XVI International Botanical Congess

The plant hormone auxin (IAA) is involved in the control of many essential developmental phenomena throughout the Plant Kingdom. Auxin levels are regulated through three main processes: biosynthesis through tryptophan-dependent and/or tryptophan-independent pathways, conjugation to sugars and/or to amino acids, and various degradation pathways. Because IAA metabolism in lower land plants was virtually unknown, we undertook a systematic and detailed investigation of IAA metabolism in liverworts, mosses, and tracheophytes using state-of-the-art techniques. Our results show that IAA biosynthesis in representative non-seed plants is carried out through a tryptophan-independent pathway. Liverworts, mosses, and tracheophytes can be differentiated by the total amount of IAA metabolites, the proportion of free and conjugated IAA, the chemical nature of their IAA conjugates, and their IAA conjugation rates. Liverworts appear to employ a biosynthesis-degradation strategy for the regulation of free IAA levels, in contrast to the conjugation-hydrolysis strategy apparently used by mosses and tracheophytes. These alternative metabolic strategies for IAA metabolism may have profound implications for developmental phenomena, which underlie the macroevolutionary differences among land plants.