Hormone Signalling Crosstalk in Plant Growth Regulation

Plant growth regulation is orchestrated by complex hormonal networks involving gibberellin and auxin signaling pathways. In this study, a comprehensive in silico approach was employed to comparatively evaluate the plant growth regulators (PGRs) forchlorfenuron (CPPU) and strigol (STG) against two key proteins from Arabidopsis thaliana: Gibberellin 3-beta-dioxygenase 2 (GA3Ox2), a rate-limiting enzyme in the biosynthesis of bioactive gibberellins, and the auxin signaling repressor IAA7. These targets were specifically selected because they represent critical regulatory nodes in two major hormonal pathways controlling plant growth: GA3Ox2 governs the final steps of gibberellin activation, while IAA7 modulates auxin-responsive gene expression through its interaction with Auxin Response Factors. Therefore, their combined analysis enables the evaluation of potential regulatory effects of PGRs on both gibberellin biosynthesis and auxin-mediated transcriptional control. Molecular docking analyses revealed that both ligands exhibited higher binding affinity toward GA3Ox2 than IAA7, with STG showing slightly more favorable binding energies (-7.91 kcal/mol for GA3Ox2 and -5.43 kcal/mol for IAA7) compared to CPPU (-7.18 and -4.79 kcal/mol, respectively). These results suggest a structural preference of both PGRs toward the gibberellin biosynthetic pathway. To further assess complex stability under near-physiological conditions, 100 ns molecular dynamics (MD) simulations were conducted using the CHARMM36m force field. Despite its slightly lower docking scores, CPPU demonstrated greater conformational stability, lower RMSD fluctuations, and more persistent hydrogen bonding patterns, particularly in complexes with IAA7. In contrast, STG induced more pronounced conformational rearrangements, although it promoted slightly more compact protein conformations in certain systems. Fourier-transform infrared (FTIR) spectroscopy supported the computational findings by confirming the presence of key functional groups responsible for hydrogen bonding and hydrophobic interactions. Collectively, the results indicate that although STG exhibits higher initial binding affinity, CPPU forms more dynamically stable complexes with both proteins. These findings suggest that CPPU may represent a more robust candidate for sustained modulation of auxin and gibberellin signaling pathways in plant growth regulation.

The root is the first plant organ to get in contact with the toxin cadmium (Cd), which is a widespread soil contaminant. Cd inhibits the growth of the primary root, but the mechanisms underlying this inhibition remain elusive. In this study, we used physiological, pharmacological and genetic approaches to investigate the roles of nitric oxide (NO) and auxin in Cd-mediated inhibition of Arabidopsis thaliana root meristem growth. Our study demonstrated that in the first 12 h of exposure, Cd inhibits primary root elongation through a decrease in the sizes of both the elongation and meristematic zones. Following Cd exposure, a decrease in auxin levels is associated with reduced PIN1/3/7 protein accumulation, but not with reduced PIN1/3/7 transcript levels. Additionally, Cd stabilized AXR3/IAA17 protein to repress auxin signalling in this Cd-mediated process. Furthermore, decreasing Cd-induced NO accumulation with either NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or NO synthase inhibitor N(ω) -nitro-l-Arg-methylester (l-NAME) compromised the Cd-mediated inhibition of root meristem development, reduction in auxin and PIN1/3/7 accumulation, as well as stabilization of AXR3/IAA17, indicating that NO participates in Cd-mediated inhibition of root meristem growth. Taken together, our data suggest that Cd inhibits root meristem growth by NO-mediated repression of auxin accumulation and signalling in Arabidopsis.

