Ethylene regulates root growth through effects on auxin biosynthesis and transport-dependent auxin distribution

Nitrate and hormonal signaling crosstalk for plant growth and development

Nitrate is an essential macronutrient for plants, a primary nitrogen source in natural and human-made ecosystems. Nitrate can also act as a signaling molecule that directs genome-wide gene expression changes with an impact on plant metabolism, physiology, growth and development. Nitrate and phytohormone signaling pathways crosstalk to modulate growth and developmental programs in a multifactorial manner. Nitrate-signaling controls plant growth and development using molecular mechanisms that involve phytohormone-signaling pathways. In contrast, many phytohormones modulate or impact nitrate signaling in interconnected pathways. In this review, we explore recent progress in our understanding of well-documented connections between nitrate and phytohormones such as auxin, cytokinin and abscisic acid. We also discuss recent studies connecting nitrate to other phytohormones such as ethylene, salicylic acid, gibberellins and brassinosteroids. While many molecular details remain to be elucidated, a number of core signaling components at the intersection between nitrate and the major hormonal pathways have been described. We focus on established interactions of nitrate and different hormonal pathways to bring about cellular, growth and developmental processes in Arabidopsis thaliana.

Cadmium stress suppresses lateral root formation by interfering with the root clock

A biological clock activated by oscillating signals, known as root clock, has been linked to lateral root (LR) formation and is essential for regular LR spacing along the primary root. However, it remains unclear how this internal mechanism is influenced by environmental factors known to affect the LR pattern. Here, we report that excessive cadmium (Cd) inhibits LR formation by disrupting the lateral root cap (LRC)-programmed cell death (PCD)-regulated root clock. Cd restricts the frequency of the oscillating signal rather than its amplitude. This could be attributed to the inhibition on meristematic activity by Cd, which resulted in decreased LRC cell number and LRC-PCD frequency. Genetic evidence further showed that LRC cell number is positively correlated with root resistance to Cd. Our study reveals root cap dynamics as a novel mechanism mediating root responses to Cd, providing insight into the signalling pathways of the root clock responding to environmental cues.

A phenotype-directed chemical screen identifies ponalrestat as an inhibitor of the plant flavin monooxygenase YUCCA in auxin biosynthesis

Plant development is regulated by both synergistic and antagonistic interactions of different phytohormones, including a complex crosstalk between ethylene and auxin. For instance, auxin and ethylene synergistically control primary root elongation and root hair formation. However, a lack of chemical agents that specifically modulate ethylene or auxin production has precluded precise delineation of the contribution of each hormone to root development. Here, we performed a chemical genetic screen based on the recovery of root growth in ethylene-related Arabidopsis mutants with constitutive "short root" phenotypes (eto1-2 and ctr1-1). We found that ponalrestat exposure recovers root elongation in these mutants in an ethylene signal-independent manner. Genetic and pharmacological investigations revealed that ponalrestat inhibits the enzymatic activity of the flavin-containing monooxygenase YUCCA, which catalyzes the rate-limiting step of the indole-3-pyruvic acid branch of the auxin biosynthesis pathway. In summary, our findings have identified a YUCCA inhibitor that may be useful as a chemical tool to dissect the distinct steps in auxin biosynthesis and in the regulation of root development.

Irrigation Combined with Aeration Promoted Soil Respiration through Increasing Soil Microbes, Enzymes, and Crop Growth in Tomato Fields

Soil respiration (Rs) is one of the major components controlling the carbon budget of terrestrial ecosystems. Aerated irrigation has been proven to increase Rs compared with the control, but the mechanisms of CO2 release remain poorly understood. The objective of this study was (1) to test the effects of irrigation, aeration, and their interaction on Rs, soil physical and biotic properties (soil water-filled pore space, temperature, bacteria, fungi, actinomycetes, microbial biomass carbon, cellulose activity, dehydrogenase activity, root morphology, and dry biomass of tomato), and (2) to assess how soil physical and biotic variables control Rs. Therefore, three irrigation levels were included (60%, 80%, and 100% of full irrigation). Each irrigation level contained aeration and control. A total of six treatments were included. The results showed that aeration significantly increased total root length, dry biomass of leaf, stem, and fruit compared with the control (p < 0.05). The positive effect of irrigation on dry biomass of leaf, fruit, and root was significant (p < 0.05). With respect to the control, greater Rs under aeration (averaging 6.2% increase) was mainly driven by soil water-filled pore space, soil bacteria, and soil fungi. The results of this study are helpful for understanding the mechanisms of soil CO2 release under aerated subsurface drip irrigation.

Inhibition of Aminotransferases by Aminoethoxyvinylglycine Triggers a Nitrogen Limitation Condition and Deregulation of Histidine Homeostasis That Impact Root and Shoot Development and Nitrate Uptake

Background and Aims: Although AVG (aminoethoxyvinylglycine) is intensely used to decipher signaling in ethylene/indol-3-acetic acid (IAA) interactions on root morphogenesis, AVG is not a specific inhibitor of aminocyclopropane-1-carboxylate synthase (ACS) and tryptophan aminotransferase (TAA) and tryptophan aminotransferase related (TAR) activities since it is able to inhibit several aminotransferases involved in N metabolism. Indeed, 1 mM glutamate (Glu) supply to the roots in plants treated with 10 μM AVG partially restores the root growth. Here, we highlight the changes induced by AVG and AVG + Glu treatments on the N metabolism impairment and root morphogenetic program. Methods: Root nitrate uptake induced by AVG and AVG + Glu treatments was measured by a differential labeling with 15NO3 - and 15Nglutamate. In parallel a profiling of amino acids (AA) was performed to decipher the impairment of AA metabolism. Key Results: 10 μM AVG treatment increases K15NO3 uptake and 15N translocation during root growth inhibition whereas 10 μM AVG + 1 mM 15Nglutamate treatment inhibits K15NO3 uptake and increases 15Nglutamate uptake during partial root growth restoration. This is explained by a nitrogen (N) limitation condition induced by AVG treatment and a N excess condition induced by AVG + Glu treatment. AA levels were mainly impaired by AVG treatment in roots, where levels of Ser, Thr, α-Ala, β-Ala, Val, Asn and His were significantly increased. His was the only amino acid for which no restoration was observed in roots and shoots after glutamate treatment suggesting important control of His homeostasis on aminotransferase network. Results were discussed in light of recent findings on the interconnection between His homeostasis and the general amino acid control system (GAAC) in eukaryotes. Conclusions: These results demonstrate that AVG concentration above 5 μM is a powerful pharmacological tool for unraveling the involvement of GAAC system or new N sensory system in morphological and metabolic changes of the roots in leguminous and non-leguminous plants.

Peg Biology: Deciphering the Molecular Regulations Involved During Peanut Peg Development

Peanut or groundnut is one of the most important legume crops with high protein and oil content. The high nutritional qualities of peanut and its multiple usage have made it an indispensable component of our daily life, in both confectionary and therapeutic food industries. Given the socio-economic significance of peanut, understanding its developmental biology is important in providing a molecular framework to support breeding activities. In peanut, the formation and directional growth of a specialized reproductive organ called a peg, or gynophore, is especially relevant in genetic improvement. Several studies have indicated that peanut yield can be improved by improving reproductive traits including peg development. Therefore, we aim to identify unifying principles for the genetic control, underpinning molecular and physiological basis of peg development for devising appropriate strategy for peg improvement. This review discusses the current understanding of the molecular aspects of peanut peg development citing several studies explaining the key mechanisms. Deciphering and integrating recent transcriptomic, proteomic, and miRNA-regulomic studies provide a new perspective for understanding the regulatory events of peg development that participate in pod formation and thus control yield.

