Tissue-specific expression of the ethylene biosynthetic machinery regulates root growth in maize

Diverse roles of ethylene in maize growth and development, and its importance in shaping plant architecture

The gaseous plant hormone ethylene is a key developmental and growth regulator, and a pivotal endogenous response signal to abiotic and biotic interactions, including stress. Much of what is known about ethylene biosynthesis, perception, and signaling comes from decades of research primarily in Arabidopsis thaliana and other eudicot model systems. In contrast, detailed knowledge on the ethylene pathway and response to the hormone is markedly limited in maize (Zea mays L.), a global cereal crop that is a major source of calories for humans and livestock, as well as a key industrial biofeedstock. Recent reports of forward screens and targeted reverse genetics have provided important insight into conserved and unique differences of the ethylene pathway and downstream responses. Natural and edited allelic variation in the promoter regions and coding sequences of ethylene biosynthesis and signaling genes alters maize shoot and root architectures, and plays a crucial role in biomass and grain yields. This review discusses recent advances in ethylene research in maize, with an emphasis on the role of ethylene in regulating growth and development of the shoot and root systems, and ultimately how this crucial hormone impacts plant architecture and grain yield.

The RhEIN3-RhARGOS-RhHYD1 module connects ethylene and sterol signaling to regulate flower opening in rose

For rose (Rosa hybrida) plants, flowering is not only a reproductive and survival strategy but also a determinant of their ornamental value. Ethylene is a critical plant hormone that accelerates flower opening, and sterols are important regulatory factors in the ethylene signaling pathway; however, the molecular regulatory mechanisms through which sterols influence that pathway are not fully understood. Here, we demonstrate that the sterol isomerase HYDRA1 (RhHYD1) participates in ethylene-induced flower opening in rose, and that AUXIN-REGULATED GENE IN ORGAN SIZE (RhARGOS) interacts with RhHYD1 and promotes its degradation. Knocking down RhHYD1 transcript levels resulted in delayed flower opening and reduced ethylene sensitivity. Conversely, knocking down RhARGOS accelerated flower opening and increased ethylene sensitivity. Additionally, we determined that ETHYLENE INSENSITIVE3 (RhEIN3), a pivotal transcription factor within the ethylene signaling pathway, binds to the RhARGOS promoter via TCTTCA motifs to promote RhARGOS transcription. In summary, our findings reveal that a RhEIN3-RhARGOS-RhHYD1 module regulates rose flower opening by connecting ethylene and sterol signaling, providing valuable insight into the mechanism underlying ethylene-promoted flower opening.

Striking a growth-defense balance: Stress regulators that function in maize development

Maize (Zea mays) cultivation is strongly affected by both abiotic and biotic stress, leading to reduced growth and productivity. It has recently become clear that regulators of plant stress responses, including the phytohormones abscisic acid (ABA), ethylene (ET), and jasmonic acid (JA), together with reactive oxygen species (ROS), shape plant growth and development. Beyond their well established functions in stress responses, these molecules play crucial roles in balancing growth and defense, which must be finely tuned to achieve high yields in crops while maintaining some level of defense. In this review, we provide an in-depth analysis of recent research on the developmental functions of stress regulators, focusing specifically on maize. By unraveling the contributions of these regulators to maize development, we present new avenues for enhancing maize cultivation and growth while highlighting the potential risks associated with manipulating stress regulators to enhance grain yields in the face of environmental challenges.

The Ethylene Biosynthetic Enzymes, 1-Aminocyclopropane-1-Carboxylate (ACC) Synthase (ACS) and ACC Oxidase (ACO): The Less Explored Players in Abiotic Stress Tolerance

Ethylene is an essential plant hormone, critical in various physiological processes. These processes include seed germination, leaf senescence, fruit ripening, and the plant's response to environmental stressors. Ethylene biosynthesis is tightly regulated by two key enzymes, namely 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO). Initially, the prevailing hypothesis suggested that ACS is the limiting factor in the ethylene biosynthesis pathway. Nevertheless, accumulating evidence from various studies has demonstrated that ACO, under specific circumstances, acts as the rate-limiting enzyme in ethylene production. Under normal developmental processes, ACS and ACO collaborate to maintain balanced ethylene production, ensuring proper plant growth and physiology. However, under abiotic stress conditions, such as drought, salinity, extreme temperatures, or pathogen attack, the regulation of ethylene biosynthesis becomes critical for plants' survival. This review highlights the structural characteristics and examines the transcriptional, post-transcriptional, and post-translational regulation of ACS and ACO and their role under abiotic stress conditions. Reviews on the role of ethylene signaling in abiotic stress adaptation are available. However, a review delineating the role of ACS and ACO in abiotic stress acclimation is unavailable. Exploring how particular ACS and ACO isoforms contribute to a specific plant's response to various abiotic stresses and understanding how they are regulated can guide the development of focused strategies. These strategies aim to enhance a plant's ability to cope with environmental challenges more effectively.

