Auxin metabolism in representative land PLANTS

The plant hormone auxin (indole-3-acetic acid, IAA) appears to control many plant developmental processes, and studies performed in seed plants suggest that IAA conjugation is the critical mechanism to regulate free IAA concentration. The purpose of this investigation is to characterize the biochemical ability of one charophyte and 23 land plants ranging from liverworts to angiosperms to produce IAA conjugates, and to study the complexity of their conjugation patterns. Actively growing tissue was incubated with ¹⁴C-IAA, after which labeled IAA conjugates were separated using thin-layer chromatography. The conjugates were analyzed using radioimaging techniques and their tentative identity assigned by co-chromatography and/or by differential hydrolysis. The charophyte and the liverworts appear unable to conjugate IAA. The mosses and the hornwort are able to conjugate IAA into a few amide and ester conjugates. The tracheophytes examined synthesize several conjugates unique to the vascular plants, indole-3-acetyl-aspartic acid (-glutamic acid) and/or indole-3-acetyl-β-1-O-glucose, as well as a variety of other amide and ester conjugates. These three conjugation patterns are correlated to the type of conducting tissue characteristic of the plants analyzed. These biochemical differences may be indicative of significative differences in the hormonal regulation in these plant groups, thus suggesting that changes in IAA regulation accompanied the major evolutionary events in land plants.