The Road to Auxin-Dependent Growth Repression and Promotion in Apical Hooks

The phytohormone auxin controls growth rates within plant tissues, but the underlying mechanisms are still largely enigmatic. The apical hook is a superb model to understand differential growth, because it displays both auxin-dependent growth repression and promotion. In this special issue on membranes, we illustrate how the distinct utilization of vesicle trafficking contributes to the spatial control of polar auxin transport, thereby pinpointing the site of growth repression in apical hooks. We moreover highlight that the transition to growth promotion is achieved by balancing inter- and intracellular auxin transport. We emphasize here that the apical hook development is a suitable model to further advance our mechanistic knowledge on plant growth regulation.

ETR1 Integrates Response to Ethylene and Cytokinins into a Single Multistep Phosphorelay Pathway to Control Root Growth

Cytokinins and ethylene control plant development via sensors from the histidine kinase (HK) family. However, downstream signaling pathways for the key phytohormones are distinct. Here we report that not only cytokinin but also ethylene is able to control root apical meristem (RAM) size through activation of the multistep phosphorelay (MSP) pathway. We found that both cytokinin and ethylene-dependent RAM shortening requires ethylene binding to ETR1 and the HK activity of ETR1. The receiver domain of ETR1 interacts with MSP signaling intermediates acting downstream of cytokinin receptors, further substantiating the role of ETR1 in MSP signaling. We revealed that both cytokinin and ethylene induce the MSP in similar and distinct cell types with ETR1-mediated ethylene signaling controlling MSP output specifically in the root transition zone. We identified members of the MSP pathway specific and common to both hormones and showed that ETR1-regulated ARR3 controls RAM size. ETR1-mediated MSP spatially differs from canonical CTR1/EIN2/EIN3 ethylene signaling and is independent of EIN2, indicating that both pathways can be spatially and functionally separated. Furthermore, we demonstrated that canonical ethylene signaling controls MSP responsiveness to cytokinin specifically in the root transition zone, presumably via regulation of ARR10, one of the positive regulators of MSP signaling in Arabidopsis.

Apocarotenoids Involved in Plant Development and Stress Response

Carotenoids are isoprenoid pigments synthesized by all photosynthetic organisms and many heterotrophic microorganisms. They are equipped with a conjugated double-bond system that builds the basis for their role in harvesting light energy and in protecting the cell from photo-oxidation. In addition, the carotenoids polyene makes them susceptible to oxidative cleavage, yielding carbonyl products called apocarotenoids. This oxidation can be catalyzed by carotenoid cleavage dioxygenases or triggered nonenzymatically by reactive oxygen species. The group of plant apocarotenoids includes important phytohormones, such as abscisic acid and strigolactones, and signaling molecules, such as β-cyclocitral. Abscisic acid is a key regulator of plant's response to abiotic stress and is involved in different developmental processes, such as seed dormancy. Strigolactone is a main regulator of plant architecture and an important signaling molecule in the plant-rhizosphere communication. β-Cyclocitral, a volatile derived from β-carotene oxidation, mediates the response of cells to singlet oxygen stress. Besides these well-known examples, recent research unraveled novel apocarotenoid growth regulators and suggests the presence of yet unidentified ones. In this review, we describe the biosynthesis and biological functions of established regulatory apocarotenoids and touch on the recently identified anchorene and zaxinone, with emphasis on their role in plant growth, development, and stress response.

Jasmonic Acid Methyl Ester Induces Xylogenesis and Modulates Auxin-Induced Xylary Cell Identity with NO Involvement

In Arabidopsis basal hypocotyls of dark-grown seedlings, xylary cells may form from the pericycle as an alternative to adventitious roots. Several hormones may induce xylogenesis, as Jasmonic acid (JA), as well as indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) auxins, which also affect xylary identity. Studies with the ethylene (ET)-perception mutant ein3eil1 and the ET-precursor 1-aminocyclopropane-1-carboxylic acid (ACC), also demonstrate ET involvement in IBA-induced ectopic metaxylem. Moreover, nitric oxide (NO), produced after IBA/IAA-treatments, may affect JA signalling and interact positively/negatively with ET. To date, NO-involvement in ET/JA-mediated xylogenesis has never been investigated. To study this, and unravel JA-effects on xylary identity, xylogenesis was investigated in hypocotyls of seedlings treated with JA methyl-ester (JAMe) with/without ACC, IBA, IAA. Wild-type (wt) and ein3eil1 responses to hormonal treatments were compared, and the NO signal was quantified and its role evaluated by using NO-donors/scavengers. Ectopic-protoxylem increased in the wt only after treatment with JAMe(10 μM), whereas in ein3eil1 with any JAMe concentration. NO was detected in cells leading to either xylogenesis or adventitious rooting, and increased after treatment with JAMe(10 μM) combined or not with IBA(10 μM). Xylary identity changed when JAMe was applied with each auxin. Altogether, the results show that xylogenesis is induced by JA and NO positively regulates this process. In addition, NO also negatively interacts with ET-signalling and modulates auxin-induced xylary identity.

Arabidopsis ANAC092 regulates auxin-mediated root development by binding to the ARF8 and PIN4 promoters

Auxin is an important plant hormone that is essential for growth and development due to its effects on organogenesis, morphogenesis, tropisms, and apical dominance. The functional diversity of auxin highlights the importance of its biosynthesis, transport, and associated responses. In this study, we show that a NAC transcription factor, ANAC092 (also named AtNAC2 and ORESARA1), known to positively regulate leaf senescence and contribute to abiotic stress responses, also affects primary root development. Plants overexpressing ANAC092 had altered root meristem lengths and shorter primary roots compared with the wild-type control. Additionally, expression of the proANAC092::GUS was strongly induced by indole-3-acetic acid. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed that the YUCCA2, PIN, and ARF expression levels were downregulated in ANAC092-overexpressing plants. Moreover, yeast one-hybrid and chromatin immunoprecipitation assays confirmed that ANAC092 binds to the promoters of AUXIN RESPONSE FACTOR 8 (ARF8) and PIN-FORMED 4 (PIN4). Furthermore, a dual-luciferase assay indicated that ANAC092 decreases ARF8 and PIN4 promoter activities. We also applied a CRISPR/Cas9 system to mutate ANAC092. The roots of three of the analyzed mutants were longer than normal. Collectively, our findings indicate that ANAC092 negatively affects root development by controlling the auxin pathway.

Tackling Plant Phosphate Starvation by the Roots

Plant responses to phosphate deprivation encompass a wide range of strategies, varying from altering root system architecture, entering symbiotic interactions to excreting root exudates for phosphorous release, and recycling of internal phosphate. These processes are tightly controlled by a complex network of proteins that are specifically upregulated upon phosphate starvation. Although the different effects of phosphate starvation have been intensely studied, the full extent of its contribution to altered root system architecture remains unclear. In this review, we focus on the effect of phosphate starvation on the developmental processes that shape the plant root system and their underlying molecular pathways.

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.

Danger-Associated Peptides Interact with PIN-Dependent Local Auxin Distribution to Inhibit Root Growth in Arabidopsis

Plant elicitor peptides (Peps) are damage/danger-associated molecular patterns that are perceived by the receptor-like kinases, PEPR1 and PEPR2, to enhance innate immunity and to inhibit root growth in Arabidopsis (Arabidopsis thaliana). Here, we show that Arabidopsis Pep1 inhibits root growth in a PEPR2-dependent manner, which is accompanied by swelling epidermal and cortex cells and root hair formation in the transition zone (TZ). These Pep1-induced changes were mimicked by exogenous auxin application and were suppressed in the auxin perception mutants transport inhibitor response1 (tir1) and tir1 afb1 afb2 Pep1-induced auxin accumulation in the TZ region preceded cell expansion in roots. Because local auxin distribution depends on PIN-type auxin transporters, we examined Pep1-PEPR-induced root growth inhibition in several pin mutants and found that pin2 was highly sensitive but pin3 was less sensitive to Pep1. The pin2 pin3 double mutant was as sensitive to Pep1 treatment as wild-type plants. Pep1 reduced the abundance of PIN2 in the plasma membrane through activating endocytosis while increasing PIN3 expression in the TZ, leading to changes in local auxin distribution and inhibiting root growth. These results suggest that Pep-PEPR signaling undergoes crosstalk with auxin accumulation to control cell expansion and differentiation in roots during immune responses.

