Imaging TCSn::GFP, a Synthetic Cytokinin Reporter, in Arabidopsis thaliana

ARR1 and ARR12 modulate arsenite toxicity responses in Arabidopsis roots by transcriptionally controlling the actions of NIP1;1 and NIP6;1

Cytokinin is central to coordinating plant adaptation to environmental stresses. Here, we first demonstrated the involvement of cytokinin in Arabidopsis responses to arsenite [As(III)] stress. As(III) treatment reduced cytokinin contents, while cytokinin treatment repressed further primary root growth in Arabidopsis plants under As(III) stress. Subsequently, we revealed that the cytokinin signaling members ARR1 and ARR12, the type-B ARABIDOPSIS RESPONSE REGULATORs, participate in cytokinin signaling-mediated As(III) responses in plants as negative regulators. A comprehensive transcriptome analysis of the arr1 and arr12 single and arr1,12 double mutants was then performed to decipher the cytokinin signaling-mediated mechanisms underlying plant As(III) stress adaptation. Results revealed important roles for ARR1 and ARR12 in ion transport, nutrient responses, and secondary metabolite accumulation. Furthermore, using hierarchical clustering and regulatory network analyses, we identified two NODULIN 26-LIKE INTRINSIC PROTEIN (NIP)-encoding genes, NIP1;1 and NIP6;1, potentially involved in ARR1/12-mediated As(III) uptake and transport in Arabidopsis. By analyzing various combinations of arr and nip mutants, including high-order triple and quadruple mutants, we demonstrated that ARR1 and ARR12 redundantly function as negative regulators of As(III) tolerance by acting upstream of NIP1;1 and NIP6;1 to modulate their function in arsenic accumulation. ChIP-qPCR, EMSA, and transient dual-LUC reporter assays revealed that ARR1 and ARR12 transcriptionally activate the expression of NIP1;1 and NIP6;1 by directly binding to their promoters and upregulating their expression, leading to increased arsenic accumulation under As(III) stress. These findings collectively provide insights into cytokinin signaling-mediated plant adaptation to excessive As(III), contributing to the development of crops with low arsenic accumulation.

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.

A century of studying plant secondary metabolism-From "what?" to "where, how, and why?"

Over the past century, early advances in understanding the identity of the chemicals that collectively form a living plant have led scientists to deeper investigations exploring where these molecules localize, how they are made, and why they are synthesized in the first place. Many small molecules are specific to the plant kingdom and have been termed plant secondary metabolites, despite the fact that they can play primary and essential roles in plant structure, development, and response to the environment. The past 100 yr have witnessed elucidation of the structure, function, localization, and biosynthesis of selected plant secondary metabolites. Nevertheless, many mysteries remain about the vast diversity of chemicals produced by plants and their roles in plant biology. From early work characterizing unpurified plant extracts, to modern integration of 'omics technology to discover genes in metabolite biosynthesis and perception, research in plant (bio)chemistry has produced knowledge with substantial benefits for society, including human medicine and agricultural biotechnology. Here, we review the history of this work and offer suggestions for future areas of exploration. We also highlight some of the recently developed technologies that are leading to ongoing research advances.

Tyrosine-sulfated peptide hormone induces flavonol biosynthesis to control elongation and differentiation in Arabidopsis primary root

In Arabidopsis roots, growth initiation and cessation are organized into distinct zones. How regulatory mechanisms are integrated to coordinate these processes and maintain proper growth progression over time is not well understood. Here, we demonstrate that the peptide hormone PLANT PEPTIDE CONTAINING SULFATED TYROSINE 1 (PSY1) promotes root growth by controlling cell elongation. Higher levels of PSY1 lead to longer differentiated cells with a shootward displacement of characteristics common to mature cells. PSY1 activates genes involved in the biosynthesis of flavonols, a group of plant-specific secondary metabolites. Using genetic and chemical approaches, we show that flavonols are required for PSY1 function. Flavonol accumulation downstream of PSY1 occurs in the differentiation zone, where PSY1 also reduces auxin and reactive oxygen species (ROS) activity. These findings support a model where PSY1 signals the developmental-specific accumulation of secondary metabolites to regulate the extent of cell elongation and the overall progression to maturation.