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

A Genome-Wide Survey of MATE Transporters in Brassicaceae and Unveiling Their Expression Profiles under Abiotic Stress in Rapeseed

The multidrug and toxic compound extrusion (MATE) protein family is important in the export of toxins and other substrates, but detailed information on this family in the Brassicaceae has not yet been reported compared to Arabidopsis thaliana. In this study, we identified 57, 124, 81, 85, 130, and 79 MATE genes in A. thaliana, Brassica napus, Brassica oleracea, Brassica rapa, Brassica juncea, and Brassica nigra, respectively, which were unevenly distributed on chromosomes owing to both tandem and segmental duplication events. Phylogenetic analysis showed that these genes could be classified into four subgroups, shared high similarity and conservation within each group, and have evolved mainly through purifying selection. Furthermore, numerous B. napusMATE genes showed differential expression between tissues and developmental stages and between plants treated with heavy metals or hormones and untreated control plants. This differential expression was especially pronounced for the Group 2 and 3 BnaMATE genes, indicating that they may play important roles in stress tolerance and hormone induction. Our results provide a valuable foundation for the functional dissection of the different BnaMATE homologs in B. napus and its parental lines, as well as for the breeding of more stress-tolerant B. napus genotypes.

Mitochondrial alternative oxidase-dependent autophagy involved in ethylene-mediated drought tolerance in Solanum lycopersicum

Mitochondrial alternative oxidase (AOX) is involved in a large number of plant physiological processes, such as growth, development and stress responses; however, the exact role of AOX in response to drought remains unclear. In our study, we provide solid evidences that the activated AOX capacity positively involved in ethylene-induced drought tolerance, in tomato (Solanum lycopersicum), accompanied by the changing level of hydrogen peroxide (H2 O2 ) and autophagy. In AOX1a-RNAi plants, the ethylene-induced drought tolerance was aggravated and associated with decreasing level of autophagy. The H2 O2 level was relatively higher in AOX1a-RNAi plants, whereas it was lower in AOX1a-overexpressing (35S-AOX1a-OE) plants after 1-(aminocarbonyl)-1-cyclopropanecarboxylic acid (ACC) pretreatment in the 14th day under drought stress. Interestingly, the accumulation of autophagosome was accompanied by the changing level of reactive oxygen species (ROS) in AOX transgenic tomato under drought stress whether or not pretreated with ACC. Pharmacological scavenging of H2 O2 accumulation in AOX1a-RNAi (aox19) stimulated autophagy acceleration under drought stress, and it seems that AOX-dependent ROS signalling is critical in triggering autophagy. Lower levels of ROS signalling positively induce autophagy activity, whereas higher ROS level would lead to rapid programmed cell death (PCD), especially in ethylene-mediated drought tolerance. Moreover, ethylene-induced autophagy during drought stress also can be through ERF5 binding to the promoters of ATG8d and ATG18h. These results demonstrated that AOX plays an essential role in ethylene-induced drought tolerance and also played important roles in mediating autophagy generation via balancing ROS level.

Tissue-specific transcriptomic profiling of Plantago major provides insights for the involvement of vasculature in phosphate deficiency responses

The vasculature of higher plants is important with transport of both nutrient and information molecules. To understand the correspondence of this tissue in molecular responses under phosphate (Pi) deficiency, Plantago major, a model plant for vasculature biology study, was chosen in our analysis. After RNA-Seq and de novo transcriptome assembly of 24 libraries prepared from the vasculature of P. major, 37,309 unigenes with a mean length of 1571 base pairs were obtained. Upon 24 h of Pi deficiency, 237 genes were shown to be differentially expressed in the vasculature of P. major. Among these genes, only 27 have been previously identified to be specifically expressed in the vasculature tissues in other plant species. Temporal expression of several marker genes associated with Pi deficiency showed that the time period of first 24 h is at the beginning stage of more dynamic expression patterns. In this study, we found several physiological processes, e.g., "phosphate metabolism and remobilization", "sucrose metabolism, loading and synthesis", "plant hormone metabolism and signal transduction", "transcription factors", and "metabolism of other minerals", were mainly involved in early responses to Pi deficiency in the vasculature. A number of vasculature genes with promising roles in Pi deficiency adaptation have been identified and deserve further functional characterization. This study clearly demonstrated that plant vasculature is actively involved in Pi deficiency responses and understanding of this process may help to create plants proficient to offset Pi deficiency.

