Polar auxin transport: an early invention

Ethylene controls three-dimensional growth involving reduced auxin levels in the moss Physcomitrium patens

The conquest of land by plants was concomitant with, and possibly enabled by, the evolution of three-dimensional (3D) growth. The moss Physcomitrium patens provides a model system for elucidating molecular mechanisms in the initiation of 3D growth. Here, we investigate whether the phytohormone ethylene, which is believed to have been a signal before land plant emergence, plays a role in 3D growth regulation in P. patens. We report ethylene controls 3D gametophore formation, based on results from exogenously applied ethylene and genetic manipulation of PpEIN2, which is a central component in the ethylene signaling pathway. Overexpression (OE) of PpEIN2 activates ethylene responses and leads to earlier formation of gametophores with fewer gametophores produced thereafter, phenocopying ethylene-treated wild-type. Conversely, Ppein2 knockout mutants, which are ethylene insensitive, show initially delayed gametophore formation with more gametophores produced later. Furthermore, pharmacological and biochemical analyses reveal auxin levels are decreased in the OE lines but increased in the knockout mutants. Our results suggest that evolutionarily, ethylene and auxin molecular networks were recruited to build the plant body plan in ancestral land plants. This might have played a role in enabling ancient plants to acclimate to the continental surfaces of the planet.

PIN-FORMED is required for shoot phototropism/gravitropism and facilitates meristem formation in Marchantia polymorpha

PIN-FORMED auxin efflux transporters, a subclass of which is plasma membrane-localised, mediate a variety of land-plant developmental processes via their polar localisation and subsequent directional auxin transport. We provide the first characterisation of PIN proteins in liverworts using Marchantia polymorpha as a model system. Marchantia polymorpha possesses a single PIN-FORMED gene, whose protein product is predicted to be plasma membrane-localised, MpPIN1. To characterise MpPIN1, we created loss-of-function alleles and produced complementation lines in both M. polymorpha and Arabidopsis. In M. polymorpha, gene expression and protein localisation were tracked using an MpPIN1 transgene encoding a translationally fused fluorescent protein. Overexpression of MpPIN1 can partially complement loss of an orthologous gene, PIN-FORMED1, in Arabidopsis. In M. polymorpha, MpPIN1 influences development in numerous ways throughout its life cycle. Most notably, MpPIN1 is required to establish gemmaling dorsiventral polarity and for orthotropic growth of gametangiophore stalks, where MpPIN1 is basally polarised. PIN activity is largely conserved within land plants, with PIN-mediated auxin flow providing a flexible mechanism to organise growth. Specifically, PIN is fundamentally linked to orthotropism and to the establishment of de novo meristems, the latter potentially involving the formation of both auxin biosynthesis maxima and auxin-signalling minima.

The birth of a giant: evolutionary insights into the origin of auxin responses in plants

The plant signaling molecule auxin is present in multiple kingdoms of life. Since its discovery, a century of research has been focused on its action as a phytohormone. In land plants, auxin regulates growth and development through transcriptional and non-transcriptional programs. Some of the molecular mechanisms underlying these responses are well understood, mainly in Arabidopsis. Recently, the availability of genomic and transcriptomic data of green lineages, together with phylogenetic inference, has provided the basis to reconstruct the evolutionary history of some components involved in auxin biology. In this review, we follow the evolutionary trajectory that allowed auxin to become the "giant" of plant biology by focusing on bryophytes and streptophyte algae. We consider auxin biosynthesis, transport, physiological, and molecular responses, as well as evidence supporting the role of auxin as a chemical messenger for communication within ecosystems. Finally, we emphasize that functional validation of predicted orthologs will shed light on the conserved properties of auxin biology among streptophytes.

Crossroads in the evolution of plant specialized metabolism

The monophyletic group of embryophytes (land plants) stands out among photosynthetic eukaryotes: they are the sole constituents of the macroscopic flora on land. In their entirety, embryophytes account for the majority of the biomass on land and constitute an astounding biodiversity. What allowed for the massive radiation of this particular lineage? One of the defining features of all land plants is the production of an array of specialized metabolites. The compounds that the specialized metabolic pathways of embryophytes produce have diverse functions, ranging from superabundant structural polymers and compounds that ward off abiotic and biotic challenges, to signaling molecules whose abundance is measured at the nanomolar scale. These specialized metabolites govern the growth, development, and physiology of land plants-including their response to the environment. Hence, specialized metabolites define the biology of land plants as we know it. And they were likely a foundation for their success. It is thus intriguing to find that the closest algal relatives of land plants, freshwater organisms from the grade of streptophyte algae, possess homologs for key enzymes of specialized metabolic pathways known from land plants. Indeed, some studies suggest that signature metabolites emerging from these pathways can be found in streptophyte algae. Here we synthesize the current understanding of which routes of the specialized metabolism of embryophytes can be traced to a time before plants had conquered land.

How was apical growth regulated in the ancestral land plant? Insights from the development of non-seed plants

Land plant life cycles are separated into distinct haploid gametophyte and diploid sporophyte stages. Indeterminate apical growth evolved independently in bryophyte (moss, liverwort, and hornwort) and fern gametophytes, and tracheophyte (vascular plant) sporophytes. The extent to which apical growth in tracheophytes co-opted conserved gametophytic gene networks, or exploited ancestral sporophytic networks, is a long-standing question in plant evolution. The recent phylogenetic confirmation of bryophytes and tracheophytes as sister groups has led to a reassessment of the nature of the ancestral land plant. Here, we review developmental genetic studies of apical regulators and speculate on their likely evolutionary history.

