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

A UPLC-MS/MS method for quantification of metabolites in the ethylene biosynthesis pathway and its biological validation in Arabidopsis

The plant hormone ethylene is of vital importance in the regulation of plant development and stress responses. Recent studies revealed that 1-aminocyclopropane-1-carboxylic acid (ACC) plays a role beyond its function as an ethylene precursor. However, the absence of reliable methods to quantify ACC and its conjugates malonyl-ACC (MACC), glutamyl-ACC (GACC), and jasmonyl-ACC (JA-ACC) hinders related research. Combining synthetic and analytical chemistry, we present the first, validated methodology to rapidly extract and quantify ACC and its conjugates using ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). Its relevance was confirmed by application to Arabidopsis mutants with altered ACC metabolism and wild-type plants under stress. Pharmacological and genetic suppression of ACC synthesis resulted in decreased ACC and MACC content, whereas induction led to elevated levels. Salt, wounding, and submergence stress enhanced ACC and MACC production. GACC and JA-ACC were undetectable in vivo; however, GACC was identified in vitro, underscoring the broad applicability of the method. This method provides an efficient tool to study individual functions of ACC and its conjugates, paving the road toward exploration of novel avenues in ACC and ethylene metabolism, and revisiting ethylene literature in view of the recent discovery of an ethylene-independent role of ACC.

Photosynthetic Activities, Phytohormones, and Secondary Metabolites Induction in Plants by Prevailing Compost Residue

Compost residue enriches soil health with the potential to enhance plant metabolism and hormonal balance, but has not yet been studied. A study was performed to determine how prevailing compost residue induces tomato (Solanum lycopersicum 'Scotia') plant morpho-physiology, phytohormones, and secondary metabolites. Plants were grown in soils with a previous history of annual (AN) and biennial (BI) compost amendments. The controls were soil without compost (C) amendment and municipal solid waste compost (MSWC) alone. The MSWC- and AN-plants had similar and significantly (p < 0.05) highest growth and photosynthetic activities compared to the BI- or C-plants. Total phenolics and lipid peroxidase activity were significantly (p < 0.001) high in BI-plants, while hydrogen peroxide and antioxidant capacity were significantly (p < 0.001) high in AN-plants. MSWC-plants recorded the highest cis-abscisic acid, followed by AN-, and then BI- and C-plants. Cis-zeatin, trans-zeatin, and isopentenyladenine ribosides were detected in the MSWC- and AN-plants but not in the BI- or C-plants. Furthermore, gibberellins GA53, GA19, and GA8 were high in the MSWC-plants, but only GA8 was detected in the AN plants and none in the others. Besides, MSWC plants exhibited the highest content of 1-aminocyclopropane-1-carboxylic acid. Conjugated salicylic acid was highest in the BI-plants, while jasmonic acid-isoleucine was highest in MSWC-plants and C plants. In conclusion, prevailing compost chemical residues upregulate plant growth, phytohormones, and metabolic compounds that can potentially increase plant growth and abiotic stress defense. Future work should investigate the flow of these compounds in plants under abiotic stress.

Integrated physiology, transcriptome and proteome analyses highlight the potential roles of multiple hormone-mediated signaling pathways involved in tapping panel dryness in rubber tree

Currently, one of the most serious threats to rubber tree is the tapping panel dryness (TPD) that greatly restricts natural rubber production. Over-tapping or excessive ethephon stimulation is regarded as the main cause of TPD occurrence. Although extensive studies have been carried out, the molecular mechanism underlying TPD remains puzzled. An attempt was made to compare the levels of endogenous hormones and the profiles of transcriptome and proteome between healthy and TPD trees. Results showed that most of endogenous hormones such as jasmonic acid (JA), 1-aminocyclopropanecarboxylic acid (ACC), indole-3-acetic acid (IAA), trans-zeatin (tZ) and salicylic acid (SA) in the barks were significantly altered in TPD-affected rubber trees. Accordingly, multiple hormone-mediated signaling pathways were changed. In total, 731 differentially expressed genes (DEGs) and 671 differentially expressed proteins (DEPs) were identified, of which 80 DEGs were identified as putative transcription factors (TFs). Further analysis revealed that 12 DEGs and five DEPs regulated plant hormone synthesis, and that 16 DEGs and six DEPs were involved in plant hormone signal transduction pathway. Nine DEGs and four DEPs participated in rubber biosynthesis and most DEGs and all the four DEPs were repressed in TPD trees. All these results highlight the potential roles of endogenous hormones, signaling pathways mediated by these hormones and rubber biosynthesis pathway in the defense response of rubber trees to TPD. The present study extends our understanding of the nature and mechanism underlying TPD and provides some candidate genes and proteins related to TPD for further research in the future.

Profiling of 1-aminocyclopropane-1-carboxylic acid and selected phytohormones in Arabidopsis using liquid chromatography-tandem mass spectrometry

BACKGROUND

Gaseous phytohormone ethylene levels are directly influenced by the production of its immediate non-volatile precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Owing to the strongly acidic character of the ACC molecule, its quantification has been difficult to perform. Here, we present a simple and straightforward validated method for accurate quantification of not only ACC levels, but also major members of other important phytohormonal classes - auxins, cytokinins, jasmonic acid, abscisic acid and salicylic acid from the same biological sample.

RESULTS

The presented technique facilitates the analysis of 15 compounds by liquid chromatography coupled with tandem mass spectrometry. It was optimized and validated for 10 mg of fresh weight plant material. The extraction procedure is composed of a minimal amount of necessary steps. Accuracy and precision were the basis for evaluating the method, together with process efficiency, recovery and matrix effects as validation parameters. The examined compounds comprise important groups of phytohormones, their active forms and some of their metabolites, including six cytokinins, four auxins, two jasmonates, abscisic acid, salicylic acid and 1-aminocyclopropane-1-carboxylic acid. The resulting method was used to examine their contents in selected Arabidopsis thaliana mutant lines.

CONCLUSION

This profiling method enables a very straightforward approach for indirect ethylene study and explores how it interacts, based on content levels, with other phytohormonal groups in plants.

H2O2 drives the transition from conchocelis to conchosporangia in the red alga Pyropia haitanensis with promotion facilitated by 1-Aminocyclopropane-1-carboxylic acid

The Bangiales represent an ancient lineage within red algae that are characterized by a life history featuring a special transitional stage from diploid to haploid known as the conchosporangia stage. However, the regulatory mechanisms governing the initiation of this stage by changes in environmental conditions are not well understood. This study analyzed the changes in phytohormones and H2O2 content during the development of conchosporangia. It also compared the gene expression changes in the early development of conchosporangia through transcriptome analysis. The findings revealed that H2O2 was shown to be the key signal initiating the transition from conchocelis to conchosporangia in Pyropia haitanensis. Phytohormone analysis showed a significant increase in 1-aminocylopropane-1-carboxylic acid (ACC) levels during conchosporangia maturation, while changes in environmental conditions were found to promote the rapid release of H2O2. H2O2 induction led to conchosporangia development, and ACC enhanced both H2O2 production and conchosporangia development. This promotive effect was inhibited by the NADPH oxidase inhibitor diphenylene iodonium and the H2O2 scavenger N, N'-dimethylthiourea. The balance of oxidative-antioxidative mechanisms was maintained by regulating the activities and transcriptional levels of enzymes involved in H2O2 production and scavenging. Transcriptome analysis in conjunction with evaluation of enzyme and transcription level changes revealed upregulation of protein and sugar synthesis along with modulation of energy supply under the conditions that induced maturation, and exogenous ACC was found to enhance the entire process. Overall, this study demonstrates that ACC enhances H2O2 promotion of the life cycle switch responsible for the transition from a vegetative conchocelis to a meiosis-preceding conchosporangia stage in Bangiales species.

The local and systemic accumulation of ethylene determines the rapid defence responses induced by flg22 in tomato (Solanum lycopersicum L.)