Phytohormones are chemical substances that in minute concentration instruct a plethora of developmental and physiological responses in plants. These signal molecules synthesized within the plant body are referred to as plant growth regulators. The available literature revealed that manipulation of phytohormonal content could be a promising approach towards augmentation of environmental stress tolerance in crop plants. They play pivotal role in acclimatization against unstable environmental extremes. Gibberellins are plant hormones affecting germination, stem elongation, flowering, abolition of dormancy, determination of sex expression, leaf and fruit senescence, and enzymatic stimulation. The uncertainty in climatic condition and over expanding population has led to a heap of abiotic stresses in plants. Salinity, high temperature, chilling, freezing, heavy metals, drought, flooding, allelochemicals, and radiation are the stresses that hinder development of plants. The perception of these stresses by plants occurs in a highly coordinated and interactive manner by triggering the activation of a myriad of elaborate signaling networks in which phytohormones play a significant role. The present review describes biosynthesis, signaling, and the potential roles of gibberellins as a tool in mitigating stress, increasing growth, development, and tolerance in plants. In future, revelations evolving the comprehensive knowledge to understand the relationship of plant growth regulators and stress conditions are discussed. This review also enlightens the latest research progress in GA signaling and its crosstalk with other hormonal pathways, underlying the multitude role of DELLA proteins with components of other hormonal signaling pathways.

The potential for exploitation of the gibberellin (GA) plant growth hormones in agriculture was a major factor driving research on the physiological function of these substances following their introduction to the West from Japan in the 1950s. As a consequence of its vital role in promoting organ growth as well as in stimulating seed germination and reproductive development, this class of hormone has assumed considerable importance as a plant growth regulator (PGR) and a wide range of applications of GAs are known. However, in terms of treated area and sales, the commercial utilisation of GAs is dwarfed by that of the growth retardants, which, as inhibitors of GA biosynthesis, function by reducing the bioactive GA content of treated plants, thereby reducing shoot elongation. Several inhibitors of GA biosynthesis are of practical relevance with important uses. The annual global PGR market accounts for approximately €700 million, of which some 50% can be assumed to be represented by growth retardants. Area- and value-wise, stem shortening in small grains and in rice production to reduce the risk of lodging is the main application of PGRs worldwide. It is estimated that some 30% of global PGR sales (equalling approximately €210 million) are represented by stem stabilisers. In addition to inhibitors of GA biosynthesis, ethylene-releasing ethephon is also used for this purpose, particularly in barley. The usage of such products is general practice in countries with intense production of wheat, barley, rye, triticale, and oats such as France, Germany and Great Britain. For instance, 92% of the winter wheat, 81% of the winter barley, 67% of the oats, and 100% of the rye acreage were treated with anti-lodging products in Great Britain in 2008. Despite this, it has been estimated that losses due to lodging cost the British wheat industry about €50 million per year and these costs are likely to be even higher to date. The application of a PGR allows for an active regulation of developmental processes. For example, control of stem elongation in cereals may be “fine-tuned” according to need, both in terms of affecting the right growth stage (by timing of treatment) and the intensity of growth reduction (by dosage). Also, but within limits, distinct parts of a plant may be targeted. It can be counter-productive in cereal production if, in addition to shoot growth, root growth is also reduced. This can be avoided by spray-applying non-systemic compounds such as prohexadione-Ca. Thus, in terms of flexibility, immediate growth control and potential specificity, PGRs can offer advantages over conventional breeding and genetic engineering approaches. On the other hand, there is wide-spread concern about the use of chemicals in crop production. In order to comply with the principles of integrated crop production better, it would be preferable if problems solved with presently existing PGRs could be overcome by introducing improved genotypes and/or by making available new and more specific PGRs with an even further reduced risk to the grower, environment and consumer. The introduction of semi-dwarfing genes into wheat and rice in the 1960s played an important role in the Green Revolution that resulted in dramatic increases in crop yield. Shorter, more stable stems were necessary to avoid lodging of the crop under high levels of nitrogenous fertiliser, but semidwarfism also improved yields by increasing grain numbers, probably by allowing more assimilate to partition into the developing spike. The dwarfing genes in both rice and wheat act on the GA-signalling system: semi-dwarf rice contains mutations in a GA-biosynthesis gene that result in reduced GA content while the reduced height (Rht) mutation in wheat compromises the ability of GA to stimulate growth. In view of the importance of the GA signalling pathway in regulating physiological processes of relevance to agriculture, it is a prime target in crop improvement programmes. To date, most interest has been focussed on the GA-biosynthetic and inactivation pathways that determine GA concentration since they have been more clearly understood than the mechanisms for GA perception and signal transduction. A number of studies describing the genetic modification of GA metabolism, either to enhance or restrict plant growth, have been published. For example, increasing GA content by ectopic expression of GA 20-oxidase genes, which encode a rate-limiting enzyme in GA biosynthesis, has been shown to result in higher biomass in tobacco and in longer fibres in aspen, resulting in improved paper-making quality. Conversely, ectopic expression of GA 2-oxidase genes, which encode GA-inactivating enzymes, provides a particularly efficient method for introducing dwarfism. These approaches are clearly effective and even though breeding by genetic engineering is not publicly acceptable in a number of countries, they highlight potential gene targets for breeding to improve plant architecture or yield. Recently, the range of targets has been potentially broadened by impressive advances in our understanding of GA signal transduction.