Rice OsDOF15 contributes to ethylene-inhibited primary root elongation under salt stress

In early seedlings, the primary root adapts rapidly to environmental changes through the modulation of endogenous hormone levels. The phytohormone ethylene inhibits primary root elongation, but the underlying molecular mechanism of how ethylene-reduced root growth is modulated in environmental changes remains poorly understood. Here, we show that a novel rice (Oryza sativa) DOF transcription factor OsDOF15 positively regulates primary root elongation by regulating cell proliferation in the root meristem, via restricting ethylene biosynthesis. Loss-of-function of OsDOF15 impaired primary root elongation and cell proliferation in the root meristem, whereas OsDOF15 overexpression enhanced these processes, indicating that OsDOF15 is a key regulator of primary root elongation. This regulation involves the direct interaction of OsDOF15 with the promoter of OsACS1, resulting in the repression of ethylene biosynthesis. The control of ethylene biosynthesis by OsDOF15 in turn regulates cell proliferation in the root meristem. OsDOF15 transcription is repressed by salt stress, and OsDOF15-mediated ethylene biosynthesis plays a role in inhibition of primary root elongation by salt stress. Thus, our data reveal how the ethylene-inhibited primary root elongation is finely controlled by OsDOF15 in response to environmental signal, a novel mechanism of plants responding to salt stress and transmitting the information to ethylene biosynthesis to restrict root elongation.

Selenium downregulates auxin and ethylene biosynthesis in rice seedlings to modify primary metabolism and root architecture

Selenium modulates the formation of primary and lateral roots through alterations in auxin and ethylene, leading to new patterns of root architecture in rice seedlings. Selenium (Se) at low concentrations can control root growth through interaction with hormone biosynthesis. Auxin and ethylene have been shown to control the root architecture, with most of the information obtained from the eudicots such Arabidopsis and Nicotiana tabacum. Here, we presented the effects of Se on auxin and ethylene pathways and examined their impact on primary metabolism and root system architecture in rice (Oryza sativa L.) seedlings. Se treatment increased elongation of primary root, but decreased the number and length of lateral roots. Se led to decreased expression of genes associated with the biosynthesis of auxin and ethylene, concomitantly with reduced production of these hormones by the roots. Moreover, Se decreased the abundance of transcripts encoding auxin transport proteins. Indole-3-acetic acid (IAA) treatment overrode the repressive effect of Se on lateral root growth. The ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)-glycine (AVG) increased elongation of primary root, whereas the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) resulted in the opposite effect. Soluble sugars accumulate in roots of rice seedlings under Se treatment. Thus, Se modulates the formation of primary and lateral roots through alterations in auxin and ethylene, leading to new patterns of root architecture in rice seedlings.

Light-induced stabilization of ACS contributes to hypocotyl elongation during the dark-to-light transition in Arabidopsis seedlings

Hypocotyl growth during seedling emergence is a crucial developmental transition influenced by light and phytohormones such as ethylene. Ethylene and light antagonistically control hypocotyl growth in either continuous light or darkness. However, how ethylene and light regulate hypocotyl growth, including seedling emergence, during the dark-to-light transition remains elusive. Here, we show that ethylene and light cooperatively stimulate a transient increase in hypocotyl growth during the dark-to-light transition via the light-mediated stabilization of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs), the rate-limiting enzymes in ethylene biosynthesis. We found that, in contrast to the known inhibitory role of light in hypocotyl growth, light treatment transiently increases hypocotyl growth in wild-type etiolated seedlings. Moreover, ACC, the direct precursor of ethylene, accentuates the effects of light on hypocotyl elongation during the dark-to-light transition. We determined that light leads to the transient elongation of hypocotyls by stabilizing the ACS5 protein during the dark-to-light transition. Furthermore, biochemical analysis of an ACS5 mutant protein bearing an alteration in the C-terminus indicated that light stabilizes ACS5 by inhibiting the degradation mechanism that acts through the C-terminus of ACS5. Our study reveals that plants regulate hypocotyl elongation during seedling establishment by coordinating light-induced ethylene biosynthesis at the post-translational level. Moreover, the stimulatory role of light on hypocotyl growth during the dark-to-light transition provides additional insights into the known inhibitory role of light in hypocotyl development.

Spatio-temporal IAA gradient is determined by interactions with ET and governs flower abscission

The abscission zone (AZ) is a specialized tissue that usually develops at the base of an organ and is highly sensitive to phytohormones, e.g., abscisic acid (ABA), ethylene (ET), and gibberellins (GAs). A current model of organ abscission assumes that the formation of an auxin gradient around the AZ area determines the time of shedding; however, that thesis is supported by studies that are primarily concerned with auxin transporters. To better understand the events underlying the progression of abscission, we focused for the first time on indole-3-acetic acid (IAA) distribution following AZ activation. We performed a series of immunolocalization studies in proximal and distal regions of floral AZ cells in yellow lupine, which is an agriculturally important legume. The examined phytohormone was abundant in natural active AZ cells, as well as above and below parts of this structure. A similar gradient of IAA was observed during the early steps of abscission, which was induced artificially by flower removal. Surprisingly, IAA was not detected in inactive AZ cells. This paper is also a consequence of our comprehensive studies concerning the phytohormonal regulation of flower abscission in yellow lupine. We present new data on interactions between IAA and ET, previously pointed out as a strong modulator of flower separation. The detailed analysis shows that disruption of the natural auxin gradient around the AZ area through the application of synthetic IAA had a positive effect on ET biosynthesis genes. We proved that these changes are accompanied by an accumulation of the ET precursor. On the other hand, exposure to ET significantly affected IAA localization in the whole AZ area in a time-dependent manner. Our results provide insight into the existence of a spatio-temporal sequential pattern of the IAA gradient related to the abscission process; this pattern is maintained by interactions with ET. We present new valuable evidence for the existence of conservative mechanisms that regulate generative organ separation and can help to improve the yield of agronomically significant species in the future.

Effects of auxin and ethylene on root growth adaptation to different ambient temperatures in Arabidopsis

As sessile organisms, plants can modify their growth strategy in response to different temperatures, however very little is known about how roots growth responds to ambient temperature change. Here, we found that high temperature-induced root elongation is dependent on light intensity and the root growth of most TAA1 loss-of-function mutants is more sensitive to higher temperatures in Arabidopsis. TAA1 encodes a tryptophan aminotransferase which involved in the indole-3-pyruvic acid (IPA) pathway of indole-3-acetic acid (IAA) biosynthesis. The root elongation in ckrc1-1(one allele mutant of TAA1) is less sensitive to lower temperatures and more sensitive to higher temperatures than that of Col-0. By comparing the regulatory mechanisms of ckrc1-1 root growth at different temperatures (17 °C, 22 °C, and 27 °C), different interactions between signals (auxin and ethylene) and the effects of downstream genes were observed at different ambient temperatures in Arabidopsis. Lower temperature-enhanced ETR1-mediated ethylene signaling did not promote the expression of CKRC1, while higher temperature-enhanced signaling did. CKRC1 had an important role in the ACC inhibition of cell elongation at 22 °C and 27 °C but not at 17 °C. CKRC1-dependent auxin biosynthesis was critical for maintaining PIN1, PIN2, and AUX1 expression at lower temperatures. CKRC1, AUX1, and PIN2 regulated root elongation by affecting different regions of the root at different temperatures in Arabidopsis. Our experimental results suggested that changes in the in vivo signals at different temperatures were multi-layered in Arabidopsis.