Plant defence responses induced by the bacterial elicitor flg22 are highly dependent on phytohormones, including gaseous ethylene (ET). While the regulatory role of ET in local defence responses to flg22 exposure has been demonstrated, its contribution to the induction of systemic responses is not clearly understood. For this consideration, we examined the effects of different ET modulators on the flg22-induced local and systemic defence progression. In our experiments, ET biosynthesis inhibitor aminoethoxyvinyl glycine (AVG) or ET receptor blocker silver thiosulphate (STS) were applied 1 h before flg22 treatments and 1 h later the rapid local and systemic responses were detected in the leaves of intact tomato plants (Solanum lycopersicum L.). Based on our results, AVG not only diminished the flg22-induced ET accumulation locally, but also in the younger leaves confirming the role of ET in the whole-plant expanding defence progression. This increase in ET emission was accompanied by increased local expression of SlACO1, which was reduced by AVG and STS. Local ET biosynthesis upon flg22 treatment was shown to positively regulate local and systemic superoxide (O2.-) and hydrogen peroxide (H2O2) production, which in turn could contribute to ET accumulation in younger leaves. Confirming the role of ET in flg22-induced rapid defence responses, application of AVG reduced local and systemic ET, O2.- and H2O2 production, whereas STS reduced it primarily in the younger leaves. Interestingly, in addition to flg22, AVG and STS induced stomatal closure alone at whole-plant level, however in the case of combined treatments together with flg22 both ET modulators reduced the rate of stomatal closure in the older- and younger leaves as well. These results demonstrate that both local and systemic ET production in sufficient amounts and active ET signalling are essential for the development of flg22-induced rapid local and systemic defence responses.

Parallels between drought and flooding: An integrated framework for plant eco-physiological responses to water stress

Drought and flooding occur at opposite ends of the soil moisture spectrum yet their resulting stress responses in plants share many similarities. Drought limits root water uptake to which plants respond with stomatal closure and reduced leaf gas exchange. Flooding limits root metabolism due to soil oxygen deficiency, which also limits root water uptake and leaf gas exchange. As drought and flooding can occur consecutively in the same system and resulting plant stress responses share similar mechanisms, a single theoretical framework that integrates plant responses over a continuum of soil water conditions from drought to flooding is attractive. Based on a review of recent literature, we integrated the main plant eco-physiological mechanisms in a single theoretical framework with a focus on plant water transport, plant oxygen dynamics, and leaf gas exchange. We used theory from the soil-plant-atmosphere continuum modeling as "backbone" for our framework, and subsequently incorporated interactions between processes that regulate plant water and oxygen status, abscisic acid and ethylene levels, and the resulting acclimation strategies in response to drought, waterlogging, and complete submergence. Our theoretical framework provides a basis for the development of mathematical models to describe plant responses to the soil moisture continuum from drought to flooding.

Comparative transcriptome profiling provides insights into the growth promotion activity of Pseudomonas fluorescens strain SLU99 in tomato and potato plants

The use of biocontrol agents with plant growth-promoting activity has emerged as an approach to support sustainable agriculture. During our field evaluation of potato plants treated with biocontrol rhizobacteria, four bacteria were associated with increased plant height. Using two important solanaceous crop plants, tomato and potato, we carried out a comparative analysis of the growth-promoting activity of the four bacterial strains: Pseudomonas fluorescens SLU99, Serratia plymuthica S412, S. rubidaea AV10, and S. rubidaea EV23. Greenhouse and in vitro experiments showed that P. fluorescens SLU99 promoted plant height, biomass accumulation, and yield of potato and tomato plants, while EV23 promoted growth in potato but not in tomato plants. SLU99 induced the expression of plant hormone-related genes in potato and tomato, especially those involved in maintaining homeostasis of auxin, cytokinin, gibberellic acid and ethylene. Our results reveal potential mechanisms underlying the growth promotion and biocontrol effects of these rhizobacteria and suggest which strains may be best deployed for sustainably improving crop yield.

Phyllosphere symbiont promotes plant growth through ACC deaminase production

Plant growth promoting bacteria can confer resistance to various types of stress and increase agricultural yields. The mechanisms they employ are diverse. One of the most important genes associated with the increase in plant biomass and stress resistance is acdS, which encodes a 1-aminocyclopropane-1-carboxylate- or ACC-deaminase. The non-proteinogenic amino acid ACC is the precursor and means of long-distance transport of ethylene, a plant hormone associated with growth arrest. Expression of acdS reduces stress induced ethylene levels and the enzyme is abundant in rhizosphere colonizers. Whether ACC hydrolysis plays a role in the phyllosphere, both as assembly cue and in growth promotion, remains unclear. Here we show that Paraburkholderia dioscoreae Msb3, a yam phyllosphere symbiont, colonizes the tomato phyllosphere and promotes plant growth by action of its ACC deaminase. We found that acdS is required for improved plant growth but not for efficient leaf colonization. Strain Msb3 readily proliferates on the leaf surface of tomato, only occasionally spreading to the leaf endosphere through stomata. The strain can also colonize the soil or medium around the roots but only spreads into the root if the plant is wounded. Our results indicate that the degradation of ACC is not just an important trait of plant growth promoting rhizobacteria but also one of leaf dwelling phyllosphere bacteria. Manipulation of the leaf microbiota by means of spray inoculation may be more easily achieved than that of the soil. Therefore, the application of ACC deaminase containing bacteria to the phyllosphere may be a promising strategy to increasing plant stress resistance, pathogen control, and harvest yields.

Endophytes in Agriculture: Potential to Improve Yields and Tolerances of Agricultural Crops

Endophytic fungi and bacteria live asymptomatically within plant tissues. In recent decades, research on endophytes has revealed that their significant role in promoting plants as endophytes has been shown to enhance nutrient uptake, stress tolerance, and disease resistance in the host plants, resulting in improved crop yields. Evidence shows that endophytes can provide improved tolerances to salinity, moisture, and drought conditions, highlighting the capacity to farm them in marginal land with the use of endophyte-based strategies. Furthermore, endophytes offer a sustainable alternative to traditional agricultural practices, reducing the need for synthetic fertilizers and pesticides, and in turn reducing the risks associated with chemical treatments. In this review, we summarise the current knowledge on endophytes in agriculture, highlighting their potential as a sustainable solution for improving crop productivity and general plant health. This review outlines key nutrient, environmental, and biotic stressors, providing examples of endophytes mitigating the effects of stress. We also discuss the challenges associated with the use of endophytes in agriculture and the need for further research to fully realise their potential.

Genome scale analysis of 1-aminocyclopropane-1-carboxylate oxidase gene family in G. barbadense and its functions in cotton fiber development

A class of proteins, 1-aminocyclopropane-1-carboxylate oxidase (ACO), is required in the final step of production of ethylene from its immediate precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Despite the crucial and regulatory role of ACO gene family in the fiber development, it has not been thoroughly analyzed and annotated in G. barbadense genome. In the present study, we have identified and characterized all isoforms of ACO gene family from genomes of Gossypium arboreum, G. barbadense, G. hirsutum and G. raimondii. Phylogenetic analysis classified all ACO proteins into six distinct groups on the basis of maximum likelihood. Gene locus analysis and circos plots indicated the distribution and relationship of these genes in cotton genomes. Transcriptional profiling of ACO isoforms in G. arboreum, G. barbadense and G. hirsutum fiber development exhibited the highest expression in G. barbadense during early fiber elongation. Moreover, the accumulation of ACC was found highest in developing fibers of G. barbadense in comparison with other cotton species. ACO expression and ACC accumulation correlated with the fiber length in cotton species. Addition of ACC to the ovule cultures of G. barbadense significantly increased fiber elongation while ethylene inhibitors hindered fiber elongation. These findings will be helpful in dissecting the role of ACOs in cotton fiber development and pave a way towards genetic manipulations for fiber quality improvement.

Ethylene Acts as a Local and Systemic Signal to Mediate UV-B-Induced Nitrate Reallocation to Arabidopsis Leaves and Roots via Regulating the ERFs-NRT1.8 Signaling Module

Nitrate is the preferred nitrogen source for plants and plays an important role in plant growth and development. Under various soil stresses, plants reallocate nitrate to roots to promote stress tolerance through the ethylene-ethylene response factors (ERFs)-nitrate transporter (NRT) signaling module. As a light signal, ultraviolet B (UV-B) also stimulates the production of ethylene. However, whether UV-B regulates nitrate reallocation in plants via ethylene remains unknown. Here, we found that UV-B-induced expression of ERF1B, ORA59, ERF104, and NRT1.8 in both Arabidopsis shoots and roots as well as nitrate reallocation from hypocotyls to leaves and roots were impaired in ethylene signaling mutants for Ethylene Insensitive2 (EIN2) and EIN3. UV-B-induced NRT1.8 expression and nitrate reallocation to leaves and roots were also inhibited in the triple mutants for ERF1B, ORA59, and ERF104. Deletion of NRT1.8 impaired UV-B-induced nitrate reallocation to both leaves and roots. Furthermore, UV-B promoted ethylene release in both shoots and roots by enhancing the gene expression and enzymatic activities of ethylene biosynthetic enzymes only in shoots. These results show that ethylene acts as a local and systemic signal to mediate UV-B-induced nitrate reallocation from Arabidopsis hypocotyls to both leaves and roots via regulating the gene expression of the ERFs-NRT1.8 signaling module.