Green Algal Models for Multicellularity

The repeated evolution of multicellularity across the tree of life has profoundly affected the ecology and evolution of nearly all life on Earth. Many of these origins were in different groups of photosynthetic eukaryotes, or algae. Here, we review the evolution and genetics of multicellularity in several groups of green algae, which include the closest relatives of land plants. These include millimeter-scale, motile spheroids of up to 50,000 cells in the volvocine algae; decimeter-scale seaweeds in the genus Ulva (sea lettuce); and very plantlike, meter-scale freshwater algae in the genus Chara (stoneworts). We also describe algae in the genus Caulerpa, which are giant, multinucleate, morphologically complex single cells. In each case, we review the life cycle, phylogeny, and genetics of traits relevant to the evolution of multicellularity, and genetic and genomic resources available for the group in question. Finally, we suggest routes toward developing these groups as model organisms for the evolution of multicellularity. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

On the Evolutionary Origins of Land Plant Auxin Biology

Indole-3-acetic acid, that is, auxin, is a molecule found in a broad phylogenetic distribution of organisms, from bacteria to eukaryotes. In the ancestral land plant auxin was co-opted to be the paramount phytohormone mediating tropic responses and acting as a facilitator of developmental decisions throughout the life cycle. The evolutionary origins of land plant auxin biology genes can now be traced with reasonable clarity. Genes encoding the two enzymes of the land plant auxin biosynthetic pathway arose in the ancestral land plant by a combination of horizontal gene transfer from bacteria and possible neofunctionalization following gene duplication. Components of the auxin transcriptional signaling network have their origins in ancestral alga genes, with gene duplication and neofunctionalization of key domains allowing integration of a portion of the preexisting transcriptional network with auxin. Knowledge of the roles of orthologous genes in extant charophycean algae is lacking, but could illuminate the ancestral functions of both auxin and the co-opted transcriptional network.

Uncharted routes: exploring the relevance of auxin movement via plasmodesmata

Auxin is an endogenous small molecule with an incredibly large impact on growth and development in plants. Movement of auxin between cells, due to its negative charge at most physiological pHs, strongly relies on families of active transporters. These proteins import auxin from the extracellular space or export it into the same. Mutations in these components have profound impacts on biological processes. Another transport route available to auxin, once the substance is inside the cell, are plasmodesmata connections. These small channels connect the cytoplasms of neighbouring plant cells and enable flow between them. Interestingly, the biological significance of this latter mode of transport is only recently starting to emerge with examples from roots, hypocotyls and leaves. The existence of two transport systems provides opportunities for reciprocal cross-regulation. Indeed, auxin levels influence proteins controlling plasmodesmata permeability, while cell-cell communication affects auxin biosynthesis and transport. In an evolutionary context, transporter driven cell-cell auxin movement and plasmodesmata seem to have evolved around the same time in the green lineage. This highlights a co-existence from early on and a likely functional specificity of the systems. Exploring more situations where auxin movement via plasmodesmata has relevance for plant growth and development, and clarifying the regulation of such transport, will be key aspects in coming years.This article has an associated Future Leader to Watch interview with the author of the paper.

Evo-physio: on stress responses and the earliest land plants

Embryophytes (land plants) can be found in almost any habitat on the Earth's surface. All of this ecologically diverse embryophytic flora arose from algae through a singular evolutionary event. Traits that were, by their nature, indispensable for the singular conquest of land by plants were those that are key for overcoming terrestrial stressors. Not surprisingly, the biology of land plant cells is shaped by a core signaling network that connects environmental cues, such as stressors, to the appropriate responses-which, thus, modulate growth and physiology. When did this network emerge? Was it already present when plant terrestrialization was in its infancy? A comparative approach between land plants and their algal relatives, the streptophyte algae, allows us to tackle such questions and resolve parts of the biology of the earliest land plants. Exploring the biology of the earliest land plants might shed light on exactly how they overcame the challenges of terrestrialization. Here, we outline the approaches and rationale underlying comparative analyses towards inferring the genetic toolkit for the stress response that aided the earliest land plants in their conquest of land.

The evolutionary origins of auxin transport: what we know and what we need to know

Auxin, represented by indole-3-acetic acid (IAA), has for a long time been studied mainly with respect to the development of land plants, and recent evidence confirms that canonical nuclear auxin signaling is a land plant apomorphy. Increasing sequential and physiological data show that the presence of auxin transport machinery pre-dates the emergence of canonical signaling. In this review, we summarize the present state of knowledge regarding the origins of auxin transport in the green lineage (Viridiplantae), integrating both data from wet lab experiments and sequence evidence on the presence of PIN-FORMED (PIN), PIN-LIKES (PILS), and AUXIN RESISTANT 1/LIKE-AUX1 (AUX1/LAX) homologs. We discuss a high divergence of auxin carrier homologs among algal lineages and emphasize the urgent need for the establishment of good molecular biology models from within the streptophyte green algae. We further postulate and discuss two hypotheses for the ancestral role of auxin in the green lineage. First, auxin was present as a by-product of cell metabolism and the evolution of its transport was stimulated by the need for IAA sequestration and cell detoxification. Second, auxin was primarily a signaling compound, possibly of bacterial origin, and its activity in the pre-plant green algae was a consequence of long-term co-existence with bacteria in shared ecological consortia.

Inhibition of endosomal trafficking by brefeldin A interferes with long-distance interaction between chloroplasts and plasma membrane transporters

The huge internodal cells of the characean green algae are a convenient model to study long-range interactions between organelles via cytoplasmic streaming. It has been shown previously that photometabolites and reactive oxygen species released by illuminated chloroplasts are transmitted to remote shaded regions where they interfere with photosynthetic electron transport and the differential activity of plasma membrane transporters, and recent findings indicated the involvement of organelle trafficking pathways. In the present study, we applied pulse amplitude-modulated microscopy and pH-sensitive electrodes to study the effect of brefeldin A (BFA), an inhibitor of vesicle trafficking, on long-distance interactions in Chara australis internodal cells. These data were compared with BFA-induced changes in organelle number, size and distribution using fluorescent dyes and confocal laser scanning microscopy. We found that BFA completely and immediately inhibited endocytosis in internodal cells and induced the aggregation of organelles into BFA compartments within 30-120 min of treatment. The comparison with the physiological data suggests that the early response, the arrest of endocytosis, is related to the attenuation of differences in surface pH, whereas the longer lasting formation of BFA compartments is probably responsible for the acceleration of the cyclosis-mediated interaction between chloroplasts. These data indicate that intracellular turnover of membrane material might be important for the circulation of electric currents between functionally distinct regions in illuminated characean internodes and that translational movement of metabolites is delayed by transient binding of the transported substances to organelles.

PIN-driven auxin transport emerged early in streptophyte evolution

PIN-FORMED (PIN) transporters mediate directional, intercellular movement of the phytohormone auxin in land plants. To elucidate the evolutionary origins of this developmentally crucial mechanism, we analysed the single PIN homologue of a simple green alga Klebsormidium flaccidum. KfPIN functions as a plasma membrane-localized auxin exporter in land plants and heterologous models. While its role in algae remains unclear, PIN-driven auxin export is probably an ancient and conserved trait within streptophytes.

