AHP6 inhibits cytokinin signaling to regulate the orientation of pericycle cell division during lateral root initiation

Cytokinin: From autoclaved DNA to two-component signaling

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

Ectopic assembly of an auxin efflux control machinery shifts developmental trajectories

Polar auxin transport in the Arabidopsis (Arabidopsis thaliana) root tip maintains high auxin levels around the stem cell niche that gradually decrease in dividing cells but increase again once they transition toward differentiation. Protophloem differentiates earlier than other proximal tissues and employs a unique auxin "canalization" machinery that is thought to balance auxin efflux with retention. It consists of a proposed activator of PIN-FORMED (PIN) auxin efflux carriers, the cAMP-, cGMP- and Calcium-dependent (AGC) kinase PROTEIN KINASE ASSOCIATED WITH BRX (PAX); its inhibitor, BREVIS RADIX (BRX); and PHOSPHATIDYLINOSITOL-4-PHOSPHATE-5-KINASE (PIP5K) enzymes, which promote polar PAX and BRX localization. Because of a dynamic PAX-BRX-PIP5K interplay, the net cellular output of this machinery remains unclear. In this study, we deciphered the dosage-sensitive regulatory interactions among PAX, BRX, and PIP5K by their ectopic expression in developing xylem vessels. The data suggest that the dominant collective output of the PAX-BRX-PIP5K module is a localized reduction in PIN abundance. This requires PAX-stimulated clathrin-mediated PIN endocytosis upon site-specific phosphorylation, which distinguishes PAX from other AGC kinases. An ectopic assembly of the PAX-BRX-PIP5K module is sufficient to cause cellular auxin retention and affects root growth vigor by accelerating the trajectory of xylem vessel development. Our data thus provide direct evidence that local manipulation of auxin efflux alters the timing of cellular differentiation in the root.

Autonomous differentiation of transgenic cells requiring no external hormone application: the endogenous gene expression and phytohormone behaviors

The ectopic overexpression of developmental regulator (DR) genes has been reported to improve the transformation in recalcitrant plant species because of the promotion of cellular differentiation during cell culture processes. In other words, the external plant growth regulator (PGR) application during the tissue and cell culture process is still required in cases utilizing DR genes for plant regeneration. Here, the effect of Arabidopsis BABY BOOM (BBM) and WUSCHEL (WUS) on the differentiation of tobacco transgenic cells was examined. We found that the SRDX fusion to WUS, when co-expressed with the BBM-VP16 fusion gene, significantly influenced the induction of autonomous differentiation under PGR-free culture conditions, with similar effects in some other plant species. Furthermore, to understand the endogenous background underlying cell differentiation toward regeneration, phytohormone and RNA-seq analyses were performed using tobacco leaf explants in which transgenic cells were autonomously differentiating. The levels of active auxins, cytokinins, abscisic acid, and inactive gibberellins increased as cell differentiation proceeded toward organogenesis. Gene Ontology terms related to phytohormones and organogenesis were identified as differentially expressed genes, in addition to those related to polysaccharide and nitrate metabolism. The qRT-PCR four selected genes as DEGs supported the RNA-seq data. This differentiation induction system and the reported phytohormone and transcript profiles provide a foundation for the development of PGR-free tissue cultures of various plant species, facilitating future biotechnological breeding.

The hidden harmony: Exploring ROS-phytohormone nexus for shaping plant root architecture in response to environmental cues

Root system architecture, encompassing lateral roots and root hairs, plays a vital in overall plant growth and stress tolerance. Reactive oxygen species (ROS) and plant hormones intricately regulate root growth and development, serving as signaling molecules that govern processes such as cell proliferation and differentiation. Manipulating the interplay between ROS and hormones has the potential to enhance nutrient absorption, stress tolerance, and agricultural productivity. In this review, we delve into how studying these processes provides insights into how plants respond to environmental changes and optimize growth patterns to better control cellular processes and stress responses in crops. We discuss various factors and complex signaling networks that may exist among ROS and phytohormones during root development. Additionally, the review highlights possible role of reactive nitrogen species (RNS) in ROS-phytohormone interactions and in shaping root system architecture according to environmental cues.

Divergent regulation of auxin responsive genes in root-knot and cyst nematodes feeding sites formed in Arabidopsis

Cysts (CNs) and root-knot nematodes (RKNs) induce specialized feeding cells, syncytia, and giant cells (GCs), respectively, within plant roots. The plant tissues around the GCs usually by respond forming a root swelling called a gall that contains the GCs. The ontogenesis of feeding cells is different. GC formation is a process of new organogenesis from vascular cells, which are still not well characterized, that differentiate into GCs. In contrast, syncytia formation involves the fusion of adjacent cells that have already differentiated. Nonetheless, both feeding sites show an auxin maximum pertinent to feeding site formation. However, data on the molecular divergences and similarities between the formation of both feeding sites regarding auxin-responsive genes are still scarce. We studied genes from the auxin transduction pathways that are crucial during gall and lateral root (LR) development in the CN interaction by using promoter-reporter (GUS/LUC)transgenic lines, as well as loss of function lines of Arabidopsis. The promoters pGATA23 and several deletions of pmiR390a were active in syncytia, as were in galls, but pAHP6 or putative up-stream regulators as ARF5/7/19 were not active in syncytia. Additionally, none of these genes seemed to play a key role during cyst nematode establishment in Arabidopsis, as the infection rates in loss of function lines did not show significant differences compared to control Col-0 plants. Furthermore, the presence of only canonical AuxRe elements in their proximal promoter regions is highly correlated with their activation in galls/GCs (AHP6, LBD16), but those promoters active in syncytia (miR390, GATA23) carry AuxRe overlapping core cis-elements for other transcription factor families (i.e., bHLH, bZIP). Strikingly, in silico transcriptomic analysis showed very few genes upregulated by auxins common to those induced in GCs and syncytia, despite the high number of upregulated IAA responsive genes in syncytia and galls. The complex regulation of auxin transduction pathways, where different members of the auxin response factor (ARF) family may interact with other factors, and the differences in auxin sensitivity, as indicated by the lower induction of the DR5 sensor in syncytia than galls, among other factors, may explain the divergent regulation of auxin responsive genes in the two types of nematode feeding sites.