Molecular mechanisms associated with microbial biostimulant-mediated growth enhancement, priming and drought stress tolerance in maize plants

Microbial-based biostimulants are emerging as effective strategies to improve agricultural productivity; however, the modes of action of such formulations are still largely unknown. Thus, herein we report elucidated metabolic reconfigurations in maize (Zea mays) leaves associated with growth promotion and drought stress tolerance induced by a microbial-based biostimulant, a Bacillus consortium. Morphophysiological measurements revealed that the biostimulant induced a significant increase in biomass and enzymatic regulators of oxidative stress. Furthermore, the targeted metabolomics approach revealed differential quantitative profiles in amino acid-, phytohormone-, flavonoid- and phenolic acid levels in plants treated with the biostimulant under well-watered, mild, and severe drought stress conditions. These metabolic alterations were complemented with gene expression and global DNA methylation profiles. Thus, the postulated framework, describing biostimulant-induced metabolic events in maize plants, provides actionable knowledge necessary for industries and farmers to confidently and innovatively explore, design and fully implement microbial-based formulations and strategies into agronomic practices for sustainable agriculture and food production.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Perspective of ACC-deaminase producing bacteria in stress agriculture

The 1-aminocyclopropane-1-carboxylate deaminase (ACCD) enzyme plays an important role in stress alleviation of both biotic and abiotic stressors in plants and thereby enhances their growth under harsh environmental conditions. In-depth analysis of AcdS gene encoding for ACC deaminase reveals its presence in diverse microorganisms including bacteria and fungi. Particularly, plant growth-promoting bacteria (PGPB) containing ACCD supports plant growth by modulating the level of 'stress ethylene' and cleaving its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) into α-ketobutyrate and ammonia, enabling PGPB to utilize ACC as a carbon and nitrogen source. The reduced synthesis of ethylene in plants further relieves the ethylene inhibition of plant growth and development, and improves plant resistance to various stressors. Therefore, the dual role of microbial ACCD makes it a cost-effective and eco-friendly biocatalyst for sustainable agricultural productions. The inducible ACCD encoding gene AcdS is differentially regulated by varying environmental conditions. Successful generation of transgenic plants with microbial AcdS gene enhanced biotic and abiotic stress tolerance in plants. In the present review, we discuss the importance of ACCD-producing PGPB for their ability to reduce ethylene production and the promotion of plant growth under stress conditions. We also highlighted the development of transgenic plants by overexpressing bacterial AcdS gene to improve their performance under stress conditions.

Crosstalk Between Ethylene and Abscisic Acid During Changes in Soil Water Content Reveals a New Role for 1-Aminocyclopropane-1- Carboxylate in Coffee Anthesis Regulation

Coffee (Coffea arabica L.) presents an asynchronous flowering regulated by an endogenous and environmental stimulus, and anthesis occurs once plants are rehydrated after a period of water deficit. We evaluated the evolution of Abscisic Acid (ABA), ethylene, 1-aminocyclopropane-1-carboxylate (ACC) content, ACC oxidase (ACO) activity, and expression analysis of the Lysine Histidine Transporter 1 (LHT1) transporter, in the roots, leaves, and flower buds from three coffee genotypes (C. arabica L. cv Oeiras, Acauã, and Semperflorens) cultivated under field conditions with two experiments. In a third field experiment, the effect of the exogenous supply of ACC in coffee anthesis was evaluated. We found an increased ACC level, low ACO activity, decreased level of ethylene, and a decreased level of ABA in all tissues from the three coffee genotypes in the re-watering period just before anthesis, and a high expression of the LHT1 in flower buds and leaves. The ethylene content and ACO activity decreased from rainy to dry period whereas the ABA content increased. A higher number of opened and G6 stage flower buds were observed in the treatment with exogenous ACC. The results showed that the interaction of ABA-ACO-ethylene and intercellular ACC transport among the leaves, buds, and roots in coffee favors an increased level of ACC that is most likely, involved as a modulator in coffee anthesis. This study provides evidence that ACC can play an important role independently of ethylene in the anthesis process in a perennial crop.

Isolation and identification of multi-trait plant growth-promoting rhizobacteria from coastal sand dune plant species of Pohang beach

Rhizobacteria are root-associated bacteria that influence plant growth by various direct and indirect mechanisms. In quest of efficient plant growth-promoting rhizobacteria (PGPR) with multiple plant growth-promoting traits, a total of 52 rhizobacterial isolates were isolated from the rhizospheric soil collected at Pohang beach, Republic of Korea. The bacterial isolates were evaluated in vitro for their plant growth-promoting traits like production of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, indole-3-acetic acid (IAA), siderophore, and phosphate solubilization activities. More than 28% of the isolates revealed all of the multi-trait plant growth-promoting activities, whereas 11.54% exhibited robust results for producing IAA, ACC deaminase, siderophore, and phosphate solubilization activities. Similarly, 36% isolates were capable for the production of IAA, siderophore, and ACC deaminase, while 32% revealed phosphate solubilization and siderophore production. The isolates with prominent multi-trait plant growth-promoting activities were identified based on 16S rRNA gene sequences and matched to Pseudomonas koreensis-(S4T10), Pseudomonas fluorescens-(S3B1), Serratia fonticola-(S1T1), Sphingobacterium multivorum-(S1B1), Brevundimonas vesicularis-(S1T13), and Arthrobacter sp.-(S2T9) with 99-100% similarity. Our results confirm that further evaluation of these PGPR (exhibiting multi-traits for plant growth promotion) is required on crop plants to reveal their pragmatic role under normal and abiotic stress conditions and add into the consortium of biofertilizers for sustainable agriculture.

Glutathione Is Required for the Early Alert Response and Subsequent Acclimation in Cadmium-Exposed Arabidopsis thaliana Plants

Pollution by cadmium (Cd) is a worldwide problem, posing risks to human health and impacting crop yield and quality. Cadmium-induced phytotoxicity arises from an imbalance between antioxidants and pro-oxidants in favour of the latter. The Cd-induced depletion of the major antioxidant glutathione (GSH) strongly contributes to this imbalance. Rather than being merely an adverse effect of Cd exposure, the rapid depletion of root GSH levels was proposed to serve as an alert response. This alarm phase is crucial for an optimal stress response, which defines acclimation later on. To obtain a better understanding on the importance of GSH in the course of these responses and how these are defined by the rapid GSH depletion, analyses were performed in the GSH-deficient cadmium-sensitive 2-1 (cad2-1) mutant. Cadmium-induced root and leaf responses related to oxidative challenge, hydrogen peroxide (H2O2), GSH, ethylene, and 1-aminocyclopropane-1-carboxylic acid (ACC) were compared between wild-type (WT) and mutant Arabidopsis thaliana plants. Although the cad2-1 mutant has significantly lower GSH levels, root GSH depletion still occurred, suggesting that the chelating capacity of GSH is prioritised over its antioxidative function. We demonstrated that responses related to GSH metabolism and ACC production were accelerated in mutant roots and that stress persisted due to suboptimal acclimation. In general, the redox imbalance in cad2-1 mutant plants and the lack of proper transient ethylene signalling contributed to this suboptimal acclimation, resulting in a more pronounced Cd effect.

The ACC deaminase-producing plant growth-promoting bacteria: Influences of bacterial strains and ACC deaminase activities in plant tolerance to abiotic stress

Global climate change results in frequent occurrences and/or long durations of abiotic stress. Field grown plants are affected by abiotic stress, and they modulate ethylene in response to abiotic stress exposure and use it as a signaling molecule in stress tolerance mechanisms. However, frequent occurrences and/or long durations of stress conditions can cause plants to induce ethylene levels higher than their thresholds, resulting in a reduction of plant growth and crop productivity. The use of plant growth-promoting bacteria (PGPB) that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase has increased in various plant species to ameliorate the deleterious effects of stress-induced ethylene and promote plant growth despite abiotic stress conditions. Unfortunately, there are restrictions that limit the use of ACC deaminase-producing PGPB to protect plants from abiotic stresses. This review describes how abiotic stress induces ethylene and how stress-induced ethylene adversely affects plant growth. In addition, this review emphasizes the importance of the compatibility of PGPB strains and specific host plants and ACC deaminase activities in the reduction of stress ethylene and the promotion of plant growth, based on the research published in the last 10 years. Moreover, due to the restrictions in PGPB use, this review highlights the potential generation of transgenic plants expressing the AcdS gene that encodes the ACC deaminase enzyme as a substitute for PGPB in the future to support and uplift agricultural sustainability and food security globally.

