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

UNLABELLED: Chlorophyll b is synthesized from chlorophyllide a , catalyzed by chlorophyllide a oxygenase ( CAO ). To examine whether reduced chlorophyll b content regulates chlorophyll (Chl) synthesis and photosynthesis, we raised CAO transgenic tobacco plants with antisense CAO expression, which had lower chlorophyll b content and, thus, higher Chl a / b ratio. Further, these plants had (i) lower chlorophyll b 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 b by antisense expression of CAO , acting at the end of Chl biosynthesis pathway, downregulates the chlorophyll b biosynthesis, resulting in decreased Chl b , total chlorophylls and increased Chl a/b . We have previously shown that the controlled up-regulation of chlorophyll b biosynthesis and decreased Chl a/b ratio by over expression of CAO enhance the rates of electron transport and CO2 assimilation in tobacco. Conversely, our data, presented here, demonstrate that-antisense expression of CAO in tobacco, which decreases Chl b biosynthesis and increases Chl a / b ratio, leads to reduced photosynthetic electron transport and carbon assimilation rates, both under low and high light. We conclude that Chl b 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.