Insights on Phytohormonal Crosstalk in Plant Response to Nitrogen Stress: A Focus on Plant Root Growth and Development

Nitrogen (N) is a vital mineral component that can restrict the growth and development of plants if supplied inappropriately. In order to benefit their growth and development, plants have complex physiological and structural responses to changes in their nitrogen supply. As higher plants have multiple organs with varying functions and nutritional requirements, they coordinate their responses at the whole-plant level based on local and long-distance signaling pathways. It has been suggested that phytohormones are signaling substances in such pathways. The nitrogen signaling pathway is closely associated with phytohormones such as auxin (AUX), abscisic acid (ABA), cytokinins (CKs), ethylene (ETH), brassinosteroid (BR), strigolactones (SLs), jasmonic acid (JA), and salicylic acid (SA). Recent research has shed light on how nitrogen and phytohormones interact to modulate physiology and morphology. This review provides a summary of the research on how phytohormone signaling affects root system architecture (RSA) in response to nitrogen availability. Overall, this review contributes to identifying recent developments in the interaction between phytohormones and N, as well as serving as a foundation for further study.

Auxin-Cytokinin Balance Shapes Maize Root Architecture by Controlling Primary Root Elongation and Lateral Root Development

The root system is responsible for water and nutrients uptake from the soil, and therefore, its extension is basic for an efficient acquisition. The maize root system is formed by different types of roots, and the lateral root branching substantially increases the surface for nutrient uptake. Therefore, the regulation of lateral root formation is fundamental in the development of root functions. Root architecture is basically controlled by auxin and cytokinins, which antagonize in the formation of lateral roots (LR) along the primary root axis, with auxin, a stimulator, and cytokinins inhibitors of LR development. This interaction has been analyzed in several zones along the primary root where LRs in different developmental stages were located. The root has been divided into several zones, such as meristem, elongation zone, and mature zone, according to the developmental processes occurring in each one. As Arabidopsis root elongated more slowly than maize root, these zones are shorter, and its delimitation is more difficult. However, these zones have previously been delimitated clearly in maize, and therefore, they analyze the effect of exogenous hormones in several LR developmental stages. The inhibitory effect of cytokinin on lateral root formation was observed in already elongated primary root zones in which initial events to form new lateral roots are taking place. Contrarily, auxin increased LR formation in the primary root segments elongated in the presence of the hormone. The inhibitory effect of cytokinin was reversed by auxin in a concentration-dependent manner when both hormones were combined. However, auxin is unable to recover LR development in primary root zones that have been previously elongated only in the presence of cytokinin. This antagonistic auxin-cytokinin effect on LR development depended on the balance between both hormones, which controls the root system architecture and determines the formation of LR during the process of initiation.

Auxin Interactions with Other Hormones in Plant Development

Auxin is a crucial growth regulator that governs plant development and responses to environmental perturbations. It functions at the heart of many developmental processes, from embryogenesis to organ senescence, and is key to plant interactions with the environment, including responses to biotic and abiotic stimuli. As remarkable as auxin is, it does not act alone, but rather solicits the help of, or is solicited by, other endogenous signals, including the plant hormones abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellic acid, jasmonates, salicylic acid, and strigolactones. The interactions between auxin and other hormones occur at multiple levels: hormones regulate one another's synthesis, transport, and/or response; hormone-specific transcriptional regulators for different pathways physically interact and/or converge on common target genes; etc. However, our understanding of this crosstalk is still fragmentary, with only a few pieces of the gigantic puzzle firmly established. In this review, we provide a glimpse into the complexity of hormone interactions that involve auxin, underscoring how patchy our current understanding is.

Function of the pseudo phosphotransfer proteins has diverged between rice and Arabidopsis

The phytohormone cytokinin plays a significant role in nearly all aspects of plant growth and development. Cytokinin signaling has primarily been studied in the dicot model Arabidopsis, with relatively little work done in monocots, which include rice (Oryza sativa) and other cereals of agronomic importance. The cytokinin signaling pathway is a phosphorelay comprised of the histidine kinase receptors, the authentic histidine phosphotransfer proteins (AHPs) and type-B response regulators (RRs). Two negative regulators of cytokinin signaling have been identified: the type-A RRs, which are cytokinin primary response genes, and the pseudo histidine phosphotransfer proteins (PHPs), which lack the His residue required for phosphorelay. Here, we describe the role of the rice PHP genes. Phylogenic analysis indicates that the PHPs are generally first found in the genomes of gymnosperms and that they arose independently in monocots and dicots. Consistent with this, the three rice PHPs fail to complement an Arabidopsis php mutant (aphp1/ahp6). Disruption of the three rice PHPs results in a molecular phenotype consistent with these elements acting as negative regulators of cytokinin signaling, including the induction of a number of type-A RR and cytokinin oxidase genes. The triple php mutant affects multiple aspects of rice growth and development, including shoot morphology, panicle architecture, and seed fill. In contrast to Arabidopsis, disruption of the rice PHPs does not affect root vascular patterning, suggesting that while many aspects of key signaling networks are conserved between monocots and dicots, the roles of at least some cytokinin signaling elements are distinct.

Dynamic cytokinin signalling landscapes during lateral root formation in Arabidopsis

By forming lateral roots, plants expand their root systems to improve anchorage and absorb more water and nutrients from the soil. Each phase of this developmental process in Arabidopsis is tightly regulated by dynamic and continuous signalling of the phytohormones cytokinin and auxin. While the roles of auxin in lateral root organogenesis and spatial accommodation by overlying cell layers have been well studied, insights on the importance of cytokinin is still somewhat limited. Cytokinin is a negative regulator of lateral root formation with versatile modes of action being activated at different root developmental zones. Here, we review the latest progress made towards our understanding of these spatially separated mechanisms of cytokinin-mediated signalling that shape lateral root initiation, outgrowth and emergence and highlight some of the enticing open questions.