The FBH family of bHLH transcription factors controls ACC synthase expression in sugarcane

Ethylene is a phytohormone involved in the regulation of several aspects of plant development and in responses to biotic and abiotic stress. The effects of exogenous application of ethylene to sugarcane plants are well characterized as growth inhibition of immature internodes and stimulation of sucrose accumulation. However, the molecular network underlying the control of ethylene biosynthesis in sugarcane remains largely unknown. The chemical reaction catalyzed by 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is an important rate-limiting step that regulates ethylene production in plants. In this work, using a yeast one-hybrid approach, we identified three basic helix-loop-helix (bHLH) transcription factors, homologs of Arabidopsis FBH (FLOWERING BHLH), that bind to the promoter of ScACS2 (Sugarcane ACS2), a sugarcane type 3 ACS isozyme gene. Protein-protein interaction assays showed that sugarcane FBH1 (ScFBH1), ScFBH2, and ScFBH3 form homo- and heterodimers in the nucleus. Gene expression analysis revealed that ScFBHs and ScACS2 transcripts are more abundant in maturing internodes during afternoon and night. In addition, Arabidopsis functional analysis demonstrated that FBH controls ethylene production by regulating transcript levels of ACS7, a homolog of ScACS2. These results indicate that ScFBHs transcriptionally regulate ethylene biosynthesis in maturing internodes of sugarcane.

Genomewide analysis of MATE-type gene family in maize reveals microsynteny and their expression patterns under aluminum treatment

Multidrug and toxic compound extrusion (MATE) proteins are a group of secondary active transporters, which widely exist in all living organisms and play important role in the detoxication of endogenous secondary metabolites and exogenous agents. However, to date, no systematic and comprehensive study of this family is reported in maize. Here, a total of 49 MATE genes (ZmMATE) were identified and divided into seven groups by phylogenetic analysis. Conserved intro-exon structures and motif compositions were investigated in these genes. Results by gene locations indicated that these genes were unevenly distributed among all 10 chromosomes. Tandem and segmental duplications appeared to contribute to the expansion and evolution of this gene family. The Ka/Ks ratios suggested that the ZmMATE has undergone large-scale purifying selection on the maize genome. Interspecies microsynteny analysis revealed that there were independent gene duplication events of 10 ZmMATE. In addition, most maize MATE genes exhibited different expression profiles in diverse tissues and developmental stages. Sixteen MATE genes were chosen for further quantitative real-time polymerase chain reaction analysis showed differential expression patterns in response to aluminum treatment. These results provide a useful clue for future studies on the identification of MATE genes and functional analysis of MATE proteins in maize.

Accumulation and Transport of 1-Aminocyclopropane-1-Carboxylic Acid (ACC) in Plants: Current Status, Considerations for Future Research and Agronomic Applications

1-aminocyclopropane-1-carboxylic acid (ACC) is a non-protein amino acid acting as the direct precursor of ethylene, a plant hormone regulating a wide variety of vegetative and developmental processes. ACC is the central molecule of ethylene biosynthesis. The rate of ACC formation differs in response to developmental, hormonal and environmental cues. ACC can be conjugated to three derivatives, metabolized in planta or by rhizobacteria using ACC deaminase, and is transported throughout the plant over short and long distances, remotely leading to ethylene responses. This review highlights some recent advances related to ACC. These include the regulation of ACC synthesis, conjugation and deamination, evidence for a role of ACC as an ethylene-independent signal, short and long range ACC transport, and the identification of a first ACC transporter. Although unraveling the complex mechanism of ACC transport is in its infancy, new questions emerge together with the identification of a first transporter. In the light of the future quest for additional ACC transporters, this review presents perspectives of the novel findings and includes considerations for future research toward applications in agronomy.

Ethylene-dependent aerenchyma formation in adventitious roots is regulated differently in rice and maize

In roots of gramineous plants, lysigenous aerenchyma is created by the death and lysis of cortical cells. Rice (Oryza sativa) constitutively forms aerenchyma under aerobic conditions, and its formation is further induced under oxygen-deficient conditions. However, maize (Zea mays) develops aerenchyma only under oxygen-deficient conditions. Ethylene is involved in lysigenous aerenchyma formation. Here, we investigated how ethylene-dependent aerenchyma formation is differently regulated between rice and maize. For this purpose, in rice, we used the reduced culm number1 (rcn1) mutant, in which ethylene biosynthesis is suppressed. Ethylene is converted from 1-aminocyclopropane-1-carboxylic acid (ACC) by the action of ACC oxidase (ACO). We found that OsACO5 was highly expressed in the wild type, but not in rcn1, under aerobic conditions, suggesting that OsACO5 contributes to aerenchyma formation in aerated rice roots. By contrast, the ACO genes in maize roots were weakly expressed under aerobic conditions, and thus ACC treatment did not effectively induce ethylene production or aerenchyma formation, unlike in rice. Aerenchyma formation in rice roots after the initiation of oxygen-deficient conditions was faster and greater than that in maize. These results suggest that the difference in aerenchyma formation in rice and maize is due to their different mechanisms for regulating ethylene biosynthesis.