Auxin EvoDevo: Conservation and Diversification of Genes Regulating Auxin Biosynthesis, Transport, and Signaling

The phytohormone auxin has been shown to be of pivotal importance in growth and development of land plants. The underlying molecular players involved in auxin biosynthesis, transport, and signaling are quite well understood in Arabidopsis. However, functional characterizations of auxin-related genes in economically important crops, specifically maize and rice, are still limited. In this article, we comprehensively review recent functional studies on auxin-related genes in both maize and rice, compared with what is known in Arabidopsis, and highlight conservation and diversification of their functions. Our analysis is illustrated by phylogenetic analysis and publicly available gene expression data for each gene family, which will aid in the identification of auxin-related genes for future research. Current challenges and future directions for auxin research in maize and rice are discussed. Developments in gene editing techniques provide powerful tools for overcoming the issue of redundancy in these gene families and will undoubtedly advance auxin research in crops.

The evolution of root branching: increasing the level of plasticity

Plant roots and root systems are indispensable for water and nutrient foraging, and are a major evolutionary achievement for plants to cope with dry land conditions. The ability of roots to branch contributes substantially to their capacity to explore the soil for water and nutrients, and led ~400 million years ago to the successful colonization of land by plants, eventually even in arid regions. During this colonization, different forms of root branching evolved, reinforcing step by step the phenotypic plasticity of the root system. Whereas the lycophytes, the most ancient land plants with roots, only branch at the root tip, ferns are able to form roots laterally in a fixed pattern along the main root. Finally, roots of seed plants show the highest phenotypic plasticity, because lateral roots can possibly, dependent on internal and/or external signals, be produced at almost any position along the main root. The competence to form lateral roots in seed plants is based on the presence of internal cell files with stem cell-like features. Despite the dissimilarities between the different clades, a number of genetic modules seem to be co-opted in order to acquire root branching capacity. In this review, starting from the lateral root pathways in seed plants, we review root branching in the different land plant lineages and discuss the hitherto described genetic modules that contribute to their root branching capacity. We try to obtain insight into how land plants have acquired an increasing root branching plasticity during evolution that contributed to the successful colonization of our planet by seed plants.

What have we learnt from studying the evolution of the arbuscular mycorrhizal symbiosis?

The arbuscular mycorrhizal (AM) symbiosis is a nearly ubiquitous association formed by most land plants. Numerous insights into the molecular mechanisms governing this symbiosis have been obtained in recent years leading to the identification of a core set of plant genes essential for successful formation of the AM symbiosis by angiosperm hosts. Recent phylogenetic analyses indicate that while the origin of some of these symbiotic genes predated the first land plants, the rest appeared through processes including de novo evolution and gene duplication that occurred specifically in the land plants. Purifying selection on this core gene set has been maintained over millions of years of plant evolution to conserve the AM symbiosis. However, several independent losses of this association have been recorded in numerous embryophyte lineages. In these lineages, potential compensatory mechanisms have been identified that could have helped these plants overcome the adversities imposed by the loss of the AM symbiosis. This review will focus on the processes governing the conservation of the AM symbiosis in the land plant lineage.

Plant evolution: landmarks on the path to terrestrial life

Contents Summary 1428 I. The singularity of plant terrestrialization 1428 II. Adaptation vs exaptation - what shaped the land plant toolkit? 1430 III. Trait mosaicism in (higher-branching) streptophyte algae 1431 IV.

CONCLUSIONS

a streptophyte algal perspective on land plant trait evolution 1432 Acknowledgements 1432 ORCID 1433 References 1433 SUMMARY: Photosynthetic eukaryotes thrive anywhere there is sunlight and water. But while such organisms are exceptionally diverse in form and function, only one phototrophic lineage succeeded in rising above its substrate: the land plants (embryophytes). Molecular phylogenetic data show that land plants evolved from streptophyte algae most closely related to extant Zygnematophyceae, and one of the principal aims of plant evolutionary biology is to uncover the key features of such algae that enabled this important transition. At the present time, however, mosaic and reductive evolution blur our picture of the closest algal ancestors of plants. Here we discuss recent progress and problems in inferring the biology of the algal progenitor of the terrestrial photosynthetic macrobiome.

Evolution and palaeophysiology of the vascular system and other means of long-distance transport

Photolithotrophic growth on land using atmospheric CO2 inevitably involves H2O vapour loss. Embryophytes greater than or equal to 100 mm tall are homoiohydric and endohydric with mass flow of aqueous solution through the xylem in tracheophytes. Structural details in Rhynie sporophytes enable modelling of the hydraulics of H2O supply to the transpiring surface, and the potential for gas exchange with the Devonian atmosphere. Xylem carrying H2O under tension involves programmed cell death, rigid cell walls and embolism repair; fossils provide little evidence on these functions other than the presence of lignin. The phenylalanine ammonia lyase essential for lignin synthesis came from horizontal gene transfer. Rhynie plants lack endodermes, limiting regulation of the supply of soil nutrients to shoots. The transfer of organic solutes from photosynthetic sites to growing and storage tissues involves mass flow through phloem in extant tracheophytes. Rhynie plants show little evidence of phloem; possible alternatives for transport of organic solutes are discussed. Extant examples of the arbuscular mycorrhizas found in Rhynie plants exchange soil-derived nutrients (especially P) for plant-derived organic matter, involving bidirectional mass flow along the hyphae. The aquatic cyanobacteria and the charalean Palaeonitella at Rhynie also have long-distance (relative to the size of the organism) transport.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

Primitive Auxin Response without TIR1 and Aux/IAA in the Charophyte Alga Klebsormidium nitens

The phytohormone auxin regulates many aspects of growth and development in land plants, but the origin and evolution of auxin signaling and response mechanisms remain largely unknown. Indeed, it remains to be investigated whether auxin-related pathways diverged before the emergence of land plants. To address this knowledge deficit, we analyzed auxin responses in the charophyte alga Klebsormidium nitens NIES-2285, whose ancestor diverged from a green algal ancestor during the evolution of land plants. This strain is the same as Klebsormidium flaccidum NIES-2285, for which the draft genome was sequenced in 2014, and was taxonomically reclassified as K. nitens This genome sequence revealed genes involved in auxin responses. Furthermore, the auxin indole-3-acetic acid (IAA) was detected in cultures of K. nitens, but K. nitens lacks the central regulators of the canonical auxin-signaling pathway found in land plants. Exogenous IAA inhibited cell division and cell elongation in K. nitens Inhibitors of auxin biosynthesis and of polar auxin transport also inhibited cell division and elongation. Moreover, exogenous IAA rapidly induced expression of a LATERAL ORGAN BOUNDARIES-DOMAIN transcription factor. These results suggest that K. nitens has acquired the part of the auxin system that regulates transcription and cell growth without the requirement for the central players that govern auxin signaling in land plants.