Connections in the cambium, receptors in the ring

In plants, pluripotent cells in meristems divide to provide cells for the formation of postembryonic tissues. The cambium is the meristem from which the vascular tissue is derived and is the main driver for secondary (radial) growth in dicots. Xylem and phloem are specified on opposing sides of the cambium, and tightly regulated cell divisions ensure their spatial separation. Peptide ligands, phytohormones, and their receptors are central to maintaining this patterning and regulating proliferation. Here, we describe recent advances in our understanding of how these signals are integrated to control vascular development and secondary growth.

The Interaction Network and Signaling Specificity of Two-Component System in Arabidopsis

Two-component systems (TCS) in plants have evolved into a more complicated multi-step phosphorelay (MSP) pathway, which employs histidine kinases (HKs), histidine-containing phosphotransfer proteins (HPts), and response regulators (RRs) to regulate various aspects of plant growth and development. How plants perceive the external signals, then integrate and transduce the secondary signals specifically to the desired destination, is a fundamental characteristic of the MSP signaling network. The TCS elements involved in the MSP pathway and molecular mechanisms of signal transduction have been best understood in the model plant Arabidopsis thaliana. In this review, we focus on updated knowledge on TCS signal transduction in Arabidopsis. We first present a brief description of the TCS elements; then, the protein–protein interaction network is established. Finally, we discuss the possible molecular mechanisms involved in the specificity of the MSP signaling at the mRNA and protein levels.

Root-knot nematodes induce gall formation by recruiting developmental pathways of post-embryonic organogenesis and regeneration to promote transient pluripotency

Root-knot nematodes (RKNs; Meloidogyne spp.) induce new post-embryogenic organs within the roots (galls) where they stablish and differentiate nematode feeding cells, giant cells (GCs). The developmental programmes and functional genes involved remain poorly defined. Arabidopsis root apical meristem (RAM), lateral root (LR) and callus marker lines, SHORT-ROOT/SHR, SCARECROW/SCR, SCHIZORIZA/SCZ, WUSCHEL-RELATED-HOMEOBOX-5/WOX5, AUXIN-RESPONSIVE-FACTOR-5/ARF5, ARABIDOPSIS-HISTIDINE PHOSPHOTRANSFER-PROTEIN-6/AHP6, GATA-TRANSCRIPTION FACTOR-23/GATA23 and S-PHASE-KINASE-ASSOCIATED-PROTEIN2B/SKP2B, were analysed for nematode-dependent expression. Their corresponding loss-of-function lines, including those for LR upstream regulators, SOLITARY ROOT/SLR/IAA14, BONDELOS/BDL/IAA12 and INDOLE-3-ACETIC-ACID-INDUCIBLE-28/IAA28, were tested for RKN resistance/tolerance. LR genes, for example ARF5 (key factor for root stem-cell niche regeneration), GATA23 (which specifies pluripotent founder cells) and AHP6 (cytokinin-signalling-inhibitor regulating pericycle cell-divisions orientation), show a crucial function during gall formation. RKNs do not compromise the number of founder cells or LR primordia but locally induce gall formation possibly by tuning the auxin/cytokinin balance in which AHP6 might be necessary. Key RAM marker genes were induced and functional in galls. Therefore, the activation of plant developmental programmes promoting transient-pluripotency/stemness leads to the generation of quiescent-centre and meristematic-like cell identities within the vascular cylinder of galls. Nematodes enlist developmental pathways of new organogenesis and/or root regeneration in the vascular cells of galls. This should determine meristematic cell identities with sufficient transient pluripotency for gall organogenesis.

Hormonal regulation of root hair growth and responses to the environment in Arabidopsis

The main functions of plant roots are water and nutrient uptake, soil anchorage, and interaction with soil-living biota. Root hairs, single cell tubular extensions of root epidermal cells, facilitate or enhance these functions by drastically enlarging the absorptive surface. Root hair development is constantly adapted to changes in the root's surroundings, allowing for optimization of root functionality in heterogeneous soil environments. The underlying molecular pathway is the result of a complex interplay between position-dependent signalling and feedback loops. Phytohormone signalling interconnects this root hair signalling cascade with biotic and abiotic changes in the rhizosphere, enabling dynamic hormone-driven changes in root hair growth, density, length, and morphology. This review critically discusses the influence of the major plant hormones on root hair development, and how changes in rhizosphere properties impact on the latter.

Role and Regulation of Cytokinins in Plant Response to Drought Stress

Cytokinins (CKs) are key phytohormones that not only regulate plant growth and development but also mediate plant tolerance to drought stress. Recent advances in genome-wide association studies coupled with in planta characterization have opened new avenues to investigate the drought-responsive expression of CK metabolic and signaling genes, as well as their functions in plant adaptation to drought. Under water deficit, CK signaling has evolved as an inter-cellular communication network which is essential to crosstalk with other types of phytohormones and their regulating pathways in mediating plant stress response. In this review, we revise the current understanding of CK involvement in drought stress tolerance. Particularly, a genetic framework for CK signaling and CK crosstalk with abscisic acid (ABA) in the precise monitoring of drought responses is proposed. In addition, the potential of endogenous CK alteration in crops towards developing drought-tolerant crops is also discussed.