Modulation of xenobiotic biotransformation system and hormonal responses in Atlantic salmon ( Salmo salar) after exposure to tributyltin (TBT)

Plant somatic cells can reprogram into differentiated embryos through somatic embryogenesis (SE) on the condition of plant growth regulators (PGRs). RNA sequencing analysis was performed to investigate transcriptional profiling on cotton redifferentiated callus that was induced by different auxin types (IAA and 2,4-D), different concentrations (0, 0.025, and 0.05 mg L−1), and different incubation times (0, 5, and 20 days). Under the 2,4-D induction effect, signal transduction pathways of plant hormones were significantly enriched in the embryogenic response stage (5 days). These results indicated that auxin signal transduction genes were necessary for the initial response of embryogenic differentiation. In the pre-embryonic initial period (20 days), the photosynthetic pathway was significantly enriched. Most differentially expressed genes (DEGs) were downregulated under the induction of 2,4-D. Upon the dose effect of IAA and 2,4-D, respectively, pathways were significantly enriched in phenylpropanoid biosynthesis, fatty acid metabolism, and carbon metabolic pathways. Therefore, primary and secondary metabolism pathways were critical in cotton SE. These results showed that complex synergistic mechanisms involving multiple cellular pathways were the causes of the induction and dose effect of auxin-induced SE. This study reveals a systematic molecular response to auxin signals and reveals the way that regulates embryogenic redifferentiation during cotton SE.

Recognition of microbial challenges leads to enhanced immunity at both the local and systemic levels. In Arabidopsis, EFR and PEPR1/PEPR2 act as the receptor for the bacterial elongation factor EF-Tu (elf18 epitope) and for the endogenous PROPEP-derived Pep epitopes, respectively. The PEPR pathway has been described to mediate defence signalling following microbial recognition. Here we show that PROPEP2/PROPEP3 induction upon pathogen challenges is robust against jasmonate, salicylate, or ethylene dysfunction. Comparative transcriptome profiling between Pep2- and elf18-treated plants points to co-activation of otherwise antagonistic jasmonate- and salicylate-mediated immune branches as a key output of PEPR signalling. Accordingly, as well as basal defences against hemibiotrophic pathogens, systemic immunity is reduced in pepr1 pepr2 plants. Remarkably, PROPEP2/PROPEP3 induction is essentially restricted to the pathogen challenge sites during pathogen-induced systemic immunity. Localized Pep application activates genetically separable jasmonate and salicylate branches in systemic leaves without significant PROPEP2/PROPEP3 induction. Our results suggest that local PEPR activation provides a critical step in connecting local to systemic immunity by reinforcing separate defence signalling pathways.