Pseudomonas 1-Aminocyclopropane-1-carboxylate (ACC) Deaminase and Its Role in Beneficial Plant-Microbe Interactions

The expression of the enzyme 1-aminocylopropane-1-carboxylate (ACC) deaminase, and the consequent modulation of plant ACC and ethylene concentrations, is one of the most important features of plant-associated bacteria. By decreasing plant ACC and ethylene concentrations, ACC deaminase-producing bacteria can overcome some of the deleterious effects of inhibitory levels of ACC and ethylene in various aspects of plant-microbe interactions, as well as plant growth and development (especially under stressful conditions). As a result, the acdS gene, encoding ACC deaminase, is often prevalent and positively selected in the microbiome of plants. Several members of the genus Pseudomonas are widely prevalent in the microbiome of plants worldwide. Due to its adaptation to a plant-associated lifestyle many Pseudomonas strains are of great interest for the development of novel sustainable agricultural and biotechnological solutions, especially those presenting ACC deaminase activity. This manuscript discusses several aspects of ACC deaminase and its role in the increased plant growth promotion, plant protection against abiotic and biotic stress and promotion of the rhizobial nodulation process by Pseudomonas. Knowledge regarding the properties and actions of ACC deaminase-producing Pseudomonas is key for a better understanding of plant-microbe interactions and the selection of highly effective strains for various applications in agriculture and biotechnology.

Interaction between polyamines and ethylene in the response to salicylic acid under normal photoperiod and prolonged darkness

The impact of salicylic acid (SA) on ethylene (ET) production and polyamine (PA) metabolism was investigated in wild type (WT) and ET receptor mutant Never ripe (Nr) tomato leaves under normal photoperiod and prolonged darkness. Nr displayed higher ET emanation compared to WT under control conditions and after SA treatments, but the ET signalling was blocked in these tissues. The accumulation of PAs was induced by 1 mM but not by 0.1 mM SA and was higher in WT than in Nr leaves. Upon 1 mM SA treatment, which caused hypersensitive response, illuminated leaves of WT showed high spermine (Spm) content in parallel with an increased expression of S-adenosylmethionine decarboxylase and Spm synthase (SlSPMS) suggesting that this process depended on the light. In Nr, however, Spm content and the expression of the SlSPMS gene were very low independently of the light conditions and SA treatments. This suggests that Spm synthesis needs functional ET perception. In WT leaves 1 mM SA enhanced putrescine (Put) synthesis by increasing the expression of Put biosynthesis genes, arginine and ornithine decarboxylases under darkness, while they were down-regulated in Nr. The activities of diamine (DAO) and polyamine oxidases (PAO), however, were generally higher in Nr compared to the WT after SA treatments. In Nr both SA applications increased the expression of SlPAO1 under normal photoperiod, while SlPAO2 was down-regulated in the dark suggesting a diverse role of PAOs in PA catabolism. These results indicated that ET could modulate the SA-induced PA metabolism in light-dependent manner.

Plant-Microbe Interaction: Aboveground to Belowground, from the Good to the Bad

Soil health and fertility issues are constantly addressed in the agricultural industry. Through the continuous and prolonged use of chemical heavy agricultural systems, most agricultural lands have been impacted, resulting in plateaued or reduced productivity. As such, to invigorate the agricultural industry, we would have to resort to alternative practices that will restore soil health and fertility. Therefore, in recent decades, studies have been directed towards taking a Magellan voyage of the soil rhizosphere region, to identify the diversity, density, and microbial population structure of the soil, and predict possible ways to restore soil health. Microbes that inhabit this region possess niche functions, such as the stimulation or promotion of plant growth, disease suppression, management of toxicity, and the cycling and utilization of nutrients. Therefore, studies should be conducted to identify microbes or groups of organisms that have assigned niche functions. Based on the above, this article reviews the aboveground and below-ground microbiomes, their roles in plant immunity, physiological functions, and challenges and tools available in studying these organisms. The information collected over the years may contribute toward future applications, and in designing sustainable agriculture.

At the Crossroads of Survival and Death: The Reactive Oxygen Species-Ethylene-Sugar Triad and the Unfolded Protein Response

Upon stress, a trade-off between plant growth and defense responses defines the capacity for survival. Stress can result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and other organelles. To cope with these proteotoxic effects, plants rely on the unfolded protein response (UPR). The involvement of reactive oxygen species (ROS), ethylene (ETH), and sugars, as well as their crosstalk, in general stress responses is well established, yet their role in UPR deserves further scrutiny. Here, a synopsis of current evidence for ROS-ETH-sugar crosstalk in UPR is discussed. We propose that this triad acts as a major signaling hub at the crossroads of survival and death, integrating information from ER, chloroplasts, and mitochondria, thereby facilitating a coordinated stress response.

Effects of Increased 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase Activity in Bradyrhizobium sp. SUTN9-2 on Mung Bean Symbiosis under Water Deficit Conditions

Bacteria exhibiting 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, which inhibits the biosynthesis of ethylene in higher plants, promote plant growth through the degradation of ethylene precursors, such as ACC. ACC deaminase activity in Bradyrhizobium sp. SUTN9-2 was enhanced by genetic engineering and adaptive laboratory evolution (ALE)-based methods. The transferal of a plasmid containing the acdR and acdS genes into SUTN9-2 was genetic engineering improved, while the ALE method was performed based on the accumulation of an adaptive bacterial population that continuously grew under specified growth conditions for a long time. ACC deaminase enzyme activity was 8.9–fold higher in SUTN9-2:pMG103::acdRS and 1.4–fold higher in SUTN9-2 (ACCDadap) than in the wild-type strain. The effects of increased activity were examined in the host plant (Vigna radiata (L.) R.Wilczek SUT1). The improved strains enhanced nodulation in early stage of plant growth. SUTN9-2:pMG103::acdRS also maintained nitrogen fixation under water deficit conditions and increased the plant biomass after rehydration. Changes in nucleotides and amino acids in the AcdS protein of SUTN9-2 (ACCDadap) were then investigated. Some nucleotides predicted to be located in the ACC-binding site were mutated. These mutations may have increased ACC deaminase activity, which enhanced both symbiotic interactions and drought tolerance and promoted recovery after rehydration more than lower ACC deaminase activity. Adaptive evolution represents a promising strategy for further applications in the field.

The regulation of ethylene biosynthesis: a complex multilevel control circuitry

The gaseous plant hormone ethylene is produced by a fairly simple two-step biosynthesis route. Despite this pathway's simplicity, recent molecular and genetic studies have revealed that the regulation of ethylene biosynthesis is far more complex and occurs at different layers. Ethylene production is intimately linked with the homeostasis of its general precursor S-adenosyl-l-methionine (SAM), which experiences transcriptional and posttranslational control of its synthesising enzymes (SAM synthetase), as well as the metabolic flux through the adjacent Yang cycle. Ethylene biosynthesis continues from SAM by two dedicated enzymes: 1-aminocyclopropane-1-carboxylic (ACC) synthase (ACS) and ACC oxidase (ACO). Although the transcriptional dynamics of ACS and ACO have been well documented, the first transcription factors that control ACS and ACO expression have only recently been discovered. Both ACS and ACO display a type-specific posttranslational regulation that controls protein stability and activity. The nonproteinogenic amino acid ACC also shows a tight level of control through conjugation and translocation. Different players in ACC conjugation and transport have been identified over the years, however their molecular regulation and biological significance is unclear, yet relevant, as ACC can also signal independently of ethylene. In this review, we bring together historical reports and the latest findings on the complex regulation of the ethylene biosynthesis pathway in plants.