Spatiotemporal Pattern of Ectopic Cell Divisions Contribute to Mis-Shaped Phenotype of Primary and Lateral Roots of katanin1 Mutant

Pattern formation, cell proliferation, and directional cell growth, are driving factors of plant organ shape, size, and overall vegetative development. The establishment of vegetative morphogenesis strongly depends on spatiotemporal control and synchronization of formative and proliferative cell division patterns. In this context, the progression of cell division and the regulation of cell division plane orientation are defined by molecular mechanisms converging to the proper positioning and temporal reorganization of microtubule arrays such as the preprophase microtubule band, the mitotic spindle and the cytokinetic phragmoplast. By focusing on the tractable example of primary root development and lateral root emergence in Arabidopsis thaliana, genetic studies have highlighted the importance of mechanisms underlying microtubule reorganization in the establishment of the root system. In this regard, severe alterations of root growth, and development found in extensively studied katanin1 mutants of A. thaliana (fra2, lue1, and ktn1-2), were previously attributed to defective rearrangements of cortical microtubules and aberrant cell division plane reorientation. How KATANIN1-mediated microtubule severing contributes to tissue patterning and organ morphogenesis, ultimately leading to anisotropy in microtubule organization is a trending topic under vigorous investigation. Here we addressed this issue during root development, using advanced light-sheet fluorescence microscopy (LSFM) and long-term imaging of ktn1-2 mutant expressing the GFP-TUA6 microtubule marker. This method allowed spatial and temporal monitoring of cell division patterns in growing roots. Analysis of acquired multidimensional data sets revealed the occurrence of ectopic cell divisions in various tissues including the calyptrogen and the protoxylem of the main root, as well as in lateral root primordia. Notably the ktn1-2 mutant exhibited excessive longitudinal cell divisions (parallel to the root axis) at ectopic positions. This suggested that changes in the cell division pattern and the occurrence of ectopic cell divisions contributed significantly to pleiotropic root phenotypes of ktn1-2 mutant. LSFM provided evidence that KATANIN1 is required for the spatiotemporal control of cell divisions and establishment of tissue patterns in living A. thaliana roots.

TRANSPORTER OF IBA1 Links Auxin and Cytokinin to Influence Root Architecture

Developmental processes that control root system architecture are critical for soil exploration by plants, allowing for uptake of water and nutrients. Conversion of the auxin precursor indole-3-butyric acid (IBA) to active auxin (indole-3-acetic acid; IAA) modulates lateral root formation. However, mechanisms governing IBA-to-IAA conversion have yet to be elucidated. We identified TRANSPORTER OF IBA1 (TOB1) as a vacuolar IBA transporter that limits lateral root formation. Moreover, TOB1, which is transcriptionally regulated by the phytohormone cytokinin, is necessary for the ability of cytokinin to exert inhibitory effects on lateral root production. The increased production of lateral roots in tob1 mutants, TOB1 transport of IBA into the vacuole, and cytokinin-regulated TOB1 expression provide a mechanism linking cytokinin signaling and IBA contribution to the auxin pool to tune root system architecture.

The Systems Biology of Lateral Root Formation: Connecting the Dots

The root system is a major determinant of a plant's access to water and nutrients. The architecture of the root system to a large extent depends on the repeated formation of new lateral roots. In this review, we discuss lateral root development from a systems biology perspective. We focus on studies combining experiments with computational modeling that have advanced our understanding of how the auxin-centered regulatory modules involved in different stages of lateral root development exert their specific functions. Moreover, we discuss how these regulatory networks may enable robust transitions from one developmental stage to the next, a subject that thus far has received limited attention. In addition, we analyze how environmental factors impinge on these modules, and the different manners in which these environmental signals are being integrated to enable coordinated developmental decision making. Finally, we provide some suggestions for extending current models of lateral root development to incorporate multiple processes and stages. Only through more comprehensive models we can fully elucidate the cooperative effects of multiple processes on later root formation, and how one stage drives the transition to the next.

A Genome-Wide Association Study Identifies New Loci Involved in Wound-Induced Lateral Root Formation in Arabidopsis thaliana

Root systems can display variable architectures that contribute to nutrient foraging or to increase the tolerance of abiotic stress conditions. Root tip excision promotes the developmental progression of previously specified lateral root (LR) founder cells, which allows to easily measuring the branching capacity of a given root as regards its genotype and/or growth conditions. Here, we describe the natural variation among 120 Arabidopsis thaliana accessions in root system architecture (RSA) after root tip excision. Wound-induced changes in RSA were associated with 19 genomic loci using genome-wide association mapping. Three candidate loci associated with wound-induced LR formation were investigated. Sequence variation in the hypothetical protein encoded by the At4g01090 gene affected wound-induced LR development and its loss-of-function mutants displayed a reduced number of LRs after root tip excision. Changes in a histidine phosphotransfer protein putatively involved in cytokinin signaling were significantly associated with LR number variation after root tip excision. Our results provide a better understanding of some of the genetic components involved in LR capacity variation among accessions.

Antagonistic activity of auxin and cytokinin in shoot and root organs

The hormones auxin and cytokinin are essential for plant growth and development. Because of the central importance of root and shoot apical meristems in plant growth, auxin/cytokinin interactions have been predominantly analyzed in relation to apical meristem formation and function. In contrast, the auxin/cytokinin interactions during organ growth have remained largely unexplored. Here, we show that a specific interaction between auxin and cytokinin operates in both the root and the shoot where it serves as an additional determinant of plant development. We found that auxin at low concentrations limits the action of cytokinin. An increase in cytokinin level counteracts this inhibitory effect and leads to an inhibition of auxin signaling. At higher concentrations of both hormones, these antagonistic interactions between cytokinin and auxin are absent. Thus, our results reveal a bidirectional and asymmetrical interaction of auxin and cytokinin beyond the bounds of apical meristems. The relation is bidirectional in that both hormones exert inhibitory effects on each other's signaling mechanisms. However, this relation is also asymmetrical because under controlled growth conditions, auxin present in nontreated plants suppresses cytokinin signaling, whereas the reverse is not the case.

Cytokinin - A Developing Story

In the past decade tremendous advances have been made in understanding the biosynthesis, perception, and signaling pathways of the plant hormone cytokinin. It also became clear that interfering with any of these steps greatly impacts all on stages of growth and development. This has recently spurted renewed effort to understand how cytokinin signaling affects developmental processes. As a result, new insights on the role of cytokinin signaling and the downstream targets during, for example, shoot apical meristem, flower, female gametophyte, stomata and vascular development are being unraveled. In this review we aim to give a comprehensive overview of recent findings on how cytokinin influences growth and development in plants, and highlight areas for future research.