Immunogold-EM analysis reveal brefeldin a-sensitive clusters of auxin in Arabidopsis root apex cells

Immunogold electron microscopy (EM) study of Arabidopsis root apices analyzed using specific IAA antibody and high-pressure freeze fixation technique allowed, for the first time, vizualization of subcellular localization of IAA in cells assembled intactly within plant tissues. Our quantitative analysis reveals that there is considerable portion of IAA gold particles that clusters within vesicles and membraneous compartments in all root apex cells. There are clear tissue-specific and developmental differences of clustered IAA in root apices. These findings have significant consequences for our understanding of this small molecule which is controlling plant growth, development and behavior.

The effect of polar auxin transport on adventitious branches formation in Gracilaria lichenoides in vitro

Seaweed tissue culture (STC) is an important micropropagation tool that has been applied for strain improvement, micropropagation and genetic engineering. Because the mechanisms associated with STC are poorly understood, its application to these organisms lags far behind that of tissue culture propagation of higher plants. Auxin, calcium (Ca²⁺ ) and hydrogen peroxide (H₂ O₂ ) fluxes all play key roles during plant growth and development. In this study, we therefore measured indole-3-acetic acid, Ca²⁺ and H₂ O₂ fluxes of Gracilaria lichenoides explants during adventitious branches (ABs) formation for the first time using noninvasive micro-test technology. We confirmed that polar auxin transport (PAT) also occurs in the marine red alga G. lichenoides. We additionally found that N-1-naphthylphthalamic acid may suppress auxin efflux via ABCB1 transporters and then inhibit ABs formation from the apical region of G. lichenoides segments. The involvement of Ca²⁺ and H₂ O₂ fluxes in PAT-mediated AB formation in G. lichenoides was also investigated. We propose that complex feedback among Ca²⁺ , H₂ O₂ and auxin signaling and response systems may occur during ABs polar formation in G. lichenoides explants, similar to that in higher plants. Our results provide innovative insights that should aid future elucidation of mechanisms operative during STC.

PIN2-like proteins may contribute to the regulation of morphogenetic processes during spermatogenesis in Chara vulgaris

KEY MESSAGE

PIN2-like auxin transporters are expressed, preferentially in a polarized manner, in antheridial cells of freshwater green alga Chara vulgaris , considered to be the closest relative of the present-day land plants. Chara vulgaris represents a group of advanced multicellular green algae that are considered as the closest relatives of the present-day land plants. A highly specialized structure of its male sex organs (antheridia) includes filaments consisting of generative cells, which after a series of synchronous divisions transform into mature sperm, and non-generative cells comprising outer shield cells, cylindrical manubria, and central complex of capitular cells from which antheridial filaments arise. Immunofluorescence observations indicate that PIN2-like proteins (PIN2-LPs), recognized by antibodies against PIN-FORMED2 (PIN2) auxin transporter in Arabidopsis thaliana, are expressed in both types of antheridial cells and, in most of them, preferentially accumulate in a polarized manner. The appearance of PIN2-LPs in germ-line cells is strictly confined to the proliferative period of spermatogenesis and their quantities increase steadily till antheridial filaments reach the 16-celled stage. An enhanced level of PIN2-LPs observed in the central cell walls separating two asynchronously developing parts of antheridial filaments (characterized by the plugged plasmodesmata) is correlated with an enhanced deposition of callose. Intense PIN2-LPs immunofluorescence maintained in the capitular cells and its altering polarity in manubria suggest a pivotal role of these cells in the regulation of auxin transport directionality during the whole time of antheridial ontogenesis. Immunohistochemical staining of IAA revealed a clear-cut correspondence between localization sites of auxins and PIN2-LPs. It seems probable then that a supplementary developmental mechanism has evolved in Chara, by which all antheridial elements may be integrated at the supra-cellular level via plasma membrane-targeted PIN2-LPs and auxin-mediated processes.

Multi-Scale Characean Experimental System: From Electrophysiology of Membrane Transporters to Cell-to-Cell Connectivity, Cytoplasmic Streaming and Auxin Metabolism

The morphology of characean algae could be mistaken for a higher plant: stem-like axes with leaf-like branchlets anchored in the soil by root-like rhizoids. However, all of these structures are made up of giant multinucleate cells separated by multicellular nodal complexes. The excised internodal cells survive long enough for the nodes to give rise to new thallus. The size of the internodes and their thick cytoplasmic layer minimize impalement injury and allow specific micro-electrode placement. The cell structure can be manipulated by centrifugation, perfusion of cell contents or creation of cytoplasmic droplets, allowing access to both vacuolar and cytoplasmic compartments and both sides of the cell membranes. Thousands of electrical measurements on intact or altered cells and cytoplasmic droplets laid down basis to modern plant electrophysiology. Furthermore, the giant internodal cells and whole thalli facilitate research into many other plant properties. As nutrients have to be transported from rhizoids to growing parts of the thallus and hormonal signals need to pass from cell to cell, Characeae possess very fast cytoplasmic streaming. The mechanism was resolved in the characean model. Plasmodesmata between the internodal cells and nodal complexes facilitate transport of ions, nutrients and photosynthates across the nodes. The internal structure was found to be similar to those of higher plants. Recent experiments suggest a strong circadian influence on metabolic pathways producing indole-3-acetic acid (IAA) and serotonin/melatonin. The review will discuss the impact of the characean models arising from fragments of cells, single cells, cell-to-cell transport or whole thalli on understanding of plant evolution and physiology.