Concurrent Metabolic Profiling and Quantification of Aromatic Amino Acids and Phytohormones in Solanum lycopersicum Plants Responding to Phytophthora capsici

Pathogenic microorganisms account for large production losses in the agricultural sector. Phytophthora capsici is an oomycete that causes blight and fruit rot in important crops, especially those in the Solanaceae family. P. capsici infection is difficult to control due to genetic diversity, arising from sexual reproduction, and resistant spores that remain dormant in soil. In this study, the metabolomics of tomato plants responding to infection by P. capsici were investigated. Non-targeted metabolomics, based on liquid chromatography coupled to mass spectrometry (LC-MS), were used with multivariate data analyses to investigate time-dependent metabolic reprogramming in the roots, stems, and leaves of stem-infected plants, over an 8 day period. In addition, phytohormones and amino acids were determined using quantitative LC-MS. Methyl salicylate and 1-aminocyclopropane-1-carboxylate were detected as major signalling molecules in the defensive response to P. capsici. As aromatic amino acid precursors of secondary metabolic pathways, both phenylalanine and tryptophan showed a continuous increase over time in all tissues, whereas tyrosine peaked at day 4. Non-targeted metabolomic analysis revealed phenylpropanoids, benzoic acids, glycoalkaloids, flavonoids, amino acids, organic acids, and fatty acids as the major classes of reprogrammed metabolites. Correlation analysis showed that metabolites derived from the same pathway, or synthesised by different pathways, could either have a positive or negative correlation. Furthermore, roots, stems, and leaves showed contrasting time-dependent metabolic reprogramming, possibly related to the biotrophic vs. necrotrophic life-stages of the pathogen, and overlapping biotic and abiotic stress signaling. As such, the targeted and untargeted approaches complemented each other, to provide a detailed view of key time-dependent metabolic changes, occurring in both the asymptomatic and symptomatic stages of infection.

Isolation and Characterization of Cold-Tolerant Hyper-ACC-Degrading Bacteria from the Rhizosphere, Endosphere, and Phyllosphere of Antarctic Vascular Plants

1-Aminociclopropane-1-carboxylate (ACC)-degrading bacteria having been widely studied for their use in alleviating abiotic stresses in plants. In the present study, we isolated and characterized ACC-degrading bacteria from the rhizosphere, phyllosphere, and endosphere of the Antarctic vascular plants Deschampsia antarctica and Colobanthus quitensis. One hundred and eighty of the 578 isolates (31%) were able to grow on minimal medium containing ACC, with 101 isolates (23, 37, and 41 endosphere-, phyllosphere- and rhizosphere-associated isolates, respectively) identified as being genetically unique by enterobacterial repetitive intergenic consensus (ERIC)-PCR. Subsequently, freeze/thaw treatments and ice-recrystallization-inhibition (IRI) activity assays were performed, the results of which revealed that 77 (13%) of cold-tolerant isolates exhibited putative ACC deaminase activity. Significant (p ≤ 0.05) differences in IRI activity were also observed between the studied plant niches. Surprisingly, all the cold-tolerant isolates showed ACC deaminase activity, independent of the plant niches, with 12 isolates showing the highest ACC deaminase activities of 13.21-39.56 mmol α KB mg protein^-1 h^-1. These isolates were categorized as 'cold-tolerant hyper-ACC-degrading bacteria', and identified as members of Pseudomonas, Serratia, and Staphylococcus genera. The results revealed the occurrence of cold-tolerant hyper-ACC-degrading bacteria in diverse plant niches of Antarctic vascular plants, that could be investigated as novel microbial inoculants to alleviate abiotic stresses in plants.

Tocopherols mutual balance is a key player for maintaining Arabidopsis thaliana growth under salt stress

Enhanced channeling carbon through pathways: shikimate/chorismate, benzenoid-phenylopropanoid or 2-C-methyl-D-erythritol 4-phosphate (MEP) provides a multitude of secondary metabolites and cell wall components and allows plants response to environmental stresses. Through the biosynthetic pathways, different secondary metabolites, like tocopherols (TCs), are bind to mutual dependencies and metabolic loops, that are not yet fully understood. We compared, in parallel, the influence of α- and γ-TCs on metabolites involved in osmoprotective/antioxidative response, and physico-chemical modification of plasma membrane and cell wall. We studied Arabidopsis thaliana Columbia ecotype (WT), mutant vte1 deficient in α- and γ-TCs, mutant vte4 over-accumulating γ-TC instead of α-TC, and transgenic line tmt over-accumulating α-TC; exposed to NaCl. The results indicate that salt stress activates β-carboxylation processes in WT plants and in plants with altered TCs accumulation. In α-TC-deficient plants, NaCl causes ACC decrease, but does not change SA, whose concentration remains higher than in α-TC accumulating plants. α/γ-TCs contents influence carbohydrates, poliamines, phenolic (caffeic, ferrulic, cinnamic) acids accumulation patterns. Salinity results in increased detection of the LM5 galactan and LM19 homogalacturonan epitopes in α-TC accumulating plants, and the LM6 arabinan and MAC207 AGP epitopes in α-TC deficient mutants. Parallel, plants with altered TCs composition show decreased both the cell turgor and elastic modulus determined at the individual cell level. α-TC deficient plants reveal lower values of cell turgor and elastic modulus, but higher cell hydraulic conductivity than α-TC accumulating plants. Under salt stress, α-TC shows stronger regulatory effect than γ-TC through the impact on chloroplastic biosynthetic pathways and ROS/osmotic-modulating compounds.

Strigolactone elevates ethylene biosynthesis in etiolated Arabidopsis seedlings

The gaseous phytohormone ethylene influences many aspects of plant life, including germination, fruit ripening, senescence, and stress responses. These diverse roles of ethylene occur in part through crosstalk with other phytohormones, which affects ethylene biosynthesis and signaling pathways. We have recently shown that the phytohormones, including gibberellic acid, abscisic acid, auxin, methyl jasmonate, and salicylic acid, regulate the stability of ACC synthases (ACSs), the rate-limiting enzymes in ethylene biosynthesis. Here, we report that treatment of etiolated Arabidopsis seedlings with strigolactone (SL) increases ethylene biosynthesis. SL does not influence ACS stability or ACS gene expression, but it increases the transcript levels of a subset of ACC oxidase (ACO) genes, thereby enhancing ethylene biosynthesis. Taken together with the results of our previous study, these findings demonstrate that most phytohormones differentially regulate ethylene biosynthesis in dark-grown Arabidopsis seedlings by affecting ACS stability and/or the transcript levels of ethylene biosynthesis genes.

Ethylene-independent signaling by the ethylene precursor ACC in Arabidopsis ovular pollen tube attraction

The phytohormone ethylene has numerous effects on plant growth and development. Its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), is a non-proteinogenic amino acid produced by ACC SYNTHASE (ACS). ACC is often used to induce ethylene responses. Here, we demonstrate that ACC exhibits ethylene-independent signaling in Arabidopsis thaliana reproduction. By analyzing an acs octuple mutant with reduced seed set, we find that ACC signaling in ovular sporophytic tissue is involved in pollen tube attraction, and promotes secretion of the pollen tube chemoattractant LURE1.2. ACC activates Ca²⁺-containing ion currents via GLUTAMATE RECEPTOR-LIKE (GLR) channels in root protoplasts. In COS-7 cells expressing moss PpGLR1, ACC induces the highest cytosolic Ca²⁺ elevation compared to all twenty proteinogenic amino acids. In ovules, ACC stimulates transient Ca²⁺ elevation, and Ca²⁺ influx in octuple mutant ovules rescues LURE1.2 secretion. These findings uncover a novel ACC function and provide insights for unraveling new physiological implications of ACC in plants.

Modes of Action of Microbial Biocontrol in the Phyllosphere

A fast-growing field of research focuses on microbial biocontrol in the phyllosphere. Phyllosphere microorganisms possess a wide range of adaptation and biocontrol factors, which allow them to adapt to the phyllosphere environment and inhibit the growth of microbial pathogens, thus sustaining plant health. These biocontrol factors can be categorized in direct, microbe-microbe, and indirect, host-microbe, interactions. This review gives an overview of the modes of action of microbial adaptation and biocontrol in the phyllosphere, the genetic basis of the mechanisms, and examples of experiments that can detect these mechanisms in laboratory and field experiments. Detailed insights in such mechanisms are key for the rational design of novel microbial biocontrol strategies and increase crop protection and production. Such novel biocontrol strategies are much needed, as ensuring sufficient and consistent food production for a growing world population, while protecting our environment, is one of the biggest challenges of our time.

