Evolutionary patterns in the auxin metabolism of green plants

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 p athway operating in the vegetative apices of the liverwort Pallavicinia lye llii, the moss Polytrichum ohioense, and the pteridophyte Selaginella kraus siana. A new methodology, which is based on the relative ability of unlabel ed tryptophan to affect the conversion of C-14-anthranilate into labeled IA A, established that the predominant IAA biosynthetic pathway(s) in all thes e plants are tryptophan-independent pathways. A second objective was to cha racterize auxin metabolism in the charophycean green alga Nitella sp. and t he hornwort Phaeoceros laevis. Gas chromatography-mass spectrometry was use d to measure the levels of free IAA and IAA conjugates, and thin-layer chro matography was used to determine the nature of IAA conjugates and the rates of IAA conjugate biosynthesis. Growing tips of Nitella thalli have low ste ady state levels of free IAA and IAA conjugates. Moreover, this alga exhibi ts very slow rates of IAA conjugate formation, which indicates that its fre e IAA level is primarily regulated via the balance between the biosynthesis of new IAA molecules and the degradation of existing molecules, as has alr eady been shown for liverworts. By contrast, apical regions of Phaeoceros t halli have much higher free IAA and IAA-amide conjugate levels under steady state conditions. This hornwort has intermediate to rapid rates of conjuga te formation, and, thus, it appears to regulate free IAA levels via the equ ilibrium between conjugate synthesis versus conjugate hydrolysis, as has re peatedly been observed for mosses and vascular plants. The data presented i n this study lead to a more comprehensive perspective on the evolutionary p atterns of auxin metabolism in green plants.