Proline Metabolism and its Functions in Development and Stress Tolerance.

Most green plants are fully autotrophic organisms and can produce their entire biomass from inorganic molecules with the help of light energy captured by photosynthesis. Energy from photosynthesis is thereby not only needed to reduce CO2 to carbohydrates but also to assimilate nitrogen, phosphorus, and sulfur from inorganic salts for the biosynthesis of proteins and nucleic acids). In contrast, most non-photosynthetic organisms, including animals and humans, depend on the uptake of organic material both as energy source and as building material. This fundamental difference exists since the development of oxygenic photosynthesis by cyanobacteria, which were later converted to endosymbiotic chloroplasts in eukaryotic algae and plants. It is therefore not surprising that despite the use of identical building blocks in all living organisms, i.e., nucleotides, amino acids, and carbohydrates, the pathways to acquire or synthesize these building blocks are not identical in distantly related groups of organisms. Knowledge about human metabolism can therefore only serve to guide investigations of regulatory and metabolic pathways in plant primary metabolism but not as a direct template.