Involvement Of Phytohormones Research Articles

The involvement of phytohormones in plant–pathogen interaction

Plant–pathogen interactions involve intricate signaling networks that coordinate the plant immune response. Recognition of pathogens through pattern recognition receptors (PRRs) triggers activation of mitogen-activated protein kinase (MAPK) pathways, initiating a cascade of defense mechanisms. Central to these responses is the synthesis of phytohormones such as salicylic acid (SA), auxins–indole-3-acetic acid (IAA), and gibberellins–gibberellic acid (GA), pivotal for immune activation. This review explores the multifaceted roles of these phytohormones in plant immunity, drawing on recent findings from Arabidopsis thaliana and Gossypium hirsutum studies. The review discusses MAPK-mediated activation of TGA1/4 (TGACG sequence-specific binding protein 1/4) transcription factors enhancing SA biosynthesis via isochorismate synthase (ICS). Increased SA levels activate NPR1, promoting gene expression in immune-related pathways including systemic acquired resistance (SAR). Concurrently, pathogen-induced IAA synthesis activates auxin-responsive genes crucial for immune responses. Elevated biosynthesis of IAA from L-tryptophan activates these genes by degrading repressor molecules. IAA acts antagonistically to SA, conserving energy during pathogen infection. Additionally, GA is vital for plant growth and development, operating DELLA (Asp–Glu–Leu–Leu–Ala) protein degradation with the formation of a complex with gibberellin insensitive dwarf 1 (GID1). Once DELLA prevents releasing GA-related response reactions, it is extremelly crucial for GA actions. In general, the review explores the intricate interplay between SA, IAA, and GA, highlighting SA's antagonistic regulation of GA signaling and the synergistic effects of auxin and GA. Understanding these hormone–mediated pathways is crucial for elucidating precise mechanisms underlying plant immunity. Insights gained could inform strategies to enhance plant resistance against pathogens, contributing to sustainable agriculture and global food security efforts.

Sulfate Availability and Hormonal Signaling in the Coordination of Plant Growth and Development.

Sulfur (S), one of the crucial macronutrients, plays a pivotal role in fundamental plant processes and the regulation of diverse metabolic pathways. Additionally, it has a major function in plant protection against adverse conditions by enhancing tolerance, often interacting with other molecules to counteract stresses. Despite its significance, a thorough comprehension of how plants regulate S nutrition and particularly the involvement of phytohormones in this process remains elusive. Phytohormone signaling pathways crosstalk to modulate growth and developmental programs in a multifactorial manner. Additionally, S availability regulates the growth and development of plants through molecular mechanisms intertwined with phytohormone signaling pathways. Conversely, many phytohormones influence or alter S metabolism within interconnected pathways. S metabolism is closely associated with phytohormones such as abscisic acid (ABA), auxin (AUX), brassinosteroids (BR), cytokinins (CK), ethylene (ET), gibberellic acid (GA), jasmonic acid (JA), salicylic acid (SA), and strigolactones (SL). This review provides a summary of the research concerning the impact of phytohormones on S metabolism and, conversely, how S availability affects hormonal signaling. Although numerous molecular details are yet to be fully understood, several core signaling components have been identified at the crossroads of S and major phytohormonal pathways.

Plant Growth Regulating Phytohormones: The Potent Targets to Alleviate Abiotic Stress in Crop Plants —A Research Update and the Way Forward

The latest modifying models of global climate have changed to be an unadorned threat to the crops production. An extensive scale of biotic and abiotic stresses frequently impacts crop plants because of their stalkless characteristics. Crop plants failures are principally triggered by abiotic factors such as salt, heat, cold, floods, ultraviolet radiation and drought. For dealing with these harsh conditions, well-established mechanisms have been developed in plants, which perform a pivotal role in comprehending stress signals and assisting ideal development responses. Remarkably, the exploitation of plant growth regulating phytohormones for decreasing the adverse effect of abiotic stress has acquired much attention in current decades. For evasion of stress at various levels, including physiological, molecular, and biochemical, an erudite method is informed to be presented by phytohormones. Thus, labeling these phytohormones plays an important function in plant growth and overall development. Phytohormones can progresses tolerance against abiotic stresses by raising seed germination, seedling growth, root growth, leaf photosynthesis and antioxidant enzymes by dropping the accretion of reactive oxygen species, malonaldehyde, and electrolyte outflow. Current findings focus on the important function of a diversity of phytohormones counting, Abscisic acid (ABA), Salicylic acid (SA), Jasmonates (JA) and Ethylene (ET) in the abiotic stress tolerance improvement of crop plants. The contribution of Cytokinins (CKs), and some other novel phytohormones such as Brassinosteroids and Strigolactones in plant development has been acknowledged under normal and stress conditions. Therefore, this review is desired to recap the new progresses in the knowledge associated to phytohormones and their association in abiotic stresses of expectation, signaling, crosstalk, and the response processes. Therefore, such a review will be helpful in improving the pathway for sustainable agriculture development via involvement of phytohormones in improvement of abiotic stress tolerance of the crop plants.

