This review represents the first effort ever to survey the entire literature on auxin (indole-3-acetic acid, IAA) action in all plants, with special emphasis on the green plant lineage, including charophytes (the green alga group closest to the land plants), bryophytes (the most basal land plants), pteridophytes (vascular non-seed plants), and seed plants. What emerges from this survey is the surprising perspective that the physiological mechanisms for regulating IAA levels and many IAA-mediated responses found in seed plants are also present in charophytes and bryophytes, at least in nascent forms. For example, the available evidence suggests that the apical regions of both charophytes and liverworts synthesize IAA via a tryptophan-independent pathway, with IAA levels being regulated via the balance between the rates of IAA biosynthesis and IAA degradation. The apical regions of all the other land plants utilize the same class of biosynthetic pathway, but they have the potential to utilize IAA conjugation and conjugate hydrolysis reactions to achieve more precise spatial and temporal control of IAA levels. The thallus tips of charophytes exhibit saturable IAA influx and efflux carriers, which are apparently not sensitive to polar IAA transport inhibitors. By contrast, two divisions of bryophyte gametophytes and moss sporophytes are reported to carry out polar IAA transport, but these groups exhibit differing sensitivities to those inhibitors. Although the IAA regulation of charophyte development has received almost no research attention, the bryophytes manifest a wide range of developmental responses, including tropisms, apical dominance, and rhizoid initiation, which are subject to IAA regulation that resembles the hormonal control over corresponding responses in seed plants. In pteridophytes, IAA regulates root initiation and vascular tissue differentiation in a manner also very similar to its effects on those processes in seed plants. Thus, it is concluded that the seed plants did not evolve de novo mechanisms for mediating IAA responses, but have rather modified pre-existing mechanisms already operating in the early land plants. Finally, this paper discusses the encouraging prospects for investigating the molecular evolution of auxin action.

A facile radiotracer method for discriminating between the activities oftryptophan-dependent and tryptophan-independent pathways for IAA biosynthesisisdescribed. This method utilized the simultaneous exposure of plants to[14C]anthranilate in the presence or absence of excess unlabeledtryptophan in order to determine if tryptophan feeding can affect the relativeenrichment of the IAA pool. Using this radiotracer method, the activities ofthetwo biosynthetic pathways were analyzed in isolated axes of germinating beanseedlings at various times after cotyledon excision. Unlabeled tryptophansuppressed [14C]anthranilate conversion into IAA in isolated axes ofdifferent ages immediately following cotyledon excision. On the other hand,tryptophan feeding did not inhibit [14C]IAA accumulation in isolatedaxes 36 or 120 h after cotyledon excision. Thus, this method wasable to resolve time-dependent differences following cotyledon excision in thebiosynthetic activities of the two pathways. Moreover, the present results lendfurther support to the emerging consensus that the tryptophan-dependent pathwayacts to maintain very high IAA levels required for mediating rapid cellproliferation in wounded tissues and, as previously shown, young embryos.

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.

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 (14)C-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.

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 14 C-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.

Sixteen rice (Oryza sativa L.) cultivars from 7 different geographical regions were grown in greenhouses at the Univ. of Maryland with and without supplemental ultraviolet-B (UV-B) radiation to determine alterations in biomass, morphology and maximum photosynthesis that would be anticipated from potential reductions in the stratospheric ozone column. A wide range of UV-B effects were observed, with the Philippines cultivar Carreon (5993) and the Sri Lankan cultivar Kurkaruppan (15449) showing the greatest decrease and increase, respectively, in total biomass with supplemental UV-B radiation. Approximately one-third of all cultivars tested showed a statistically significant decrease in total biomass with UV-B radiation. For these sensitive cultivars, leaf area and tiller number were also significantly reduced. Photosynthetic capacity as determined by oxygen evolution declined for some cultivars, but the correlation between changes in photosynthesis and biomass with increasing UV-B was equivocal. Results from this experiment indicate that: (1) a number of rice cultivars are sensitive to potential increases in UV-B radiation; and (2) the diversity exhibited by rice in response to increased levels of UV-B suggests that selective breeding might be successfully used to develop UV-B-tolerant rice cultivars.