Saline and Arid Soils: Impact on Bacteria, Plants, and their Interaction

Salinity and drought are the most important abiotic stresses hampering crop growth and yield. It has been estimated that arid areas cover between 41% and 45% of the total Earth area worldwide. At the same time, the world’s population is going to soon reach 9 billion and the survival of this huge amount of people is dependent on agricultural products. Plants growing in saline/arid soil shows low germination rate, short roots, reduced shoot biomass, and serious impairment of photosynthetic efficiency, thus leading to a substantial loss of crop productivity, resulting in significant economic damage. However, plants should not be considered as single entities, but as a superorganism, or a holobiont, resulting from the intimate interactions occurring between the plant and the associated microbiota. Consequently, it is very complex to define how the plant responds to stress on the basis of the interaction with its associated plant growth-promoting bacteria (PGPB). This review provides an overview of the physiological mechanisms involved in plant survival in arid and saline soils and aims at describing the interactions occurring between plants and its bacteriome in such perturbed environments. The potential of PGPB in supporting plant survival and fitness in these environmental conditions has been discussed.

Expression profiling of the genes encoding ABA route components and the ACC oxidase isozymes in the senescing leaves of Populus tremula

Abscisic acid (ABA) triggers and regulates, while ethylene modulates autumn leaf senescence. The expression profiles of genes encoding ABA route components and the ACC oxidase isozymes were investigated in Populus tremula during the early and moderate stages of autumn leaf senescence. The targets of interest were Ptre-HAB1-like genes (Ptre-HAB1, Ptre-HAB3a and Ptre-HAB3b), the subclass 3 of Ptre-SnRK2s genes (Ptre-SnRK2.6a, Ptre-SnRK2.6b and Ptre-SnRK2.6b) and Ptre-RbohD1, Ptre-RbohF1, and Ptre-RbohF2 genes encoding the poplar components, which are counterparts of the ABA route key regulators or the counterparts of its secondary messengers, such as Homology to ABA-insensitive 1 (HAB1), Sucrose non-fermenting 1-related protein kinases 2 (SnRK2s) or Respiratory burst oxidase D and Respiratory burst oxidase F (RbohD and RbohF, respectively) in Arabidopsis, and Ptre-ACO3, Ptre-ACO5, and Ptre-ACO6 genes encoding ACC oxidase isozymes involved in ethylene biosynthesis. The fold change in their expression levels enabled to distinguish the distinct expression patterns for the following pairs of genes: Ptre-HAB3a and Ptre-SnRK2.6a, Ptre-HAB3b and Ptre-SnRK2.2, and Ptre-HAB1 and Ptre-SnRK2.6b, where each pair involves the genes encoding the negative and positive regulators of ABA route, respectively. Among the investigated genes, the fold change of expression was the highest for Ptre-ACO3, Ptre-ACO6, and Ptre-SnRK2.6b genes during both the studied stages, and additionally for Ptre-HAB1 and Ptre-RbohD1 genes during the moderate stage. In contrast, the Ptre-RbohF1 and Ptre-RbohF2 genes exhibited only the transient upregulation at the early stage of senescence. In an in vitro study, the ability of protein kinases Ptre-SnRK2.6a and Ptre-SnRK2.6b to phosphorylate the N-terminal regions of Ptre-RbohD1 and Ptre-RbohF2 was studied; the activity of Ptre-SnRK2.6b against the studied Ptre-Rbohs was noticeably lower than that exhibited by Ptre-SnRK2.6a. It seems that despite the high similarity of their polypeptides, Ptre-SnRK2.6a and Ptre-SnRK2.6b may play different biological roles; nonetheless, it requires in vivo confirmation. Surprisingly, the highest protein kinase activity against the Ptre-Rbohs was detected in the heterologous reaction with AT-SnRK2.6/OST1 which suggests that the discussed interactions are evolutionary conserved.

Identification of Quantitative Trait Loci Controlling Ethylene Production in Germinating Seeds in Maize (Zea mays L.)

Plant seed germination is a crucial developmental event that has significant effects on seedling establishment and yield production. This process is controlled by multiple intrinsic signals, particularly phytohormones. The gaseous hormone ethylene stimulates seed germination; however, the genetic basis of ethylene production in maize during seed germination remains poorly understood. In this study, we quantified the diversity of germination among 14 inbred lines representing the parental materials corresponding to multiple recombinant inbred line (RIL) mapping populations. Quantitative trait loci (QTLs) controlling ethylene production were then identified in germinating seeds from an RIL population constructed from two parental lines showing differences in both germination speed and ethylene production during germination. To explore the possible genetic correlations of ethylene production with other traits, seed germination and seed weight were evaluated using the same batch of samples. On the basis of high-density single nucleotide polymorphism-based genetic linkage maps, we detected three QTLs for ethylene production in germinating seeds, three QTLs for seed germination, and four QTLs for seed weight, with each QTL explaining 5.8%–13.2% of the phenotypic variation of the trait. No QTLs were observed to be co-localized, suggesting that the genetic bases underlying the three traits are largely different. Our findings reveal three chromosomal regions responsible for ethylene production during seed germination, and provide a valuable reference for the future investigation of the genetic mechanism underlying the role of the stress hormone ethylene in maize germination control under unfavourable external conditions.

Sexual Reproduction via a 1-Aminocyclopropane-1-Carboxylic Acid-Dependent Pathway Through Redox Modulation in the Marine Red Alga Pyropia yezoensis (Rhodophyta)

The transition from the vegetative to sexually reproductive phase is the most dynamic change to occur during a plant's life cycle. In the present study, we showed that the ethylene precursor 1-aminocylopropane-1-carboxylic acid (ACC) induces sexual reproduction in the marine red alga Pyropia yezoensis independently from ethylene. Exogenous application of ACC, which contains a three membered carbocyclic ring, promoted the formation of spermatia and carporspores in gametophytes, whereas ethephon, an ethylene-releasing compound, did not stimulate sexual reproduction. In addition, an ACC analog, 1-aminocyclobutane-1-carboxylic acid (ACBC), which contains a four membered carbocyclic ring, promoted sexual reproduction and enhanced tolerance to oxidative stress in the same manner as ACC, but 1-aminocyclopentane-1-carboxylic acid (cycloleucine; which contains a cyclopentane ring) did not. The application of ACC increased the generation of reactive oxygen species (ROS) and induced the expression of PyRboh gene encoding NADPH oxidase. ACC also stimulated the synthesis of ascorbate (AsA) by inducing transcripts of PyGalLDH, which encodes galactono-1,4-lactone dehydrogenase, the catalyst for the final enzymatic step of the AsA biosynthetic pathway. Conversely, ACC caused a decrease in the synthesis of glutathione (GSH) by repressing transcripts of PyGCL, which encodes glutamate cysteine ligase, the catalyst for the rate-limiting step in the formation of GSH. These results suggest a possible role played by ACC as a signaling molecule independent from ethylene in the regulation of sexual reproduction through alterations to the redox state in P. yezoensis.

A comparison between water uptake and root length density in winter wheat: effects of root density and rhizosphere properties

BACKGROUND AND AIMS

We aim to quantify the variation in root distribution in a set of 35 experimental wheat lines. We also compared the effect of variation in hydraulic properties of the rhizosphere on water uptake by roots.

METHODS

We measured the root length density and soil drying in 35 wheat lines in a field experiment. A 3D numerical model was used to predict soil drying profiles with the different root length distributions and compared with measured soil drying. The model was used to test different scenarios of the hydraulic properties of the rhizosphere.

RESULTS

We showed that wheat lines with no detectable differences in root length density can induce soil drying profiles with statistically significant differences. Our data confirmed that a root length density of at least 1 cm/cm³ is needed to drain all the available water in soil. In surface layers where the root length density was far greater than 1 cm/cm³ water uptake was independent of rooting density due to competition for water. However, in deeper layers where root length density was less than 1 cm/cm³, water uptake by roots was proportional to root density.

CONCLUSION

In a set of wheat lines with no detectable differences in the root length density we found significant differences in water uptake. This may be because small differences in root density at depth can result in larger differences in water uptake or that the hydraulic properties of the rhizosphere can greatly affect water uptake.