Comparative transcriptome analysis of high- and low-embryogenic Hevea brasiliensis genotypes reveals involvement of phytohormones in somatic embryogenesis

BackgroundRubber plant (Hevea brasiliensis) is one of the major sources of latex. Somatic embryogenesis (SE) is a promising alterative to its propagation by grafting and seed. Phytohormones have been shown to influence SE in different plant species. However, limited knowledge is available on the role of phytohormones in SE in Hevea. The anther cultures of two Hevea genotypes (Yunyan 73477-YT and Reken 628-RT) with contrasting SE rate were established and four stages i.e., anthers (h), anther induced callus (y), callus differentiation state (f), and somatic embryos (p) were studied. UPLC-ESI-MS/MS and transcriptome analyses were used to study phytohormone accumulation and related expression changes in biosynthesis and signaling genes.ResultsYT showed higher callus induction rate than RT. Of the two genotypes, only YT exhibited successful SE. Auxins, cytokinins (CKs), abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), gibberellins (GAs), and ethylene (ETH) were detected in the two genotypes. Indole-3-acetic acid (IAA), CKs, ABA, and ETH had notable differences in the studied stages of the two genotypes. The differentially expressed genes identified in treatment comparisons were majorly enriched in MAPK and phytohormone signaling, biosynthesis of secondary metabolites, and metabolic pathways. The expression changes in IAA, CK, ABA, and ETH biosynthesis and signaling genes confirmed the differential accumulation of respective phytohormones in the two genotypes.ConclusionThese results suggest potential roles of phytohormones in SE in Hevea.

Insights on the stem elongation of spur-type bud sport mutant of ‘Red Delicious’ apple

The decreased capacity of auxin-, CTK-, and BR-mediated cell division and cell enlargement pathways, combined with the enhanced capacity of GA and ETH-, JA-, ABA-, SA-mediated stress-resistant pathways were presumed to be the crucial reasons for the formation of spur-type 'Red Delicious' mutants. Vallee Spur', which exhibit short internodes and compact tree shape, is the fourth generation of the spur-type bud sport mutant of 'Red Delicious'. However, the underlying molecular mechanism of these properties remains unclear. Here, comparative phenotypic, full-length transcriptome and phytohormone analyses were performed between 'Red Delicious' (NSP) and 'Vallee Spur' (SP). The new shoot internode length of NSP was ˃ 1.53-fold higher than that of the SP mutant. Cytological analysis showed that the stem cells of the SP mutant were smaller and more tightly arranged relative to the NSP. By Iso-Seq, a total of 1426 differentially expressed genes (DEGs) were detected, including 808 upregulated and 618 downregulated genes in new shoot apex with 2 leaves of the SP mutant. Gene expressions involved in auxin, cytokinin (CTK), and brassinosteroid (BR) signal transduction were mostly downregulated in the SP mutant, whereas those involved in gibberellin (GA), ethylene (ETH), jasmonate (JA), ABA, and salicylic acid (SA) signal transduction were mostly upregulated. The overall thermogram analysis of hormone levels in the shoot apex carrying two leaves detected by LC-MS/MS absolute quantification showed that the levels of IAA-Asp, IAA, iP7G, OPDA, and 6-deoxyCS were significantly upregulated in the SP mutant, while the remaining 28 hormones were significantly downregulated. It is speculated that the decreased capacity of auxin, CTK, and BR-mediated cell division and cell enlargement pathways is crucial for the formation of the SP mutant. GA and stress-resistant pathways of ETH, JA, ABA, and SA also play vital roles in stem elongation. These results highlight the involvement of phytohormones in the formation of stem elongation occurring in 'Red Delicious' spur-type bud sport mutants and provide information for exploring its biological mechanism.

Phytohormones regulate the abiotic stress: An overview of physiological, biochemical, and molecular responses in horticultural crops.