Interplay of Auxin and Cytokinin in Lateral Root Development

The spacing and distribution of lateral roots are critical determinants of plant root system architecture. In addition to providing anchorage, lateral roots explore the soil to acquire water and nutrients. Over the past several decades, we have deepened our understanding of the regulatory mechanisms governing lateral root formation and development. In this review, we summarize these recent advances and provide an overview of how auxin and cytokinin coordinate the regulation of lateral root formation and development.

Spatial regularity control of phyllotaxis pattern generated by the mutual interaction between auxin and PIN1

Phyllotaxis, the arrangement of leaves on a plant stem, is well known because of its beautiful geometric configuration, which is derived from the constant spacing between leaf primordia. This phyllotaxis is established by mutual interaction between a diffusible plant hormone auxin and its efflux carrier PIN1, which cooperatively generate a regular pattern of auxin maxima, small regions with high auxin concentrations, leading to leaf primordia. However, the molecular mechanism of the regular pattern of auxin maxima is still largely unknown. To better understand how the phyllotaxis pattern is controlled, we investigated mathematical models based on the auxin-PIN1 interaction through linear stability analysis and numerical simulations, focusing on the spatial regularity control of auxin maxima. As in previous reports, we first confirmed that this spatial regularity can be reproduced by a highly simplified and abstract model. However, this model lacks the extracellular region and is not appropriate for considering the molecular mechanism. Thus, we investigated how auxin maxima patterns are affected under more realistic conditions. We found that the spatial regularity is eliminated by introducing the extracellular region, even in the presence of direct diffusion between cells or between extracellular spaces, and this strongly suggests the existence of an unknown molecular mechanism. To unravel this mechanism, we assumed a diffusible molecule to verify various feedback interactions with auxin-PIN1 dynamics. We revealed that regular patterns can be restored by a diffusible molecule that mediates the signaling from auxin to PIN1 polarization. Furthermore, as in the one-dimensional case, similar results are observed in the two-dimensional space. These results provide a great insight into the theoretical and molecular basis for understanding the phyllotaxis pattern. Our theoretical analysis strongly predicts a diffusible molecule that is pivotal for the phyllotaxis pattern but is yet to be determined experimentally.

Cytokinin signaling in plant development

The phytohormone cytokinin plays diverse roles in plant development, influencing many agriculturally important processes, including growth, nutrient responses and the response to biotic and abiotic stresses. Cytokinin levels in plants are regulated by biosynthesis and inactivation pathways. Cytokinins are perceived by membrane-localized histidine-kinase receptors and are transduced through a His-Asp phosphorelay to activate a family of transcription factors in the nucleus. Here, and in the accompanying poster, we summarize the current understanding of cytokinin metabolism, transport and signaling, and discuss how this phytohormone regulates changes in gene expression to mediate its pleiotropic effects.

Root Development in Medicago truncatula: Lessons from Genetics to Functional Genomics

This decade introduced "omics" approaches, such as genomics, transcriptomics, proteomics, and metabolomics in association with reverse and forward genetic approaches, developed earlier, to try to identify molecular pathways involved in the development or in the response to environmental conditions as well as in animals and plants. This review summarizes studies that utilized "omics" strategies to unravel the root development in the model legume Medicago truncatula and how external factors such as soil mineral status or the presence of bacteria and fungi affect root system architecture in this species. We also compare these "omics" data to the knowledges concerning the Arabidopsis thaliana root development, nowadays considered as the model of allorhiz root systems. However, unlike legumes, this species is unable to interact with soil nitrogen-fixing rhizobia and arbuscular-mycorrhizal (AM) fungi to develop novel root-derived symbiotic structures. Differences in root organization, development, and regulatory pathways between these two model species have been highlighted.

Plants under Stress: Involvement of Auxin and Cytokinin

Plant growth and development are critically influenced by unpredictable abiotic factors. To survive fluctuating changes in their environments, plants have had to develop robust adaptive mechanisms. The dynamic and complementary actions of the auxin and cytokinin pathways regulate a plethora of developmental processes, and their ability to crosstalk makes them ideal candidates for mediating stress-adaptation responses. Other crucial signaling molecules responsible for the tremendous plasticity observed in plant morphology and in response to abiotic stress are reactive oxygen species (ROS). Proper temporal and spatial distribution of ROS and hormone gradients is crucial for plant survival in response to unfavorable environments. In this regard, the convergence of ROS with phytohormone pathways acts as an integrator of external and developmental signals into systemic responses organized to adapt plants to their environments. Auxin and cytokinin signaling pathways have been studied extensively. Nevertheless, we do not yet understand the impact on plant stress tolerance of the sophisticated crosstalk between the two hormones. Here, we review current knowledge on the function of auxin and cytokinin in redirecting growth induced by abiotic stress in order to deduce their potential points of crosstalk.

A Standardized Method to Assess Infection Rates of Root-Knot and Cyst Nematodes in Arabidopsis thaliana Mutants with Alterations in Root Development Related to Auxin and Cytokinin Signaling

Plant parasitic nematodes cause a great impact in agricultural systems. The search for effective control methods is partly based on the understanding of underlying molecular mechanisms leading to the formation of nematode feeding sites. In this respect, crosstalk of hormones such as auxins and cytokinins (IAA, CK) between the plant and the nematode seems to be crucial. Thence, the study of loss of function or overexpressing lines with altered IAA and CK functioning is entailed. Those lines frequently show developmental defects in the number, position and/or length of the lateral roots what could generate a bias in the interpretation of the nematode infection parameters. Here we present a protocol to assess differences in nematode infectivity with the lowest interference of root architecture phenotypes in the results. Thus, tailored growth conditions and normalization parameters facilitate the standardized phenotyping of nematode infection.