Cyanide produced with ethylene by ACS and its incomplete detoxification by β-CAS in mango inflorescence leads to malformation

Malformation of mango inflorescences (MMI) disease causes severe economic losses worldwide. Present research investigates the underlying causes of MMI. Results revealed significantly higher levels of cyanide, a by-product of ethylene biosynthesis, in malformed inflorescences (MI) of mango cultivars. There was a significant rise in ACS transcripts, ACS enzyme activity and cyanide and ethylene levels in MI as compared to healthy inflorescences (HI). Significant differences in levels of methionine, phosphate, S-adenosyl-L-methionine, S-adenosyl-L-homocysteine, ascorbate and glutathione, and activities of dehydroascorbate reductase and glutathione reductase were seen in MI over HI. Further, a lower expression of β-cyanoalanine synthase (β-CAS) transcript was associated with decreased cellular β-CAS activity in MI, indicating accumulation of unmetabolized cyanide. TEM studies showed increased gum-resinosis and necrotic cell organelles, which might be attributed to unmetabolized cyanide. In field trials, increased malformed-necrotic-inflorescence (MNI) by spraying ethrel and decreased MNI by treating with ethylene inhibitors (silver and cobalt ions) further confirmed the involvement of cyanide in MMI. Implying a role for cyanide in MMI at the physiological and molecular level, this study will contribute to better understanding of the etiology of mango inflorescence malformation, and also help manipulate mango varieties genetically for resistance to malformation.

Disruption and Overexpression of the Gene Encoding ACC (1-Aminocyclopropane-1-Carboxylic Acid) Deaminase in Soil-Borne Fungal Pathogen Verticillium dahliae Revealed the Role of ACC as a Potential Regulator of Virulence and Plant Defense

It has been suggested that some microorganisms, including plant growth-promoting rhizobacteria, manipulate the level of ethylene in plants by degrading 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene precursor, into α-ketobutyrate and ammonia, using ACC deaminase (ACCd). Here, we investigated whether ACCd of Verticillium dahliae, a soil-borne fungal pathogen of many important crops, is involved in causing vascular wilt disease. Overexpression of the V. dahliae gene encoding this enzyme, labeled as ACCd, significantly increased virulence in both tomato and eggplant, while disruption of ACCd reduced virulence. Both types of mutant produced more ethylene than a wild-type (70V-WT) strain, although they significantly differed in ACC content. Overexpression strains lowered ACC levels in the roots of infected plants, while the amount of ACC in the roots of plants infected with deletion mutants increased. To test the hypothesis that ACC acts as a signal for controlling defense, roots of WT and Never-ripe (Nr) tomato plants were treated with ACC before V. dahliae inoculation. Plants pretreated with ACC displayed less severe symptoms than untreated controls. Collectively, our results suggest a novel role of ACC as a regulator of both plant defense and pathogen virulence.

Expression profiling of CTR1-like and EIN2-like genes in buds and leaves of Populus tremula, and in vitro study of the interaction between their polypeptides

In Arabidopsis, the serine/threonine protein kinase Constitutive Triple Response 1 (CTR1) and Ethylene Insensitive 2 polypeptide (EIN2) functions are key negative and positive components, respectively, in the ethylene signalling route. Here, we report on an in silico study of members of the CTR1-like and EIN2-like polypeptide families from poplars. The expression of CTR1-like and EIN2-like genes such as Ptre-CTR1, Ptre-CTR3 and Ptre-EIN2a was studied in Populus tremula buds and leaves in response to dehydration, various light conditions and under senescence. In buds under dehydration, the maximal fold-change of the Ptre-CTR1, Ptre-CTR3 and Ptre-EIN2a expression level recorded almost identical values. This suggests that maintenance of a constant ratio between the transcript levels of genes encoding positive and negative ethylene signalling components is required under stress. The expression of the studied genes was 1.4-to 3-fold higher in response to darkness, but 4.5- to 51.2-fold and 21.6- to 51.2-fold higher under the early and moderate leaf senescence, respectively. It is worth noting that the senescence-related Ptre-EIN2a and Ptre-CTR3a expression profiles were very similar. Using in vitro assays, we demonstrated the ability of the catalytic domain of Ptre-CTR1 to phosphorylate the Ptre-EIN2a-like polypeptide, which is similar to that in Arabidopsis. The target substrate, the Ptre-CEND2a polypeptide (C-terminal part of Ptre-EIN2a), was only phosphorylated by the protein kinase Ptre-CTR1 and not by Ptre-CTR3. Moreover, the addition of Ptre-CTR3 polypeptides (-CTR3a or -CTR3b forms) to the reaction mixture had an inhibitory effect on Ptre-CTR1 auto- and trans-phosphorylation. In contrast to Ptre-CTR1, Ptre-CTR3 may act as a positive regulator in ethylene signalling in poplar; however, this hypothesis requires in vivo confirmation. Thus, the ethylene signalling route in poplar seems to be under the control of certain additional mechanisms which have not been reported in Arabidopsis.

Aminocyclopropane-1-carboxylic acid is a key regulator of guard mother cell terminal division in Arabidopsis thaliana

Stomata have a critical function in the exchange of gases and water vapor between plants and their environment. Stomatal development is under the rigorous control of many regulators. The last step of development is the terminal division of guard mother cells (GMC) into two guard cells (GC). It is still unclear how the symmetric division of GMCs is regulated. Here, we show that the ethylene precursor aminocyclopropane-1-carboxylic acid (ACC) is required for the symmetric division of GMCs into GCs in Arabidopsis. Exogenous application of the ACC biosynthesis inhibitor aminoethoxyvinylglycine (AVG) induced the formation of single guard cells (SGCs). Correspondingly, an acs octuple-mutant with extremely low endogenous ACC also developed SGCs, and exogenous ACC dramatically decreased the number of SGCs in this mutant whereas exogenous ethephon (which is gradually converted into ethylene) had no effect. Furthermore, neither blocking of endogenous ethylene synthesis nor disruption of ethylene signaling transduction could induce the production of SGCs. Further investigation indicated that ACC promoted the division of GMCs in fama-1 and flp-1myb88 mutants whereas AVG inhibited it. Moreover, ACC positively regulated the expression of CDKB1;1 and CYCA2;3 in the fama-1 and flp-1myb88 mutants. The SGC number was not affected by ACC or AVG in cdkb1;11;2 and cyca2;234 mutants. Taken together, the results demonstrate that ACC itself, but not ethylene, positively modulates the symmetric division of GMCs in a manner that is dependent on CDKB1s and CYCA2s.

The Ethylene Precursor ACC Affects Early Vegetative Development Independently of Ethylene Signaling

The plant hormone ethylene plays a pivotal role in virtually every aspect of plant development, including vegetative growth, fruit ripening, senescence, and abscission. Moreover, it acts as a primary defense signal during plant stress. Being a volatile, its immediate biosynthetic precursor, 1-aminocyclopropane-1-carboxylic acid, ACC, is generally employed as a tool to provoke ethylene responses. However, several reports propose a role for ACC in parallel or independently of ethylene signaling. In this study, pharmacological experiments with ethylene biosynthesis and signaling inhibitors, 2-aminoisobutyric acid and 1-methylcyclopropene, as well as mutant analyses demonstrate ACC-specific but ethylene-independent growth responses in both dark- and light-grown Arabidopsis seedlings. Detection of ethylene emanation in ethylene-deficient seedlings by means of laser-based photoacoustic spectroscopy further supports a signaling role for ACC. In view of these results, future studies employing ACC as a proxy for ethylene should consider ethylene-independent effects as well. The use of multiple knockout lines of ethylene biosynthesis genes will aid in the elucidation of the physiological roles of ACC as a signaling molecule in addition to its function as an ethylene precursor.

1-Aminocyclopropane 1-Carboxylic Acid and Its Emerging Role as an Ethylene-Independent Growth Regulator

1-Aminocyclopropane 1-carboxylic acid (ACC) is the direct precursor of the plant hormone ethylene. ACC is synthesized from S-adenosyl-L-methionine (SAM) by ACC synthases (ACSs) and subsequently oxidized to ethylene by ACC oxidases (ACOs). Exogenous ACC application has been used as a proxy for ethylene in numerous studies as it is readily converted by nearly all plant tissues to ethylene. However, in recent years, a growing body of evidence suggests that ACC plays a signaling role independent of the biosynthesis. In this review, we briefly summarize our current knowledge of ACC as an ethylene precursor, and present new findings with regards to the post-translational modifications of ACS proteins and to ACC transport. We also summarize the role of ACC in regulating plant development, and its involvement in cell wall signaling, guard mother cell division, and pathogen virulence.