Enzyme-Less Growth in Chara and Terrestrial Plants

Enzyme-less chemistry appears to control the growth rate of the green alga Chara corallina. The chemistry occurs in the wall where a calcium pectate cycle determines both the rate of wall enlargement and the rate of pectate deposition into the wall. The process is the first to indicate that a wall polymer can control how a plant cell enlarges after exocytosis releases the polymer to the wall. This raises the question of whether other species use a similar mechanism. Chara is one of the closest relatives of the progenitors of terrestrial plants and during the course of evolution, new wall features evolved while pectate remained one of the most conserved components. In addition, charophytes contain auxin which affects Chara in ways resembling its action in terrestrial plants. Therefore, this review considers whether more recently acquired wall features require different mechanisms to explain cell expansion.

Auxin effects on ion transport in Chara corallina

The plant hormone auxin has been widely studied with regard to synthesis, transport, signaling and functions among the land plants while there is still a lack of knowledge about the possible role for auxin regulation mechanisms in algae with "plant-like" structures. Here we use the alga Chara corallina as a model to study aspects of auxin signaling. In this respect we measured auxin on membrane potential changes and different ion fluxes (K(+), H(+)) through the plasma membrane. Results showed that auxin, mainly IAA, could hyperpolarize the membrane potential of C. corallina internodal cells. Ion flux measurements showed that the auxin-induced membrane potential change may be based on the change of K(+) permeability and/or channel activity rather than through the activation of proton pumps as known in land plants.

Modelling the dynamics of polar auxin transport in inflorescence stems of Arabidopsis thaliana

The polar transport of the plant hormone auxin has been the subject of many studies, several involving mathematical modelling. Unfortunately, most of these models have not been experimentally verified. Here we present experimental measurements of long-distance polar auxin transport (PAT) in segments of inflorescence stems of Arabidopsis thaliana together with a descriptive mathematical model that was developed from these data. It is based on a general advection-diffusion equation for auxin density, as suggested by the chemiosmotic theory, but is extended to incorporate both immobilization of auxin and exchange with the surrounding tissue of cells involved in PAT, in order to account for crucial observations. We found that development of the present model assisted effectively in the analysis of experimental observations. As an example, we discuss the analysis of a quadruple mutant for all four AUX1/LAX1-LAX3 influx carriers genes. We found a drastic change in the parameters governing the exchange of PAT channels with the surrounding tissue, whereas the velocity was still of the order of magnitude of the wild type. In addition, the steady-state flux of auxin through the PAT system of the mutant did not exhibit a saturable component, as we found for the wild type, suggesting that the import carriers are responsible for the saturable component in the wild type. In the accompanying Supplementary data available at JXB online, we describe in more detail the data-driven development of the model, review and derive predictions from a mathematical model of the chemiosmotic theory, and explore relationships between parameters in our model and processes and parameters at the cellular level.

Charophytes: Evolutionary Giants and Emerging Model Organisms

Charophytes are the group of green algae whose ancestral lineage gave rise to land plants in what resulted in a profoundly transformative event in the natural history of the planet. Extant charophytes exhibit many features that are similar to those found in land plants and their relatively simple phenotypes make them efficacious organisms for the study of many fundamental biological phenomena. Several taxa including Micrasterias, Penium, Chara, and Coleochaete are valuable model organisms for the study of cell biology, development, physiology and ecology of plants. New and rapidly expanding molecular studies are increasing the use of charophytes that in turn, will dramatically enhance our understanding of the evolution of plants and the adaptations that allowed for survival on land. The Frontiers in Plant Science series on "Charophytes" provides an assortment of new research reports and reviews on charophytes and their emerging significance as model plants.

Keeping it all together: auxin-actin crosstalk in plant development

Polar auxin transport and the action of the actin cytoskeleton are tightly interconnected, which is documented by the finding that auxin transporters reach their final destination by active movement of secretory vesicles along F-actin tracks. Moreover, auxin transporter polarity and flexibility is thought to depend on transporter cycling that requires endocytosis and exocytosis of vesicles. In this context, we have reviewed the current literature on an involvement of the actin cytoskeleton in polar auxin transport and identify known similarities and differences in its structure, function and dynamics in comparison to non-plant organisms. By describing how auxin modulates actin expression and actin organization and how actin and its stability affects auxin-transporter endocytosis and recycling, we discuss the current knowledge on regulatory auxin-actin feedback loops. We focus on known effects of auxin and of auxin transport inhibitors on the stability and organization of actin and examine the functionality of auxin and/or auxin transport inhibitor-binding proteins with respect to their suitability to integrate auxin/auxin transport inhibitor action. Finally, we indicate current difficulties in the interpretation of organ, time and concentration-dependent auxin/auxin transport inhibitor treatments and formulate simple future experimental guidelines.

PIN proteins and the evolution of plant development

Many aspects of development in the model plant Arabidopsis thaliana involve regulated distribution of the hormone auxin by the PIN-FORMED (PIN) family of auxin efflux carriers. The role of PIN-mediated auxin transport in other plants is not well understood, but studies in a wider range of species have begun to illuminate developmental mechanisms across land plants. In this review, I discuss recent progress in understanding the evolution of PIN-mediated auxin transport, and its role in development across the green plant lineage. I also discuss the idea that changes in auxin biology led to morphological novelty in plant development: currently available evidence suggests major innovations in auxin transport are rare and not associated with the evolution of new developmental mechanisms.

