Abstract

To meet the challenge of growing population in India food production need to be increased. Mostly, C3 plants are underachievers. Plants with C4 photosynthesis are not only efficient in carbon assimilation, but they also have an advantage under unusual growth conditions. In C4 photosynthesis, the primary CO2 fixation is catalyzed by phosphoenolpyruvate carboxylase (PEPC). It is show that overexpression of Zea mays (Zm) PEPC cDNA, under the control of 35S promoter, in Arabidopsis thaliana and resulted in higher ZmPEPC gene expression, ~7-10 fold higher protein abundance and ~7-11 fold increase in PEPC activity in the transgenic lines than that in the vector control. Further, the PEPC overexpressed transgenic plants had higher chlorophyll content, enhanced electron transport rate (ETR), lower non-photochemical quenching (NPQ) of chlorophyll (Chl) a fluorescence, and a higher performance index than the vector control. Consistent with these observations, the rate of CO2 assimilation, the starch content and the dry weight of PEPC overexpressed plants increased by 14-18%, 10-18% and 6.5-16% respectively.  We have also overexpressed both PEP Carboxylase and PEP Carboxykinase in Arabidopsis thaliana to have increased CO2 concentration in the vicinity of Rubisco.  Overexpression of these enzymes leads to higher electron transport, carbon assimilation, increased biomass coupled with better water use efficiency.  Carbonic anhydrase (CA) catalyzes the inter-conversion of CO2 and bicarbonate used by the primary carboxylating enzymes of C3 and C4 plants respectively. In this study cytosolic carbonic anhydrase (b-CA3) of the C4 dicot Flaveria bidentis was overexpressed in C3 Arabidopsis thaliana to enhance its photosynthetic efficiency. Overexpression of b-CA3 cDNA resulted in ~2 fold higher CA protein abundance and ~50-65% increase in CA activity. Phosphoenolpyruvate carboxylase (PEPC) plays an anaplerotic role of replenishing the tricarboxylic acid cycle with certain intermediates to meet the demand of carbon skeletons for the synthesis of organic acids and amino acids in C3 plants. Due to higher concentration of HCO3- in CA overexpressors, their PEPC activity increased generating more oxaloacetic acid and amino acids. Consequently, their total protein content increased resulting in higher Chl synthesis. Due to their increased chlorophyll and protein content the transgenics had enhanced ETR and lower NPQ of chlorophyll a fluorescence. Therefore, CO2 assimilation, starch content, plant fresh weight and dry weight increased by 10-20% in CA overexpressors. Transgenic plants had lower stomatal conductance, reduce transpiration rate and higher water use efficiency. These approaches are being replicated in rice (Oryza sativa) to have increased photosynthesis, plant productivity and grain yield.