Engineering high Zn in tomato shoots through expression of AtHMA4 involves tissue-specific modification of endogenous genes

Background

To increase the Zn level in shoots, AtHMA4 was ectopically expressed in tomato under the constitutive CaMV 35S promoter. However, the Zn concentration in the shoots of transgenic plants failed to increase at all tested Zn levels in the medium. Modification of Zn root/shoot distribution in tomato expressing 35S::AtHMA4 depended on the concentration of Zn in the medium, thus indicating involvement of unknown endogenous metal-homeostasis mechanisms. To determine these mechanisms, those metal-homeostasis genes that were expressed differently in transgenic and wild-type plants were identified by microarray and RT-qPCR analysis using laser-assisted microdissected RNA isolated from two root sectors: (epidermis + cortex and stele), and leaf sectors (upper epidermis + palisade parenchyma and lower epidermis + spongy parenchyma).

Results

Zn-supply-dependent modification of Zn root/shoot distribution in AtHMA4-tomato (increase at 5 μM Zn, no change at 0.5 μM Zn) involved tissue-specific, distinct from that in the wild type, expression of tomato endogenous genes. First, it is suggested that an ethylene-dependent pathway underlies the detected changes in Zn root/shoot partitioning, as it was induced in transgenic plants in a distinct way depending on Zn exposure. Upon exposure to 5 or 0.5 μM Zn, in the epidermis + cortex of the transgenics’ roots the expression of the Strategy I Fe-uptake system (ethylene-dependent LeIRT1 and LeFER) was respectively lower or higher than in the wild type and was accompanied by respectively lower or higher expression of the identified ethylene genes (LeNR, LeACO4, LeACO5) and of LeChln. Second, the contribution of LeNRAMP2 expression in the stele is shown to be distinct for wild-type and transgenic plants at both Zn exposures. Ethylene was also suggested as an important factor in a pathway induced in the leaves of transgenic plants by high Zn in the apoplast, which results in the initiation of loading of the excess Zn into the mesophyll of “Zn accumulating cells”.

Conclusions

In transgenic tomato plants, the export activity of ectopically expressed AtHMA4 changes the cellular Zn status, which induces coordinated tissue-specific responses of endogenous ethylene-related genes and metal transporters. These changes constitute an important mechanism involved in the generation of the metal-related phenotype of transgenic tomato expressing AtHMA4.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2990-x) contains supplementary material, which is available to authorized users.

Appearance and elaboration of the ethylene receptor family during land plant evolution

Ethylene is perceived following binding to endoplasmic reticulum-localized receptors, which in Arabidopsis thaliana, include ETR1, ERS1, EIN4, ETR2, and ERS2. These receptors fall into two subfamilies based on conservation of features within their histidine kinase domain. Subfamily 1 contains ETR1 and ERS1 whereas subfamily 2 contains EIN4, ETR2, and ERS2. Because ethylene receptors are found only in plants, this raises questions of when each receptor evolved. Here it is shown that subfamily 1 receptors encoded by a multigene family are present in all charophytes examined, these being most homologous to ETR1 based on their evolutionary relationship as well as containing histidine kinase and receiver domains. In charophytes and Physcomitrella patens, one or more gene family members contain the intron characteristic of subfamily 2 genes, indicating the first step in subfamily 2 receptor evolution. ERS1 homologs appear in basal angiosperm species after Amborella trichopoda and, in some early and basal angiosperm species and monocots in general, it is the only subfamily 1 receptor present. Distinct EIN4 and ETR2 homologs appear only in core eudicots and ERS2 homologs appear only in the Brassicaceae, suggesting it is the most recent receptor to evolve. These findings show that a subfamily 1 receptor had evolved and a subfamily 2 receptor had begun to evolve in plants prior to the colonization of land and only these two existed up to the appearance of the first basal angiosperm. The appearance of ERS2 in the Brassicaceae suggests ongoing evolution of the ethylene receptor family.

