Dhawan Gunjan, Rao Basuthkar Jagadeeshwar
Department of Biology, Indian Institute of Science Education and Research (IISER) Tirupati, Tirupati, Andhra Pradesh, India.
Department of Animal Biology, School of Life Sciences, University of Hyderabad, Hyderabad, Telangana, India.
Front Plant Sci. 2025 Jul 17;16:1622214. doi: 10.3389/fpls.2025.1622214. eCollection 2025.
In photosynthetic organisms, the inter-organellar coordination between photosynthetic and respiratory processes is fundamental for maintaining cellular energy homeostasis. These organelles not only operate distinct bioenergetic pathways but also engage in extensive metabolic cross-talk to balance energy production, redox status and metabolic demands, especially under fluctuating light-dark conditions. Such coordination has been widely appreciated across plant and algal systems but it remains relatively less explored, particularly in synchronous cultures where cellular rhythms can be more precisely examined. Our study investigated the coordination in photosynthetic and mitochondrial activity during (12:12 h) light-dark cycle using synchronized cultures. Live cell confocal imaging revealed light-dark-dependent mitochondrial morphology transitions from fragmented to intermediate to tubular forms by the end of 12 hr of light period, which reverses sharply through 6 and 12 hr of dark. Concurrently, chloroplast show novel transitions from an intact cup (light) to a distorted and punctured structure (dark), which gets reversed in light phase. Spatial mapping showed tubular mitochondria positioned peripherally to the chloroplast cup in light, whereas fragmented and intermediate mitochondria were diffused around distorted chloroplast in dark, which again gets reversed in light. Functional analysis using 77K spectroscopy and photosynthetic protein levels (PsaA and D1) reflected that PSI/PSII fluorescence ratio remains stable in continuous light condition but increased exceptionally in continuous dark, which led to transient changes observed in fluorescence ratio in light/dark-dependent manner in synchronous cultures. Mitochondrial activity, measured using Seahorse flux analyzer, showed basal oxygen consumption rate in continuous light and a marked reduction in continuous dark condition, resulting in dynamic changes observed in light-dark cycle, indicating a coordination in the organellar function. Further, Target of rapamycin (TOR) kinase activity was essential to maintain inter-organellar coupling as evidenced by subdued fluctuations following TOR kinase inhibition. The study, for the first time, argues for (12:12 h) light-dark cycle-mediated coupled dynamics between mitochondria and chloroplast in , offering new insights into the temporal regulation of cellular energy dynamics.
在光合生物中，光合过程与呼吸过程之间的细胞器间协调对于维持细胞能量稳态至关重要。这些细胞器不仅运行不同的生物能量途径，还进行广泛的代谢相互作用，以平衡能量产生、氧化还原状态和代谢需求，特别是在光暗条件波动的情况下。这种协调在植物和藻类系统中已得到广泛认可，但仍相对较少被探索，特别是在可以更精确检查细胞节律的同步培养物中。我们的研究使用同步培养物研究了（12:12小时）光暗循环期间光合和线粒体活性的协调。活细胞共聚焦成像显示，在光照期12小时结束时，线粒体形态从碎片化转变为中间态再转变为管状，这种转变在黑暗6小时和12小时时急剧逆转。同时，叶绿体显示出从完整杯状（光照）到扭曲和穿孔结构（黑暗）的新转变，这种转变在光期逆转。空间映射显示，光照条件下管状线粒体位于叶绿体杯的周边，而黑暗中碎片化和中间态线粒体在扭曲的叶绿体周围扩散，这种情况在光照下再次逆转。使用77K光谱和光合蛋白水平（PsaA和D1）进行的功能分析表明，PSI/PSII荧光比率在连续光照条件下保持稳定，但在连续黑暗中异常增加，这导致同步培养物中荧光比率以光暗依赖的方式出现短暂变化。使用海马通量分析仪测量线粒体活性，结果显示连续光照下的基础氧消耗率以及连续黑暗条件下的显著降低，导致在光暗循环中观察到动态变化，表明细胞器功能存在协调。此外，雷帕霉素靶蛋白（TOR）激酶活性对于维持细胞器间耦合至关重要，TOR激酶抑制后波动减弱证明了这一点。该研究首次论证了（12:12小时）光暗循环介导的线粒体与叶绿体之间的耦合动力学，为细胞能量动态的时间调节提供了新见解。
