Photoadaptive resource allocation strategies: How CAM plant Ottelia alismoides optimizes carbon-nitrogen metabolism...

Crassulacean acid metabolism (CAM) in aquatic plants exhibits unique photoadaptive strategies to cope with the spectrally dynamic underwater environment. This study investigated how monochromatic red (R) and blue (B) light regulate CAM-related carbon-nitrogen (C-N) coupling in the facultative CAM plant Ottelia alismoides, which possesses three carbon-concentrating mechanisms. Our analyses revealed that both monochromatic red (665 nm) and blue (440 nm) light sustained robust circadian rhythmicity in organic acid flux and carbohydrate turnover, but exhibited divergent regulatory patterns. Under blue light, nocturnal acidity accumulation surged by 101 %, accompanied by a 94 % increase in daytime decarboxylation efficiency, despite paradoxical reductions in PEPC (Phosphoenolpyruvate carboxylase) and Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) activities (21 % and 10 % decreases, respectively). In contrast, red light upregulated PEPC and Rubisco activities but reduced malate accumulation, reflecting a carbon-channeling homeostasis strategy that prioritizes photosynthetic continuity. Additionally, red light reinforced respiratory-carbon coupling through nocturnal TCA cycle intensification. Spectrum-dependent nitrogen partitioning was also prominent, with blue light increasing GS (glutamine synthetase) activity during the day (13 % increase) and NADP-GDH (NADP-glutamate dehydrogenase) activity at night (43 % increase), along with a 75 % increase in free amino acid (FAA) biosynthesis. Our findings provide the first evidence of wavelength-partitioned resource allocation in the aquatic CAM plant O. alismoides, which utilizes decoupled wavelength responses. This novel adaptation likely enhances metabolic flexibility in water columns with varying spectral compositions. Future research should focus on characterizing the photoreceptor networks and downstream signaling cascades that govern this specialized photoregulation.