Molecular and hormonal regulation of plant responses to waterlogging stress: From fundamental mechanisms to potential strategies of crop tolerance engineering.

Plants have evolved numerous mechanisms to cope with abiotic stresses such as salinity, drought, high temperatures, chilling, heavy metal stress as well as UV or mechanical wounding. To survive in unfavorable conditions, plants have developed perception of external signals which leads to induction of defense mechanisms. Abiotic stress conditions, individually or in combination, require a set of specific acclimation responses, tailored to the definite needs of the plant, and a combination of two or more different stresses might require a response that is also equally specific. Phytohormones are essential for the ability of plants to adapt to changing environments, by mediating growth, development, nutrient allocation, and source/sink transitions. The phytohormones engaged in the defense responses to abiotic stresses are abscisic acid (ABA), ethylene (ET), auxin (IAA), gibberellic acid (GA), cytokinins (CKs), jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BRs), and polyamines (PAs). Much evidence has implicated the key role of ABA, ET, IAA, CKs, JA, and SA in plant signaling pathways in plant responses to abiotic stresses, whereas the defensive mechanisms induced by some phytohormones, such as GA, BRs, and PAs, are less well studied. Phytohormones function as signal molecules in regulation of the expression of defensive genes and modification of enzyme activity. These phytohormones can act separately or coordinate with other signaling pathways in a complex network. Cross-talk between the different hormones results in synergetic or antagonistic interactions that play crucial roles in the response of plants to abiotic stress.

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

1 - Hormone function in plants

Transcriptome profiling analysis revealed co-regulation of multiple pathways in jujube during infection by ‘Candidatus Phytoplasma ziziphi’

Barley is the major raw material for the malting and brewing industries. Seed germination is a fundamental process for malting and is affected by physical and chemical factors, especially plant hormones. It is well recognized that the phytohormones gibberellic acid (GA) and abscisic acid (ABA) are the primary hormones that antagonistically regulate barley germination. Other plant hormones, including auxin (IAA), ethylene (ET), brassinosteroid (BR), jasmonic acid (JA), salicylic acid (SA), cytokinins (CTKs), and strigolactone (SLs) also regulate seed germination by mediating the ABA/GA balance. In this study, the effects of eight hormones added during barley germination were investigated. An efficient design of the experimental method was employed to assess the effect of eight factors (α-amylase, β-amylase, limit dextrinase, β-glucanase, wort sugar profile, FAN, and β-glucan content) on epicotyl and root elongation. Almost all studied phytohormones played a significant role on barley epicotyl elongation. GA, ET, BA (benzylaminopurine), and zeatin (ZEA) exerted an increased effect, while ABA, SA, and BR showed a decreased effect for α-amylase, β-amylase, limit dextrinase, and β-glucanase activity. Results showed that in addition to GA and ABA, that SA, ET, BR, BA, ZEA, and JA also had significant effects on malt quality. The balance between these phytohormones is vital to the malting quality of barley. Future applications of these results could improve current recognition about the effects of phytohormones on malting and provide concrete ideas for enhancing malt quality.

Melatonin: Discovery, biosynthesis, phytohormones crosstalk, and roles in agricultural crops under abiotic stress conditions

Brassinosteroids (BRs) are key phytohormones involved in the regulation of major processes of cell metabolism that guide plant growth. In the past decades, new evidence has made it clear that BRs also play a key role in the orchestration of plant responses to many abiotic and biotic stresses. In the present work, we analyzed the impact of foliar treatment with 24-epicastasterone (ECS) on the endogenous content of major phytohormones (auxins, salicylic acid, jasmonic acid, and abscisic acid) and their intermediates in soybean leaves 7 days following the treatment. Changes in the endogenous content of phytohormones have been identified and quantified by LC/MS. The obtained results point to a clear role of ECS in the upregulation of auxin content (indole-3-acetic acid, IAA) and downregulation of salicylic, jasmonic, and abscisic acid levels. These data confirm that under optimal conditions, ECS in tested concentrations of 0.25 µM and 1 µM might promote growth in soybeans by inducing auxin contents. Benzoic acid (a precursor of salicylic acid (SA)), but not SA itself, has also been highly accumulated under ECS treatment, which indicates an activation of the adaptation strategies of cell metabolism to possible environmental challenges.