Recent changing patterns of global climate have turned out to be a severe hazard to the horticulture crops production. A wide range of biotic and abiotic stresses often affect plants due to their sessile nature. Horticultural crop losses are mainly caused by abiotic factors such as drought, salt, heat, cold, floods, and ultraviolet radiation. For coping up with these adversities, well-developed mechanisms have been evolved in plants, which play a role in perceiving stress signals and enabling optimal growth responses. Interestingly, the use of phytohormones for suppressing the impact of abiotic stress has gained much attention in recent decades. For circumvention of stress at various levels, including physiological, molecular, as well as biochemical, a sophisticated mechanism is reported to be provided by the phytohormones, thus labeling these phytohormones a significant role in plant growth and development. Phytohormones can improves tolerance against abiotic stresses by increasing seed germination, seedling growth, leaf photosynthesis, root growth, and antioxidant enzymes and reducing the accumulation of reactive oxygen species, malonaldehyde, and electrolyte leakage. Recent discoveries highlight the significant role of a variety of phytohormones including melatonin (MEL), Gamma-aminobutyric acid (GABA), jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BRs), and strigolactones (SLs) in abiotic stress tolerance enhancement of horticultural plants. Thus, current review is aimed to summarize the developmental concepts regarding role of phytohormones in abiotic-stress mitigation, mainly in horticultural crops, along with the description of recent studies which identified the role of different phytohormones in stressed environments. Hence, such a review will help in paving the path for sustainable agriculture growth via involvement of phytohormones in enhancement of abiotic stress tolerance of horticultural crops.

Dynamic formation and transcriptional regulation mediated by phytohormones during chalkiness formation in rice

BackgroundRice (Oryza sativa L.) Chalkiness, the opaque part in the kernel endosperm formed by loosely piled starch and protein bodies. Chalkiness is a complex quantitative trait regulated by multiple genes and various environmental factors. Phytohormones play important roles in the regulation of chalkiness formation but the underlying molecular mechanism is still unclear at present.ResultsIn this research, Xiangzaoxian24 (X24, pure line of indica rice with high-chalkiness) and its origin parents Xiangzaoxian11 (X11, female parent, pure line of indica rice with high-chalkiness) and Xiangzaoxian7 (X7, male parent, pure line of indica rice with low-chalkiness) were used as materials. The phenotype, physiological and biochemical traits combined with transcriptome analysis were conducted to illustrate the dynamic process and transcriptional regulation of rice chalkiness formation. Impressively, phytohormonal contents and multiple phytohormonal signals were significantly different in chalky caryopsis, suggesting the involvement of phytohormones, particularly ABA and auxin, in the regulation of rice chalkiness formation, through the interaction of multiple transcription factors and their downstream regulators.ConclusionThese results indicated that chalkiness formation is a dynamic process associated with multiple genes, forming a complex regulatory network in which phytohormones play important roles. These results provided informative clues for illustrating the regulatory mechanisms of chalkiness formation in rice.

Phytohormone profile and CiFL1 expression in young seedlings of Cichorium intybus L. var sativum exposed to high temperature in relation to vernalization and de-vernalization processes

Cichorium intybus is a biennial plant species forming a taproot and a leaf rosette during the first year and which requires low temperature during vernalization to flower during the second year of its cycle. Heat stress, however, is known to induce premature flowering during the first year. The present work aims to compare the effect of heat treatment (38 °C during 3 days) and vernalization (4 °C during 6 weeks) on the phytohormonal profile and the expression of two chicory genes (CiFL1 and CiFT) homologous to the Arabidopsis flowering genes FLOWERING LOCUS C (FLC) and FLOWERING LOCUS T (FT). In Arabidopsis, FLC and FT code, respectively, for a repressor and an activator of flowering. Although CiFL1 was inhibited by vernalization, its expression was increased by heat treatment in both vernalized and non-vernalized seedlings while high temperature unexpectedly inhibited FT expression in vernalized seedlings. Vernalization induced a modification in the hormonal profile of C. intybus (cv. Melci) in a sense of a decrease in ethylene production, abscisic acid and total jasmonates content, while the level of salicylic and benzoic acids as well as polyamine spermidine increased. A limited effect on the gibberellins’ profile was observed. The subsequent impact of heat stress on phytohormone content was different in non-vernalized and vernalized seedlings, with a higher abscisic acid and jasmonates and a lower 1-aminocyclopropane-1-carboxylic acid concentration in the former than in the latter. These data are discussed in relation to the putative involvement of phytohormones in stress-induced flowering. It appears that heat stress implies different pathways than vernalization to hasten flowering process.