Editing of the OsACS locus alters phosphate deficiency-induced adaptive responses in rice seedlings

Phosphate (Pi) deficiency severely influences the growth and reproduction of plants. To cope with Pi deficiency, plants initiate morphological and biochemical adaptive responses upon sensing low Pi in the soil, and the plant hormone ethylene plays a crucial role during this process. However, how regulation of ethylene biosynthesis influences the Pi-induced adaptive responses remains unclear. Here, we determine the roles of rice 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS), the rate-limiting enzymes in ethylene biosynthesis, in response to Pi deficiency. Through analysis of tissue-specific expression of OsACS in response to Pi deficiency and OsACS mutants generated by CRISPR/Cas9 [clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9] genome editing, we found that two members of the OsACS family, i.e. OsACS1 and OsACS2, are involved but differed in their importance in controlling the remodeling of root system architecture, transcriptional regulation of Pi starvation-induced genes, and cellular phosphorus homeostasis. Interestingly, in contrast to the known inhibitory role of ethylene on root elongation, both OsACS mutants, especially OsACS1, almost fail to promote lateral root growth in response to Pi deficiency, demonstrating a stimulatory role for ethylene in lateral root development under Pi-deficient conditions. Together, this study provides new insights into the roles of ethylene in Pi deficiency response in rice seedlings and the isoform-specific function of OsACS genes in this process.

CONSTITUTIVE TRIPLE RESPONSE1 and PIN2 act in a coordinate manner to support the indeterminate root growth and meristem cell proliferating activity in Arabidopsis seedlings

The plant hormone ethylene induces auxin biosynthesis and transport and modulates root growth and branching. However, its function on root stem cells and the identity of interacting factors for the control of meristem activity remains unclear. Genetic analysis for primary root growth in wild-type (WT) Arabidopsis thaliana seedlings and ethylene-related mutants showed that the loss-of-function of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) inhibits cell division and elongation. This phenotype is associated with an increase in the expression of the auxin transporter PIN2 and a drastic decrease in the expression of key factors for stem cell niche maintenance such as PLETHORA1, SHORT ROOT and SCARECROW. While the root stem cell niche is affected in ctr1 mutants, its maintenance is severely compromised in the ctr1-1eir1-1(pin2) double mutant, in which an evident loss of proliferative capacity of the meristematic cells leads to a fully differentiated root meristem shortly after germination. Root traits affected in ctr1-1 mutants could be restored in ctr1-1ein2-1 double mutants. These results reveal that ethylene perception via CTR1 and EIN2 in the root modulates the proliferative capacity of root stem cells via affecting the expression of genes involved in the two major pathways, AUX-PIN-PLT and SCR-SHR, which are key factors for proper root stem cell niche maintenance.

Analysis of Differentially Expressed Genes in Tissues of Camellia sinensis during Dedifferentiation and Root Redifferentiation

Tissue culture is very important for identifying the gene function of Camellia sinensis (L.) and exploiting novel germplasm through transgenic technology. Regeneration system of tea plant has been explored but not been well established since the molecular mechanism of tea plant regeneration is not clear yet. In this study, transcriptomic analysis was performed in the initial explants of tea plant and their dedifferentiated and redifferentiated tissues. A total of 93,607 unigenes were obtained through de novo assembly, and 7,193 differentially expressed genes (DEGs) were screened out from the 42,417 annotated unigenes. Much more DEGs were observed during phase transition rather than at growth stages of callus. Our KOG and KEGG analysis, and qPCR results confirmed that phase transition of tea plant was closely related to the mechanism that regulate expression of genes encoding the auxin- and cytokinin-responsive proteins, transcription factor MYB15 and ethylene-responsive transcription factor ERF RAP2-12. These findings provide a reliable foundation for elucidating the mechanism of the phase transition and may help to optimize the regeneration system by regulating the gene expression pattern.

Auxin Modulated Initiation of Lateral Roots Is Linked to Pericycle Cell Length in Maize

Auxin is essential for the regulation of root system architecture by controlling primary root elongation and lateral root (LR) formation. Exogenous auxin has been reported to inhibit primary root elongation and promote the formation of LRs. In this study, LR formation in the Zea mays primary root was quantitatively evaluated after exogenous auxin treatment by comparing the effects of auxin on two selected zones elongated either before or after auxin application. We determined two main variables in both zones: the LR density per unit of root length (LRD), and the mean phloem pericycle cell length. The total number of phloem pericycle cells (PPCs) per unit of root length was then calculated. Considering that each LR primordium is initiated from four founder cells (FCs), the percentage of PPCs (%PPC) that behave as FCs in a specific root zone was estimated by dividing the number of pericycle cells by four times the LRD. This index was utilized to describe LR initiation. Root zones elongated in the presence of a synthetic auxin (1-naphthalene acetic acid, NAA) at low concentrations (0.01 μM) showed reduced cell length and increased LRD. However, a high concentration of NAA (0.1 μM) strongly reduced both cell length and LRD. In contrast, both low and high levels of NAA stimulated LRD in zones elongated before auxin application. Analysis of the percentage of FCs in the phloem pericycle in zones elongated in the presence or absence of NAA showed that low concentrations of NAA increased the %PFC, indicating that LR initiation is promoted at new sites; however, high concentrations of NAA elicited a considerable reduction in this variable in zones developed in the presence of auxin. As these zones are composed of short pericycle cells, we propose that short pericycle cells are incapable to participate in LR primordium initiation and that auxin modulated initiation of LRs is linked to pericycle cell length.

The Coordination of Ethylene and Other Hormones in Primary Root Development

The primary root is the basic component of root systems, initiates during embryogenesis and develops shortly after germination, and plays a key role in early seedling growth and survival. The phytohormone ethylene shows significant inhibition of the growth of primary roots. Recent findings have revealed that the inhibition of ethylene in primary root elongation is mediated via interactions with phytohormones, such as auxin, abscisic acid, gibberellin, cytokinins, jasmonic acid, and brassinosteroids. Considering that Arabidopsis and rice are the model plants of dicots and monocots, as well as the fact that hormonal crosstalk in primary root growth has been extensively investigated in Arabidopsis and rice, a better understanding of the mechanisms in Arabidopsis and rice will increase potential applications in other species. Therefore, we focus our interest on the emerging studies in the research of ethylene and hormone crosstalk in primary root development in Arabidopsis and rice.

Light Modulates Ethylene Synthesis, Signaling, and Downstream Transcriptional Networks to Control Plant Development

The inhibition of hypocotyl elongation by ethylene in dark-grown seedlings was the basis of elegant screens that identified ethylene-insensitive Arabidopsis mutants, which remained tall even when treated with high concentrations of ethylene. This simple approach proved invaluable for identification and molecular characterization of major players in the ethylene signaling and response pathway, including receptors and downstream signaling proteins, as well as transcription factors that mediate the extensive transcriptional remodeling observed in response to elevated ethylene. However, the dark-adapted early developmental stage used in these experiments represents only a small segment of a plant's life cycle. After a seedling's emergence from the soil, light signaling pathways elicit a switch in developmental programming and the hormonal circuitry that controls it. Accordingly, ethylene levels and responses diverge under these different environmental conditions. In this review, we compare and contrast ethylene synthesis, perception, and response in light and dark contexts, including the molecular mechanisms linking light responses to ethylene biology. One powerful method to identify similarities and differences in these important regulatory processes is through comparison of transcriptomic datasets resulting from manipulation of ethylene levels or signaling under varying light conditions. We performed a meta-analysis of multiple transcriptomic datasets to uncover transcriptional responses to ethylene that are both light-dependent and light-independent. We identified a core set of 139 transcripts with robust and consistent responses to elevated ethylene across three root-specific datasets. This "gold standard" group of ethylene-regulated transcripts includes mRNAs encoding numerous proteins that function in ethylene signaling and synthesis, but also reveals a number of previously uncharacterized gene products that may contribute to ethylene response phenotypes. Understanding these light-dependent differences in ethylene signaling and synthesis will provide greater insight into the roles of ethylene in growth and development across the entire plant life cycle.