The 2HA line of Medicago truncatula has characteristics of an epigenetic mutant that is weakly ethylene insensitive

BACKGROUND

The Medicago truncatula 2HA seed line is highly embryogenic while the parental line Jemalong rarely produces embryos. The 2HA line was developed from one of the rare Jemalong regenerates and this method for obtaining a highly regenerable genotype in M. truncatula is readily reproducible suggesting an epigenetic mechanism. Microarray transcriptomic analysis showed down regulation of an ETHYLENE INSENSITIVE 3-like gene in 2HA callus which provided an approach to investigating epigenetic regulation of genes related to ethylene signalling and the 2HA phenotype. Ethylene is involved in many developmental processes including somatic embryogenesis (SE) and is associated with stress responses.

RESULTS

Microarray transcriptomic analysis showed a significant number of up-regulated transcripts in 2HA tissue culture, including nodule and embryo specific genes and transposon-like genes, while only a few genes were down-regulated, including an EIN3-like gene we called MtEIL1. This reduced expression was associated with ethylene insensitivity of 2HA plants that was further investigated. The weak ethylene insensitivity affected root and nodule development. Sequencing of MtEIL1 found no difference between 2HA and wild-type plants. DNA methylation analysis of MtEIL1 revealed significant difference between 2HA and wild-type plants. Tiling arrays demonstrated an elevated level of miRNA in 2HA plants that hybridised to the antisense strand of the MtEIL1 gene. AFLP-like methylation profiling revealed more differences in DNA methylation between 2HA and wild-type. Segregation analysis demonstrated the recessive nature of the eil1 phenotype and the dominant nature of the SE trait.

CONCLUSIONS

We have demonstrated that EIL1 of Medicago truncatula (MtEIL1) is epigenetically silenced in the 2HA seed line. The possible cause is an elevated level of miRNA that targets its 3'UTR and is also associated with DNA methylation of MtEIL1. Down regulation of MtEIL1 makes it possible to form nodules in the presence of ethylene and affects root growth under normal conditions. Segregation analysis showed no association between MtEIL1 expression and SE in culture but the role and mechanism of ethylene signalling in the process of plant regeneration through SE requires further investigation. The work also suggests that epigenetic changes to a particular gene induced in culture can be fixed in regenerated plants.

Chromium stress response effect on signal transduction and expression of signaling genes in rice

Hexavalent chromium [Cr(VI)] is a non-essential metal for normal plants and is toxic to plants at high concentrations. However, signaling pathways and molecular mechanisms of its action on cell function and gene expression remain elusive. In this study, we found that Cr(VI) induced endogenous reactive oxygen species (ROS) generation and Ca(2+) accumulation and activated NADPH oxidase and calcium-dependent protein kinase. We investigated global transcriptional changes in rice roots by microarray analysis. Gene expression profiling indicated activation of abscisic acid-, ethylene- and jasmonic acid-mediated signaling and inactivation of gibberellic acid-related pathways in Cr(VI) stress-treated rice roots. Genes encoding signaling components such as the protein kinases domain of unknown function 26, receptor-like cytoplasmic kinase, LRK10-like kinase type 2 and protein phosphatase 2C, as well as transcription factors WRKY and apetala2/ethylene response factor were predominant during Cr(VI) stress. Genes involved in vesicle trafficking were subjected to functional characterization. Pretreating rice roots with a vesicle trafficking inhibitor, brefeldin A, effectively reduced Cr(VI)-induced ROS production. Suppression of the vesicle trafficking gene, Exo70, by virus-induced gene silencing strategies revealed that vesicle trafficking is required for mediation of Cr(VI)-induced ROS production. Taken together, these findings shed light on the molecular mechanisms in signaling pathways and transcriptional regulation in response to Cr stress in plants.

1-aminocyclopropane-1-carboxylic acid (ACC) in plants: more than just the precursor of ethylene!

Ethylene is a simple two carbon atom molecule with profound effects on plants. There are quite a few review papers covering all aspects of ethylene biology in plants, including its biosynthesis, signaling and physiology. This is merely a logical consequence of the fascinating and pleiotropic nature of this gaseous plant hormone. Its biochemical precursor, 1-aminocyclopropane-1-carboxylic acid (ACC) is also a fairly simple molecule, but perhaps its role in plant biology is seriously underestimated. This triangularly shaped amino acid has many more features than just being the precursor of the lead-role player ethylene. For example, ACC can be conjugated to three different derivatives, but their biological role remains vague. ACC can also be metabolized by bacteria using ACC-deaminase, favoring plant growth and lowering stress susceptibility. ACC is also subjected to a sophisticated transport mechanism to ensure local and long-distance ethylene responses. Last but not least, there are now a few exciting studies where ACC has been reported to function as a signal itself, independently from ethylene. This review puts ACC in the spotlight, not to give it the lead-role, but to create a picture of the stunning co-production of the hormone and its precursor.

