Cytokinin oxidase gene expression in maize is localized to the vasculature, and is induced by cytokinins, abscisic acid, and abiotic stress

Heat stress and sexual reproduction in maize: unveiling the most pivotal factors and the greatest opportunities

The escalation in the intensity, frequency, and duration of high-temperature (HT) stress is currently unparalleled, which aggravates the challenges for crop production. Yet, the stage-dependent responses of reproductive organs to HT stress at the morphological, physiological, and molecular levels remain inadequately explored in pivotal staple crops. This review synthesized current knowledge regarding the mechanisms by which HT stress induces abnormalities and aberrations in reproductive growth and development, as well as by which it alters the morphology and function of florets, flowering patterns, and the processes of pollination and fertilization in maize (Zea mays L.). We identified the stage-specific sensitivities to HT stress and accurately defined the sensitive period from a time scale of days to hours. The microspore tetrad phase of pollen development and anthesis (especially shortly after pollination) are most sensitive to HT stress, and even brief temperature spikes during these stages can lead to significant kernel loss. The impetuses behind the heat-induced impairments in seed set are closely related to carbon, reactive oxygen species, phytohormone signals, ion (e.g. Ca2+) homeostasis, plasma membrane structure and function, and others. Recent advances in understanding the genetic mechanisms underlying HT stress responses during maize sexual reproduction have been systematically summarized.

Genomic insights into CKX genes: key players in cotton fibre development and abiotic stress responses

Cytokinin oxidase/dehydrogenase (CKX), responsible for irreversible cytokinin degradation, also controls plant growth and development and response to abiotic stress. While the CKX gene has been studied in other plants extensively, its function in cotton is still unknown. Therefore, a genome-wide study to identify the CKX gene family in the four cotton species was conducted using transcriptomics, quantitative real-time PCR (qRT-PCR) and bioinformatics. As a result, in G. hirsutum and G. barbadense (the tetraploid cotton species), 87 and 96 CKX genes respectively and 62 genes each in G. arboreum and G. raimondii, were identified. Based on the evolutionary studies, the cotton CKX gene family has been divided into five distinct subfamilies. It was observed that CKX genes in cotton have conserved sequence logos and gene family expansion was due to segmental duplication or whole genome duplication (WGD). Collinearity and multiple synteny studies showed an expansion of gene families during evolution and purifying selection pressure has been exerted. G. hirsutum CKX genes displayed multiple exons/introns, uneven chromosomal distribution, conserved protein motifs, and cis-elements related to growth and stress in their promoter regions. Cis-elements related to resistance, physiological metabolism and hormonal regulation were identified within the promoter regions of the CKX genes. Expression analysis under different stress conditions (cold, heat, drought and salt) revealed different expression patterns in the different tissues. Through virus-induced gene silencing (VIGS), the GhCKX34A gene was found to improve cold resistance by modulating antioxidant-related activity. Since GhCKX29A is highly expressed during fibre development, we hypothesize that the increased expression of GhCKX29A in fibres has significant effects on fibre elongation. Consequently, these results contribute to our understanding of the involvement of GhCKXs in both fibre development and response to abiotic stress.

Phytohormonal modulation of the drought stress in soybean: outlook, research progress, and cross-talk

Phytohormones play vital roles in stress modulation and enhancing the growth of plants. They interact with one another to produce programmed signaling responses by regulating gene expression. Environmental stress, including drought stress, hampers food and energy security. Drought is abiotic stress that negatively affects the productivity of the crops. Abscisic acid (ABA) acts as a prime controller during an acute transient response that leads to stomatal closure. Under long-term stress conditions, ABA interacts with other hormones, such as jasmonic acid (JA), gibberellins (GAs), salicylic acid (SA), and brassinosteroids (BRs), to promote stomatal closure by regulating genetic expression. Regarding antagonistic approaches, cytokinins (CK) and auxins (IAA) regulate stomatal opening. Exogenous application of phytohormone enhances drought stress tolerance in soybean. Thus, phytohormone-producing microbes have received considerable attention from researchers owing to their ability to enhance drought-stress tolerance and regulate biological processes in plants. The present study was conducted to summarize the role of phytohormones (exogenous and endogenous) and their corresponding microbes in drought stress tolerance in model plant soybean. A total of n=137 relevant studies were collected and reviewed using different research databases.

Cytokinin Translocation to, and Biosynthesis and Metabolism within, Cereal and Legume Seeds: Looking Back to Inform the Future

Early in the history of cytokinins, it was clear that Zea mays seeds contained not just trans-zeatin, but its nucleosides and nucleotides. Subsequently, both pods and seeds of legumes and cereal grains have been shown to contain a complex of cytokinin forms. Relative to the very high quantities of cytokinin detected in developing seeds, only a limited amount appears to have been translocated from the parent plant. Translocation experiments, and the detection of high levels of endogenous cytokinin in the maternal seed coat tissues of legumes, indicates that cytokinin does not readily cross the maternal/filial boundary, indicating that the filial tissues are autonomous for cytokinin biosynthesis. Within the seed, trans-zeatin plays a key role in sink establishment and it may also contribute to sink strength. The roles, if any, of the other biologically active forms of cytokinin (cis-zeatin, dihydrozeatin and isopentenyladenine) remain to be elucidated. The recent identification of genes coding for the enzyme that leads to the biosynthesis of trans-zeatin in rice (OsCYP735A3 and 4), and the identification of a gene coding for an enzyme (CPN1) that converts trans-zeatin riboside to trans-zeatin in the apoplast, further cements the key role played by trans-zeatin in plants.

GhCKX14 responding to drought stress by modulating antioxi-dative enzyme activity in Gossypium hirsutum compared to CKX family genes

BACKGROUND

Cytokinin oxidase/dehydrogenase (CKX) plays a vital role in response to abiotic stress through modulating the antioxidant enzyme activities. Nevertheless, the biological function of the CKX gene family has yet to be reported in cotton.

RESULT

In this study, a total of 27 GhCKXs were identified by the genome-wide investigation and distributed across 18 chromosomes. Phylogenetic tree analysis revealed that CKX genes were clustered into four clades, and most gene expansions originated from segmental duplications. The CKXs gene structure and motif analysis displayed remarkably well conserved among the four groups. Moreover, the cis-acting elements related to the abiotic stress, hormones, and light response were identified within the promoter regions of GhCKXs. Transcriptome data and RT-qPCR showed that GhCKX genes demonstrated higher expression levels in various tissues and were involved in cotton's abiotic stress and phytohormone response. The protein-protein interaction network indicates that the CKX family probably participated in redox regulation, including oxidoreduction or ATP levels, to mediate plant growth and development. Functionally identified via virus-induced gene silencing (VIGS) found that the GhCKX14 gene improved drought resistance by modulating the antioxidant-related activitie.

CONCLUSIONS

In this study, the CKX gene family members were analyzed by bioinformatics, and validates the response of GhCKX gene to various phytohormone treatment and abiotic stresses. Our findings established the foundation of GhCKXs in responding to abiotic stress and GhCKX14 in regulating drought resistance in cotton.