Anti-stress phytohormones impact on proteome profile of green gram (Vigna radiata) under salt toxicity

Anti-stress phytohormones impact on proteome profile of green gram (Vigna radiata) under salt toxicity

Effect of Major Tea Insect Attack on Formation of Quality-Related Nonvolatile Specialized Metabolites in Tea ( Camellia sinensis) Leaves.

Insect attack is known to induce a high accumulation of volatile metabolites in tea ( Camellia sinensis). However, little information is available concerning the effect of insect attack on tea quality-related nonvolatile specialized metabolites. This study aimed to investigate the formation of characteristic nonvolatile specialized metabolites in tea leaves in response to attack by major tea insects, namely, tea green leafhoppers and tea geometrids, and determine the possible involvement of phytohormones in metabolite formation resulting from insect attack. Both tea green leafhopper and tea geometrid attacks increased the jasmonic acid and salicylic acid contents. The abscisic acid content was only increased under tea green leafhopper attack, perhaps due to special continuous piercing-sucking wounding. Tea green leafhopper attack induced the formation of theaflavins from catechins under the action of polyphenol oxidase, while tea geometrid attack increased the l-theanine content. Exogenous phytohormone treatments can affect the caffeine and catechin contents. These results will help to determine the influence of major tea pest insects on important tea quality-related metabolites and enhance understanding of the relationship of phytohormones and quality-related nonvolatile metabolite formation in tea exposed to tea pest insect attacks.

Stem Cell Maintenance and Abiotic Stress Response in Shoot Apical Meristem for Developmental Plasticity

The shoot apical meristem (SAM) serves as a non-drying reservoir of pluripotent stem cells to supply new daughter cells forming above-ground tissues and organs such as leaves, stems, flowers and fruits throughout the life cycle of plants. Accordingly, the homeostasis control of stem cell division and differentiation must be an essential core mechanism for harmonic growth and development of plants as multicellular higher eukaryotes. Unlike animals, plants are sessile organisms and thus constantly face environmental factors, including abiotic stresses. Therefore, post-embryonic development derived from stem cells in the SAM likely interacts with surrounding abiotic stresses for plant adaptation and plastic development. For this reason, this review provides the most recent findings regarding comprehensive signaling networks involved in stem cell maintenance in the SAM, and then describes how stem cell signaling is related with abiotic stress response through involvement of phytohormones and reactive oxygen species in the SAM.

Involvement of phytohormones in root elongation activity of isoprothiolane in Arabidopsis.

Isoprothiolane (diisopropyl 1,3-dithiolan-2-ylidenemalonate, IPT), an active ingredient of "Fuji-one," has been used as a plant growth regulating agent to control non-parasitic damping-off (MURENAE disease) of rice seedlings. To understant plant growth regulating activity of IPT, its effect on root development was investigated in Arabidopsis. IPT enhanced root elongation at a lower concentration (12.5 µg/mL) but suppressed it at a higher concentration (75 µg/mL). Analysis using phytohormone-related mutants and chemical inhibitors revealed that the enhancement of root elongation by IPT required auxin, jasmonic acid, and ethylene signal transduction. Activation of the signal transduction mediated by these three phytohormones was confirmed by gene expression analysis. More detailed mechanisms of IPT's effect on root development were demonstrated via investigation using Arabidopsis and chemical inhibitors.

Phytochrome and Phytohormones: Working in Tandem for Plant Growth and Development.

Being sessile organisms, plants need to continually adapt and modulate their rate of growth and development in accordance with the changing environmental conditions, a phenomenon referred to as plasticity. Plasticity in plants is a highly complex process that involves a well-coordinated interaction between different signaling pathways, the spatiotemporal involvement of phytohormones and cues from the environment. Though research studies are being carried out over the years to understand how plants perceive the signals from changing environmental conditions and activate plasticity, such remain a mystery to be resolved. Among all environmental cues, the light seems to be the stand out factor influencing plant growth and development. During the course of evolution, plants have developed well-equipped signaling system that enables regulation of both quantitative and qualitative differences in the amount of perceived light. Light influences essential developmental switches in plants ranging from germination or transition to flowering, photomorphogenesis, as well as switches in response to shade avoidances and architectural changes occurring during phototropism. Abscisic acid (ABA) is controlling seed germination and is regulated by light. Furthermore, circadian clock adds another level of regulation to plant growth by integrating light signals with different hormonal pathways. MYB96 has been identified as a regulator of circadian gating of ABA-mediated responses in plants by binding to the TIMING OF CAB EXPRESSION 1(TOC1) promoter. This review will present a representative regulatory model, highlight the successes achieved in employing novel strategies to dissect the levels of interaction and provide perspective for future research on phytochrome-phytohormones relationships toward facilitating plant growth, development, and function under abiotic-biotic stresses.