As above, so below: Auxin's role in lateral organ development

Organogenesis requires the coordination of many highly-regulated developmental processes, including cell fate determination, cell division and growth, and cell-cell communication. For tissue- and organ-scale coordination, a network of regulators enables molecular events in individual cells to translate into multicellular changes in structure and functional capacity. One recurrent theme in plant developmental networks is a central role for plant hormones, especially auxin. Here, we focus first on describing recent advances in understanding lateral root development, one of the best-studied examples of auxin-mediated organogenesis. We then use this framework to examine the parallel process of emergence of lateral organs in the shoot-a process called phyllotaxy. This comparison reveals a high degree of conservation, highlighting auxin's pivotal role determining overall plant architecture.

Cytokinin Synthesis, Signaling, and Function--Advances and New Insights

The plant hormones referred to as cytokinins are chemical signals that control numerous developmental processes throughout the plant life cycle, including gametogenesis, root meristem specification, vascular development, shoot and root growth, meristem homeostasis, senescence, and more. In addition, they mediate responses to environmental cues such as light, stress, and nutrient conditions. The core mechanistics of cytokinin metabolism and signaling have been elucidated, but more layers of regulation, additional functions, and interactions with other signals are continuously discovered and described. In this chapter, we recapitulate the highlights of over 100 years of cytokinin research covering its isolation, the elucidation of phosphorelay signaling, and how cytokinin functions in various developmental contexts including its interaction with other pathways. Additionally, given cytokinin's paracrine signaling mechanism, we postulate that cellular exporters for cytokinins exist.

Use of a Probabilistic Motif Search to Identify Histidine Phosphotransfer Domain-Containing Proteins

The wealth of newly obtained proteomic information affords researchers the possibility of searching for proteins of a given structure or function. Here we describe a general method for the detection of a protein domain of interest in any species for which a complete proteome exists. In particular, we apply this approach to identify histidine phosphotransfer (HPt) domain-containing proteins across a range of eukaryotic species. From the sequences of known HPt domains, we created an amino acid occurrence matrix which we then used to define a conserved, probabilistic motif. Examination of various organisms either known to contain (plant and fungal species) or believed to lack (mammals) HPt domains established criteria by which new HPt candidates were identified and ranked. Search results using a probabilistic motif matrix compare favorably with data to be found in several commonly used protein structure/function databases: our method identified all known HPt proteins in the Arabidopsis thaliana proteome, confirmed the absence of such motifs in mice and humans, and suggests new candidate HPts in several organisms. Moreover, probabilistic motif searching can be applied more generally, in a manner both readily customized and computationally compact, to other protein domains; this utility is demonstrated by our identification of histones in a range of eukaryotic organisms.

Strigolactones spatially influence lateral root development through the cytokinin signaling network

Strigolactones are important rhizosphere signals that act as phytohormones and have multiple functions, including modulation of lateral root (LR) development. Here, we show that treatment with the strigolactone analog GR24 did not affect LR initiation, but negatively influenced LR priming and emergence, the latter especially near the root-shoot junction. The cytokinin module ARABIDOPSIS HISTIDINE KINASE3 (AHK3)/ARABIDOPSIS RESPONSE REGULATOR1 (ARR1)/ARR12 was found to interact with the GR24-dependent reduction in LR development, because mutants in this pathway rendered LR development insensitive to GR24. Additionally, pharmacological analyses, mutant analyses, and gene expression analyses indicated that the affected polar auxin transport stream in mutants of the AHK3/ARR1/ARR12 module could be the underlying cause. Altogether, the data reveal that the GR24 effect on LR development depends on the hormonal landscape that results from the intimate connection with auxins and cytokinins, two main players in LR development.

Cytokinin as a positional cue regulating lateral root spacing in Arabidopsis

The root systems of plants have developed adaptive architectures to exploit soil resources. The formation of lateral roots (LRs) contributes to root system architecture. Roots of plants with a lower cytokinin status form LR primordia (LRP) in unusually close proximity, indicating a role for the hormone in regulating the positioning of LRs along the main root axis. Data obtained from cytokinin-synthesis mutants of Arabidopsis thaliana combined with gene expression analysis indicate that cytokinin synthesis by IPT5 and LOG4 occurring early during LRP initiation generates a local cytokinin signal abbreviating LRP formation in neighbouring pericycle cells. In addition, IPT3, IPT5, and IPT7 contribute to cytokinin synthesis in the vicinity of existing LRP, thus suppressing initiation of new LRs. Interestingly, mutation of CYP735A genes required for trans-zeatin biosynthesis caused strong defects in LR positioning, indicating an important role for this cytokinin metabolite in regulating LR spacing. Further it is shown that cytokinin and a known regulator of LR spacing, the receptor-like kinase ARABIDOPSIS CRINKLY4 (ACR4), operate in a non-hierarchical manner but might exert reciprocal control at the transcript level. Taken together, the results suggest that cytokinin acts as a paracrine hormonal signal in regulating root system architecture.

Morphological Characteristics, Anatomical Structure, and Gene Expression: Novel Insights into Cytokinin Accumulation during Carrot Growth and Development

Cytokinins have been implicated in normal plant growth and development. These bioactive molecules are essential for cell production and expansion in higher plants. Carrot is an Apiaceae vegetable with great value and undergoes significant size changes over the process of plant growth. However, cytokinin accumulation and its potential roles in carrot growth have not been elucidated. To address this problem, carrot plants at five stages were collected, and morphological and anatomical characteristics and expression profiles of cytokinin-related genes were determined. During carrot growth and development, cytokinin levels were the highest at the second stage in the roots, whereas relatively stable levels were observed in the petioles and leaves. DcCYP735A2 showed high expression at stage 2 in the roots, which may contribute largely to the higher cytokinin level at this stage. However, expression of most metabolic genes did not follow a pattern similar to that of cytokinin accumulation, indicating that cytokinin biosynthesis was regulated through a complex network. Genes involved in cytokinin signal perception and transduction were also integrated to normal plant growth and development. The results from the present work suggested that cytokinins may regulate plant growth in a stage-dependent manner. Our work would shed novel insights into cytokinin accumulation and its potential roles during carrot growth. Further studies regarding carrot cytokinins may be achieved by modification of the genes involved in cytokinin biosynthesis, inactivation, and perception.