Transcriptome reprogramming, epigenetic modifications and alternative splicing orchestrate the tomato root response to the beneficial fungus Trichoderma harzianum

Beneficial interactions of rhizosphere microorganisms are widely exploited for plant biofertilization and mitigation of biotic and abiotic constraints. To provide new insights into the onset of the roots-beneficial microorganisms interplay, we characterised the transcriptomes expressed in tomato roots at 24, 48 and 72 h post inoculation with the beneficial fungus Trichoderma harzianum T22 and analysed the epigenetic and post-trascriptional regulation mechanisms. We detected 1243 tomato transcripts that were differentially expressed between Trichoderma-interacting and control roots and 83 T. harzianum transcripts that were differentially expressed between the three experimental time points. Interaction with Trichoderma triggered a transcriptional response mainly ascribable to signal recognition and transduction, stress response, transcriptional regulation and transport. In tomato roots, salicylic acid, and not jasmonate, appears to have a prominent role in orchestrating the interplay with this beneficial strain. Differential regulation of many nutrient transporter genes indicated a strong effect on plant nutrition processes, which, together with the possible modifications in root architecture triggered by ethylene/indole-3-acetic acid signalling at 72 h post inoculation may concur to the well-described growth-promotion ability of this strain. Alongside, T. harzianum-induced defence priming and stress tolerance may be mediated by the induction of reactive oxygen species, detoxification and defence genes. A deeper insight into gene expression and regulation control provided first evidences for the involvement of cytosine methylation and alternative splicing mechanisms in the plant-Trichoderma interaction. A model is proposed that integrates the plant transcriptomic responses in the roots, where interaction between the plant and beneficial rhizosphere microorganisms occurs.

Auxin response factor 6A regulates photosynthesis, sugar accumulation, and fruit development in tomato

Auxin response factors (ARFs) are involved in auxin-mediated transcriptional regulation in plants. In this study, we performed functional characterization of SlARF6A in tomato. SlARF6A is located in the nucleus and exhibits transcriptional activator activity. Overexpression of SlARF6A increased chlorophyll contents in the fruits and leaves of tomato plants, whereas downregulation of SlARF6A decreased chlorophyll contents compared with those of wild-type (WT) plants. Analysis of chloroplasts using transmission electron microscopy indicated increased sizes of chloroplasts in SlARF6A-overexpressing plants and decreased numbers of chloroplasts in SlARF6A-downregulated plants. Overexpression of SlARF6A increased the photosynthesis rate and accumulation of starch and soluble sugars, whereas knockdown of SlARF6A resulted in opposite phenotypes in tomato leaves and fruits. RNA-sequence analysis showed that regulation of SlARF6A expression altered the expression of genes involved in chlorophyll metabolism, photosynthesis and sugar metabolism. SlARF6A directly bound to the promoters of SlGLK1, CAB, and RbcS genes and positively regulated the expression of these genes. Overexpression of SlARF6A also inhibited fruit ripening and ethylene production, whereas downregulation of SlARF6A increased fruit ripening and ethylene production. SlARF6A directly bound to the SAMS1 promoter and negatively regulated SAMS1 expression. Taken together, these results expand our understanding of ARFs with regard to photosynthesis, sugar accumulation and fruit development and provide a potential target for genetic engineering to improve fruit nutrition in horticulture crops.

Involvement of Pyridoxine/Pyridoxamine 5'-Phosphate Oxidase (PDX3) in Ethylene-Induced Auxin Biosynthesis in the Arabidopsis Root

As sessile organisms, plants have evolved to adjust their growth and development to environmental changes. It has been well documented that the crosstalk between different plant hormones plays important roles in the coordination of growth and development of the plant. Here, we describe a novel recessive mutant, mildly insensitive to ethylene (mine), which displayed insensitivity to the ethylene precursor, ACC (1-aminocyclopropane-1-carboxylic acid), in the root under the dark-grown conditions. By contrast, mine roots exhibited a normal growth response to exogenous IAA (indole-3-acetic acid). Thus, it appears that the growth responses of mine to ACC and IAA resemble those of weak ethylene insensitive (wei) mutants. To understand the molecular events underlying the crosstalk between ethylene and auxin in the root, we identified the MINE locus and found that the MINE gene encodes the pyridoxine 5'-phosphate (PNP)/pyridoxamine 5'-phosphate (PMP) oxidase, PDX3. Our results revealed that MINE/PDX3 likely plays a role in the conversion of the auxin precursor tryptophan to indole-3-pyruvic acid in the auxin biosynthesis pathway, in which TAA1 (TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1) and its related genes (TRYPTOPHAN AMINOTRANSFERASE RELATED 1 and 2; TAR1 and TAR2) are involved. Considering that TAA1 and TARs belong to a subgroup of PLP (pyridoxal-5'-phosphate)-dependent enzymes, we propose that PLP produced by MINE/PDX3 acts as a cofactor in TAA1/TAR-dependent auxin biosynthesis induced by ethylene, which in turn influences the crosstalk between ethylene and auxin in the Arabidopsis root.

Multiple Links between HD-Zip Proteins and Hormone Networks

HD-Zip proteins are unique to plants, and contain a homeodomain closely linked to a leucine zipper motif, which are involved in dimerization and DNA binding. Based on homology in the HD-Zip domain, gene structure and the presence of additional motifs, HD-Zips are divided into four families, HD-Zip I–IV. Phylogenetic analysis of HD-Zip genes using transcriptomic and genomic datasets from a wide range of plant species indicate that the HD-Zip protein class was already present in green algae. Later, HD-Zips experienced multiple duplication events that promoted neo- and sub-functionalizations. HD-Zip proteins are known to control key developmental and environmental responses, and a growing body of evidence indicates a strict link between members of the HD-Zip II and III families and the auxin machineries. Interactions of HD-Zip proteins with other hormones such as brassinolide and cytokinin have also been described. More recent data indicate that members of different HD-Zip families are directly involved in the regulation of abscisic acid (ABA) homeostasis and signaling. Considering the fundamental role of specific HD-Zip proteins in the control of key developmental pathways and in the cross-talk between auxin and cytokinin, a relevant role of these factors in adjusting plant growth and development to changing environment is emerging.

Supraoptimal Cytokinin Content Inhibits Rice Seminal Root Growth by Reducing Root Meristem Size and Cell Length via Increased Ethylene Content

Cytokinins (CKs), a class of phytohormone, regulate root growth in a dose-dependent manner. A certain threshold content of CK is required for rapid root growth, but supraoptimal CK content inhibits root growth, and the mechanism of this inhibition remains unclear in rice. In this study, treatments of lovastatin (an inhibitor of CK biosynthesis) and kinetin (KT; a synthetic CK) were found to inhibit rice seminal root growth in a dose-dependent manner, suggesting that endogenous CK content is optimal for rapid growth of the seminal root in rice. KT treatment strongly increased ethylene level by upregulating the transcription of ethylene biosynthesis genes. Ethylene produced in response to exogenous KT inhibited rice seminal root growth by reducing meristem size via upregulation of OsIAA3 transcription and reduced cell length by downregulating transcription of cell elongation-related genes. Moreover, the effects of KT treatment on rice seminal root growth, root meristem size and cell length were rescued by treatment with aminoethoxyvinylglycine (an inhibitor of ethylene biosynthesis), which restored ethylene level and transcription levels of OsIAA3 and cell elongation-related genes. Supraoptimal CK content increases ethylene level by promoting ethylene biosynthesis, which in turn inhibits rice seminal root growth by reducing root meristem size and cell length.

Deciphering Auxin-Ethylene Crosstalk at a Systems Level

The auxin and ethylene pathways cooperatively regulate a variety of developmental processes in plants. Growth responses to ethylene are largely dependent on auxin, the key regulator of plant morphogenesis. Auxin, in turn, is capable of inducing ethylene biosynthesis and signaling, making the interaction of these hormones reciprocal. Recent studies discovered a number of molecular events underlying auxin-ethylene crosstalk. In this review, we summarize the results of fine-scale and large-scale experiments on the interactions between the auxin and ethylene pathways in Arabidopsis. We integrate knowledge on molecular crosstalk events, their tissue specificity, and associated phenotypic responses to decipher the crosstalk mechanisms at a systems level. We also discuss the prospects of applying systems biology approaches to study the mechanisms of crosstalk between plant hormones.