Dissection of Developmental Programs and Regulatory Modules Directing Endosperm Transfer Cell and Aleurone Identity in the Syncytial Endosperm of Barley

Endosperm development in barley starts with the formation of a multinucleate syncytium, followed by cellularization in the ventral part of the syncytium generating endosperm transfer cells (ETCs) as first differentiating subdomain, whereas aleurone (AL) cells will originate from the periphery of the enclosing syncytium. Positional signaling in the syncytial stage determines cell identity in the cereal endosperm. Here, we performed a morphological analysis and employed laser capture microdissection (LCM)-based RNA-seq of the ETC region and the peripheral syncytium at the onset of cellularization to dissect developmental and regulatory programs directing cell specification in the early endosperm. Transcriptome data revealed domain-specific characteristics and identified two-component signaling (TCS) and hormone activities (auxin, ABA, ethylene) with associated transcription factors (TFs) as the main regulatory links for ETC specification. On the contrary, differential hormone signaling (canonical auxin, gibberellins, cytokinin) and interacting TFs control the duration of the syncytial phase and timing of cellularization of AL initials. Domain-specific expression of candidate genes was validated by in situ hybridization and putative protein-protein interactions were confirmed by split-YFP assays. This is the first transcriptome analysis dissecting syncytial subdomains of cereal seeds and provides an essential framework for initial endosperm differentiation in barley, which is likely also valuable for comparative studies with other cereal crops.

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

Cloning of Three Cytokinin Oxidase/Dehydrogenase Genes in Bambusa oldhamii

Cytokinin oxidase/dehydrogenase (CKX) catalyzes the irreversible breakdown of active cytokinins, which are a class of plant hormones that regulate cell division. According to conserved sequences of CKX genes from monocotyledons, PCR primers were designed to synthesize a probe for screening a bamboo genomic library. Cloned results of three genes encoding cytokinin oxidase were named as follows: BoCKX1, BoCKX2, and BoCKX3. In comparing the exon-intron structures among the above three genes, there are three exons and two introns in BoCKX1 and BoCKX3 genes, whereas BoCKX2 contains four exons and three introns. The amino acid sequence of BoCKX2 protein shares 78% and 79% identity with BoCKX1 and BoCKX3 proteins, respectively. BoCKX1 and BoCKX3 genes are particularly closely related given that the amino acid and nucleotide sequence identities are more than 90%. These three BoCKX proteins carried putative signal peptide sequences typical of secretion pathway, and a GHS-motif was found at N-terminal flavin adenine dinucleotide (FAD) binding domain, suggesting that BoCKX proteins might covalently conjugate with an FAD cofactor through a predicted histidine residue.

Genome-wide identification of the soybean cytokinin oxidase/dehydrogenase gene family and its diverse roles in response to multiple abiotic stress

Cytokinin oxidase/dehydrogenase (CKX) irreversibly degrades cytokinin, regulates growth and development, and helps plants to respond to environmental stress. Although the CKX gene has been well characterized in various plants, its role in soybean remains elusive. Therefore, in this study, the evolutionary relationship, chromosomal location, gene structure, motifs, cis-regulatory elements, collinearity, and gene expression patterns of GmCKXs were analyzed using RNA-seq, quantitative real-time PCR (qRT-PCR), and bioinformatics. We identified 18 GmCKX genes from the soybean genome and grouped them into five clades, each comprising members with similar gene structures and motifs. Cis-acting elements involved in hormones, resistance, and physiological metabolism were detected in the promoter regions of GmCKXs. Synteny analysis indicated that segmental duplication events contributed to the expansion of the soybean CKX family. The expression profiling of the GmCKXs genes using qRT-PCR showed tissue-specific expression patterns. The RNA-seq analysis also indicated that GmCKXs play an important role in response to salt and drought stresses at the seedling stage. The responses of the genes to salt, drought, synthetic cytokinin 6-benzyl aminopurine (6-BA), and the auxin indole-3-acetic acid (IAA) at the germination stage were further evaluated by qRT-PCR. Specifically, the GmCKX14 gene was downregulated in the roots and the radicles at the germination stage. The hormones 6-BA and IAA repressed the expression levels of GmCKX1, GmCKX6, and GmCKX9 genes but upregulated the expression levels of GmCKX10 and GmCKX18 genes. The three abiotic stresses also decreased the zeatin content in soybean radicle but enhanced the activity of the CKX enzymes. Conversely, the 6-BA and IAA treatments enhanced the CKX enzymes' activity but reduced the zeatin content in the radicles. This study, therefore, provides a reference for the functional analysis of GmCKXs in soybean in response to abiotic stresses.

A spatiotemporal transcriptomic network dynamically modulates stalk development in maize

Maize (Zea mays) is an important cereal crop with suitable stalk formation which is beneficial for acquiring an ideal agronomic trait to resist lodging and higher planting density. The elongation pattern of stalks arises from the variable growth of individual internodes driven by cell division and cell expansion comprising the maize stalk. However, the spatiotemporal dynamics and regulatory network of the maize stalk development and differentiation process remain unclear. Here, we report spatiotemporally resolved transcriptomes using all internodes of the whole stalks from developing maize at the elongation and maturation stages. We identified four distinct groups corresponding to four developmental zones and nine specific clusters with diverse spatiotemporal expression patterns among individual internodes of the stalk. Through weighted gene coexpression network analysis, we constructed transcriptional regulatory networks at a fine spatiotemporal resolution and uncovered key modules and candidate genes involved in internode maintenance, elongation, and division that determine stalk length and thickness in maize. Further CRISPR/Cas9-mediated knockout validated the function of a cytochrome P450 gene, ZmD1, in the regulation of stalk length and thickness as predicted by the WGCN. Collectively, these results provide insights into the high genetic complexity of stalk development and the potentially valuable resources with ideal stalk lengths and widths for genetic improvements in maize.

Evolutionary expansion and expression dynamics of cytokinin-catabolizing CKX gene family in the modern amphidiploid mustard (Brassica sp.)

Plant cytokinins (CKs) promote development and physiological processes, drought tolerance, root architecture, and ultimately crop productivity. Biologically active CKs (iP, tZ, and cZ) are precisely maintained in the vegetative and floral tissues through their irreversible degradation by developmentally regulated CK-catabolizing cytokinin oxidase/dehydrogenase (CKX) enzyme. A meta-analysis of CKX proteins was performed through an exhaustive exploration of multiple genome databases of cyanobacteria, bryophyte, monocot and eudicot plants to reveal the intricate evolutionary profiles of CKX enzymes specific to the family Brassicaceae. At least 175 unique paralogous/orthologous CKX sequences were successfully retrieved and phylogenetically clustered into distinct groups. Observations of structural divergences among paralogous sequences compared to their orthologs indicated that the progenitor CKX sequence had been subjected to massive structural modifications, possibly as a result of the evolutionary split between monocots and eudicots. An analysis of dN/dS comparisons of orthologous genes revealed that segmental CKX gene duplications have evolved primarily under purifying selection. Further, 24 CKX genes with conserved signature domain were identified in the amphidiploid Brassica juncea genome (AABB; 2n = 36). Genetic evolution of paralogous and orthologous genes was largely responsible for the expansion of CKX homoeologs in the amphidiploid Brassica genomes. Also, comparative analyses of 1.5 kb-long upstream regulatory regions of BjCKX genes identified various development- and stress-responsive elements. Spatial and temporal expression profiles of CKX genes were primarily attributed to their structural diversity observed in the 5'-regulatory regions along with species evolution. This data suggested that CKX duplicate genes had partitioned their spatial expression (= function) during evolution. These findings illustrated the evolutionary importance of CKX genes during plant development, and also suggested their deployment for future crop improvement programs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03294-0.

Genome-Wide Analysis and Evolutionary Perspective of the Cytokinin Dehydrogenase Gene Family in Wheat (Triticum aestivum L.)