GA(3) enhances root responsiveness to exogenous IAA by modulating auxin transport and signalling in Arabidopsis

We used auxin-signalling mutants, auxin transport mutants, and auxin-related marker lines to show that exogenously applied GA enhances auxin-induced root inhibition by affecting auxin signalling and transport. Variation in root elongation is valuable when studying the interactions of phytohormones. Auxins influence the biosynthesis and signalling of gibberellins (GAs), but the influence of GAs on auxins in root elongation is poorly understood. This study was conducted to investigate the effect of GA3 on Arabidopsis root elongation in the presence of auxin. Root elongation was inhibited in roots treated with both IAA and GA3, compared to IAA alone, and the effect was dose dependent. Further experiments showed that GA3 could modulate auxin signalling based on root elongation in auxin-signalling mutants and the expression of auxin-responsive reporters. The GA3-enhanced inhibition of root elongation observed in the wild type was not found in the auxin-signalling mutants tir1-1 and axr1-3. GA3 increased DR5::GUS expression in the root meristem and elongation zones, and IAA2::GUS in the columella. The DR5rev::GFP signal was enhanced in columella cells of the root caps and in the elongation zone in GA3-treated seedling roots. A reduction was observed in the stele of PAC-treated roots. We also examined the effect of GA3 on auxin transport. The enhanced responsiveness caused by GA3 was not observed in the auxin influx mutant aux1-7 or the efflux mutant eir1-1. Additional molecular data demonstrated that GA3 could promote auxin transport via AUX1 and PIN proteins. However, GA3-induced PIN gene expression did not fully explain GA-enhanced PIN protein accumulation. These results suggest that GA3 is involved in auxin-mediated primary root elongation by modulating auxin signalling and transport, and thus enhances root responsiveness to exogenous IAA.

Insights into the origin and evolution of the plant hormone signaling machinery

Plant hormones modulate plant growth, development, and defense. However, many aspects of the origin and evolution of plant hormone signaling pathways remain obscure. Here, we use a comparative genomic and phylogenetic approach to investigate the origin and evolution of nine major plant hormone (abscisic acid, auxin, brassinosteroid, cytokinin, ethylene, gibberellin, jasmonate, salicylic acid, and strigolactone) signaling pathways. Our multispecies genome-wide analysis reveals that: (1) auxin, cytokinin, and strigolactone signaling pathways originated in charophyte lineages; (2) abscisic acid, jasmonate, and salicylic acid signaling pathways arose in the last common ancestor of land plants; (3) gibberellin signaling evolved after the divergence of bryophytes from land plants; (4) the canonical brassinosteroid signaling originated before the emergence of angiosperms but likely after the split of gymnosperms and angiosperms; and (5) the origin of the canonical ethylene signaling pathway postdates shortly the emergence of angiosperms. Our findings might have important implications in understanding the molecular mechanisms underlying the emergence of land plants.

Circadian changes in endogenous concentrations of indole-3-acetic acid, melatonin, serotonin, abscisic acid and jasmonic acid in Characeae (Chara australis Brown)

Giant-celled Characeae (Chara australis Brown), grown for 4 months on 12/12 hr day/night cycle and summer/autumn temperatures, exhibited distinct concentration maxima in auxin (indole-3-acetic acid; IAA), melatonin and serotonin about 4 hr after subjective daybreak. These concentration peaks persisted after 3 day pretreatment in continuous darkness: confirming a circadian rhythm, rather than a response to "light on." The plants pretreated for 3 d in continuous light exhibited several large IAA concentration maxima throughout the 24 hr. The melatonin and serotonin concentrations decreased and were less synchronized with IAA. Chara plants grown on 9/15 hr day/night cycle for 4 months and winter/spring temperatures contained much smaller concentrations of IAA, melatonin and serotonin. The IAA concentration maxima were observed in subjective dark phase. Serotonin concentration peaks were weakly correlated with those of IAA. Melatonin concentration was low and mostly independent of circadian cycle. The "dark" IAA concentration peaks persisted in plants treated for 3 d in the dark. The plants pretreated for 3 d in the light again developed more IAA concentration peaks. In this case the concentration maxima in melatonin and serotonin became more synchronous with those in IAA. The abscisic acid (ABA) and jasmonic acid (JA) concentrations were also measured in plants on winter regime. The ABA concentration did not exhibit circadian pattern, while JA concentration peaks were out of phase with those of IAA. The data are discussed in terms of crosstalk between metabolic pathways.

Direction of illumination controls gametophyte orientation in seedless plants and related algae

The environmental influences that determine dorsiventral or axial gametophyte orientation are unknown for most modern seedless plants. To fill this gap, an experimental laboratory system was employed to evaluate the relative effects of light direction and gravity on body orientation of the dorsiventral green alga Coleochaete orbicularis, and gametophytes of liverworts Blasia pusilla and Marchantia polymorpha, early-diverging moss Sphagnum compactum, and fern Ceratopteris richardii, the latter functioning as experimental control. Replicate clonal cultures were experimentally illuminated only from above, only from below, or from multiple directions, with the same near-saturation PAR level for periods brief enough to minimize nutrient limitation effects, and orientation of new growth was evaluated. For all species tested, direction of illumination exerted stronger control over gametophyte body orientation than gravity. When illuminated only from below: 1) axial Sphagnum gametophores that had initially grown into an overlying air space inverted growth by 180°, burrowing into the substrate; 2) new growth of dorsiventral Blasia, Marchantia, and Ceratopteris gametophytes-whose ventral rhizoids initially penetrated agar substrate and dorsal surfaces initially faced overlying airspace-twisted 180° so that ventral surfaces bearing rhizoids faced overlying air space and rhizoids extended into the air; and 3) Coleochaete lost typical dorsiventral organization and diagnostic dorsal hairs. Direction of illumination also exerted stronger control over orientation of liverwort new growth than surface contact did. These results indicate that early land plants likely inherited light-directed gametophyte body orientation from ancestral streptophyte algae and suggest a mechanism for reorientation of gametophyte-dominant land plants after spatial disturbance.

Plasma membrane-targeted PIN proteins drive shoot development in a moss

Background

Plant body plans arise by the activity of meristematic growing tips during development and radiated independently in the gametophyte (n) and sporophyte (2n) stages of the life cycle during evolution. Although auxin and its intercellular transport by PIN family efflux carriers are primary regulators of sporophytic shoot development in flowering plants, the extent of conservation in PIN function within the land plants and the mechanisms regulating bryophyte gametophytic shoot development are largely unknown.

Results

We have found that treating gametophytic shoots of the moss Physcomitrella patens with exogenous auxins and auxin transport inhibitors disrupts apical function and leaf development. Two plasma membrane-targeted PIN proteins are expressed in leafy shoots, and pin mutants resemble plants treated with auxins or auxin transport inhibitors. PIN-mediated auxin transport regulates apical cell function, leaf initiation, leaf shape, and shoot tropisms in moss gametophytes. pin mutant sporophytes are sometimes branched, reproducing a phenotype only previously seen in the fossil record and in rare natural moss variants.

Conclusions

Our results show that PIN-mediated auxin transport is an ancient, conserved regulator of shoot development.

