Auxin metabolism in mosses and liverworts

The plant hormone auxin (indole-3-acetic acid, IAA) is involved in the cont rol of many phenomena during plant development. By characterizing steady-st ate free and conjugated IAA levels using a stable isotope dilution method c oupled with gas chromatography- selected ion monitoring- mass spectrometry, this paper provides a detailed characterization of IAA metabolism in five liverworts, four mosses, and two tracheophytes. Long-term IAA conjugation p atterns were monitored by incubating actively growing tissue with C-14-IAA and then analyzing the de novo synthesis of IAA conjugates with radioimagin g techniques. The liverworts, mosses, and tracheophytes can be differentiat ed by the total amount of IAA metabolites, the proportion of free and conju gated IAA, the chemical nature of their IAA conjugates, and the rates of IA A conjugation. Our tentative conclusion is that the liverworts appear to em ploy a biosynthesis-degradation strategy for the regulation of free IAA lev els, in contrast to the conjugation-hydrolysis strategy apparently used by the mosses and tracheophytes. Such alternative metabolic strategies may hav e profound implications for macroevolutionary processes in these plant grou ps.