Cytokinin dehydrogenase (CKX; EC.1.5.99.12) regulates the level of cytokinin (CK) in plants and is involved in CK regulatory activities. In different plants, a small gene family encodes CKX proteins with varied numbers of members. These genes are expanded in the genome mainly due to segmental duplication events. Despite their biological importance, CKX genes in Triticum aestivum have yet to be studied in depth. A total of 11 CKX subfamilies were identified with similar gene structures, motifs, domains, cis-acting elements, and an average signal peptide of 25 amino acid length was found. Introns, ranging from one to four, were present in the coding regions at a similar interval in major CKX genes. Putative cis-elements such as abscisic acid, auxin, salicylic acid, and low-temperature-, drought-, and light-responsive cis-regulatory elements were found in the promoter region of majority CKX genes. Variation in the expression pattern of CKX genes were identified across different tissues in Triticum. Phylogenetic analysis shows that the same subfamily of CKX clustered into a similar clade that reflects their evolutionary relationship. We performed a genome-wide identification of CKX family members in the Triticum aestivum genome to get their chromosomal location, gene structure, cis-element, phylogeny, synteny, and tissue- and stage-specific expression along with gene ontology. This study has also elaborately described the tissue- and stage-specific expression and is the resource for further analysis of CKX in the regulation of biotic and abiotic stress resistance, growth, and development in Triticum and other cereals to endeavor for higher production and proper management.

Auxin-Cytokinin Cross Talk in Somatic Embryogenesis of Coffea canephora

Cytokinins (CK) are plant growth regulators involved in multiple physiological processes in plants. One less studied aspect is CK homeostasis (HM). The primary genes related to HM are involved in biosynthesis (IPT), degradation (CKX), and signaling (ARR). This paper demonstrates the effect of auxin (Aux) and CK and their cross talk in a Coffea canephora embryogenic system. The transcriptome and RT-qPCR suggest that Aux in pre-treatment represses biosynthesis, degradation, and signal CK genes. However, in the induction, there is an increase of genes implicated in the CK perception/signal, indicating perhaps, as in other species, Aux is repressing CK, and CK are inducing per se genes involved in its HM. This is reflected in the endogenous concentration of CK; pharmacology experiments helped study the effect of each plant growth regulator in our SE system. We conclude that the Aux–CK balance is crucial to directing somatic embryogenesis in C. canephora.

Distinct Hormone Signalling-Modulation Activities Characterize Two Maize Endosperm-Specific Type-A Response Regulators

ZmTCRR1 and 2 are type-A response regulators expressed in the maize endosperm transfer cells (TC). While type-B response regulators transcriptionally control canonical type-A response regulators, as part of the cytokinin signal transduction mechanism, the ZmTCRRs are regulated by ZmMRP1, a master regulator of TC identity. In addition, the corresponding proteins are not detected in the TC, accumulating in the inner endosperm cells instead. These features suggest these molecules are not involved in classical, cell-autonomous, cytokinin signalling pathways. Using transgenic Arabidopsis plants ectopically expressing these genes, we have shown that ZmTCRR1 and 2 can modulate auxin and cytokinin signalling, respectively. In Arabidopsis, the ectopic expression of ZmTCRR2 blocked, almost completely, cytokinin perception. Given the conservation of these signalling pathways at the molecular level, our results suggest that the ZmTCRRs modulate cytokinin and auxin perception in the inner endosperm cells.

Overexpression of ZmIPT2 gene delays leaf senescence and improves grain yield in maize

Cytokinins (CTKs) are a major phytohormone group that are significant in the promotion of cellular division, growth, and divergence. Isopentenyl transferase (IPT) regulates a rate-limiting step in plant CTK synthesis, promotes the synthesis of isopentenyl adenonucleotides from 5-AMP and isopentenyl pyrophosphate, and then converts both these chemicals into various CTKs. Here, the full-length cDNA of ZmIPT2, which encodes 322 amino acids, was isolated and was introduced into a maize inbred line by Agrobacterium-mediated transformation. In both controlled environments and field experiments, the overexpression of ZmIPT2 gene in the transformed plants delayed leaf senescence. Compared to the receptor line, the transgenic maize lines retained higher chlorophyll levels, photosynthetic rates, and cytokinin content for an extended period of time, and produced significantly higher grain yield by a margin of 17.71-20.29% under normal field planting conditions. Subsequently, ten possible genes that interacted with ZmIPT2 were analyzed by qRT-PCR, showing that the expression pattern of GRMZM2G022904 was consistent with ZmIPT2 expression. Through comprehensive analysis, we screened for transgenic lines with stable inheritance of ZmIPT2 gene, clear functional efficiency, and significant yield improvement, in order to provide theoretical basis and material support for the breeding of new high-yield transgenic maize varieties.

Cereal Endosperms: Development and Storage Product Accumulation

The persistent triploid endosperms of cereal crops are the most important source of human food and animal feed. The development of cereal endosperms progresses through coenocytic nuclear division, cellularization, aleurone and starchy endosperm differentiation, and storage product accumulation. In the past few decades, the cell biological processes involved in endosperm formation in most cereals have been described. Molecular genetic studies performed in recent years led to the identification of the genes underlying endosperm differentiation, regulatory network governing storage product accumulation, and epigenetic mechanism underlying imprinted gene expression. In this article, we outline recent progress in this area and propose hypothetical models to illustrate machineries that control aleurone and starchy endosperm differentiation, sugar loading, and storage product accumulations. A future challenge in this area is to decipher the molecular mechanisms underlying coenocytic nuclear division, endosperm cellularization, and programmed cell death.

Cytokinin Oxygenase/Dehydrogenase Inhibitors: An Emerging Tool in Stress Biotechnology Employed for Crop Improvement

In order to meet the global challenges of food security, one of the foremost solutions lies in enhancing the crop productivity. This can be attained by considering key plant hormones such as cytokinins as agrochemicals as cytokinins in particular are known to control the essential processes of the plants. Even though, it has already been established since 1980s that the enzyme, cytokinin oxidase/dehydrogenase (CKO/CKX) deactivates cytokinins; the potential applications of manipulating these enzymes have mostly been speculated to have a high potential in the biotechnology industry and spreads to agriculture, horticulture and agroforestry. The enzyme is critical in maintaining a balanced level of cytokinins in plants. However, it is yet to be fully established that inhibiting this enzyme can be the constant source of improvement in the productivity of plants, even though success has been obtained in some economically important plant species. Furthermore, the impact efficiency of this enzyme may vary from plant to plant, which needs to be evaluated employing tissue culture and other extrinsic applications. This review intends to cover the relevant studies addressing any biological activity of this enzyme in the current context and any associated biotechnological applications specific to enhanced grain yield, abiotic stress tolerance, delayed senescence and in vitro organogenesis among various plants and not only cereals. Moreover, our study will identify the present gaps in research with respect to many important food crops, which will be useful for researchers who are actively involved in providing a foundation for a variety of genetically improved plants achieved through this manner. In addition to this, other ways of engineering the amount of cytokinin levels appropriate for signaling also needs to be analyzed in order to extend the benefits of cytokinin biology to other crops too. The application of these inhibitors can be considered among the best alternates as well as addition to genetically modified plants for overcoming the gaps in crop demand.

Green revolution to grain revolution: Florigen in the frontiers

Burgeoning human population dents, globally, the brimming buffer stock as well as gain in food grain production. However, an imminent global starvation was averted through precise scientific intervention and pragmatic policy changes in the 1960s and was eulogized as the "Green Revolution". Miracle rice and wheat obtained through morphometric changes in the ideotype of these two crops yielded bumper harvest that nucleated in Asia and translated into Latin America. The altered agronomic traits in these two crops were the result of tinkering with the phyto-hormone "Gibberellin'. Recently, another plant hormone 'Cytokinin' has gained prominence for its involvement in the grain revolution in rice and other field crops. Suo moto homeostasis of CK by the cytokinin oxidase enzyme governs the cardinal shoot apical meristem that produces new flowering primordia thereby enhancing grain number. Similarly, the flowering hormone 'Florigen' impacts sympodia formation, flowering, and fruit production in tomato. The role of heterozygosity induced heterosis by florigen in revolutionizing tomato production and cellular homeostasis of CK by CK oxidising enzyme (CKX) in enhancing rice production has been path-breaking. This review highlights role of phytohormones in grain revolution and crop specific fine-tuning of gibberellins, cytokinins and florigen to accomplish maximum yield potential in field crops.

