Reduced expression of chlorophyllide oxygenase (CAO) decreases the metabolic flux for chlorophyll synthesis and downregulates photosynthesis in tobacco plants.

Unlabelled: Chlorophyll is synthesized from chlorophyllide , catalyzed by chlorophyllide oxygenase (). To examine whether reduced chlorophyll content regulates chlorophyll (Chl) synthesis and photosynthesis, we raised transgenic tobacco plants with antisense expression, which had lower chlorophyll content and, thus, higher Chl / ratio. Further, these plants had (i) lower chlorophyll and total Chl content, whether they were grown under low or high light; (ii) decreased steady-state levels of chlorophyll biosynthetic intermediates, due, perhaps, to a feedback-controlled reduction in enzyme expressions/activities; (iii) reduced electron transport rates in their intact leaves, and reduced Photosystem (PS) I, PS II and whole chain electron transport activities in their isolated thylakoids; (iv) decreased carbon assimilation in plants grown under low or high light. We suggest that reduced synthesis of chlorophyll by antisense expression of , acting at the end of Chl biosynthesis pathway, downregulates the chlorophyll b biosynthesis, resulting in decreased Chl , total chlorophylls and increased Chl . We have previously shown that the controlled up-regulation of chlorophyll biosynthesis and decreased Chl ratio by over expression of enhance the rates of electron transport and CO assimilation in tobacco. Conversely, our data, presented here, demonstrate that-antisense expression of in tobacco, which decreases Chl biosynthesis and increases Chl / ratio, leads to reduced photosynthetic electron transport and carbon assimilation rates, both under low and high light. We conclude that Chl modulates photosynthesis; its controlled down regulation/ up regulation decreases/ increases light-harvesting, rates of electron transport, and carbon assimilation.

Supplementary Information: The online version contains supplementary material available at 10.1007/s12298-023-01395-5.