E151 (sym15), a pleiotropic mutant of pea (Pisum sativum L.), displays low nodule number, enhanced mycorrhizae, delayed lateral root emergence, and high root cytokinin levels

In legumes, the formation of rhizobial and mycorrhizal root symbioses is a highly regulated process which requires close communication between plant and microorganism. Plant mutants that have difficulties establishing symbioses are valuable tools for unravelling the mechanisms by which these symbioses are formed and regulated. Here E151, a mutant of Pisum sativum cv. Sparkle, was examined to characterize its root growth and symbiotic defects. The symbioses in terms of colonization intensity, functionality of micro-symbionts, and organ dominance were compared between the mutant and wild type. The endogenous cytokinin (CK) and abscisic acid (ABA) levels and the effect of the exogenous application of these two hormones were determined. E151 was found to be a low and delayed nodulator, exhibiting defects in both the epidermal and cortical programmes though a few mature and functional nodules develop. Mycorrhizal colonization of E151 was intensified, although the fungal functionality was impaired. Furthermore, E151 displayed an altered lateral root (LR) phenotype compared with that of the wild type whereby LR emergence is initially delayed but eventually overcome. No differences in ABA levels were found between the mutant and the wild type, but non-inoculated E151 exhibited significantly high CK levels. It is hypothesized that CK plays an essential role in differentially mediating the entry of the two micro-symbionts into the cortex; whereas it would inhibit the entry of the rhizobia in that tissue, it would promote that of the fungus. E151 is a developmental mutant which may prove to be a useful tool in further understanding the role of hormones in the regulation of beneficial root symbioses.

Cell-Type-Specific Cytokinin Distribution within the Arabidopsis Primary Root Apex

Cytokinins (CKs) play a crucial role in many physiological and developmental processes at the levels of individual plant components (cells, tissues, and organs) and by coordinating activities across these parts. High-resolution measurements of intracellular CKs in different plant tissues can therefore provide insights into their metabolism and mode of action. Here, we applied fluorescence-activated cell sorting of green fluorescent protein (GFP)-marked cell types, combined with solid-phase microextraction and an ultra-high-sensitivity mass spectrometry (MS) method for analysis of CK biosynthesis and homeostasis at cellular resolution. This method was validated by series of control experiments, establishing that protoplast isolation and cell sorting procedures did not greatly alter endogenous CK levels. The MS-based method facilitated the quantification of all the well known CK isoprenoid metabolites in four different transgenic Arabidopsis thaliana lines expressing GFP in specific cell populations within the primary root apex. Our results revealed the presence of a CK gradient within the Arabidopsis root tip, with a concentration maximum in the lateral root cap, columella, columella initials, and quiescent center cells. This distribution, when compared with previously published auxin gradients, implies that the well known antagonistic interactions between the two hormone groups are cell type specific.

Cytokinin-auxin crosstalk in cell type specification

Auxin and cytokinin affect cell fate specification transcriptionally and non-transcriptionally, and their roles have been characterised in several founder cell specification and activation contexts. Similarly to auxin, local cytokinin synthesis and response gradients are instructive, and the roles of ARABIDOPSIS RESPONSE REGULATOR 7/15 (ARR7/15) and the negative cytokinin response regulator ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6, as well as auxin signalling via MONOPTEROS/BODENLOS, are functionally conserved across different developmental processes. Auxin and cytokinin crosstalk is tissue- and context-specific, and may be synergistic in the shoot apical meristem (SAM) but antagonistic in the root. We review recent advances in understanding the interactions between auxin and cytokinin in pivotal developmental processes, and show that feedback complexity and the multistep nature of specification processes argue against a single morphogenetic signal.

The yin-yang of hormones: cytokinin and auxin interactions in plant development

The phytohormones auxin and cytokinin interact to regulate many plant growth and developmental processes. Elements involved in the biosynthesis, inactivation, transport, perception, and signaling of these hormones have been elucidated, revealing the variety of mechanisms by which signal output from these pathways can be regulated. Recent studies shed light on how these hormones interact with each other to promote and maintain plant growth and development. In this review, we focus on the interaction of auxin and cytokinin in several developmental contexts, including its role in regulating apical meristems, the patterning of the root, the development of the gynoecium and female gametophyte, and organogenesis and phyllotaxy in the shoot.

Two rice authentic histidine phosphotransfer proteins, OsAHP1 and OsAHP2, mediate cytokinin signaling and stress responses in rice

Cytokinin plays an important role in plant development and stress tolerance. Studies of Arabidopsis (Arabidopsis thaliana) have demonstrated that cytokinin acts through a two-component system that includes a histidine (His) kinase, a His phosphotransfer protein (HP), and a response regulator. Phylogenetic analyses have revealed the conservation of His kinases but lineage-specific expansion of HPs and response regulators in rice (Oryza sativa). However, whether the functions of rice HPs have diverged remains unknown. In this study, two rice authentic HPs (OsAHP1 and OsAHP2) were knocked down simultaneously via RNA interference (RNAi), and the transgenic OsAHP-RNAi plants exhibited phenotypes expected for a deficiency in cytokinin signaling, including dwarfism with reduced internode lengths, enhanced lateral root growth, early leaf senescence, and reduced tiller numbers and fertility under natural conditions. The OsAHP-RNAi seedlings were also hyposensitive to exogenous cytokinin. Furthermore, OsAHP-RNAi seedlings were hypersensitive to salt treatment but resistant to osmotic stress relative to wild-type plants. These results indicate that OsAHPs function as positive regulators of the cytokinin signaling pathway and play different roles in salt and drought tolerance in rice.