Phytohormone profiles in non-transformed and AtCKX transgenic centaury (Centaurium erythraea Rafn) shoots and roots in response to salinity stress in vitro

Plant hormones regulate numerous developmental and physiological processes. Abiotic stresses considerably affect production and distribution of phytohormones as the stress signal triggers. The homeostasis of plant hormones is controlled by their de novo synthesis and catabolism. The aim of this work was to analyse the contents of total and individual groups of endogenous cytokinins (CKs) as well as indole-3-acetic acid (IAA) in AtCKX overexpressing centaury plants grown in vitro on graded NaCl concentrations (0, 50, 100, 150, 200 mM). The levels of endogenous stress hormones including abscisic acid (ABA), salicylic acid (SA) and jasmonic acid (JA) were also detected. The elevated contents of total CKs were found in all analysed centaury shoots. Furthermore, increased amounts of all five CK groups, as well as enhanced total CKs were revealed on graded NaCl concentrations in non-transformed and AtCKX roots. All analysed AtCKX centaury lines exhibited decreased amounts of endogenous IAA in shoots and roots. Consequently, the IAA/bioactive CK forms ratios showed a significant variation in the shoots and roots of all AtCKX lines. In shoots and roots of both non-transformed and AtCKX transgenic centaury plants, salinity was associated with an increase of ABA and JA and a decrease of SA content.

Plant Growth Regulators INCYDE and TD-K Underperform in Cereal Field Trials

Using plant growth regulators to alter cytokinin homeostasis with the aim of enhancing endogenous cytokinin levels has been proposed as a strategy to increase yields in wheat and barley. The plant growth regulators INCYDE and CPPU inhibit the cytokinin degrading enzyme cytokinin oxidase/dehydrogenase (CKX), while TD-K inhibits the process of senescence. We report that the application of these plant growth regulators in wheat and barley field trials failed to enhance yields, or change the components of yields. Analyses of the endogenous cytokinin content showed a high concentration of trans-zeatin (tZ) in both wheat and barley grains at four days after anthesis, and statistically significant, but probably biologically insignificant, increases in cisZ-O-glucoside, along with small decreases in cZ riboside (cZR), dihydro Z (DHZ), and DHZR and DHZOG cytokinins, following INCYDE application to barley at anthesis. We discuss possible reasons for the lack of efficacy of the three plant growth regulators under field conditions and comment on future approaches to manipulating yield in the light of the strong homeostatic mechanisms controlling endogenous cytokinin levels.

Leaf size modulation by cytokinins in sesame plants

Phytohormones play important roles in controlling leaf size and in the modulation of various stress responses, including drought. In this study, hormone profiling analyses by ultra high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry (UHPLC-MS/MS) was performed in leaves collected at three stages of active leaf growth to unravel which phytohormones modulate leaf size in sesame (Sesamum indicum L.) plants, an important oil-rich crop. Furthermore, endogenous contents of phytohormones were measured in parallel to various stress markers in sesame plants exposed to mild water deficit conditions by withholding water in potted plants for one week. Results revealed a major role of cytokinins and auxin in the modulation of leaf growth in sesame plants (which increased by 21.5 and 2.1-fold, respectively, with leaf growth), as well as a putative antagonistic response between jasmonic acid and salicylic acid during leaf development. Furthermore, growth arrest during water deficit stress appeared to be modulated by cytokinins, the endogenous contents of which decreased (by 48%) in parallel with ABA increases (by 59%). Reductions in the contents of the active cytokinin trans-zeatin occurred in parallel with increases in isopentenyladenine contents under drought, which suggests a partial metabolic limitation in cytokinin biosynthesis in leaves upon water deficit stress. These results provide useful information for the hormonal modulation of leaf size and the improvement of leaf growth and production in sesame plants through manipulation of the levels of key regulatory phytohormones.

Crosstalk between phytohormones and secondary metabolites in the drought stress tolerance of crop plants: A review

Drought stress negatively affects crop performance and weakens global food security. It triggers the activation of downstream pathways, mainly through phytohormones homeostasis and their signaling networks, which further initiate the biosynthesis of secondary metabolites (SMs). Roots sense drought stress, the signal travels to the above-ground tissues to induce systemic phytohormones signaling. The systemic signals further trigger the biosynthesis of SMs and stomatal closure to prevent water loss. SMs primarily scavenge reactive oxygen species (ROS) to protect plants from lipid peroxidation and also perform additional defense-related functions. Moreover, drought-induced volatile SMs can alert the plant tissues to perform drought stress mitigating functions in plants. Other phytohormone-induced stress responses include cell wall and cuticle thickening, root and leaf morphology alteration, and anatomical changes of roots, stems, and leaves, which in turn minimize the oxidative stress, water loss, and other adverse effects of drought. Exogenous applications of phytohormones and genetic engineering of phytohormones signaling and biosynthesis pathways mitigate the drought stress effects. Direct modulation of the SMs biosynthetic pathway genes or indirect via phytohormones' regulation provides drought tolerance. Thus, phytohormones and SMs play key roles in plant development under the drought stress environment in crop plants.

OsRR6, a type-A response regulator in rice, mediates cytokinin, light and stress responses when over-expressed in Arabidopsis

Plants have evolved a complex network of components that sense and respond to diverse signals. In the present study, we have characterized OsRR6, a type-A response regulator, which is part of the two-component sensor-regulator machinery in rice. The expression of OsRR6 is induced by exogenous cytokinin and various abiotic stress treatments, including drought, cold and salinity stress. Organ-specific expression analysis revealed that its expression is high in anther and low in shoot apical meristem. The Arabidopsis plants constitutively expressing OsRR6 (OsRR6^OX) exhibited reduced cytokinin sensitivity, adventitious root formation and enhanced anthocyanin accumulation in seeds. OsRR6^OX plants were more tolerant to drought and salinity conditions when compared to wild-type. The hypocotyl growth in OsRR6^OX seedlings was significantly inhibited under red, far-red and blue-light conditions and also a decline in transcript levels of OsRR6 was observed in rice under the above monochromatic as well as white light treatments. Transcriptome profiling revealed that the genes associated with defense responses and anthocyanin metabolism are up-regulated in OsRR6^OX seedlings. Comparative transcriptome analysis showed that the genes associated with phenylpropanoid and triterpenoid biosynthesis are enriched among differentially expressed genes in OsRR6^OX seedlings of Arabidopsis, which is in conformity with reanalysis of the transcriptome data performed in rice transgenics for OsRR6. Further, genes like DREB1A/CBF3, COR15A, KIN1, ERD10 and RD29A are significantly upregulated in OsRR6^OX seedlings when subjected to ABA and abiotic stress treatments. Thus, a negative regulator of cytokinin signaling, OsRR6, plays a positive role in imparting abiotic stress tolerance.

A Comparison of the Transcriptomes of Cowpeas in Response to Two Different Ionizing Radiations

In this study, gene expression changes in cowpea plants irradiated by two different types of radiation: proton-beams and gamma-rays were investigated. Seeds of the Okdang cultivar were exposed to 100, 200, and 300 Gy of gamma-rays and proton-beams. In transcriptome analysis, the 32, 75, and 69 differentially expressed genes (DEGs) at each dose of gamma-ray irradiation compared with that of the control were identified. A total of eight genes were commonly up-regulated for all gamma-ray doses. However, there were no down-regulated genes. In contrast, 168, 434, and 387 DEGs were identified for each dose of proton-beam irradiation compared with that of the control. A total of 61 DEGs were commonly up-regulated for all proton-beam doses. As a result of GO and KEGG analysis, the ranks of functional categories according to the number of DEGs were not the same in both treatments and were more diverse in terms of pathways in the proton-beam treatments than gamma-ray treatments. The number of genes related to defense, photosynthesis, reactive oxygen species (ROS), plant hormones, and transcription factors (TF) that were up-/down-regulated was higher in the proton beam treatment than that in gamma ray treatment. Proton-beam treatment had a distinct mutation spectrum and gene expression pattern compared to that of gamma-ray treatment. These results provide important information on the mechanism for gene regulation in response to two ionizing radiations in cowpeas.