Dopamine Inhibits Arabidopsis Growth through Increased Oxidative Stress and Auxin Activity

Like some bacterial species and all animals, plants synthesize dopamine and react to its exogenous applications. Despite dopamine’s widespread presence and activity in plants, its role in plant physiology is still poorly understood. Using targeted experimentation informed by the transcriptomic response to dopamine exposure, we identify three major effects of dopamine. First, we show that dopamine causes hypersensitivity to auxin indole-3-acetic acid by enhancing auxin activity. Second, we show that dopamine increases oxidative stress, which can be mitigated with glutathione. Third, we find that dopamine downregulates iron uptake mechanisms, leading to a decreased iron content—a response possibly aimed at reducing DA-induced oxidative stress. Finally, we show that dopamine-induced auxin sensitivity is downstream of glutathione biosynthesis, indicating that the auxin response is likely a consequence of DA-induced oxidative stress. Collectively, our results show that exogenous dopamine increases oxidative stress, which inhibits growth both directly and indirectly by promoting glutathione-biosynthesis-dependent auxin hypersensitivity.

Cytokinin signaling regulates two-stage inflorescence arrest in Arabidopsis

To maximize reproductive success, flowering plants must correctly time entry and exit from the reproductive phase. While much is known about mechanisms that regulate initiation of flowering, end-of-flowering remains largely uncharacterized. End-of-flowering in Arabidopsis (Arabidopsis thaliana) consists of quasi-synchronous arrest of inflorescences, but it is unclear how arrest is correctly timed with respect to environmental stimuli and reproductive success. Here, we showed that Arabidopsis inflorescence arrest is a complex developmental phenomenon, which includes the arrest of the inflorescence meristem (IM), coupled with a separable "floral arrest" of all unopened floral primordia; these events occur well before visible inflorescence arrest. We showed that global inflorescence removal delays both IM and floral arrest, but that local fruit removal only delays floral arrest, emphasizing their separability. We tested whether cytokinin regulates inflorescence arrest, and found that cytokinin signaling dynamics mirror IM activity, while cytokinin treatment can delay both IM and floral arrest. We further showed that gain-of-function cytokinin receptor mutants can delay IM and floral arrest; conversely, loss-of-function mutants prevented the extension of flowering in response to inflorescence removal. Collectively, our data suggest that the dilution of cytokinin among an increasing number of sink organs leads to end-of-flowering in Arabidopsis by triggering IM and floral arrest.

Ultrastructural and hormonal changes related to harmaline-induced treatment in Arabidopsis thaliana (L.) Heynh. root meristem

Harmaline is an indole alkaloid with demonstrated phytotoxicity and recognized pharmacological applications. However, no information is available concerning its mode of action on plant metabolism. Therefore, the present work evaluated bioherbicide mode of action of harmaline on plant metabolism of Arabidopsis thaliana (L.) Heynh. Harmaline induced a strong inhibitory activity on root growth of treated seedlings, reaching IC₅₀ and IC₈₀ values of 14 and 29 μM, respectively. Treated roots were shorter and thicker than control and were characterized by a shorter root meristem size and an increase of root hairs production. Harmaline induced ultrastructural changes such as increment of cell wall thickness, higher density and condensation of mitochondria and vacuolization, appearance of cell wall deposits, increment of Golgi secretory activity and higher percentage of aberrant nuclei. The ethylene inhibitor AgNO₃ reversed high root hair appearance and increment of root thickness, and pTCSn::GFP transgenic line showed fluorescence cytokinin signal in stele zone after harmaline treatment that was absent in control, whereas the auxin signal in the transgenic line DR5 was significantly reduced by the treatment. All these results suggest that the mode of action of harmaline could be involving auxin, ethylene and cytokinin synergic/antagonistic action.