Auxin plays a crucial role throughout the entire life cycle of plants. The auxin/indole-3-acetic acid (Aux/IAA) gene family serves as a negative regulator of auxin response and is one of the earliest auxin-responsive gene families. It regulates the expression of auxin-responsive genes by specifically binding to auxin response factors. This review summarizes the protein structural characteristics of the Aux/IAA gene family and its typical and atypical transduction mechanisms in auxin signaling. Additionally, it examines the role of Aux/IAA in regulating plant growth and development, as well as its function in modulating plant resistance to abiotic stress through hormonal signaling pathways. Our findings indicate that the Aux/IAA gene family plays a significant role in plant growth and development, as well as in abiotic stress resistance. However, research on the functional roles of the Aux/IAA gene family in crops such as rice, wheat, and maize remains relatively scarce. Furthermore, we identified key questions and proposed new research directions regarding the Aux/IAA gene family, aiming to provide insights for future research on plant hormone signaling and molecular breeding in crop design.

Ubiquitin, hormones and biotic stress in plants.

As global climate change intensifies, the occurrence and severity of various abiotic stresses will significantly threaten plant health and productivity. Drought stress (DS) is a formidable obstacle, disrupting normal plant functions through specific morphological, physiological, biochemical, and molecular mechanisms. Understanding how plants navigate DS is paramount to mitigating its adverse effects. In response to DS, plants synthesize or accumulate various plant growth regulators (PGRs), including phytohormones, neurotransmitters, gasotransmitters, and polyamines, which present promising sustainable green chemical strategies to adapt or tolerate stress conditions. These PGRs orchestrate crucial plant structure and function adjustments, activating defense systems and modulating cellular-level responses, transcript levels, transcription factors, metabolic genes, and stress-responsive candidate proteins. However, the efficacy of these molecules in mitigating DS depends on the plant species, applied PGR dose, treatment type, duration of DS exposure, and growth stages. Thus, exploring the integrated impact of PGRs on enhancing plant fitness and DS tolerance is crucial for global food security and sustainable agriculture. This review investigates plant responses to DS, explains the potential of exogenously applied diverse PGRs, dissects the complex chemistry among PGRs, and sheds light on omics approaches for harnessing the molecular basis of DS tolerance. This updated review delivers comprehensive mechanistic insights for leveraging various PGRs to enhance overall plant fitness under DS conditions.

Auxin signaling, transport, and regulation during adventitious root formation

Ethephon-regulated maize internode elongation associated with modulating auxin and gibberellin signal to alter cell wall biosynthesis and modification

Physiology and biochemistry of algae: Edited by Ralph A. Lewin, Scripps Institution of Oceanography, University of California, La Jolla, Calif. Academic Press, New York and London. 1962. xxviii, 929 pp. price $32.00

AUXOLOGY: When auxin meets plant evo-devo

Plant development requires regulation of both cell division and differentiation. The class 1 KNOTTED1-like homeobox (KNOX) genes such as knotted1 (kn1) in maize (Zea mays) and SHOOTMERISTEMLESS in Arabidopsis (Arabidopsis thaliana) play a role in maintaining shoot apical meristem indeterminacy, and their misexpression is sufficient to induce cell division and meristem formation. KNOX overexpression experiments have shown that these genes interact with the cytokinin, auxin, and gibberellin pathways. The L1 layer has been shown to be necessary for the maintenance of indeterminacy in the underlying meristem layers. This work explores the possibility that the L1 affects meristem function by disrupting hormone transport pathways. The semidominant Extra cell layers1 (Xcl1) mutation in maize leads to the production of multiple epidermal layers by overproduction of a normal gene product. Meristem size is reduced in mutant plants and more cells are incorporated into the incipient leaf primordium. Thus, Xcl1 may provide a link between L1 division patterns, hormonal pathways, and meristem maintenance. We used double mutants between Xcl1 and dominant KNOX mutants and showed that Xcl1 suppresses the Kn1 phenotype but has a synergistic interaction with gnarley1 and rough sheath1, possibly correlated with changes in gibberellin and auxin signaling. In addition, double mutants between Xcl1 and crinkly4 had defects in shoot meristem maintenance. Thus, proper L1 development is essential for meristem function, and XCL1 may act to coordinate hormonal effects with KNOX gene function at the shoot apex.