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Auxin metabolism in mosses and liverworts¹

²Department of Cell Biology and Molecular Genetics, University of Maryland at College Park,College Park, Maryland 20742-5815; and ³Horticultural Crops Quality Laboratory, Beltsville Agricultural Center, Agricultural Research Service,United States Department of Agriculture, Beltsville, Maryland 20705-2350

The plant hormone auxin (indole-3-acetic acid, IAA) is involved in the control of many phenomena during plant development. By characterizing steady-state free and conjugated IAA levels using a stable isotope dilution method coupled 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 patterns were monitored by incubating actively growing tissue with ¹⁴C-IAA and then analyzing the de novo synthesis of IAA conjugates with radioimaging techniques. The 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 the rates of IAA conjugation. Our tentative conclusion is that the 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 the mosses and tracheophytes. Such alternative metabolic strategies may have profound implications for macroevolutionary processes in these plant groups.

Key Words: auxin • auxin conjugates • auxin metabolism • indole-3-acetic acid • liverworts • mosses • tracheophytes • vascular tissue

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