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N⁶-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.

Targeting Cytokinin Homeostasis in Rapid Cycling Brassica rapa with Plant Growth Regulators INCYDE and TD-K

Modifying the cytokinin content in plants is a means of improving plant productivity. Here, we report the development and biological activity of compound TD-K (1-(furan-2-ylmethyl)-3-(1,2,3-thiadiazol-5-yl)urea)which is related to thidiazuron. TD-K—which exhibited extremely high antisenescence activity in the wheat leaf bioassay—and INCYDE (2-chloro-6-(3-methoxyphenyl)aminopurine)—a plant growth regulator reported to inhibit cytokinin oxidase/dehydrogenase (CKX), an enzyme involved in the degradation of the plant hormone cytokinin—were selected for investigation of their effects on the model plant Rapid Cycling Brassica rapa (RCBr). We monitored the expression of BrCKX and isopentenyl transferase (BrIPT), which codes for the key cytokinin biosynthesis enzyme, in developing leaves following INCYDE and TD-K application. Growth room experiments revealed that INCYDE increased RCBr seed yield per plant, but only when applied multiple times and when grown in 5 mM KNO₃. Expression in control leaves showed transient, high levels of expression of BrCKX and BrIPT at true leaf appearance. Following INCYDE application, there was a rapid and strong upregulation of BrCKX3, and a transient downregulation of BrIPT1 and BrIPT3. Interestingly, the upregulation of BrCKX3 persisted in a milder form throughout the course of the experiment (16 days). TD-K also upregulated BrCKX3. However, in contrast to INCYDE, this effect disappeared after two days. These results suggest that both compounds (CKX inhibitor and cytokinin TD-K) influenced cytokinin homeostasis in RCBr leaves, but with different mechanisms.

Crosstalk Pathway between Trehalose Metabolism and Cytokinin Degradation for the Determination of the Number of Berries per Bunch in Grapes

In grapes, the number of flowers per inflorescence determines the compactness of grape bunches. Grape cultivars with tight bunches and thin-skinned berries easily undergo berry splitting, especially in growing areas with heavy rainfall during the grapevine growing season, such as Japan. We report herein that grape cytokinin oxidase/dehydrogenase 5 (VvCKX5) determines the number of berries per inflorescence in grapes. The number of berries per bunch was inversely proportional to the VvCKX5 expression level in juvenile inflorescences among the cultivars tested. VvCKX5 overexpression drastically decreased the number of flower buds per inflorescence in Arabidopsis plants, suggesting that VvCKX5 might be one of the negative regulators of the number of flowers per inflorescence in grapes. Similarly, the overexpression of grape sister of ramose 3 (VvSRA), which encodes trehalose 6-phosphate phosphatase that catalyzes the conversion of trehalose-6-phosphate into trehalose, upregulated AtCKX7 expression in Arabidopsis plants, leading to a decrease in the number of flower buds per Arabidopsis inflorescence. VvCKX5 gene expression was upregulated in grapevine cultured cells and juvenile grape inflorescences treated with trehalose. Finally, injecting trehalose into swelling buds nearing bud break using a microsyringe decreased the number of berries per bunch by half. VvCKX5 overexpression in Arabidopsis plants had no effect on the number of secondary inflorescences from the main inflorescence, and similarly trehalose did not affect pedicel branching on grapevine inflorescences, suggesting that VvCKX5, as well as VvSRA-mediated trehalose metabolism, regulates flower formation but not inflorescence branching. These findings may provide new information on the crosstalk between VvSRA-mediated trehalose metabolism and VvCKX-mediated cytokinin degradation for determining the number of berries per bunch. Furthermore, this study is expected to contribute to the development of innovative cultivation techniques for loosening tight bunches.

Transcriptome and phytohormone changes associated with ethylene-induced onion bulb dormancy

Control of dormancy and sprouting in onion bulbs is commercially important for postharvest management. Although ethylene application is sometimes used to extend dormancy, the underlying mechanisms regulating dormancy transition remain unclear. Since the sprout leaves emerge from the bulb baseplate, we used this tissue to assess the impact of ethylene treatment and storage time on the hormone profile and the transcriptome. Reads from 30 libraries were assembled and annotated, with 94,840 unigenes retained after filtering. The de novo transcriptome assembly was of high quality and continuity (N50: 1809 bp, GC content: 36.21 %), and was used to analyse differential expression and Gene Onotologies. Across two years, applied ethylene resulted in delayed dormancy break and reduced post-dormancy sprout vigour. Ethylene supplementation enhanced endogenous ethylene production and caused a transient climacteric-like increase in respiration. Significant changes in hormone and associated transcript profiles occurred through storage and in response to ethylene. In particular, abscisic acid (ABA) and its metabolite phaseic acid (PA) increased under ethylene during the longer dormancy period; however, cytokinin increases observed during storage appeared largely independent of ethylene treatment. Several hormone-related transcripts showed differential expression over time and/or in response to ethylene. Expression of ethylene biosynthesis (ACO), receptor (EIN4) and transcription factor (EIL3) genes were modified by ethylene, as were ABA biosynthesis genes such NCED, and cytokinin biosynthesis genes such as LOG and CKX. We conclude that ethylene substantially modifies expression of genes in several phytohormone pathways, and some of these changes may underlie the dormancy-extending effects of exogenous ethylene.

Reproductive load modulates drought stress response but does not compromise recovery in an invasive plant during the Mediterranean summer

Despite summer drought may challenge plant survival in Mediterranean-type ecosystems, the role of reproductive load on drought stress and recovery has been poorly studied in invasive plants, most particularly under natural field conditions. In this study, a highly plastic clonal invasive species, Carpobrotus edulis was used to explore a putative differential response to drought between reproductive (senescent) ramets and non-reproductive ramets. Furthermore, fruit removal was used to assess how alterations on the source-sink dynamics influence plant performance during drought stress and recovery. We examined the variations in chloroplast pigments, antioxidants, lipid peroxidation and cytokinins in leaves of non-reproductive and reproductive ramets (either with intact or fruit-removed ramets) in response to summer drought stress and recovery after rains under Mediterranean field conditions. Results showed that although both ramet types within a C. edulis patch recovered at the end of the summer, increased photoprotective investment was found in leaves from reproductive ramets, thus indicating an increased photoprotective demand associated with reproduction at the ramet level. This response was associated with differentiated cytokinin contents in leaves of reproductive ramets compared to those of non-reproductive ramets. Although leaf senescence was not reversed by the fruit removal, leaves recovered their chlorophyll content after rainfall during late summer in parallel with the accumulation of cytokinins. In conclusion, C. edulis shows a huge plasticity in drought stress responses with a marked compartmentation at the ramet level, which helps at least in part to an efficient recovery from unpredictable water shortage periods in the current frame of climate change.