A cellular analysis of meristem activity at the end of flowering points to cytokinin as a major regulator of proliferative arrest in Arabidopsis

In monocarpic plants, all reproductive meristem activity arrests and flower production ceases after the production of a certain number of fruits. This proliferative arrest (PA) is an evolutionary adaptation that ensures nutrient availability for seed production. Moreover, PA is a process of agronomic interest because it affects the duration of the flowering period and therefore fruit production. While our knowledge of the inputs and genetic factors controlling the initiation of the flowering period is extensive, little is known about the regulatory pathways and cellular events that participate in the end of flowering and trigger PA. Here, we characterize with high spatiotemporal resolution the cellular and molecular changes related to cell proliferation and meristem activity in the shoot apical meristem throughout the flowering period and PA. Our results suggest that cytokinin (CK) signaling repression precedes PA and that this hormone is sufficient to prevent and revert the process. We have also observed that repression of known CK downstream factors, such as type B cyclins and WUSCHEL (WUS), correlates with PA. These molecular changes are accompanied by changes in cell size and number likely caused by the cessation of cell division and WUS activity during PA. Parallel assays in fruitfull (ful) mutants, which do not undergo PA, have revealed that FUL may promote PA via repression of these CK-dependent pathways. Moreover, our data allow to define two phases, based on the relative contribution of FUL, that lead to PA: an early reduction of CK-related events and a late blocking of these events.

Sensors for the quantification, localization and analysis of the dynamics of plant hormones

Plant hormones play important roles in plant growth and development and physiology, and in acclimation to environmental changes. The hormone signaling networks are highly complex and interconnected. It is thus important to not only know where the hormones are produced, how they are transported and how and where they are perceived, but also to monitor their distribution quantitatively, ideally in a non-invasive manner. Here we summarize the diverse set of tools available for quantifying and visualizing hormone distribution and dynamics. We provide an overview over the tools that are currently available, including transcriptional reporters, degradation sensors, and luciferase and fluorescent sensors, and compare the tools and their suitability for different purposes.

flasher, a novel mutation in a glucosinolate modifying enzyme, conditions changes in plant architecture and hormone homeostasis

Meristem function is underpinned by numerous genes that affect hormone levels, ultimately controlling phyllotaxy, the transition to flowering and general growth properties. Class I KNOX genes are major contributors to this process, promoting cytokinin biosynthesis but repressing gibberellin production to condition a replication competent state. We identified a suppressor mutant of the KNOX1 mutant brevipedicellus (bp) that we termed flasher (fsh), which promotes stem and pedicel elongation, suppresses early senescence, and negatively affects reproductive development. Map-based cloning and complementation tests revealed that fsh is due to an E40K change in the flavin monooxygenase GS-OX5, a gene encoding a glucosinolate (GSL) modifying enzyme. In vitro enzymatic assays revealed that fsh poorly converts substrate to product, yet the levels of several GSLs are higher in the suppressor line, implicating FSH in feedback control of GSL flux. FSH is expressed predominantly in the vasculature in patterns that do not significantly overlap those of BP, implying a non-cell autonomous mode of meristem control via one or more GSL metabolites. Hormone analyses revealed that cytokinin levels are low in bp, but fsh restores cytokinin levels to near normal by activating cytokinin biosynthesis genes. In addition, jasmonate levels in the fsh suppressor are significantly lower than in bp, which is likely due to elevated expression of JA inactivating genes. These observations suggest the involvement of the GSL pathway in generating one or more negative effectors of growth that influence inflorescence architecture and fecundity by altering the balance of hormonal regulators.