Auxin Controlled by Ethylene Steers Root Development

Roots are important plant ground organs, which absorb water and nutrients to control plant growth and development. Phytohormones have been known to play a crucial role in the regulation of root growth, such as auxin and ethylene, which are central regulators of this process. Recent findings have revealed that root development and elongation regulated by ethylene are auxin dependent through alterations of auxin biosynthesis, transport and signaling. In this review, we focus on the recent advances in the study of auxin and auxin⁻ethylene crosstalk in plant root development, demonstrating that auxin and ethylene act synergistically to control primary root and root hair growth, but function antagonistically in lateral root formation. Moreover, ethylene modulates auxin biosynthesis, transport and signaling to fine-tune root growth and development. Thus, this review steps up the understanding of the regulation of auxin and ethylene in root growth.

Root Gravitropism Is Regulated by a Crosstalk between para-Aminobenzoic Acid, Ethylene, and Auxin

Plants respond to gravitational force through directional growth along the gravity vector. Although auxin is the central component of the root graviresponse, it works in concert with other plant hormones. Here, we show that the folate precursor para-aminobenzoic acid (PABA) is a key modulator of the auxin-ethylene interplay during root gravitropism in Arabidopsis (Arabidopsis thaliana). In gravistimulated roots, PABA promotes an asymmetric auxin response, which causes the asymmetric growth responsible for root curvature. This activity requires the auxin response transcription factors AUXIN RESPONSE FACTOR7 (ARF7) and ARF19 as well as ethylene biosynthesis and signaling, indicating that PABA activity requires both auxin and ethylene pathways. Similar to ethylene, exogenous PABA reverses the agravitropic root growth of the auxin transport mutant pin-formed2 (pin2) and the auxin biosynthetic double mutant with loss of function of weak ethylene insensitive (wei) genes, wei8wei2, but not the pin2wei8wei2 triple mutant. This finding suggests that PABA regulates the ethylene-dependent reciprocal compensation between auxin transport and biosynthesis. Furthermore, manipulation of endogenous free PABA levels by modulating the expression of the gene encoding its glucosylation enzyme, UDP-GLYCOSYL TRANSFERASE75B1, impacts the root graviresponse, suggesting that endogenous free PABA levels may play a crucial role in modulating the auxin-ethylene cross talk necessary for root gravitropism.

HOMEOBOX PROTEIN52 Mediates the Crosstalk between Ethylene and Auxin Signaling during Primary Root Elongation by Modulating Auxin Transport-Related Gene Expression

The gaseous hormone ethylene participates in many physiological processes in plants. Ethylene-inhibited root elongation involves PIN-FORMED2 (PIN2)-mediated basipetal auxin transport, but the molecular mechanisms underlying the regulation of PIN2 function by ethylene (and therefore auxin distribution) are poorly understood. Here, we report that the plant-specific and ethylene-responsive HD-Zip gene HB52 is involved in ethylene-mediated inhibition of primary root elongation in Arabidopsis thaliana Biochemical and genetic analyses demonstrated that HB52 is ethylene responsive and acts downstream of ETHYLENE-INSENSITIVE3 (EIN3). HB52 knockdown mutants displayed an ethylene-insensitive phenotype during primary root elongation, while its overexpression resulted in short roots, as observed in ethylene-treated plants. In addition, root auxin distribution and gravitropism were impaired in HB52 knockdown and overexpression lines. Consistent with these findings, in vitro and in vivo binding experiments showed that HB52 regulates the expression of auxin transport-related genes, including PIN2, WAVY ROOT GROWTH1 (WAG1), and WAG2 by physically binding to their promoter regions. These findings suggest that HB52 functions in the ethylene-mediated inhibition of root elongation by modulating the expression of auxin transport components downstream of EIN3, revealing a mechanism in which HB52 acts as an important node in the crosstalk between ethylene and auxin signaling during plant growth and development.

Local Auxin Biosynthesis Is a Key Regulator of Plant Development

Auxin is a major phytohormone that controls numerous aspects of plant development and coordinates plant responses to the environment. Morphogenic gradients of auxin govern cell fate decisions and underlie plant phenotypic plasticity. Polar auxin transport plays a central role in auxin maxima generation. The discovery of the exquisite spatiotemporal expression patterns of auxin biosynthesis genes of the WEI8/TAR and YUC families suggested that local auxin production may contribute to the formation of auxin maxima. Herein, we systematically addressed the role of local auxin biosynthesis in plant development and responses to the stress phytohormone ethylene by manipulating spatiotemporal patterns of WEI8. Our study revealed that local auxin biosynthesis and transport act synergistically and are individually dispensable for root meristem maintenance. In contrast, flower fertility and root responses to ethylene require local auxin production that cannot be fully compensated for by transport in the generation of morphogenic auxin maxima.

TaWRKY51 promotes lateral root formation through negative regulation of ethylene biosynthesis in wheat (Triticum aestivum L.)

Common wheat (Triticum aestivum L.) is an important staple food crop worldwide. Lateral roots (LRs), as the major component of root architecture, affect water and nutrient uptake in wheat. The phytohormone ethylene is known to affect LR formation; however, the factor(s) modulating ethylene during this process have not yet been elucidated in wheat. Here we identified wheat TaWRKY51 as a key factor that functions in LR formation by modulating ethylene biosynthesis. Wheat TaWRKY51RNA interference lines (TaWRKY51-RNAi) and the homozygous mutants tawrky51-2a and tawrky51-2b all produced fewer LRs than the wild type and negative transgenic plants, whereas the TaWRKY51 overexpression lines (TaWRKY51-OE) had the opposite phenotype. Transcription analysis revealed that 1-aminocyclopropane-1-carboxylic acid synthase (ACS) genes (TaACS2, TaACS7 and TaACS8) involved in ethylene biosynthesis were downregulated in TaWRKY51-OE lines but upregulated in TaWRKY51-RNAi lines. The rate of ethylene production also decreased in TaWRKY51-OE lines but increased in TaWRKY51-RNAi lines compared with their respective negative transgenic controls. Electrophoretic mobility shift and transient expression assays revealed that TaWRKY51 inhibits the expression of ACS genes by binding to the W-box cis-element present in their promoter region. Moreover, overexpression of ACS2 or exogenous application of 1-aminocyclopropane-1-carboxylic acid reversed the phenotype of enhanced LR number in TaWRKY51-OE Arabidopsis lines, and overexpression of TaWRKY51 in the ethylene-overproducing mutant eto1-1 rescued its LR defect phenotype. In addition, genetic evidence demonstrates that TaWRKY51-regulated LR formation is also dependent on ethylene and auxin signaling pathways. Our findings reveal a molecular genetic mechanism by which a WRKY gene coordinates ethylene production and LR formation in wheat.

Volatile Compounds Emitted by Diverse Verticillium Species Enhance Plant Growth by Manipulating Auxin Signaling

Some volatile compounds (VC) play critical roles in intra- and interspecies interactions. To investigate roles of VC in fungal ecology, we characterized how VC produced by Verticillium spp., a group of broad-host-range soilborne fungal pathogens, affect plant growth and development. VC produced by 19 strains corresponding to 10 species significantly enhanced the growth of Arabidopsis thaliana and Nicotiana benthamiana. Analysis of VC produced by four species revealed the presence of diverse compounds, including those previously shown to affect plant growth. Using A. thaliana, we investigated the mechanism underpinning plant growth enhancement by Verticillium dahliae VC. Allometric analysis indicated that VC caused preferential resource allocation for root growth over shoot growth. Growth responses of A. thaliana mutants defective in auxin or ethylene signaling suggested the involvement of several components of auxin signaling, with TIR3 playing a key role. AUX1, TIR1, and AXR1 were also implicated but appeared to play lesser roles. Inhibition of auxin efflux using 1-naphthylphthalamic acid blocked VC-mediated growth enhancement. Spatial and temporal expression patterns of the auxin-responsive reporter DR5::GUS indicated that the activation of auxin signaling occurred before enhanced plant growth became visible. Results from this study suggest critical yet overlooked roles of VC in Verticillium ecology and pathology.