Mechanizmy obronne roślin drzewiastych uruchamiane w odpowiedzi na atak patogenów

This paper presents a comprehensive study on the mechanisms of tree responses to pathogens. We describe natural resistance concerning the presence of different types of barriers protecting plants from invasion and also give them strength after a pathogen attack. We put emphasis on induced resistance functioning both locally and systemically and involvement of phytohormones signaling networks. Systemic acquired resistance (SAR) involved the action of salicylic acid and H202 and accumulation of pathogenesis-related proteins. In turn, jasmonates and ethylene are signaling molecules in the induced resistance (SIR). All these substances play a crucial role in the forest management and can be applied in the tree protection strategies based on the natural and synthetic active compounds.

Comprehensive Transcriptome Analysis of Phytohormone Biosynthesis and Signaling Genes in the Flowers of Chinese Chinquapin (Castanea henryi).

Chinese chinquapin (Castanea henryi) nut provides a rich source of starch and nutrients as food and feed, but its yield is restricted by a low ratio of female to male flowers. Little is known about the developmental programs underlying sex differentiation of the flowers. To investigate the involvement of phytohormones during sex differentiation, we described the morphology of male and female floral organs and the cytology of flower sex differentiation, analyzed endogenous levels of indole-3-acetic acid (IAA), gibberellins (GAs), cytokinins (CKs), and abscisic acid (ABA) in the flowers, investigated the effects of exogenous hormones on flower development, and evaluated the expression profiles of genes related to biosyntheses and signaling pathways of these four hormones using RNA-Seq combined with qPCR. Morphological results showed that the flowers consisted of unisexual and bisexual catkins, and could be divided into four developmental stages. HPLC results showed that CK accumulated much more in the female flowers than that in the male flowers, GA and ABA showed the opposite results, while IAA did not show a tendency. The effects of exogenous hormones on sex differentiation were consistent with those of endogenous hormones. RNA-Seq combined with qPCR analyses suggest that several genes may play key roles in hormone biosynthesis and sex differentiation. This study presents the first comprehensive report of phytohormone biosynthesis and signaling during sex differentiation of C. henryi, which should provide a foundation for further mechanistic studies of sex differentiation in Castanea Miller species and other nonmodel plants.

Cell cycle genes are activated earlier than respiratory genes during release of grapevine buds from endodormancy

ABSTRACTSingle-bud cuttings of Vitis vinifera L exposed to forced growing conditions were used to investigate the involvement of phytohormones, abscisic acid (ABA), auxin (Aux) and cytokinin (CK) in the release of buds from the ED and in bud-sprouting. This artificial system imitates and hastens the natural sprouting that occurs in spring. Temporal expression analysis of genes related to phytohormones synthesis, showed an early drop in the expression of ABA biosynthesis gene that preceded an increase in Aux and CK biosynthesis genes. Bud-break is headed by the activation of all structures of the latent bud, especially the differentiation of the inflorescence and the development of the early stages of floral organs. Therefore, resumption of cell division and increases in respiration are essential for the activation of the bud. Temporal expression analysis of the cell cycle and respiration genes indicate that an increase in cell division go before the increase in respiration. These results, together with results indicating that the cell cycle genes are upregulated by Aux and CK, suggest that the events before the bud-break, start with a reduction in ABA content, followed by an increase in the content of Aux and CK, which activates the machinery of the cell cycle, which eventually would cause an increase in respiration.

植物切断組織の癒合における植物ホルモンおよび転写因子の役割(奨励賞,受賞業績)

植物切断組織の癒合における植物ホルモンおよび転写因子の役割(奨励賞,受賞業績)

A first insight into the involvement of phytohormones pathways in coffee resistance and susceptibility to Colletotrichum kahawae.