Stem cell ageing of the root apical meristem of Arabidopsis thaliana

Plants form new organs from pluripotent stem cells throughout their lives and under changing environmental conditions. In the Arabidopsis root meristem, a pool of stem cells surrounding a stem cell organizer, named Quiescent Center (QC), gives rise to the specific root tissues. Among them, the columella stem cell niche that gives rise to the gravity-sensing columella cells has been used as a model system to study stem cell regulation at the young seedling stage. However, little is known about the changes of the stem cell niche during later development. Here, we report that the columella stem cell niche undergoes pronounced histological and molecular reorganization as the plant progresses towards the adult stage. Commonly-used reporters for cellular states undergo re-patterning after an initial juvenile meristem phase. Furthermore, the responsiveness to the plant hormone abscisic acid, an integrator of stress response, strongly decreases. Many ageing effects are reminiscent of the loss-of-function phenotype of the central stem cell regulator WOX5 and can be explained by gradually decreasing WOX5 expression levels during ageing. Our results show that the architecture and central regulatory components of the root stem cell niche are already highly dynamic within the first weeks of development.

Asymmetric distribution of cytokinins determines root hydrotropism in Arabidopsis thaliana

The phenomenon of plant root tips sensing moisture gradient in soil and growing towards higher water potential is designated as root hydrotropism, which is critical for plants to survive when water is a limited factor. Molecular mechanisms regulating such a fundamental process, however, are largely unknown. Here we report our identification that cytokinins are key signaling molecules directing root growth orientation in a hydrostimulation (moisture gradient) condition. Lower water potential side of the root tip shows more cytokinin response relative to the higher water potential side. Consequently, two cytokinin downstream type-A response regulators, ARR16 and ARR17, were found to be up-regulated at the lower water potential side, causing increased cell division in the meristem zone, which allows the root to bend towards higher water potential side. Genetic analyses indicated that various cytokinin biosynthesis and signaling mutants, including the arr16 arr17 double mutant, are significantly less responsive to hydrostimulation. Consistently, treatments with chemical inhibitors interfering with either cytokinin biosynthesis or cell division completely abolished root hydrotropic response. Asymmetrically induced expression of ARR16 or ARR17 effectively led to root bending in both wild-type and miz1, a previously known hydrotropism-defective mutant. These data demonstrate that asymmetric cytokinin distribution is a primary determinant governing root hydrotropism.

Role of cis-zeatin in root responses to phosphate starvation

Phosphate (Pi) is an essential nutrient for all organisms. Roots are underground organs, but the majority of the root biology studies have been done on root systems growing in the presence of light. Root illumination alters the Pi starvation response (PSR) at different intensities. Thus, we have analyzed morphological, transcriptional and physiological responses to Pi starvation in dark-grown roots. We have identified new genes and pathways regulated by Pi starvation that were not described previously. We also show that Pi-starved plants increase the cis-zeatin (cZ) : trans-zeatin (tZ) ratio. Transcriptomic analyses show that tZ preferentially represses cell cycle and PSR genes, whereas cZ induces genes involved in cell and root hair elongation and differentiation. In fact, cZ-treated seedlings show longer root system as well as longer root hairs compared with tZ-treated seedlings, increasing the total absorbing surface. Mutants with low cZ concentrations do not allocate free Pi in roots during Pi starvation. We propose that Pi-starved plants increase the cZ : tZ ratio to maintain basal cytokinin responses and allocate Pi in the root system to sustain its growth. Therefore, cZ acts as a PSR hormone that stimulates root and root hair elongation to enlarge the root absorbing surface and to increase Pi concentrations in roots.

Cytokinin functions as an asymmetric and anti-gravitropic signal in lateral roots

Directional organ growth allows the plant root system to strategically cover its surroundings. Intercellular auxin transport is aligned with the gravity vector in the primary root tips, facilitating downward organ bending at the lower root flank. Here we show that cytokinin signaling functions as a lateral root specific anti-gravitropic component, promoting the radial distribution of the root system. We performed a genome-wide association study and reveal that signal peptide processing of Cytokinin Oxidase 2 (CKX2) affects its enzymatic activity and, thereby, determines the degradation of cytokinins in natural Arabidopsis thaliana accessions. Cytokinin signaling interferes with growth at the upper lateral root flank and thereby prevents downward bending. Our interdisciplinary approach proposes that two phytohormonal cues at opposite organ flanks counterbalance each other's negative impact on growth, suppressing organ growth towards gravity and allow for radial expansion of the root system.

