要，因为在水相环境中气体扩散缓慢会限制光合作用中 CO2 的可用性，从而引发 ROS 形成。