Understanding the molecular mechanisms underlying coffee-pathogen interactions are of key importance to aid disease resistance breeding efforts. In this work the expression of genes involved in salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) pathways were studied in hypocotyls of two coffee varieties challenged with the hemibiotrophic fungus Colletotrichum kahawae, the causal agent of Coffee Berry Disease. Based on a cytological analysis, key time-points of the infection process were selected and qPCR was used to evaluate the expression of phytohormones biosynthesis, reception and responsive-related genes. The resistance to C. kahawae was characterized by restricted fungal growth associated with early accumulation of phenolic compounds in the cell walls and cytoplasmic contents, and deployment of hypersensitive reaction. Similar responses were detected in the susceptible variety, but in a significantly lower percentage of infection sites and with no apparent effect on disease development. Gene expression analysis suggests a more relevant involvement of JA and ET phytohormones than SA in this pathosystem. An earlier and stronger activation of the JA pathway observed in the resistant variety, when compared with the susceptible one, seems to be responsible for the successful activation of defense responses and inhibition of fungal growth. For the ET pathway, the down or non-regulation of ET receptors in the resistant variety, together with a moderate expression of the responsive-related gene ERF1, indicates that this phytohormone may be related with other functions besides the resistance response. However, in the susceptible variety, the stronger activation of ERF1 gene at the beginning of the necrotrophic phase, suggests the involvement of ET in tissue senescence. As far as we know, this is the first attempt to unveil the role of phytohormones in coffee-C. kahawae interactions, thus contributing to deepen our understanding on the complex mechanisms of plant signaling and defense.

Phytohormone Involvement in the Ustilago maydis- Zea mays Pathosystem: Relationships between Abscisic Acid and Cytokinin Levels and Strain Virulence in Infected Cob Tissue.

Ustilago maydis is the causative agent of common smut of corn. Early studies noted its ability to synthesize phytohormones and, more recently these growth promoting substances were confirmed as cytokinins (CKs). Cytokinins comprise a group of phytohormones commonly associated with actively dividing tissues. Lab analyses identified variation in virulence between U. maydis dikaryon and solopathogen infections of corn cob tissue. Samples from infected cob tissue were taken at sequential time points post infection and biochemical profiling was performed using high performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI MS/MS). This hormone profiling revealed that there were altered levels of ABA and major CKs, with a marked reduction in CK glucosides, increases in methylthiol CKs and a particularly dramatic increase in cisZ CK forms, in U. maydis infected tissue. These changes were more pronounced in the more virulent dikaryon relative to the solopathogenic strain suggesting a role for cytokinins in moderating virulence during biotrophic infection. These findings highlight the fact that U. maydis does not simply mimic a fertilized seed but instead reprograms the host tissue. Results underscore the suitability of the Ustilago maydis– Zea mays model as a basis for investigating the control of phytohormone dynamics during biotrophic infection of plants.

Ammonium regulates embryogenic potential in Cucurbita pepo through pH-mediated changes in endogenous auxin and abscisic acid

When supplied as the sole nitrogen source, ammonium can induce somatic embryogenesis in some plant species, including pumpkin. To get an insight into the involvement of phytohormones in this adaptive developmental response to nitrogen availability, the status of endogenous abscisic acid (ABA), indole-3-acetic acid (IAA), and their conjugates was analyzed in an ammonium-induced embryogenic pumpkin culture. Results were compared with endogenous hormone status in an embryogenic culture induced by exogenously supplied 2,4-dichlorophenoxyacetic acid (2,4-D). In the NH4 +-induced somatic embryogenic culture, the maximum concentration of ABA was measured after 21 days of cultivation, while in the 2,4-D-induced culture ABA peaked after 7 days of cultivation, which suggested a possible activation of different ABA-regulated stress responses. The NH4 +-induced embryogenic culture was also characterized by increased accumulation of endogenous IAA to the level similar or higher than the one induced by 2,4-D. A considerable decrease (threefold) in endogenous IAA content was observed after buffering the NH4 +-medium with MES or after nitrate re-supply. In contrast, buffering the NH4 +-medium increased endogenous ABA by five times. Significant ammonium- and pH-related changes were also detected in ascorbate and glutathione redox status, and discussed regarding the corresponding changes in cell size and mitotic activity in the cultures. Furthermore, a negative correlation between free IAA content and the activity of ascorbate peroxidase (EC 1.11.1.11) and auxin-sensitive peroxidase CpAPRX (EC 1.11.1.7) was observed. The results strongly suggest a mediating role of external pH in the regulation of endogenous phytohormones and the ascorbate–glutathione cycle during ammonium-induced acquisition of embryogenic competence in pumpkin.