Ethylene and the responses of plants to phosphate deficiency

This review considers the evidence that ethylene biosynthesis is up-regulated by locally-generated signals in response to a change in external P supply, where the hormone then mediates, with auxin, changes in root system architecture. Subsequent changes in endogenous P evoke systemic responses whereby ethylene again is important in inducing some of the key signature changes observed in P-deprived tissues (eg. phosphate transporter and acid phosphatase up-regulation).

The consideration as to how plants uptake and transport phosphorus (P) is of significant agronomic and economic importance, in part driven by finite reserves of rock phosphate. Our understanding of these mechanisms has been greatly advanced, particularly with respect to the responses of plants to P deficiency and the genetic dissection of the signalling involved. Further, the realization that there are two tiers of transcriptional responses, the local, in which inorganic P (Pi) acts as an external signal independent of the endogenous P level, and the systemic involving root–shoot signalling, has now added a dimension of both clarity and complexity. Notwithstanding, it is now clear that the hormone ethylene plays a key role in mediating both levels of responses. This review, therefore, covers the role of ethylene in terms of mediating responses to P deficiency. The evidence that Pi supply regulates ethylene biosynthesis and sensitivity, and that this, in turn, regulates changes in root system architecture and in Pi-deprivation responses is examined here. While ethylene is the focus, the key interactions with auxin are also assessed, but interactions with the other hormone groups, which have recently been reviewed, are not covered. The emerging view that ethylene is a multi-faceted hormone in terms of mediating responses to P deficiency invites the dissection of the transcriptional cues that mediate changes in ethylene biosynthesis and/or sensitivity. Knowledge of the nature of such cues will subsequently reveal more of the underpinning interactions that govern P responses and provide avenues for the production of germplasm with an improved phosphate use efficiency.

Transcriptomic changes and signalling pathways induced by arsenic stress in rice roots

Arsenic (As) is considered the most common toxic metalloid, but its molecular mode of action is not well understood. We investigated whether arsenate [As(V)] can induce intracellular reactive oxygen species production and calcium oscillation in rice roots. To better understand the molecular basis of plant cell responses to As, we performed a large-scale analysis of the rice transcriptome during As(V) stress. As(V) induced genes involved in abiotic stress, detoxification pathways and secondary metabolic process. Genes involved in secondary cell wall biogenesis, cell cycle and oligopeptide transport were mainly downregulated. Genes encoding signalling components such as receptor-like cytoplasmic kinases protein kinase, APETALA2/ethylene response factor, heat shock factor, MYB and zinc-finger protein expressed in inflorescence meristem transcription factors were increased in expression. The expression of GARP-G2-like and C3H transcription factors was specifically modulated by As(V) stress. The predominant families of As(V)-regulated transporters belonged to the ATP-binding cassette superfamily and telurite-resistance/dicarboxylate transporters. Several factors involved in signaling, such as mitogen-activated protein kinase (MAPK), MAPK kinase kinase and calcium-dependent protein kinase (CDPK), were also upregulated. Moreover, As(V) markedly increased the activity of MAPKs and CDPK-like kinases, and CDPK and NADPH oxidases were involved in As-induced MAPK activation. Further characterization of these As(V)-responsive genes and signalling pathways may help better understand the mechanisms of metalloid uptake, tolerance and detoxification in plants.

Potential impact of CO2 leakage from carbon capture and storage systems on field bean (Vicia faba)

Capture and geological storage of carbon dioxide (CO(2)) has been suggested to be essential to reduce emissions to the atmosphere and aid mitigation of global climate change. However, leakage from transport pipelines or carbon capture and storage (CCS) reservoirs may pose risks to vegetation and contribute to rising atmospheric concentrations [CO(2)]. This study examined effects on seedling emergence and growth when field bean plants (Vicia faba cv. Wizard) grown under field conditions were subjected to elevated soil [CO(2)] for 39 days after planting. The strong negative correlation between soil [CO(2)] and [O(2)] during the injection period created a hypoxic soil environment under conditions of elevated soil [CO(2)]. The damaging impact of this treatment became apparent early in the experiment as no seeds exposed to soil [CO(2)] >50% emerged, even after injection was discontinued. Some seeds exposed to soil [CO(2)] <50% produced seedlings, but many did not survive. Seedling emergence and survival in the gassed plots was greatest at [CO(2)] of 5-20%, but root and shoot growth was reduced relative to control plants. Seedling emergence and growth were negatively related to soil [CO(2)] and positively related to [O(2)], although it is not known which was more important in inducing the observed effects. These findings suggest that leakage of CO(2) from transport pipelines or CCS sites may greatly reduce seedling emergence and crop establishment in the vicinity of such infrastructures.

