Decarboxylation of indoleacetic acid illuminates its evolution into a plant hormone

Indoleacetic acid (IAA), more commonly known as auxin, proved an enigmatic molecule in its evolutionary journey to becoming a plant hormone. Researchers sought to illuminate this mystery by investigating IAA response and metabolism in both land plants and their closest relatives, streptophyte algae. Through chemical treatments, phytohormone profiling, and metabolomics, the team discovered that streptophyte algae lacked plant-like auxin responses and the conventional IAA metabolism pathways.

However, the study unveiled a novel metabolic pathway in both streptophyte algae and land plants: IAA side-chain decarboxylation. This process operates in both kingdoms, playing a crucial role in cellular IAA metabolism. Moreover, decarboxylation contributed to IAA's photolability, meaning it easily decomposed upon exposure to light. This led to the formation of cytotoxic intermediates, which also accumulated within the cells when exposed to simulated sunlight.

Collectively, these findings provide strong evidence that IAA decarboxylation is a significant, conserved metabolic pathway shared by both land plants and streptophyte algae. The researchers furthermore propose that the metabolic challenges posed by IAA's inherent instability and indirect cytotoxicity likely played a key role in its eventual selection as a plant hormone.