Silencing of TaCKX1 Mediates Expression of Other TaCKX Genes to Increase Yield Parameters in Wheat

TaCKX, Triticum aestivum (cytokinin oxidase/dehydrogenase) family genes influence the development of wheat plants by the specific regulation of cytokinin content in different organs. However, their detailed role is not known. The TaCKX1, highly and specifically expressed in developing spikes and in seedling roots, was silenced by RNAi-mediated gene silencing via Agrobacterium tumefaciens and the effect of silencing was investigated in 7 DAP (days after pollination) spikes of T₁ and T₂ generations. Various levels of TaCKX1 silencing in both generations influence different models of co-expression with other TaCKX genes and parameters of yield-related traits. Only a high level of silencing in T₂ resulted in strong down-regulation of TaCKX11 (3), up-regulation of TaCKX2.1, 2.2, 5, and 9 (10), and a high yielding phenotype. This phenotype is characterized by a higher spike number, grain number, and grain yield, but lower thousand grain weight (TGW). The content of most of cytokinin forms in 7 DAP spikes of silenced T₂ lines increased from 23% to 76% compared to the non-silenced control. The CKs cross talk with other phytohormones. Each of the tested yield-related traits is regulated by various up- or down-regulated TaCKX genes and phytohormones. The coordinated effect of TaCKX1 silencing on the expression of other TaCKX genes, phytohormone levels in 7 DAP spikes, and yield-related traits in silenced T₂ lines is presented.

A cytokinin-activation enzyme-like gene improves grain yield under various field conditions in rice

CRISPR-edited variants at the 3'-end of OsLOGL5's coding sequence (CDS), significantly increased rice grain yield under well-watered, drought, normal nitrogen, and low nitrogen field conditions at multiple geographical locations. Cytokinins impact numerous aspects of plant growth and development. This study reports that constitutive ectopic overexpression of a rice cytokinin-activation enzyme-like gene, OsLOGL5, significantly reduced primary root growth, tiller number, and yield. Conversely, mutations at the 3'-end of OsLOGL5 CDS resulted in normal rice plant morphology but with increased grain yield under well-watered, drought, normal nitrogen, and low nitrogen field conditions at multiple geographical locations. Six gene edited variants (Edit A to F) were created and tested in the field. Edit-B and Edit-F plants increased, but Edit-D and Edit-E plants decreased, the panicle number per plant. All OsLOGL5-edited plants significantly increased seed setting rate, total grain numbers, full-filled grain numbers per panicle, and thousand seed weight under drought conditions, suggesting that OsLOGL5 is likely involved in the regulation of both seed development and grain filling processes. Our results indicate that the C-terminal end of OsLOGL5 protein plays an important role in regulating rice yield improvement under different abiotic stress conditions, and OsLOGL5 is important for rice yield enhancement and stability.

Cytokinin dehydrogenase: a genetic target for yield improvement in wheat

The plant hormone group, the cytokinins, is implicated in both qualitative and quantitative components of yield. Cytokinins have opposing actions in shoot and root growth-actions shown to involve cytokinin dehydrogenase (CKX), the enzyme that inactivates cytokinin. We revise and provide unambiguous names for the CKX gene family members in wheat, based on the most recently released wheat genome database, IWGSC RefSeq v1.0 & v2.0. We review expression data of CKX gene family members in wheat, revealing tissue-specific gene family member expression as well as sub-genome-specific expression. Manipulation of CKX in cereals shows clear impacts on yield, root growth and orientation, and Zn nutrition, but this also emphasizes the necessity to unlink promotive effects on grain yield from negative effects of cytokinin on root growth and uptake of mineral nutrients, particularly Zn and Fe. Wheat is the most widely grown cereal crop globally, yet is under-research compared with rice and maize. We highlight gaps in our knowledge of the involvement of CKX for wheat. We also highlight the necessity for accurate analysis of endogenous cytokinins, acknowledging why this is challenging, and provide examples where inadequate analyses of endogenous cytokinins have led to unjustified conclusions. We acknowledge that the allohexaploid nature of bread wheat poses challenges in terms of uncovering useful mutations. However, we predict TILLING followed by whole-exome sequencing will uncover informative mutations and we indicate the potential for stacking mutations within the three genomes to modify yield components. We model a wheat ideotype based on CKX manipulation.

Phytohormone and Transcriptomic Analysis Reveals Endogenous Cytokinins Affect Kiwifruit Growth under Restricted Carbon Supply

Following cell division, fruit growth is characterized by both expansion through increases in cell volume and biomass accumulation in cells. Fruit growth is limited by carbon starvation; however, the mechanism controlling fruit growth under restricted carbohydrate supply is poorly understood. In a previous study using red-fleshed kiwifruit, we showed that long-term carbon starvation had detrimental effects on carbohydrate, anthocyanin metabolism, and fruit growth. To elucidate the mechanisms underlying the reduction in fruit growth during kiwifruit development, we integrated phytohormone profiling with transcriptomic and developmental datasets for fruit under high or low carbohydrate supplies. Phytohormone profiling of the outer pericarp tissue of kiwifruit showed a 6-fold reduction in total cytokinin concentrations in carbon-starved fruit, whilst other hormones were less affected. Principal component analysis visualised that cytokinin composition was distinct between fruit at 16 weeks after mid bloom, based on their carbohydrate supply status. Cytokinin biosynthetic genes (IPT, CYP735A) were significantly downregulated under carbon starvation, in agreement with the metabolite data. Several genes that code for expansins, proteins involved in cell wall loosening, were also downregulated under carbon starvation. In contrast to other fleshy fruits, our results suggest that cytokinins not only promote cell division, but also drive fruit cell expansion and growth in kiwifruit.

The Mode of Cytokinin Functions Assisting Plant Adaptations to Osmotic Stresses

Plants respond to abiotic stresses by activating a specific genetic program that supports survival by developing robust adaptive mechanisms. This leads to accelerated senescence and reduced growth, resulting in negative agro-economic impacts on crop productivity. Cytokinins (CKs) customarily regulate various biological processes in plants, including growth and development. In recent years, cytokinins have been implicated in adaptations to osmotic stresses with improved plant growth and yield. Endogenous CK content under osmotic stresses can be enhanced either by transforming plants with a bacterial isopentenyl transferase (IPT) gene under the control of a stress inducible promoter or by exogenous application of synthetic CKs. CKs counteract osmotic stress-induced premature senescence by redistributing soluble sugars and inhibiting the expression of senescence-associated genes. Elevated CK contents under osmotic stress antagonize abscisic acid (ABA) signaling and ABA mediated responses, delay leaf senescence, reduce reactive oxygen species (ROS) damage and lipid peroxidation, improve plant growth, and ameliorate osmotic stress adaptability in plants.

Knockout of the HvCKX1 or HvCKX3 Gene in Barley (Hordeum vulgare L.) by RNA-Guided Cas9 Nuclease Affects the Regulation of Cytokinin Metabolism and Root Morphology

Barley is among four of the most important cereal crops with respect to global production. Increasing barley yields to desired levels can be achieved by the genetic manipulation of cytokinin content. Cytokinins are plant hormones that regulate many developmental processes and have a strong influence on grain yield. Cytokinin homeostasis is regulated by members of several multigene families. CKX genes encode the cytokinin oxidase/dehydrogenase enzyme, which catalyzes the irreversible degradation of cytokinin. Several recent studies have demonstrated that the RNAi-based silencing of CKX genes leads to increased grain yields in some crop species. To assess the possibility of increasing the grain yield of barley by knocking out CKX genes, we used an RNA-guided Cas9 system to generate ckx1 and ckx3 mutant lines with knockout mutations in the HvCKX1 and HvCKX3 genes, respectively. Homozygous, transgene-free mutant lines were subsequently selected and analyzed. A significant decrease in CKX enzyme activity was observed in the spikes of the ckx1 lines, while in the ckx3 lines, the activity remained at a similar level to that in the control plants. Despite these differences, no changes in grain yield were observed in either mutant line. In turn, differences in CKX activity in the roots between the ckx1 and ckx3 mutants were reflected via root morphology. The decreased CKX activity in the ckx1 lines corresponded to greater root length, increased surface area, and greater numbers of root hairs, while the increased CKX activity in the ckx3 mutants gave the opposite results. RNA-seq analysis of the spike and root transcriptomes revealed an altered regulation of genes controlling cytokinin metabolism and signaling, as well as other genes that are important during seed development, such as those that encode nutrient transporters. The observed changes suggest that the knockout of a single CKX gene in barley may be not sufficient for disrupting cytokinin homeostasis or increasing grain yields.