Ethylene in vegetative development: a tale with a riddle

The vegetative development of plants is strongly dependent on the action of phytohormones. For over a century, the effects of ethylene on plants have been studied, illustrating the profound impact of this gaseous hormone on plant growth, development and stress responses. Ethylene signaling is under tight self-control at various levels. Feedback regulation occurs on both biosynthesis and signaling. For its role in developmental processes, ethylene has a close and reciprocal relation with auxin, another major determinant of plant architecture. Here, we discuss, in view of novel findings mainly in the reference plant Arabidopsis, how ethylene is distributed and perceived throughout the plant at the organ, tissue and cellular levels, and reflect on how plants benefit from the complex interaction of ethylene and auxin, determining their shape. Furthermore, we elaborate on the implications of recent discoveries on the control of ethylene signaling.

Plant hormone interactions: innovative targets for crop breeding and management

Here we highlight how both the root and shoot environment impact on whole plant hormone balance, particularly under stresses such as soil drying, and relate hormone ratios and relative abundances to processes influencing plant performance and yield under both mild and more severe stress. We discuss evidence (i) that abscisic acid (ABA) and ethylene act antagonistically on grain-filling rate amongst other yield-impacting processes; (ii) that ABA's effectiveness as an agent of stomatal closure can be modulated by coincident ethylene or cytokinin accumulation; and (iii) that enhanced cytokinin production can increase growth and yield by improving foliar stay-green indices under stress, and by improving processes that impact grain-filling and number, and that this can be the result of altered relative abundances of cytokinin and ABA (and other hormones). We describe evidence and novel processes whereby these phenomena are/could be amenable to manipulation through genetic and management routes, such that plant performance and yield can be improved. We explore the possibility that a range of ABA-ethylene and ABA-cytokinin relative abundances could represent targets for breeding/managing for yield resilience under a spectrum of stress levels between severe and mild, and could circumvent some of the pitfalls so far encountered in the massive research effort towards breeding for increases in the complex trait of yield.

The root as a drill: an ethylene-auxin interaction facilitates root penetration in soil

Plant roots forage the soil for water and nutrients and overcome the soil's physical compactness. Roots are endowed with a mechanism that allows them to penetrate and grow in dense media such as soil. However, the molecular mechanisms underlying this process are still poorly understood. The nature of the media in which roots grow adds to the difficulty to in situ analyze the mechanisms underlying root penetration. Inhibition of ethylene perception by application of 1-methyl cyclopropene (1-MCP) to tomato seedlings nearly abolished the root penetration in Soilrite. The reversal of this process by auxin indicated operation of an auxin-ethylene signaling pathway in the regulation of root penetration. The tomato pct1-2 mutant that exhibits an enhanced polar transport of auxin required higher doses of 1-MCP to inhibit root penetration, indicating a pivotal role of auxin transport in this process. In this update we provide a brief review of our current understanding of molecular processes underlying root penetration in higher plants.

Tomato root penetration in soil requires a coaction between ethylene and auxin signaling

During seed germination, emerging roots display positive gravitropism and penetrate into the soil for nutrition and anchorage. Tomato (Solanum lycopersicum) seeds germinated in the presence of 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action, failed to insert roots into Soilrite and grew in the air, forming loops. Time-lapse video imaging showed that 1-MCP-grown root tips retained positive gravitropism and made contact with the surface of Soilrite but failed to penetrate into the Soilrite. Time-course studies revealed that the effect of 1-MCP was most prominent when seed imbibition and germination were carried out in the continual presence of 1-MCP. Conversely, 1-MCP was ineffective when applied postgermination after penetration of roots in the Soilrite. Furthermore, treatment with 1-MCP caused a reduction in DR5::β-glucuronidase auxin-reporter activity and modified the expression of SlIAA3 and SlIAA9 transcripts, indicating interference with auxin signaling. The reduced ethylene perception mutant, Never-ripe, displayed decreased ability for root penetration, and the enhanced polar auxin transport mutant, polycotyledon, showed a nearly normal root penetration in the presence of 1-MCP, which could be reversed by application of auxin transport inhibitors. Our results indicate that during tomato seed germination, a coaction between ethylene and auxin is required for root penetration into the soil.

Transcriptional profile of maize roots under acid soil growth

BACKGROUND

Aluminum (Al) toxicity is one of the most important yield-limiting factors of many crops worldwide. The primary symptom of Al toxicity syndrome is the inhibition of root growth leading to poor water and nutrient absorption. Al tolerance has been extensively studied using hydroponic experiments. However, unlike soil conditions, this method does not address all of the components that are necessary for proper root growth and development. In the present study, we grew two maize genotypes with contrasting tolerance to Al in soil containing toxic levels of Al and then compared their transcriptomic responses.

