Systems-wide Analysis Revealed Shared and Unique Responses to Moderate and Acute High Temperatures in the Green Alga Chlamydomonas reinhardtii. Systems-wide Analysis Revealed Shared and Unique Responses to Moderate and Acute High Temperatures in the Green Alga Chlamydomonas reinhardtii

Different intensities of high temperatures affect the growth of photosynthetic cells in nature. To elucidate the underlying mechanisms, we cultivated the unicellular green alga Chlamydomonas reinhardtii under highly controlled photobioreactor conditions and revealed systems-wide shared and unique responses to 24-hour moderate (35oC) and acute (40oC) high temperatures and subsequent recovery at 25oC. We identified transcripts/proteins with unique differential regulation at 35oC, uncovering previously overlooked novel elements in response to moderate high temperature. Heat at 35oC increased transcripts/proteins involved in gluconeogensis/glyoxylate-cycle for carbon uptake, promoted growth, and increased starch accumulation. Heat at 40oC inhibited growth, resulting in carbon uptake over usage and increased starch accumulation. Heat at 35oC transiently arrested the cell cycle followed by partial synchronization while 40oC inhibited DNA replication and arrested the cell cycle. Both high temperatures induced photoprotection, while 40oC decreased photosynthetic efficiencies, distorted thylakoid/pyrenoid ultrastructure, and affected the carbon concentrating mechanism. We demonstrated increased transcript/protein correlation during heat, which decreased during recovery, suggesting reduced post-transcriptional regulation during heat may help coordinate heat tolerance activities efficiently. During recovery after both treatments, transcripts/proteins related to DNA synthesis increased while those involved in photosynthetic light reactions decreased. We propose down-regulating photosynthetic light reactions during DNA replication benefits cell cycle resumption by reducing ROS production. Our results provide potential targets to increase thermotolerance in algae and crops. Overall design: Transcriptomic time-course analysis of wild-type Chlamydomonas reinhardtii cells exposed to 24-hours of moderate (35C) or acute (40C) high temperature, followed by a 48-hour recovery at 25C.

自然界中，不同强度的高温会影响光合细胞的生长。为阐明其潜在机制，本研究在严格可控的光生物反应器（photobioreactor）条件下培养单细胞绿藻莱茵衣藻（Chlamydomonas reinhardtii），解析了其对24小时中度（35℃）与急性（40℃）高温以及后续25℃恢复过程的系统水平共享与特异性响应。我们鉴定出在35℃下呈现独特差异调控的转录本与蛋白质，发现了此前被忽视的中度高温响应新元件。35℃高温可上调参与糖异生/乙醛酸循环（gluconeogensis/glyoxylate-cycle）以摄取碳源的转录本与蛋白质，促进细胞生长并增加淀粉积累；40℃高温则抑制细胞生长，导致碳摄入大于消耗并提升淀粉积累量。35℃高温会短暂阻滞细胞周期，随后引发部分细胞同步化；而40℃高温可抑制DNA复制，完全阻滞细胞周期。两种高温均能诱导光保护响应，但40℃高温会降低光合效率、扭曲类囊体/淀粉核（thylakoid/pyrenoid）超微结构，并影响碳浓缩机制（carbon concentrating mechanism）。我们发现高温阶段转录本与蛋白质的相关性升高，而恢复阶段该相关性下降，提示高温期间转录后调控（post-transcriptional regulation）的减弱可有效协调热耐受相关活性。在两种处理后的恢复阶段，参与DNA合成的转录本与蛋白质表达量上升，而与光合光反应相关的分子表达量下降。我们提出，在DNA复制过程中下调光合光反应可通过减少活性氧（Reactive Oxygen Species, ROS）生成，助力细胞周期恢复。本研究结果为提升藻类与作物的热耐受性提供了潜在靶点。实验设计：对野生型莱茵衣藻（Chlamydomonas reinhardtii）细胞进行转录组时序分析，将其暴露于24小时中度（35℃）或急性（40℃）高温环境后，于25℃下进行48小时恢复培养。