Hormonal regulation of cereal endosperm development with a focus on rice (Oryza sativa)

The endosperm of cereal grain forms the staple diet for most of the world's population, and feeds much of their stock. Grain size and quality are determined largely by events taking place during coenocytic nuclear division, endosperm cellularisation and cell differentiation, and the production of storage molecules. Thus, understanding the complex signalling processes occurring at each of these steps is essential for maintaining and improving our food supply. Here, we critically review evidence for the effects of phytohormones on grain size, as well as hormone homeostasis, signalling and crosstalk. We focus on rice endosperm due to the importance of rice as a food crop and a model grass, as well as its relative neglect in recent reviews; however, data from other cereals are also discussed due to strong evidence for conserved signalling networks operating during grain development. Discussion is restricted to auxin, cytokinin, ethylene, abscisic acid and gibberellin. Our review highlights the need for accurate hormone determinations combined with information on gene expression. We present evidence for separate, localised signalling roles for auxin at different stages of grain development and highlight key research questions for other hormones where much less data are available.

Abscisic acid promotes root system development in birch tissue culture: a comparison to aspen culture and conventional rooting-related growth regulators

The research aim was to assess the effects of the plant hormone abscisic acid (ABA) and the growth regulator paclobutrazol (PBZ) on root system development during the in vitro culture of different birch and aspen genotypes. The studied genotypes involved two aspen (Populus tremula and Populus tremuloides × P. tremula) and two silver birch (Betula pendula) trees, with one of the birches characterized by its inability to root in vitro. For experiments, apical shoot segments were cultured on nutrient medium enriched with either ABA or PBZ. Additionally, the analysis of the endogenous hormones in shoots developed on hormone-free medium was conducted by high-performance liquid chromatography. The endogenous concentration of auxin indole-3-acetic acid was much higher in the aspens than that in the birches, while the highest concentration of ABA was found in the root-forming birch. The culturing of this birch genotype on medium enriched with ABA resulted in an increased root length and a higher number of lateral roots without any negative effect on either shoot growth or adventitious root (AR) formation, although these two processes were largely inhibited by ABA in the aspens. Meanwhile, PBZ promoted AR formation in both aspen and birch cultures but impaired secondary root formation and shoot growth in birches. These results suggest the use of ABA for the in vitro rooting of birches and PBZ for the rooting of aspens.

Drought affects abortion of reproductive organs by exacerbating developmentally driven processes via expansive growth and hydraulics

Abortion of reproductive organs is a major limiting factor of yield under water deficit, but is also a trait selected for by evolutionary processes. The youngest reproductive organs must be prone to abortion so older organs can finish their development in case of limited resources. Water deficit increases natural abortion via two developmentally driven processes, namely a signal from the first fertilized ovaries and a simultaneous arrest of the expansive growth of all ovaries at a precise stage. In maize (Zea mays) subjected to water deficits typically encountered in dryland agriculture, these developmental mechanisms account for 90% of drought-associated abortion and are irreversible 3 d after silk emergence. Consistently, transcripts and enzyme activities suggest that the molecular events associated with abortion affect expansive growth in silks whereas ovaries maintain a favourable carbon status. Abortion due to carbon starvation is only observed for severe drought scenarios occurring after silking. Both kinetic and genetic evidence indicates that vegetative and reproductive structures share a partly common hydraulic control of expansive growth. Hence, the control of expansive growth of reproductive structures probably has a prominent effect on abortion for mild water deficits occurring at flowering time, while carbon starvation dominates in severe post-flowering drought scenarios.

Biochemical and molecular characterisation of exogenous cytokinin application on grain filling in rice

BACKGROUND

Poor filling of grains in the basal spikelets of large size panicles bearing numerous spikelets has been a major limitation in attempts to increase the rice production to feed the world's increasing population. Considering that biotechnological intervention could play important role in overcoming this limitation, the role of cytokinin in grain filling was investigated based on the information on cell proliferating potential of the hormone and reports of its high accumulation in immature seeds.

RESULTS

A comparative study considering two rice varieties differing in panicle compactness, lax-panicle Upahar and compact-panicle OR-1918, revealed significant difference in grain filling, cytokinin oxidase (CKX) activity and expression, and expression of cell cycle regulators and cytokinin signaling components between the basal and apical spikelets of OR-1918, but not of Upahar. Exogenous application of cytokinin (6-Benzylaminopurine, BAP) to OR-1918 improved grain filling significantly, and this was accompanied by a significant decrease in expression and activity of CKX, particularly in the basal spikelets where the activity of CKX was significantly higher than that in the apical spikelets. Cytokinin application also resulted in significant increase in expression of cell cycle regulators like cyclin dependent kinases and cyclins in the basal spikelets that might be facilitating cell division in the endosperm cells by promoting G1/S phase and G2/M phase transition leading to improvement in grain filling. Expression studies of type-A response regulator (RR) component of cytokinin signaling indicated possible role of OsRR3, OsRR4 and OsRR6 as repressors of CKX expression, much needed for an increased accumulation of CK in cells. Furthermore, the observed effect of BAP might not be solely because of it, but also because of induced synthesis of trans-zeatin (tZ) and N⁶-(Δ²-isopentenyl)adenine (iP), as reflected from accumulation of tZR (tZ riboside) and iPR (iP riboside), and significantly enhanced expression of an isopentenyl transferase (IPT) isoform.

CONCLUSION

The results suggested that seed-specific overexpression of OsRR4 and OsRR6, and more importantly of IPT9 could be an effective biotechnological intervention towards improving the CK level of the developing caryopses leading to enhanced grain filling in rice cultivars bearing large panicles with numerous spikelets, and thereby increasing their yield potential.

Knockdown of an inflorescence meristem-specific cytokinin oxidase - OsCKX2 in rice reduces yield penalty under salinity stress condition

Cytokinins play a significant role in determining grain yield in plants. Cytokinin oxidases catalyse irreversible degradation of cytokinins and hence modulate cellular cytokinin levels. Here, we studied the role of an inflorescence meristem-specific rice cytokinin oxidase - OsCKX2 - in reducing yield penalty under salinity stress conditions. We utilized an RNAi-based approach to study the function of OsCKX2 in maintaining grain yield under salinity stress condition. Ultra-performance liquid chromatography-based estimation revealed a significant increase in cytokinins in the inflorescence meristem of OsCKX2-knockdown plants. To determine if there exists a correlation between OsCKX2 levels and yield under salinity stress condition, we assessed the growth, physiology and grain yield of OsCKX2-knockdown plants vis-à-vis the wild type. OsCKX2-knockdown plants showed better vegetative growth, higher relative water content and photosynthetic efficiency and reduced electrolyte leakage as compared with the wild type under salinity stress. Importantly, we found a negative correlation between OsCKX2 expression and plant productivity as evident by assessment of agronomical parameters such as panicle branching, filled grains per plant and harvest index both under control and salinity stress conditions. These results suggest that OsCKX2, via controlling cytokinin levels, regulates floral primordial activity modulating rice grain yield under normal as well as abiotic stress conditions.

Genome-Wide Identification and Expression Profiling of Cytokinin Oxidase/Dehydrogenase (CKX) Genes Reveal Likely Roles in Pod Development and Stress Responses in Oilseed Rape (Brassica napus L.)

