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Abstract

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.

Diversity of phyllotaxis in land plants in reference to the shoot apical meristem structure

“…To fully elucidate and understand the reason(s) for and regulation of this spatio-temporal patterning would require detailed examinations of this process in all groups of land plants, and possibly in their ancestors as well. Of particular interest would be to trace the origin and evolution of auxin-regulated phyllotaxis formation from the angiosperms back through the ancestral lineages, as auxin appears to be a factor common to all land plants [138,139].…”

Paralogous Radiations of PIN Proteins with Multiple Origins of Noncanonical PIN Structure

“…Alterations in auxin transport are associated with morphological differences both within (Bennett et al 2006) and between (Barkoulas et al 2008) species, and auxin has also been shown to underpin morphological convergence in leaf and rooting (Pires et al 2013) functions. Many developmental roles of auxin transport are conserved across land plants (Cooke et al 2002; Sanders and Langdale 2013), and disruption of transport can disrupt the development of major organ systems (Galweiler et al 1998). These data suggest a strong potential contribution of auxin transport to morphological change in plant evolution.…”

Control of cytokinin and auxin homeostasis in cyanobacteria and algae

“…Endogenous IAA has been detected in cyanobacteria (Sergeeva et al , 2002; Hussain et al , 2010; Mazhar et al , 2013) as well as in brown algae (Le Bail et al , 2010), red algae (Ashen et al , 1999; Yokoya et al , 2010) and green algae (Mazur et al , 2001; Cooke et al , 2002; Stirk et al , 2013). Likewise, other auxin precursors and metabolites represented by tryptophan, anthranilate, IAM, indole-3-ethanol and IAOx were reported in substantial amounts in algae (Yokoya et al , 2010; Stirk et al , 2013, 2014), while in some cyanobacteria the species IBA was found as a predominant metabolite (Hashtroudi et al , 2013).…”