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PIN proteins have polar plasma membrane localizations in the moss Physcomitrella

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PIN-mediated auxin transport drives gametophytic shoot development in Physcomitrella

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PIN-mediated auxin transport suppresses branching in Physcomitrella sporophytes

Green algae and the origins of multicellularity in the plant kingdom

The green lineage of chlorophyte algae and streptophytes form a large and diverse clade with multiple independent transitions to produce multicellular and/or macroscopically complex organization. In this review, I focus on two of the best-studied multicellular groups of green algae: charophytes and volvocines. Charophyte algae are the closest relatives of land plants and encompass the transition from unicellularity to simple multicellularity. Many of the innovations present in land plants have their roots in the cell and developmental biology of charophyte algae. Volvocine algae evolved an independent route to multicellularity that is captured by a graded series of increasing cell-type specialization and developmental complexity. The study of volvocine algae has provided unprecedented insights into the innovations required to achieve multicellularity.

Microgravity induces changes in microsome-associated proteins of Arabidopsis seedlings grown on board the international space station

The "GENARA A" experiment was designed to monitor global changes in the proteome of membranes of Arabidopsis thaliana seedlings subjected to microgravity on board the International Space Station (ISS). For this purpose, 12-day-old seedlings were grown either in space, in the European Modular Cultivation System (EMCS) under microgravity or on a 1 g centrifuge, or on the ground. Proteins associated to membranes were selectively extracted from microsomes and identified and quantified through LC-MS-MS using a label-free method. Among the 1484 proteins identified and quantified in the 3 conditions mentioned above, 80 membrane-associated proteins were significantly more abundant in seedlings grown under microgravity in space than under 1 g (space and ground) and 69 were less abundant. Clustering of these proteins according to their predicted function indicates that proteins associated to auxin metabolism and trafficking were depleted in the microsomal fraction in µg space conditions, whereas proteins associated to stress responses, defence and metabolism were more abundant in µg than in 1 g indicating that microgravity is perceived by plants as a stressful environment. These results clearly indicate that a global membrane proteomics approach gives a snapshot of the cell status and its signaling activity in response to microgravity and highlight the major processes affected.

Cellular Auxin Transport in Algae

The phytohormone auxin is one of the main directors of plant growth and development. In higher plants, auxin is generated in apical plant parts and transported from cell-to-cell in a polar fashion. Auxin is present in all plant phyla, and the existence of polar auxin transport (PAT) is well established in land plants. Algae are a group of relatively simple, autotrophic, photosynthetic organisms that share many features with land plants. In particular, Charophyceae (a taxon of green algae) are closest ancestors of land plants. In the study of auxin function, transport and its evolution, the algae form an interesting research target. Recently, proof for polar auxin transport in Chara species was published and auxin related research in algae gained more attention. In this review we discuss auxin transport in algae with respect to land plants and suggest directions for future studies.

Characean internodal cells as a model system for the study of cell organization

Giant internodal cells of characean green algae have been widely used for studying cellular physiology. This review emphasizes their significance for understanding cytoarchitecture and cytoplasmic reorganization. The cytoarchitecture of internodal cells undergoes pronounced, cytoskeleton-dependent changes during development and in response to environmental cues. Under bright light, internodes develop alternating bands of acid and alkaline pH at their surface that correlate with the differential size and abundance of cortical organelles and, in the genus Chara, with the size and distribution of convoluted plasma membrane domains known as charasomes. Wounding induces responses ranging from chloroplast detachment to deposition of wound walls. These properties and the possibility for mechanical manipulation make the internodal cell ideal for exploring plasma membrane domains, organelle interactions, vesicle trafficking, and local cell wall deposition. The significance of this model system will further increase with the application of molecular biological methods in combination with metabolomics and proteomics.

Heterotrimeric G proteins in green algae: an early innovation in the evolution of the plant lineage

Heterotrimeric G-proteins (G-proteins, hereafter) are important signaling components in all eukaryotes. The absence of these proteins in the sequenced genomes of Chlorophyaceaen green algae has raised questions about their evolutionary origin and prevalence in the plant lineage. The existence of G-proteins has often been correlated with the acquisition of embryophytic life-cycle and/or terrestrial habitats of plants which occurred around 450 million years ago. Our discovery of functional G-proteins in Chara braunii, a representative of the Charophycean green algae, establishes the existence of this conserved signaling pathway in the most basal plants and dates it even further back to 1-1.5 billion years ago. We have now identified the sequence homologs of G-proteins in additional algal families and propose that green algae represent a model system for one of the most basal forms of G-protein signaling known to exist to date. Given the possible differences that exist between plant and metazoan G-protein signaling mechanisms, such basal organisms will serve as important resources to trace the evolutionary origin of proposed mechanistic differences between the systems as well as their plant-specific functions.

Canalization: what the flux?

Polarized transport of the hormone auxin plays crucial roles in many processes in plant development. A self-organizing pattern of auxin transport--canalization--is thought to be responsible for vascular patterning and shoot branching regulation in flowering plants. Mathematical modeling has demonstrated that membrane localization of PIN-FORMED (PIN)-family auxin efflux carriers in proportion to net auxin flux can plausibly explain canalization and possibly other auxin transport phenomena. Other plausible models have also been proposed, and there has recently been much interest in producing a unified model of all auxin transport phenomena. However, it is our opinion that lacunae in our understanding of auxin transport biology are now limiting progress in developing the next generation of models. Here we examine several key areas where significant experimental advances are necessary to address both biological and theoretical aspects of auxin transport, including the possibility of a unified transport model.

Biophysical and size-dependent perspectives on plant evolution

Physical laws and processes have profoundly influenced plant evolution. Their effects are invariably size dependent and thus subject to scaling as well as biophysical analyses even though these effects differ depending upon the fluid (water or air) in which plants evolve. Although organisms cannot obviate the effects of physical laws and processes, the consequences of these effects can be altered by ontogenetic or phylogenetic alterations in geometry, shape, or orientation as well as in body size. These assertions are examined using theoretical insights and empirical data drawn from extant and fossil plants pertinent to four evolutionary transitions: (1) the evolution of multicellularity, (2) the transition from an aquatic to an aerial habitat, (3) the evolution of vascular tissues, and (4) the evolution of secondary growth by the independent acquisition of cambia. This examination shows how physical laws limit phenotypic expression, but how they also simultaneously provide alternative, potentially adaptive possibilities.