Roles of exogenous plant growth regulators on phytoextraction of Cd/Pb/Zn by Sedum alfredii Hance in contaminated soils

Due to sessile nature of plants, they face a variety of biotic and abiotic stresses during their life cycle. These stresses are responsible for disturbed cellular processes that adversely affect their growth and yield. To cope with these stresses, plants have developed various physiological mechanisms at cellular level that result in a change in morphology and help them to tolerate these environmental changes and respond to these changes with an optimal response. These responses of plants toward environmental stresses are both dynamic and complex that help them complete their life cycle rapidly under these stressful environmental conditions. The defense response of the plants to these stresses starts through variations in different molecular events with the involvement of different signaling molecules including the phytohormones. Phytohormones are small low-molecular-weight endogenous molecules that play important roles in different defense responses of plants against biotic as well as abiotic stresses. Along with their role in defense signaling, they also regulate various physiological, growth, and developmental processes in plants. These phytohormones include the auxins, cytokinins (CKs), gibberellins (GAs), ethylene (ET), jasmonic acid (JA), abscisic acid (ABA), salicylic acid (SA), and brassinosteroids (BRs) tocotrienols, triacontanols, and polyamines which are important ones that have ability to help plants to respond to different environmental stresses through their specific signaling properties. These signaling defense responses are the results of the interaction of various genes, helping the phytohormones to be involved in almost all cellular metabolic processes due to their specific modulations in the activities of these genes. All these phytohormones have their specific roles, e.g., auxin is involved in the regulation of differentiation and plant growth, cytokinin is responsible for cell division, gibberellin is responsible for stem elongation, seed germination, dormancy, senescence, and flower development, ethylene is involved in fruit ripening, and ABA has stress tolerance ability. The stress tolerance mechanism of plant is much complex that also has the involvement of other phytohormones, including the brassinosteroids (BRs), jasmonic acid (JA), salicylic acid (SA), polyamines (spermine, spermidine, putrescine, and thermospermine), tocotrienols, triacontanols (TRAI) as newly discovered ones. All the phytohormones help plants to survive under adverse environmental conditions with their specific roles in various growth, developmental, and physiological processes either through their endogenous accumulation or by exogenous application (foliar spray or seed priming) where the optimal concentration for the stress response is not sufficient. The exogenous use of these phytohormones has been increasing in crop plants with their economic value for obtaining the desired characters along with better production. The information given in the chapter will be helpful for plant growers and researchers to understand the mechanism of action of these phytohormones for better growth and production under changing environmental conditions.

Flowering is the adaptability of plants in response to the environment, which is regulated by the complex flowering control network formed by a variety of exogenous and endogenous signals. Plant hormones, the most important endogenous signal participants, play important roles in the process of plant flowering. Recent reports reveal the pivotal roles of hormones in the epigenetic regulation and flowering promotion pathway. In addition, synergistic or antagonistic interaction has been observed among many hormones. Numerous hormones have been found to be involved in the regulation of the multiple flowering development regulation and signaling pathways mediated by DELLA protein in the gibberellin (GA) pathway. In this review, we summarize the recent advances ofthe flowering mechanisms related to GA pathway and discuss the effects of abscisic acid (ABA), auxin (IAA), cytokinin (CTK), salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) on flowering, including their cross-regulation with DELLA, miRNAs, and transcription factor (TFs). This review provides a reference for further comprehensive analysis of the hormone-regulated network of plant flower formation.