The AHP6 cytokinin signaling inhibitor mediates an auxin-cytokinin crosstalk that regulates the timing of organ initiation at the shoot apical meristem

Phyllotaxis, the spatio-temporal pattern of organogenesis at the shoot apical meristem, emerges in large part from inhibitory fields consisting in auxin-depleted areas centered on organs. We recently demonstrated the existence of an additional hormone-based inhibitory field generated by Arabidopsis Histidine Phosphotransfer Protein 6 (AHP6), an inhibitor of cytokinin signaling. We have shown that the spatio-temporal distribution of AHP6 in the meristem is essential for optimizing the rhythmicity of organ initiation. Here, we further analyzed AHP6 expression using fluorescent whole mount mRNA in situ hybridization and demonstrate a precise control of AHP6 level and expression domain over time. While we previously showed a regulation of AHP6 directly downstream of auxin, we show here that AHP6 transcription is unlikely influenced by cytokinin distribution in the meristem. Finally, we provide evidence that cytokinins and auxin might act synergistically during organ initiation, providing a plausible explanation for how AHP6 regulates phyllotaxis.

The AHP6 cytokinin signaling inhibitor mediates an auxin-cytokinin crosstalk that regulates the timing of organ initiation at the shoot apical meristem

Phyllotaxis, the spatio-temporal pattern of organogenesis at the shoot apical meristem, emerges in large part from inhibitory fields consisting in auxin-depleted areas centered on organs. We recently demonstrated the existence of an additional hormone-based inhibitory field generated by Arabidopsis Histidine Phosphotransfer Protein 6 (AHP6), an inhibitor of cytokinin signaling. We have shown that the spatio-temporal distribution of AHP6 in the meristem is essential for optimizing the rhythmicity of organ initiation. Here, we further analyzed AHP6 expression using fluorescent whole mount mRNA in situ hybridization and demonstrate a precise control of AHP6 level and expression domain over time. While we previously showed a regulation of AHP6 directly downstream of auxin, we show here that AHP6 transcription is unlikely influenced by cytokinin distribution in the meristem. Finally, we provide evidence that cytokinins and auxin might act synergistically during organ initiation, providing a plausible explanation for how AHP6 regulates phyllotaxis.

Cytokinins

Cytokinins are N (6) substituted adenine derivatives that affect many aspects of plant growth and development, including cell division, shoot initiation and growth, leaf senescence, apical dominance, sink/source relationships, nutrient uptake, phyllotaxis, and vascular, gametophyte, and embryonic development, as well as the response to biotic and abiotic factors. Molecular genetic studies in Arabidopsis have helped elucidate the mechanisms underlying the function of this phytohormone in plants. Here, we review our current understanding of cytokinin biosynthesis and signaling in Arabidopsis, the latter of which is similar to bacterial two-component phosphorelays. We discuss the perception of cytokinin by the ER-localized histidine kinase receptors, the role of the AHPs in mediating the transfer of the phosphoryl group from the receptors to the response regulators (ARRs), and finally the role of the large ARR family in cytokinin function. The identification and genetic manipulation of the genes involved in cytokinin metabolism and signaling have helped illuminate the roles of cytokinins in Arabidopsis. We discuss these diverse roles, and how other signaling pathways influence cytokinin levels and sensitivity though modulation of the expression of cytokinin signaling and metabolic genes.

Lateral root formation and growth of Arabidopsis is redundantly regulated by cytokinin metabolism and signalling genes

The plant root system is important for the uptake of water and nutrients and the anchoring of plants in the soil. Lateral roots (LRs) contribute considerably to root system architecture. Their post-embryonic formation is regulated by hormones and environmental cues. The hormone cytokinin influences LR formation and growth in Arabidopsis thaliana on different levels by disturbing cell division activity and pattern formation. This includes inhibition of the first formative cell division of pericycle founder cells and inhibition of the outgrowth of young LR primordia. Mutant analysis revealed that the cytokinin biosynthesis genes IPT3 and IPT5 and all three cytokinin receptor genes (AHK2, AHK3, and CRE1/AHK4) act redundantly during LR initiation. Mutation of AHK2 and AHK3 caused increased auxin sensitivity of LR formation, corroborating the functional relevance of auxin-cytokinin interaction during LR formation. In contrast, LR development of cytokinin receptor mutants in response to other hormones was mostly similar to that of the wild type, which is consistent with separate response pathways. A noticeable exception was an increased sensitivity of LR elongation to brassinolide in ahk2 ahk3 mutants indicating antagonistic action of cytokinin and brassinosteroid. It is proposed that the multilevel redundancy of the cytokinin system in modulating LR formation reflects its role in mediating environmental cues.

The Arabidopsis HP6 gene is expressed in Medicago truncatula lateral roots and root nodule primordia

Expression patterns of orthologous genes can be similar between distantly related species, suggesting that developmental programs can be conserved between organisms. Here, we show that the promoter of AHP6, a gene which is involved in Arabidopsis lateral root development, also drives the expression of the reporter GUS gene in lateral roots of Medicago truncatula suggesting that similar regulatory elements are involved in lateral root organogenesis in these species. Interestingly, the AHP6 promoter was able to drive GUS expression in root nodules and nodule primordia, structures that are absent in Arabidopsis. We found two AHP6 orthologous genes in the M. truncatula genome and we speculate that these putative cytokinin inhibitors may play a role during lateral root and nodule development in this species.

Symplastic intercellular connectivity regulates lateral root patterning

Cell-to-cell communication coordinates the behavior of individual cells to establish organ patterning and development. Although mobile signals are known to be important in lateral root development, the role of plasmodesmata (PD)-mediated transport in this process has not been investigated. Here, we show that changes in symplastic connectivity accompany and regulate lateral root organogenesis in Arabidopsis. This connectivity is dependent upon callose deposition around PD affecting molecular flux through the channel. Two plasmodesmal-localized β-1,3 glucanases (PdBGs) were identified that regulate callose accumulation and the number and distribution of lateral roots. The fundamental role of PD-associated callose in this process was illustrated by the induction of similar phenotypes in lines with altered callose turnover. Our results show that regulation of callose and cell-to-cell connectivity is critical in determining the pattern of lateral root formation, which influences root architecture and optimal plant performance.