A rapid screening method to detect specific inhibitors of pyruvate orthophosphate dikinase as leads for C4 plant-selective herbicides

Plants using the C-4 photosynthetic pathway are highly represented among the world's worst weeds, with only 4 C-4 species being agriculturally productive (maize, sorghum, millet, and sugar cane). With the C-4 acid cycle operating as a biochemical appendage of C-3 photosynthesis, the additional enzymes involved in C-4 photosynthesis represent an attractive target for the development of weed-specific herbicides. The rate-limiting enzyme of this metabolic pathway is pyruvate orthophosphate dikinase (PPDK). PPDK, coupled with phosphoenolpyruvate carboxylase and nicotinamide adenine dinucleotide-malate dehydrogenase. was used to develop a microplate-based assay to detect inhibitors of enzymes of the C-4 acid cycle. The resulting assay had a Z' factor of 0.61, making it a high-quality assay able to reliably identify active test samples. Organic extracts of 6679 marine macroscopic organisms were tested within the assay, and 343 were identified that inhibited the 3 enzyme-coupled reaction. A high confirmation rate was achieved, with 95% of these hit extracts proving active again upon retesting. Sequential addition of phosphoenolpyruvate and oxaloacetate to the assay facilitated identification of 83 extracts that specifically inhibited PPDK.