Dhawan, G.; Rao, B. J.

In photosynthetic organisms, inter-organellar coordination between mitochondria and chloroplast, particularly in synchronous cultures, has been widely appreciated but relatively less understood. Our study investigated the coordination in photosynthetic and mitochondrial activity during (12:12 h) light-dark cycle in Chlamydomonas reinhardtii. 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 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 rhythmic oscillation 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 rhythmic changes in light-dark cycle, indicating a coordinated rhythmicity in organellar function. Further, Target of rapamycin (TOR) kinase activity was essential to maintain inter-organellar coupled rhythmicity as evidenced by subdued rhythmicity following TOR kinase inhibition. The study, for the first time, argues for (12:12 h) light-dark cycle-mediated coupled rhythmicity between mitochondria and chloroplast in C. reinhardtii.