The origins of multicellular organisms

Multicellularity has evolved in several eukaryotic lineages leading to plants, fungi, and animals. Theoretically, in each case, this involved (1) cell-to-cell adhesion with an alignment-of-fitness among cells, (2) cell-to-cell communication, cooperation, and specialization with an export-of-fitness to a multicellular organism, and (3) in some cases, a transition from "simple" to "complex" multicellularity. When mapped onto a matrix of morphologies based on developmental and physical rules for plants, these three phases help to identify a "unicellular ⇒ colonial ⇒ filamentous (unbranched ⇒ branched) ⇒ pseudoparenchymatous ⇒ parenchymatous" morphological transformation series that is consistent with trends observed within each of the three major plant clades. In contrast, a more direct "unicellular ⇒ colonial or siphonous ⇒ parenchymatous" series is observed in fungal and animal lineages. In these contexts, we discuss the roles played by the cooptation, expansion, and subsequent diversification of ancestral genomic toolkits and patterning modules during the evolution of multicellularity. We conclude that the extent to which multicellularity is achieved using the same toolkits and modules (and thus the extent to which multicellularity is homologous among different organisms) differs among clades and even among some closely related lineages.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

Conserved transport mechanisms but distinct auxin responses govern shoot patterning in Selaginella kraussiana

To provide a comparative framework to understand the evolution of auxin regulation in vascular plants, the effect of perturbed auxin homeostasis was examined in the lycophyte Selaginella kraussiana. Polar auxin transport was measured by tracing tritiated IAA in excised shoots. Shoots were cultured in the presence of auxin efflux inhibitors and exogenous auxin, and developmental abnormalities were documented. Auxin transport in Selaginella shoots is exclusively basipetal, as in angiosperms. Perturbed auxin transport results in the loss of meristem maintenance and abnormal shoot architecture. Dichotomous root branching in Selaginella appears to be regulated by an antagonistic relationship between auxin and cytokinin. The results suggest that basipetal polar auxin transport occurred in the common ancestor of lycophytes and euphyllophytes. Although the mechanisms of auxin transport appear to be conserved across all vascular plants, distinct auxin responses govern shoot growth and development in lycophytes and euphyllophytes.

Polar Expansion Dynamics in the Plant Kingdom: A Diverse and Multifunctional Journey on the Path to Pollen Tubes

Polar expansion is a widespread phenomenon in plants spanning all taxonomic groups from the Charophycean Green Algae to pollen tubes in Angiosperms and Gymnosperms. Current data strongly suggests that many common features are shared amongst cells displaying polar growth mechanics including changes to the structural features of localized regions of the cell wall, mobilization of targeted secretion mechanisms, employment of the actin cytoskeleton for directing secretion and in many cases, endocytosis and coordinated interaction of multiple signal transduction mechanisms prompted by external biotic and abiotic cues. The products of polar expansion perform diverse functions including delivery of male gametes to the egg, absorption, anchorage, adhesion and photo-absorption efficacy. A comparative analysis of polar expansion dynamics is provided with special emphasis on those found in early divergent plants.

AUCSIA: an ancestral green plant miniprotein and the emergence of auxin transport

Aucsia is a green plant gene family. In Angiosperms, Aucsia genes control several aspects of auxin biology, including polar auxin transport. AUCSIA miniproteins are produced via splicing of three exons. The first two exons span the conserved AUCSIA motif, while the third exon(s) encodes the more variable carboxyterminal end. AUCSIA presence in green algae indicates that the Aucsia gene family predated the emergence of land plants and the complex auxin biology of Angiosperms. In algae, however, AUCSIA might have been involved in a primitive auxin biology, when auxin was just a simple metabolite, probably noxious at high concentrations, and consequently pump out via the ancestral auxin exporters, i.e., ABCB1/19 homologs. This speculative scenario implies that in green algae AUCSIA is involved in controlling the ABCB-dependent efflux of noxious metabolites, including auxin. Such speculative hypothesis might be tested in living green algae.

Polar auxin transport in relation to long-distance transport of nutrients in the Charales

This paper examines the significance of the recent demonstration of polar auxin transport (PAT) in the green macroalga Chara (Charophyceae: Charales) and, especially, options for explaining some features of PAT in the Charales. The occurrence of PAT in the Charales shows that PAT originated in the algal ancestors of the embryophytes (liverworts, mosses, hornworts, and vascular plants), although it is not yet known if PAT occurs elsewhere in the Charophyceae or in other algae. While in the embryophytes PAT occurs in parenchymatously constructed structures which commonly also have xylem and phloem (or their bryophyte analogues) as long-distance transport processes in parallel to PAT, in Chara corallina PAT shares the pathway for long-distance transport of nutrients though the parenchymatously constructed nodal complexes and the single giant cells of the internode. The speed of auxin movement of PAT is much more rapid than that attributable to diffusion and of the same order as the rate of cytoplasmic streaming in the giant internodal cells, yet complete inhibition of streaming by the inhibitor cytochalasin H does not slow down auxin transport. Explanations for this phenomenon are sought in the operation of other mechanochemical motors, dynein-tubulin and kinesin-tubulin, as alternatives to the myosin-actin system which powers cytoplasmic streaming. Experiments in which microtubules are disrupted, for example by colchicine, could show if one of the tubulin-based motors is involved. If these motors are involved, some mechanism is needed to amplify the speeds known for the motors to explain the order of magnitude higher speeds seen for auxin transport.

Auxology: when auxin meets plant evo-devo

Auxin is implicated throughout plant growth and development. Although the effects of this plant hormone have been recognized for more than a century, it is only in the past two decades that light has been shed on the molecular mechanisms that regulate auxin homeostasis, signaling, transport, crosstalk with other hormonal pathways as well as its roles in plant development. These discoveries established a molecular framework to study the role of auxin in land plant evolution. Here, we review recent advances in auxin biology and their implications in both micro- and macro-evolution of plant morphology. By analogy to the term 'hoxology', which refers to the critical role of HOX genes in metazoan evolution, we propose to introduce the term 'auxology' to take into account the crucial role of auxin in plant evo-devo.