The MADS transcription factor CmANR1 positively modulates root system development by directly regulating CmPIN2 in chrysanthemum

Plant root systems are essential for many physiological processes, including water and nutrient absorption. MADS-box transcription factor (TF) genes have been characterized as the important regulators of root development in plants; however, the underlying mechanism is largely unknown, including chrysanthemum. Here, it was found that the overexpression of CmANR1, a chrysanthemum MADS-box TF gene, promoted both adventitious root (AR) and lateral root (LR) development in chrysanthemum. Whole transcriptome sequencing analysis revealed a series of differentially expressed unigenes (DEGs) in the roots of CmANR1-transgenic chrysanthemum plants compared to wild-type plants. Functional annotation of these DEGs by alignment with Gene Ontology (GO) terms and biochemical pathway Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that CmANR1 TF exhibited "DNA binding" and "catalytic" activity, as well as participated in "phytohormone signal transduction". Both chromatin immunoprecipitation-polymerase chain reaction (ChIP-PCR) and gel electrophoresis mobility shift assays (EMSA) indicated the direct binding of CmPIN2 to the recognition site CArG-box motif by CmANR1. Finally, a firefly luciferase imaging assay demonstrated the transcriptional activation of CmPIN2 by CmANR1 in vivo. Overall, our results provide novel insights into the mechanisms of MADS-box TF CmANR1 modulation of both AR and LR development, which occurs by directly regulating auxin transport gene CmPIN2 in chrysanthemum.

GmGRP-like gene confers Al tolerance in Arabidopsis

Aluminium (Al) toxicity restrains water and nutrient uptake and is toxic to plant roots, ultimately inhibiting crop production. Here, we isolated and characterized a soybean glycine-rich protein-like gene (GmGRPL) that is mainly expressed in the root and that is regulated by Al treatment. Overexpression of GmGRPL can alleviate Al-induced root growth inhibition in Arabidopsis. The levels of IAA and ethylene in GmGRPL-overexpressing hairy roots were lower than those in control and RNA interference-exposed GmGRPL hairy roots with or without Al stress, which were mainly regulated by TAA1 and ACO, respectively. In transgenic soybean hairy roots, the MDA, H2O2 and O2-·content in GmGRPL-overexpressing hairy roots were less than that in control and RNA interference-exposed GmGRPL hairy roots under Al stress. In addition, IAA and ACC can enhance the expression level of the GmGRPL promoter with or without Al stress. These results indicated that GmGRPL can alleviate Al-induced root growth inhibition by regulating the level of IAA and ethylene and improving antioxidant activity.

Jasmonate promotes auxin-induced adventitious rooting in dark-grown Arabidopsis thaliana seedlings and stem thin cell layers by a cross-talk with ethylene signalling and a modulation of xylogenesis

BACKGROUND

Adventitious roots (ARs) are often necessary for plant survival, and essential for successful micropropagation. In Arabidopsis thaliana dark-grown seedlings AR-formation occurs from the hypocotyl and is enhanced by application of indole-3-butyric acid (IBA) combined with kinetin (Kin). The same IBA + Kin-treatment induces AR-formation in thin cell layers (TCLs). Auxin is the main inducer of AR-formation and xylogenesis in numerous species and experimental systems. Xylogenesis is competitive to AR-formation in Arabidopsis hypocotyls and TCLs. Jasmonates (JAs) negatively affect AR-formation in de-etiolated Arabidopsis seedlings, but positively affect both AR-formation and xylogenesis in tobacco dark-grown IBA + Kin TCLs. In Arabidopsis the interplay between JAs and auxin in AR-formation vs xylogenesis needs investigation. In de-etiolated Arabidopsis seedlings, the Auxin Response Factors ARF6 and ARF8 positively regulate AR-formation and ARF17 negatively affects the process, but their role in xylogenesis is unknown. The cross-talk between auxin and ethylene (ET) is also important for AR-formation and xylogenesis, occurring through EIN3/EIL1 signalling pathway. EIN3/EIL1 is the direct link for JA and ET-signalling. The research investigated JA role on AR-formation and xylogenesis in Arabidopsis dark-grown seedlings and TCLs, and the relationship with ET and auxin. The JA-donor methyl-jasmonate (MeJA), and/or the ET precursor 1-aminocyclopropane-1-carboxylic acid were applied, and the response of mutants in JA-synthesis and -signalling, and ET-signalling investigated. Endogenous levels of auxin, JA and JA-related compounds, and ARF6, ARF8 and ARF17 expression were monitored.

RESULTS

MeJA, at 0.01 μM, enhances AR-formation, when combined with IBA + Kin, and the response of the early-JA-biosynthesis mutant dde2-2 and the JA-signalling mutant coi1-16 confirmed this result. JA levels early change during TCL-culture, and JA/JA-Ile is immunolocalized in AR-tips and xylogenic cells. The high AR-response of the late JA-biosynthesis mutant opr3 suggests a positive action also of 12-oxophytodienoic acid on AR-formation. The crosstalk between JA and ET-signalling by EIN3/EIL1 is critical for AR-formation, and involves a competitive modulation of xylogenesis. Xylogenesis is enhanced by a MeJA concentration repressing AR-formation, and is positively related to ARF17 expression.

CONCLUSIONS

The JA concentration-dependent role on AR-formation and xylogenesis, and the interaction with ET opens the way to applications in the micropropagation of recalcitrant species.

Mutualistic fungus Phomopsis liquidambari increases root aerenchyma formation through auxin-mediated ethylene accumulation in rice (Oryza sativa L.)

The fungal endophyte Phomopsis liquidambari can improve nitrification rates and alter the abundance and composition of ammonia-oxidizers in the soil rhizosphere of rice. Aerenchyma is related to oxygen transport efficiency and contributes to the enhanced rhizospheric nitrification under flooding conditions. However, whether and how P. liquidambari affects aerenchyma formation is largely unknown. We therefore conducted pot and hydroponic experiments to investigate the changes of aerenchyma area, ethylene and indole-3-acetic acid (IAA) levels in rice with or without P. liquidambari infection. Our results showed that the larger aerenchyma area in rice roots with P. liquidambari inoculation was associated with markedly up-regulated expression of genes related to aerenchyma formation. Meanwhile, P. liquidambari inoculation substantially elevated root porosity (POR) and radial oxygen loss (ROL), leading to the enhancement of oxidation-reduction potential (ORP) under pot condition. Besides, P. liquidambari significantly increased IAA and ethylene levels in rice by stimulating the expression of genes involved in auxin and ethylene biosyntheses. Furthermore, auxin that partly acting upstream of ethylene signalling played an essential role in P. liquidambari-promoted aerenchyma formation. These results verified the direct contribution of P. liquidambari in promoting aerenchyma formation via the accumulation of IAA and ethylene in rice roots, which provides a constructive suggestion for improving hypoxia tolerance through plant-endophyte interactions.

Ethylene-induced microtubule reorientation is essential for fast inhibition of root elongation in Arabidopsis

Microtubule reorientation is a long-standing observation that has been implicated in regulating the inhibitory effect of ethylene on axial elongation of plant cells. However, the signaling mechanism underlying ethylene-induced microtubule reorientation has remained elusive. Here, we reveal, by live confocal imaging and kinetic root elongation assays, that the time courses of ethylene-induced microtubule reorientation and root elongation inhibition are highly correlated, and that microtubule reorientation is required for the full responsiveness of root elongation to ethylene treatment. Our genetic analysis demonstrated that the effect of ethylene on microtubule orientation and root elongation is mainly transduced through the canonical linear ethylene signaling pathway. By using pharmacological and genetic analyses, we demonstrate further that the TIR1/AFBs-Aux/IAAs-ARFs auxin signaling pathway, but not the ABP1-ROP6-RIC1 auxin signaling branch, is essential for ethylene-induced microtubule reorientation and root elongation inhibition. Together, these findings offer evidence for the functional significance and elucidate the signaling mechanism for ethylene-induced microtubule reorientation in fast root elongation inhibition in Arabidopsis.

