Photorespiration's evolution resulted from the assembly of enzymes with diverse origins, specifically from cyanobacteria and heterotrophic bacteria.
Photorespiration, a metabolic pathway that occurs in plants when Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) binds to oxygen instead of carbon dioxide, has evolved through a fascinating process of acquiring enzymes from different bacterial sources. This complex pathway, essential for recycling a two-carbon compound produced during the oxygenation reaction, features enzymes with diverse evolutionary histories.
Origins of Photorespiration Enzymes
Phylogenetic analyses of the enzymes involved in photorespiration reveal a dual origin:
- Cyanobacterial Ancestry: Some photorespiratory enzymes originated from cyanobacteria, the ancestors of chloroplasts. This is consistent with the endosymbiotic theory, which posits that chloroplasts, the sites of photosynthesis, were once free-living cyanobacteria engulfed by eukaryotic cells.
- Heterotrophic Bacterial Ancestry: Other enzymes in the photorespiratory pathway are derived from heterotrophic bacteria, the ancestors of mitochondria in plant cells. This suggests that, similar to chloroplasts, mitochondria also originated through endosymbiosis.
This mosaic origin highlights the evolutionary history of photorespiration as a pathway assembled from components acquired from different sources during the evolution of plant cells. The compartmentalization of photorespiration across chloroplasts, peroxisomes, and mitochondria underscores the importance of endosymbiotic events in shaping plant metabolism.
In summary, the evolution of photorespiration involved the acquisition of enzymes from both cyanobacteria (leading to chloroplasts) and heterotrophic bacteria (leading to mitochondria), illustrating a complex interplay of endosymbiotic events and the recruitment of existing enzymatic functions to build a novel metabolic pathway.