The abscission of fruits has a significant impact on yield, which in turn has a corresponding effect on economic benefits. In order to better understand the molecular mechanism of early coconut fruit abscission, the morphological and structural characteristics, cell wall hydrolysis and oxidase activities, phytohormones, and transcriptomes were analyzed in the abscission zone (AZ) from early-abscised coconut fruits (AFs) and non-abscised coconut fruits (CFs). These results indicated that the weight and water content of AFs are significantly lower than those of CFs, and the color of AFs is a grayish dark red, with an abnormal AZ structure. Cellulase (CEL), polygalacturonase (PG), pectinesterase (PE), and peroxidase (POD) activities were significantly lower than those of CFs. The levels of auxin (IAA), gibberellin (GA), cytokinins (CKs), and brassinosteroid (BR) in AFs were significantly lower than those in CFs. However, the content of abscisic acid (ABA), ethylene (ETH), jasmonic acid (JA), and salicylic acid (SA) in AFs was significantly higher than in CFs. The transcriptome analysis results showed that 3601 DEGs were functionally annotated, with 1813 DEGs upregulated and 1788 DEGs downregulated. Among these DEGs, many genes were enriched in pathways such as plant hormone signal transduction, carbon metabolism, peroxisome, pentose and gluconate interconversion, MAPK signaling pathway—plant, and starch and sucrose metabolism. Regarding cell wall remodeling-related genes (PG, CEL, PE, POD, xyloglucan endoglucosidase/hydrogenase (XTH), expansin (EXP), endoglucanase, chitinase, and beta-galactosidase) and phytohormone-related genes (IAA, GA, CKs, BR, ABA, JA, SA, and ETH) were significantly differentially expressed in the AZ of AFs. Additionally, BHLH, ERF/AP2, WRKY, bZIP, and NAC transcription factors (TFs) were significantly differently expressed, reflecting their crucial role in regulating the abscission process. This study’s results revealed the molecular mechanism of early fruit abscission in coconuts. This provided a new reference point for further research on coconut organ development and abscission.

The worldwide agriculture industry is facing increasing problems due to rapid population increase and increasingly unfavorable weather patterns. In order to reach the projected food production targets, which are essential for guaranteeing global food security, innovative and sustainable agricultural methods must be adopted. Conventional approaches, including traditional breeding procedures, often cannot handle the complex and simultaneous effects of biotic pressures such as pest infestations, disease attacks, and nutritional imbalances, as well as abiotic stresses including heat, salt, drought, and heavy metal toxicity. Applying phytohormonal approaches, particularly those involving hormonal crosstalk, presents a viable way to increase crop resilience in this context. Abscisic acid (ABA), gibberellins (GAs), auxin, cytokinins, salicylic acid (SA), jasmonic acid (JA), ethylene, and GA are among the plant hormones that control plant stress responses. In order to precisely respond to a range of environmental stimuli, these hormones allow plants to control gene expression, signal transduction, and physiological adaptation through intricate networks of antagonistic and constructive interactions. This review focuses on how the principal hormonal signaling pathways (in particular, ABA-ET, ABA-JA, JA-SA, and ABA-auxin) intricately interact and how they affect the plant stress response. For example, ABA-driven drought tolerance controls immunological responses and stomatal behavior through antagonistic interactions with ET and SA, while using SnRK2 kinases to activate genes that react to stress. Similarly, the transcription factor MYC2 is an essential node in ABA-JA crosstalk and mediates the integration of defense and drought signals. Plants' complex hormonal crosstalk networks are an example of a precisely calibrated regulatory system that strikes a balance between growth and abiotic stress adaptation. ABA, JA, SA, ethylene, auxin, cytokinin, GA, and BR are examples of central nodes that interact dynamically and context-specifically to modify signal transduction, rewire gene expression, and change physiological outcomes. To engineer stress-resilient crops in the face of shifting environmental challenges, a systems-level view of these pathways is provided by a combination of enrichment analyses and STRING-based interaction mapping. These hormonal interactions are directly related to the United Nations Sustainable Development Goals (SDGs), particularly SDGs 2 (Zero Hunger), 12 (Responsible Consumption and Production), and 13 (Climate Action). This review emphasizes the potential of biotechnologies to use hormone signaling to improve agricultural performance and sustainability by uncovering the molecular foundations of hormonal crosstalk. Increasing our understanding of these pathways presents a strategic opportunity to increase crop resilience, reduce environmental degradation, and secure food systems in the face of increasing climate unpredictability.