Evidence for the Involvement of Auxin, Ethylene and ROS Signaling During Primary Root Inhibition of Arabidopsis by the Allelochemical Benzoic Acid

Allelopathy is mediated by plant-derived secondary metabolites (allelochemicals) which are released by donor plants and affect the growth and development of receptor plants. The plant root is the first organ which senses soil allelochemicals this results in the production of a shorter primary root. However, the mechanisms underlying this process remain elusive. Here, we report that a model allelochemical benzoic acid (BA) inhibited primary root elongation of Arabidopsis seedlings by reducing the sizes of both the meristem and elongation zones, and that auxin signaling affected this process. An increase in auxin level in the root tips was associated with increased expression of auxin biosynthesis genes and auxin polar transporter AUX1 and PIN2 genes under BA stress. Mutant analyses demonstrated that AUX1 and PIN2 rather than PIN1 were required for the inhibition of primary root elongation during BA exposure. Furthermore, BA stimulated ethylene evolution, whereas blocking BA-induced ethylene signaling with an ethylene biosynthesis inhibitor (Co2+), an ethylene perception antagonist (1-methylcyclopropene) or ethylene signaling mutant lines etr1-3 and ein3eil1 compromised BA-mediated inhibition of root elongation and up-regulation of auxin biosynthesis-related genes together with AUX1 and PIN2, indicating that ethylene signal was involved in auxin-mediated inhibition of primary root elongation during BA stress. Further analysis revealed that the BA-induced reactive oxygen species (ROS) burst contributed to BA-mediated root growth inhibition without affecting auxin and ethylene signals. Taken together, our results reveal that the allelochemical BA inhibits root elongation by increasing auxin accumulation via stimulation of auxin biosynthesis and AUX1/PIN2-mediated auxin transport via stimulation of ethylene production and an auxin/ethylene-independent ROS burst.

Hormonal regulation in adventitious roots and during their emergence under waterlogged conditions in wheat

To gain insights into the molecular mechanisms underlying hormonal regulation in adventitious roots and during their emergence under waterlogged conditions in wheat, the present study investigated transcriptional regulation of genes related to hormone metabolism and transport in the root and stem node tissues. Waterlogging-induced inhibition of axile root elongation and lateral root formation, and promotion of surface adventitious and axile root emergence and aerenchyma formation are associated with enhanced expression levels of ethylene biosynthesis genes, ACS7 and ACO2, in both tissues. Inhibition of axile root elongation is also related to increased root indole acetic acid (IAA) and jasmonate (JA) levels that are associated with up-regulation of specific IAA biosynthesis/transport (TDC, YUC1, and PIN9) and JA metabolism (LOX8, AOS1, AOC1, and JAR1) genes, and transcriptional alteration of gibberellin (GA) metabolism genes (GA3ox2 and GA2ox8). Adventitious root emergence from waterlogged stem nodes is associated with increased levels of IAA and GA but decreased levels of cytokinin and abscisic acid (ABA), which are regulated through the expression of specific IAA biosynthesis/transport (TDC, YUC1, and PIN9), cytokinin metabolism (IPT5-2, LOG1, CKX5, and ZOG2), ABA biosynthesis (NCED1 and NCED2), and GA metabolism (GA3ox2 and GA2ox8) genes. These results enhance our understanding of the molecular mechanisms underlying the adaptive response of wheat to waterlogging.

Auxin Acts Downstream of Ethylene and Nitric Oxide to Regulate Magnesium Deficiency-Induced Root Hair Development in Arabidopsis thaliana

This study examines the association of auxin with ethylene and nitric oxide (NO) in regulating the magnesium (Mg) deficiency-induced root hair development in Arabidopsis thaliana. With Mg deficiency, both ethylene and NO promoted the elevation of root auxin levels in roots by inducing the expression of AUXIN-RESISTANT1 (AUX1), PIN-FORMED 1 (PIN1) and PIN2 transporters. In turn, auxin stimulated ethylene and NO production by activating the activities of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO), ACC synthase (ACS), nitrate reductase (NR) and NO synthase-like (NOS-L). These processes constituted an NO/ethylene-auxin feedback loop. Interestingly, however, the roles of ethylene and NO in regulating Mg deficiency-induced root hair development required the action of auxin, but not vice versa. In summary, these results suggest that Mg deficiency induces a positive interaction between the accumulation of auxin and ethylene/NO in roots, with auxin acting downstream of ethylene and NO signals to regulate Mg deficiency-induced root hair morphogenesis.

The tomato Ethylene Response Factor Sl-ERF.B3 integrates ethylene and auxin signaling via direct regulation of Sl-Aux/IAA27

Plant growth and development is coordinated by complex networks of interacting hormones, and cross-talk between ethylene and auxin signaling is essential for a wide range of plant developmental processes. Nevertheless, the molecular links underlying the interaction between the two hormones remain poorly understood. In order to decipher the cross-talk between the Ethylene Response Factor Sl-ERF.B3 and Sl-IAA27, mediating ethylene and auxin signaling, respectively, we combined reverse genetic approaches, physiological methods, transactivation experiments and electrophoretic mobility shift assays. Sl-ERF.B3 is responsive to both ethylene and auxin and ectopic expression of its dominant repressor version (ERF.B3-SRDX) results in impaired sensitivity to auxin with phenotypes recalling those previously reported for Sl-IAA27 downregulated tomato lines. The expression of Sl-IAA27 is dramatically reduced in the ERF.B3-SRDX lines and Sl-ERF.B3 is shown to regulate the expression of Sl-IAA27 via direct binding to its promoter. The data support a model in which the ethylene-responsive Sl-ERF.B3 integrates ethylene and auxin signaling via regulation of the expression of the auxin signaling component Sl-IAA27. The study uncovers a molecular mechanism that links ethylene and auxin signaling in tomato.

Effects of overproduced ethylene on the contents of other phytohormones and expression of their key biosynthetic genes

Ethylene is involved in regulation of various aspects of plant growth and development. Physiological and genetic analyses have indicated the existence of crosstalk between ethylene and other phytohormones, including auxin, cytokinin (CK), abscisic acid (ABA), gibberellin (GA), salicylic acid (SA), jasmonic acid (JA), brassinosteroid (BR) and strigolactone (SL) in regulation of different developmental processes. However, the effects of ethylene on the biosynthesis and contents of these hormones are not fully understood. Here, we investigated how overproduction of ethylene may affect the contents of other plant hormones using the ethylene-overproducing mutant ethylene-overproducer 1 (eto1-1). The contents of various hormones and transcript levels of the associated biosynthetic genes in the 10-day-old Arabidopsis eto1-1 mutant and wild-type (WT) plants were determined and compared. Higher levels of CK and ABA, while lower levels of auxin, SA and GA were observed in eto1-1 plants in comparison with WT, which was supported by the up- or down-regulation of their biosynthetic genes. Although we could not quantify the BR and SL contents in Arabidopsis, we observed that the transcript levels of the potential rate-limiting BR and SL biosynthetic genes were increased in the eto1-1 versus WT plants, suggesting that BR and SL levels might be enhanced by ethylene overproduction. JA level was not affected by overproduction of ethylene, which might be explained by unaltered expression level of the proposed rate-limiting JA biosynthetic gene allene oxide synthase. Taken together, our results suggest that ET affects the levels of auxin, CK, ABA, SA and GA, and potentially BR and SL, by influencing the expression of genes involved in the rate-limiting steps of their biosynthesis.