Identification of Lysine Histidine Transporter 2 as an 1-Aminocyclopropane Carboxylic Acid Transporter in Arabidopsis thaliana by Transgenic Complementation Approach

1-Aminocyclopropane-1-carboxylic acid (ACC), a biosynthetic precursor of ethylene, has long been proposed to act as a mobile messenger in higher plants. However, little is known about the transport system of ACC. Recently, our genetic characterization of an ACC-resistant mutant with normal ethylene sensitivity revealed that lysine histidine transporter 1 (LHT1) functions as a transporter of ACC. As amino acid transporters might have broad substrate specificity, we hypothesized that other amino acid transporters including LHT1 paralogs might have the ACC-transporter activity. Here, we took a gain-of-function approach by transgenic complementation of lht1 mutant with a selected set of amino acid transporters. When we introduced transgene into the lht1 mutant, the transgenic expression of LHT2, but not of LHT3 or amino acid permease 5 (AAP5), restored the ACC resistance phenotype of the lht1 mutant. The result provides genetic evidence that some, if not all, amino acid transporters in Arabidopsis can function as ACC transporters. In support, when expressed in Xenopus laevis oocytes, both LHT1 and LHT2 exhibited ACC-transporting activity, inducing inward current upon addition of ACC. Interestingly, the transgenic expression of LHT2, but not of LHT3 or AAP5, could also suppress the early senescence phenotypes of the lht1 mutant. Taking together, we propose that plants have evolved a multitude of ACC transporters based on amino acid transporters, which would contribute to the differential distribution of ACC under various spatiotemporal contexts.

AtDAT1 Is a Key Enzyme of D-Amino Acid Stimulated Ethylene Production in Arabidopsis thaliana

D-Enantiomers of proteinogenic amino acids (D-AAs) are found ubiquitously, but the knowledge about their metabolism and functions in plants is scarce. A long forgotten phenomenon in this regard is the D-AA-stimulated ethylene production in plants. As a starting point to investigate this effect, the Arabidopsis accession Landsberg erecta (Ler) got into focus as it was found defective in metabolizing D-AAs. Combining genetics and molecular biology of T-DNA insertion lines and natural variants together with biochemical and physiological approaches, we could identify AtDAT1 as a major D-AA transaminase in Arabidopsis. Atdat1 loss-of-function mutants and Arabidopsis accessions with defective AtDAT1 alleles were unable to produce the metabolites of D-Met, D-Ala, D-Glu, and L-Met. This result corroborates the biochemical characterization, which showed highest activity of AtDAT1 using D-Met as a substrate. Germination of seedlings in light and dark led to enhanced growth inhibition of atdat1 mutants on D-Met. Ethylene measurements revealed an increased D-AA stimulated ethylene production in these mutants. According to initial working models of this phenomenon, D-Met is preferentially malonylated instead of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). This decrease of ACC degradation should then lead to the increase of ethylene production. We could observe a reciprocal relation of malonylated methionine and ACC upon D-Met application and significantly more malonyl-methionine in atdat1 mutants. Unexpectedly, the malonyl-ACC levels did not differ between mutants and wild type. With AtDAT1, the first central enzyme of plant D-AA metabolism was characterized biochemically and physiologically. The specific effects of D-Met on ACC metabolism, ethylene production, and plant development of dat1 mutants unraveled the impact of AtDAT1 on these processes; however, they are not in full accordance to previous working models. Instead, our results imply the influence of additional factors or processes on D-AA-stimulated ethylene production, which await to be uncovered.

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.

Transient induction of a subset of ethylene biosynthesis genes is potentially involved in regulation of grapevine bud dormancy release

Transient increases in ethylene biosynthesis, achieved by tight regulation of transcription of specific ACC oxidase and ACC synthase genes, play a role in activation of grapevine bud dormancy release. The molecular mechanisms regulating dormancy release in grapevine buds are as yet unclear. It has been hypothesized that its core involves perturbation of respiration which induces an interplay between ethylene and ABA metabolism that removes repression and allows regrowth. Roles for hypoxia and ABA metabolism in this process have been previously supported. The potential involvement of ethylene biosynthesis in regulation of dormancy release, which has received little attention so far, is now explored. Our results indicate that (1) ethylene biosynthesis is induced by hydrogen cyanamide (HC) and azide (AZ), known artificial stimuli of dormancy release, (2) inhibitors of ethylene biosynthesis and signalling antagonize dormancy release by HC/AZ treatments, (3) ethylene application induces dormancy release, (4) there are two sets of bud-expressed ethylene biosynthesis genes which are differentially regulated, (5) only one set is transiently upregulated by HC/AZ and during the natural dormancy cycle, concomitant with changes in ethylene levels, and (6) levels of ACC oxidase transcripts and ethylene sharply decrease during natural dormancy release, whereas ACC accumulates. Given these results, we propose that transient increases in ethylene biosynthesis prior to dormancy release, achieved primarily by regulation of transcription of specific ACC oxidase genes, play a role in activation of dormancy release.

Ethylene production and signaling in tomato (Solanum lycopersicum) pollen grains is responsive to heat stress conditions

Tomato pollen grains have the capacity for ethylene production, possessing specific components of the ethylene-biosynthesis and -signaling pathways, being affected/responsive to high-temperature conditions. Exposure of plants to heat stress (HS) conditions reduces crop yield and quality, mainly due to sensitivity of pollen grains. Recently, it was demonstrated that ethylene, a gaseous plant hormone, plays a significant role in tomato pollen heat-tolerance. It is not clear, however, whether, or to what extent, pollen grains are dependent on the capacity of the surrounding anther tissues for ethylene synthesis and signaling, or can synthesize this hormone and possess an active signaling pathway. The aim of this work was (1) to investigate if isolated, maturing and mature, tomato pollen grains have the capacity for ethylene production, (2) to find out whether pollen grains possess an active ethylene-biosynthesis and -signaling pathway and characterize the respective tomato pollen components at the transcript level, (3) to look into the effect of short-term HS conditions. Results from accumulation studies showed that pollen, anthers, and flowers produced ethylene and HS affected differentially ethylene production by (rehydrated) mature pollen, compared to anthers and flowers, causing elevated ethylene levels. Furthermore, several ethylene synthesis genes were expressed, with SlACS3 and SlACS11 standing out as highly HS-induced genes of the pollen ethylene biosynthesis pathway. Specific components of the ethylene-signaling pathway as well as several ethylene-responsive factors were expressed in pollen, with SlETR3 (ethylene receptor; named also NR, for never ripe) and SlCTR2 (constitutive triple response2) being HS responsive. This work shows that tomato pollen grains have the capacity for ethylene production, possessing active ethylene-biosynthesis and -signaling pathways, highlighting specific pollen components that serve as a valuable resource for future research on the role of ethylene in pollen thermotolerance.

Effect of Exogenous General Plant Growth Regulators on the Growth of the Duckweed Lemna minor

Gibberellic acid (GA₃), indole-3-acetic acid (IAA), salicylic acid (SA), abscidic acid (ABA), jasmonic acid (JA) 1-amino cyclopropane-1-carboxylic acid (ACC) and aminoethoxyvinylglycine (AVG) are popular growth regulators of plants. However, the effects of their exogenous addition on the biomass production of aquatic plants, including Lemnoideae plants, "duckweeds," are largely unknown. In this study, the growth of Lemna minor was tested for 10 d in Hoagland medium containing each compound at different concentrations of 0-50 μM. GA₃, IAA, and SA were found to have no apparent positive effect on the growth at all concentrations tested. Conversely, ACC and JA moderately and AVG and ABA severely inhibited the growth of L. minor. Among the tested compounds, ascorbic acid had an apparent growth-promoting effect.

Plant Growth Promotion Under Water: Decrease of Waterlogging-Induced ACC and Ethylene Levels by ACC Deaminase-Producing Bacteria

Some plant growth-promoting bacteria encode for 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which facilitates plant growth and development by lowering the level of stress ethylene under waterlogged conditions. The substrate ACC is the immediate precursor for ethylene synthesis in plants; while bacterial ACC deaminase hydrolyzes this compound into α-ketobutyrate and ammonia to mitigate the adverse effects of the stress caused by ethylene exposure. Here, the structure and function of ACC deaminase, ethylene biosynthesis and waterlogging response, waterlogging and its consequences, role of bacterial ACC deaminase under waterlogged conditions, and effect of this enzyme on terrestrial and riparian plants are discussed.