RESULTS

When grown in acid soil containing toxic levels of Al, the Al-sensitive genotype (S1587-17) showed greater root growth inhibition, more Al accumulation and more callose deposition in root tips than did the tolerant genotype (Cat100-6). Transcriptome profiling showed a higher number of genes differentially expressed in S1587-17 grown in acid soil, probably due to secondary effects of Al toxicity. Genes involved in the biosynthesis of organic acids, which are frequently associated with an Al tolerance response, were not differentially regulated in both genotypes after acid soil exposure. However, genes related to the biosynthesis of auxin, ethylene and lignin were up-regulated in the Al-sensitive genotype, indicating that these pathways might be associated with root growth inhibition. By comparing the two maize lines, we were able to discover genes up-regulated only in the Al-tolerant line that also presented higher absolute levels than those observed in the Al-sensitive line. These genes encoded a lipase hydrolase, a retinol dehydrogenase, a glycine-rich protein, a member of the WRKY transcriptional family and two unknown proteins.

CONCLUSIONS

This work provides the first characterization of the physiological and transcriptional responses of maize roots when grown in acid soil containing toxic levels of Al. The transcriptome profiles highlighted several pathways that are related to Al toxicity and tolerance during growth in acid soil. We found several genes that were not found in previous studies using hydroponic experiments, increasing our understanding of plant responses to acid soil. The use of two germplasms with markedly different Al tolerances allowed the identification of genes that are a valuable tool for assessing the mechanisms of Al tolerance in maize in acid soil.

Exogenously induced expression of ethylene biosynthesis, ethylene perception, phospholipase D, and Rboh-oxidase genes in broccoli seedlings

In higher plants, copper ions, hydrogen peroxide, and cycloheximide have been recognized as very effective inducers of the transcriptional activity of genes encoding the enzymes of the ethylene biosynthesis pathway. In this report, the transcriptional patterns of genes encoding the 1-aminocyclopropane-1-carboxylate synthases (ACSs), 1-aminocyclopropane-1-carboxylate oxidases (ACOs), ETR1, ETR2, and ERS1 ethylene receptors, phospholipase D (PLD)-alpha1, -alpha2, -gamma1, and -delta, and respiratory burst oxidase homologue (Rboh)-NADPH oxidase-D and -F in response to these inducers in Brassica oleracea etiolated seedlings are shown. ACS1, ACO1, ETR2, PLD-gamma1, and RbohD represent genes whose expression was considerably affected by all of the inducers used. The investigations were performed on the seedlings with (i) ethylene insensitivity and (ii) a reduced level of the PLD-derived phosphatidic acid (PA). The general conclusion is that the expression of ACS1, -3, -4, -5, -7, and -11, ACO1, ETR1, ERS1, and ETR2, PLD-gamma 1, and RbohD and F genes is undoubtedly under the reciprocal cross-talk of the ethylene and PA(PLD) signalling routes; both signals affect it in concerted or opposite ways depending on the gene or the type of stimuli. The results of these studies on broccoli seedlings are in agreement with the hypothesis that PA may directly affect the ethylene signal transduction pathway via an inhibitory effect on CTR1 (constitutive triple response 1) activity.

Expression of the ethylene biosynthetic machinery in maize roots is regulated in response to hypoxia

Ethylene regulates plant growth in response to many adverse environmental conditions, including the induction of aerenchyma, i.e. the formation of air spaces, in flooded roots in an effort to maintain oxygen levels. In this work, quantitative RT-PCR and in situ RNA hybridization were used to determine how the expression of the ethylene biosynthetic machinery in maize roots is spatially and temporally regulated following exposure to 4% oxygen (i.e. hypoxia) for up to 24 h, conditions that induced aerenchyma formation in the fully-expanded region of the root and reduced cytoplasmic density throughout the root. Expression of ACC oxidase, the ethylene forming enzyme, was observed in the root cap, protophloem sieve elements, and companion cells associated with metaphloem sieve elements. Exposure to 4% oxygen induced ACC oxidase expression in these cell types as well as in the root cortex. ACC synthase, which generates the ethylene precursor, was expressed in the root cap and the cortex and its expression was induced in cortical cells following low oxygen treatment. The induction of expression of the ethylene biosynthetic machinery was accompanied by an induction of ethylene evolution and a reduced rate of root growth. These results suggest that maize roots respond to conditions of hypoxia by inducing the spatially restricted expression of the ethylene biosynthetic machinery, resulting in increased ethylene production.