Comparison of efficiency and time to regeneration of Agrobacterium-mediated transformation methods in Medicago truncatula

BACKGROUND

Tissue culture transformation of plants has an element of art to it, with protocols passed on between labs but often not directly compared. As Medicago truncatula has become popular as a model system for legumes, rapid transformation is critical, and many protocols exist, with varying results.

RESULTS

The M. truncatula ecotypes, R108 and A17, were utilized to compare the effect of a modification to a previously used protocol based on shoot explants on the percentage of transformed plants produced from calli. This percentage was then compared to that of two additional transformation protocols based on root explants in the R108 ecotype. Variations in embryonic tissue sources, media components, time for transformation, and vectors were analyzed.

CONCLUSIONS

While no A17 transgenic plants were obtained, transgenic plantlets from the R108 ecotype were produced in as little as 4 months with a comparison of the two widely studied ecotypes under a single set of conditions. While the protocols tested gave similar results in percentage of transformed plants produced, considerations of labor and time to transgenics that vary between the root explant protocols tested were discovered. These considerations may influence which protocol to choose for introducing a single transgene versus creating lines with multiple mutations utilizing a CRISPR/Cas9 construct.

Sense and sensibility: the use of fluorescent protein-based genetically encoded biosensors in plants

Fluorescent protein-based biosensors are providing us with an unprecedented, quantitative view of the dynamic nature of the cellular networks that lie at the heart of plant biology. Such bioreporters can visualize the spatial and temporal kinetics of cellular regulators such as Ca²⁺ and H⁺, plant hormones and even allow membrane transport activities to be monitored in real time in living plant cells. The fast pace of their development is making these tools increasingly sensitive and easy to use and the rapidly expanding biosensor toolkit offers great potential for new insights into a wide range of plant regulatory processes. We suggest a checklist of controls that should help avoid some of the more cryptic issues with using these bioreporter technologies.

Arabidopsis CKI1 mediated two-component signaling in the specification of female gametophyte

Cytokinin independent 1 (CKI1) is a histidine kinase involved in the two-component signaling pathway and acts as a master regulator of central cell specification via CKI1-mediated two-component signaling. In this study, the dynamic distribution of two-component system (TCS) signals was primarily investigated during Arabidopsis embryo sac development. TCS signals were stably detected in female gametophytes cells from the megaspore stage all through to the mature embryo sac stage. CKI1 acts as the primary activator of the TCS signaling pathway in embryo sacs. Accordingly, focusing on CKI1, two alternate models are proposed for female gametophyte cell fate specification. In the first model, CKI1 co-determines the central cell fate in combination with a hypothetical X factor at the micropylar pole, and in the alternate model, CKI1 alone determines the central cell fate.

Transient Transactivation Studies in Nicotiana benthamiana Leaves

Transitory gene expression systems in Nicotiana benthamiana leaves, in combination with the use of gene silencing suppressors as the p19 or HC-pro proteins that allow for elevated levels of gene expression, have proven to be a highly versatile tool to analyze transcriptional function of DNA binding factors in the activated or repressed expression of their gene targets. This experimental setup uses Agrobacterium-mediated infection to deliver the various DNA constructs into the cell, and offers the advantage with respect to mesophyll protoplast transfection procedures that it entails a much easier protocol, in addition to preserving the intact leaf tissue, thus being more amenable to the study of wound and stress signaling pathways or to the functional analyses of regulators that respond to Ca+2 signatures. Furthermore, by using reporter constructs based on the LUCIFERASE (LUC) gene, which does not require a destructive determination assay, this expression system can be used to test for changes in gene activity over time or in response to various treatments, thus providing a comprehensive understanding of the signaling pathways that modulate activity of the expressed regulators and therefore their in vivo function in the control of the analyzed promoter.