Plant hormones can act in synergistic and antagonistic ways in response to biotic and abiotic stresses and in plant growth and development. Thus, a technique is needed to simultaneously determine the distributions and concentrations of several plant hormones. Previously, we reported that localizations of two plant hormones [cytokinin (CK) and abscisic acid (ABA)] can be simultaneously visualized in a plant tissue using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). In MALDI-MS, however, self-ionization of an organic matrix occasionally interferes with ionizations of small molecules (<500 m/z) including most plant hormones. Another technique, nanoparticle-assisted laser desorption/ionization (Nano-PALDI), can avoid matrix self-ionization using nanoparticles to assist the ionization of analytes. Here, we compared the ionization efficiencies of common plant hormones by MALDI-MS and Nano-PALDI-MS. For the comparison, we prepared a standard mix of seven plant hormones [ABA, auxin (IAA), brassinosteroid (Br), two CKs (trans-zeatin, tZ, and 6-(γ,γ-dimethylallylamino) purine, iP), jasmonic acid, and salicylic acid (SA)], an ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC), and a heavy hydrogen-labeled ABA (D6-ABA). Basic MALDI-MS detected all compounds except IAA, Br, and D6-ABA, while Nano-PALDI-MS detected all nine compounds. By Nano-PALDI-MS imaging (MSI), each of the abovementioned hormones and ACC were also detected in root cross sections of rice which were incubated in the hormone mix for 2 h. In the elongation zone of untreated roots, Nano-PALDI-MSI revealed high levels of ABA and CKs in the outer part of roots and much lower levels in the stele, but Br, SA, and ACC were broadly distributed in the cross section. IAA seemed to be distributed in the epidermis, cortex, and stele. Multiple-hormone imaging using Nano-PALDI-MS has great potential for investigating the roles of hormone signaling in crop development and stress responses.

The synergistic effect of caffeic, jasmonic and salicylic acids and abscisic acid was highly effective in reducing the phytotoxic effect of Ni in isabgol plants by promoting growth, physiological processes and nutrient metabolism. This study was necessitated by the need to formulate an effective plan to make plants more tolerant to heavy metal-contaminated soil. The experiment was performed in a pot test using a completely randomized design with four replications. Nickel stress (100mg/kg soil) significantly reduced root length (49.49%), shoot length (44.59%), root fresh weight (60.05%), shoot fresh weight (55.85%) and chlorophyll content (52.66%) compared to the unstressed control. The synergistic application of caffeic acid (1mM), jasmonic acid (100µM), salicylic acid (1mM) and abscisic acid (50µM) significantly mitigated these effects, increasing root length, shoot length by 21%1%, root fresh weight by 97%7% and shoot fresh weight by 59%9%, 21%1%, 97%7% and 45.31%, respectively, relative to the Ni-stressed control. This treatment also enhanced the activities of antioxidant enzymes, including superoxide dismutase (31.98%), peroxidase (46.24%), catalase (35.98%), ascorbate peroxidase (73.39%), glutathione peroxidase (62.94%) and glutathione reductase (64.76%), as well as non-enzymatic antioxidants such as ascorbic acid (48.57%), anthocyanins (64.40%), β-cyanin (27.83%), β-xanthin (42.13%), phenolic content (33.51%) and flavonoid content (38.70%). It also reduced Ni accumulation in roots, shoots and seeds by 33.72%, 36.31% and 49.69%, respectively, while improving the uptake of essential nutrients, such as Fe (30.66%), Mn (35.52%), Zn (32.98%) and N (14.99%). Photosynthetic efficiency, membrane stability and relative water content were also restored, leading to a 51.16% increase in seed yield. The synergistic interaction of these compounds enhances stress signaling, redox balance and metabolic regulation more effectively than their individual applications. Therefore, the combined exogenous application of caffeic acid, jasmonic acid, salicylic and abscisic acid is recommended as a sustainable strategy to improve Isabgol production in nickel-contaminated environments.

Chapter 6 - The Role of Growth Regulators in Senescence