Evolutionary Patterns in the Auxin Metabolism of Green Plants

Abstract

The metabolism, transport, and action of the hormone auxin (indole‐3‐acetic acid; IAA) is thought to regulate the morphological processes responsible for generating the characteristic body plans of green plants. One objective of this study was to determine the general class of the IAA biosynthetic pathway operating in the vegetative apices of the liverwort Pallavicinia lyellii, the moss Polytrichum ohioense, and the pteridophyte Selaginella kraussiana. A new methodology, which is based on the relative ability of unlabeled tryptophan to affect the conversion of 14C‐anthranilate into labeled IAA, established that the predominant IAA biosynthetic pathway(s) in all these plants are tryptophan‐independent pathways. A second objective was to characterize auxin metabolism in the charophycean green alga Nitella sp. and the hornwort Phaeoceros laevis. Gas chromatography–mass spectrometry was used to measure the levels of free IAA and IAA conjugates, and thin‐layer chromatography was used to determine the nature of IAA conjugates and the rates of IAA conjugate biosynthesis. Growing tips of Nitella thalli have low steady state levels of free IAA and IAA conjugates. Moreover, this alga exhibits very slow rates of IAA conjugate formation, which indicates that its free IAA level is primarily regulated via the balance between the biosynthesis of new IAA molecules and the degradation of existing molecules, as has already been shown for liverworts. By contrast, apical regions of Phaeoceros thalli have much higher free IAA and IAA‐amide conjugate levels under steady state conditions. This hornwort has intermediate to rapid rates of conjugate formation, and, thus, it appears to regulate free IAA levels via the equilibrium between conjugate synthesis versus conjugate hydrolysis, as has repeatedly been observed for mosses and vascular plants. The data presented in this study lead to a more comprehensive perspective on the evolutionary patterns of auxin metabolism in green plants.