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Summary: The most productive C4 food and biofuel crops, such as Saccharum officinarum (sugarcane), Sorghum bicolor (sorghum) and Zea mays (maize), all use NADP‐ME‐type C4 photosynthesis. Despite high productivities, these crops fall well short of the theoretical maximum solar conversion efficiency of 6%. Understanding the basis of these inefficiencies is key for bioengineering and breeding strategies to increase the sustainable productivity of these major C4 crops. Photosynthesis is studied predominantly at steady state in saturating light. In field stands of these crops light is continually changing, and often with rapid fluctuations. Although light may change in a second, the adjustment of photosynthesis may take many minutes, leading to inefficiencies. We measured the rates of CO2 uptake and stomatal conductance of maize, sorghum and sugarcane under fluctuating light regimes. The gas exchange results were combined with a new dynamic photosynthesis model to infer the limiting factors under non‐steady‐state conditions. The dynamic photosynthesis model was developed from an existing C4 metabolic model for maize and extended to include: (i) post‐translational regulation of key photosynthetic enzymes and their temperature responses; (ii) dynamic stomatal conductance; and (iii) leaf energy balance. Testing the model outputs against measured rates of leaf CO2 uptake and stomatal conductance in the three C4 crops indicated that Rubisco activase, the pyruvate phosphate dikinase regulatory protein and stomatal conductance are the major limitations to the efficiency of NADP‐ME‐type C4 photosynthesis during dark‐to‐high light transitions. We propose that the level of influence of these limiting factors make them targets for bioengineering the improved photosynthetic efficiency of these key crops. [ABSTRACT FROM AUTHOR]

In C4 plants, the pyruvate phosphate dikinase regulatory protein (PDRP) regulates the C4 pathway enzyme pyruvate phosphate dikinase (PPDK) in response to changes in incident light intensity. In maize (Zea mays) leaves, two distinct isoforms of PDRP are expressed, ZmPDRP1 and ZmPDRP2. The properties and C4 function of the ZmPDRP1 isoform are well understood. However, the PDRP2 isoform has only recently been identified and its properties and function(s) in maize leaves are unknown. We therefore initiated an investigation into the maize PDRP2 isoform by performing a side by side comparison of its enzyme properties and cell-specific distribution with PDRP1. In terms of enzyme functionality, PDRP2 was found to possess the same protein kinase-specific activity as PDRP1. However, the PDRP2 isoform was found to lack the phosphotransferase activity of the bifunctional PDRP1 isoform except when PDRP2 in the assays is elevated 5- to 10-fold. A primarily immuno-based approach was used to show that PDRP1 is strictly expressed in mesophyll cells and PDRP2 is strictly expressed in bundle sheath strand cells (BSCs). Additionally, using in situ immunolocalization, we establish a regulatory target for PDRP2 by showing a significant presence of C4 PPDK in BSC chloroplasts. However, a metabolic role for PPDK in this compartment is obscure, assuming PPDK accumulating in this compartment would be irreversibly inactivated each dark cycle by a monofunctional PDRP2. [ABSTRACT FROM AUTHOR]

Abstract: Pyruvate, orthophosphate dikinase (PPDK; E.C. 2.7.9.1) catalyzes the synthesis of the primary inorganic carbon acceptor, phosphoenolpyruvate in the C4 photosynthetic pathway and is reversibly regulated by light. PPDK regulatory protein (RP), a bifunctional serine/threonine kinase-phosphatase, catalyzes both the ADP-dependent inactivation and the Pi-dependent activation of PPDK. Attempts to clone the RP have to date proven unsuccessful. A bioinformatics approach was taken to identify the nucleotide and amino acid sequence of the protein. Based on previously established characteristics including molecular mass, known inter- and intracellular location, functionality, and low level of expression, available databases were interrogated to ultimately identify a single candidate gene. In this paper, we describe the nucleotide and deduced amino acid sequence of this gene and establish its identity as maize PPDK RP by in vitro analysis of its catalytic properties via the cloning and expression of the recombinant protein. [Copyright &y& Elsevier]

Pyruvate, orthophosphate dikinase (PPDK, E.C. 2.7.9.1) is a cardinal carbon-assimilating, stromal enzyme of the C4 photosynthetic pathway. Like several other photosynthetic pathway enzymes, its activity is strictly and reversibly regulated by light. This regulation is conferred by the PPDK regulatory protein (RP), a bifunctional protein kinase/phosphatase that catalyzes the ADP-/Pi-dependent, reversible phosphorylation of an active-site threonine residue. In this minireview, we highlight how plastidic PPDK in leaves and developing seeds of C3 plants is regulated in an identical manner as C4 PPDK via a putative C3-RP isoform. Additionally, we also detail the progress in research concerning C4 RP, since this highly unusual regulatory enzyme was last reviewed nearly two decades ago. [Copyright &y& Elsevier]