Cytokinin oxidase/dehydrogenases (CKXs) play a critical role in the irreversible degradation of cytokinins, thereby regulating plant growth and development. Brassica napus is one of the most widely cultivated oilseed crops worldwide. With the completion of whole-genome sequencing of B. napus, genome-wide identification and expression analysis of the BnCKX gene family has become technically feasible. In this study, we identified 23 BnCKX genes and analyzed their phylogenetic relationships, gene structures, conserved motifs, protein subcellular localizations, and other properties. We also analyzed the expression of the 23 BnCKX genes in the B. napus cultivar Zhong Shuang 11 ('ZS11') by quantitative reverse-transcription polymerase chain reaction (qRT-PCR), revealing their diverse expression patterns. We selected four BnCKX genes based on the results of RNA-sequencing and qRT-PCR and compared their expression in cultivated varieties with extremely long versus short siliques. The expression levels of BnCKX5-1, 5-2, 6-1, and 7-1 significantly differed between the two lines and changed during pod development, suggesting they might play roles in determining silique length and in pod development. Finally, we investigated the effects of treatment with the synthetic cytokinin 6-benzylaminopurine (6-BA) and the auxin indole-3-acetic acid (IAA) on the expression of the four selected BnCKX genes. Our results suggest that regulating BnCKX expression is a promising way to enhance the harvest index and stress resistance in plants.

Diluted seawater affects phytohormone receptors and maintains the protonema stage in Physcomitrella patens

Due to its highly efficient homologous recombination ability and unusual evolutionary position, the moss Physcomitrella patens has begun to attract more attention in genetic and evolutionary studies. Protonema, the filament stage of the gametophyte, is of great significance in P. patens protoplast isolation. Moreover, protonema is widely used in genetic engineering. However, difficulties in the induction and state maintenance of protonema restrict its wider application. In this work, protonema was induced efficiently in a diluted seawater medium, and the filamentous state was maintained without further cell differentiation. The developmental process of the protonema resumed, progressing to bud assembly and gametophore formation after transfer to freshwater medium. In addition, a transcriptome analysis showed that plant hormone signal transduction pathways were downregulated when protonema was grown in diluted seawater medium. Consistent with the transcriptome results, the protonema failed to respond to the addition of indole-3-acetic acid and 6-benzylaminopurine to the diluted seawater medium. Based on these results, we concluded that diluted seawater medium blocks the differentiation of protonema. This result could provide a novel insight to benefit future protonema production.

Regulation of maize kernel weight and carbohydrate metabolism by abscisic acid applied at the early and middle post-pollination stages in vitro

Abscisic acid (ABA) accumulates in plants under drought stress, but views on the role of ABA in kernel formation and abortion are not unified. The response of the developing maize kernel to exogenous ABA was investigated by excising kernels from cob sections at four days after pollination and culturing in vitro with different concentrations of ABA (0, 5, 10, 100μM). When ABA was applied at the early post-pollination stage (EPPS), significant weight loss was observed at high ABA concentration (100μM), which could be attributed to jointly affected sink capacity and activity. Endosperm cells and starch granules were decreased significantly with high concentration, and ABA inhibited the activities of soluble acid invertase and acid cell wall invertase, together with earlier attainment of peak values. When ABA was applied at the middle post-pollination stage (MPPS), kernel weight was observably reduced with high concentration and mildly increased with low concentration, which was regulated due to sink activity. The inhibitory effect of high concentration and the mild stimulatory effect of low concentration on sucrose synthase and starch synthase activities were noted, but a peak level of ADP-glucose pyrophosphorylase (AGPase) was stimulated in all ABA treatments. Interestingly, AGPase peak values were advanced by low concentration and postponed by high concentration. In addition, compared with the control, the weight of low ABA concentration treatments were not statistically significant at the two stages, whereas weight loss from high concentration applied at EPPS was considerably obvious compared with that of the MPPS, but neither led to kernel abortion. The temporal- and dose-dependent impacts of ABA reveal a complex process of maize kernel growth and development.

Signaling in Early Maize Kernel Development

Developing the next plant generation within the seed requires the coordination of complex programs driving pattern formation, growth, and differentiation of the three main seed compartments: the embryo (future plant), the endosperm (storage compartment), representing the two filial tissues, and the surrounding maternal tissues. This review focuses on the signaling pathways and molecular players involved in early maize kernel development. In the 2 weeks following pollination, functional tissues are shaped from single cells, readying the kernel for filling with storage compounds. Although the overall picture of the signaling pathways regulating embryo and endosperm development remains fragmentary, several types of molecular actors, such as hormones, sugars, or peptides, have been shown to be involved in particular aspects of these developmental processes. These molecular actors are likely to be components of signaling pathways that lead to transcriptional programming mediated by transcriptional factors. Through the integrated action of these components, multiple types of information received by cells or tissues lead to the correct differentiation and patterning of kernel compartments. In this review, recent advances regarding the four types of molecular actors (hormones, sugars, peptides/receptors, and transcription factors) involved in early maize development are presented.

Differential Gene Expression in the Meristem and during Early Fruit Growth of Pisum sativum L. Identifies Potential Targets for Breeding

For successful molecular breeding it is important to identify targets to the gene family level, and in the specific species of interest, in this case Pisum sativum L. The cytokinins have been identified as a key breeding target due to their influence on plant architecture, and on seed size and sink activity. We focused on the cytokinin biosynthetic gene family (the IPTs) and the gene family key to the destruction of cytokinins (the CKXs), as well as other gene families potentially affected by changing cytokinin levels. These included key meristem genes (WUS and BAM1) and the transporter gene families, sucrose transporters (SUTs) and amino acid permeases (AAPs). We used reverse transcription quantitative PCR (RT-qPCR) to monitor gene expression in the vegetative meristem and in pre- and post-fertilisation young pea fruits. PsWUS expression was specific to the shoot apical meristem while PsBAM1 was highly expressed in the shoot apical meristem (SAM) but was also expressed at a low level in the young fruit. Differential expression was shown between genes and within gene families for IPT, CKX, SUT, and AAP. PsCKX7 showed strong gene family member-specific expression in the SAM, and was also expressed in young pea fruits. We suggest that PsCKX7 is a potential target for downregulation via molecular breeding or gene editing.

Endogenous trans-zeatin content in plants with different metal-accumulating ability: a field survey

A field survey was conducted to evaluate soil metal pollution and endogenous trans-zeatin content in the leaves of plants growing at six sites in a metal-polluted area located in Gejiu, Yunnan, China. Five plant species were collected, and the physicochemical properties and concentrations of five metals in the soil were analyzed. The trans-zeatin content in plant leaves was measured by high-performance liquid chromatography. Based on the Nemerow pollution index, the six sites were classified into four levels of pollution (i.e., low, medium, high, and severely high). The degree of soil metal pollution was cadmium (Cd) > arsenic (As) > lead (Pb) > zinc (Zn) > copper (Cu). The leaf trans-zeatin content in Pteris vittata (an arsenic hyperaccumulator) increased significantly by 98.6 % in soil with a severely high level of pollution compared with soil at a low level of pollution. However, in non-hyperaccumulators Bidens pilosa var. radiata and Ageratina adenophora, a significant decrease in leaf trans-zeatin content of 35.6 and 87.6 %, respectively, was observed. The leaf trans-zeatin content in Artemisia argyi also decreased significantly by 73.6 % in high metal-polluted soil compared with that in medium metal-polluted soil. Furthermore, significant correlations were observed between leaf trans-zeatin content in Pteris vittata and As, Pb, and Cd concentrations in the soil; however, either no correlation or a negative one was observed in the other plant species. Therefore, a high content of trans-zeatin in the leaves of Pteris vittata may play an important role in its normal growth and tolerance to metals.

Genetic and epigenetic control of transfer cell development in plants

The inter-cellular translocation of nutrients in plant is mediated by highly specialized transfer cells (TCs). TCs share similar functional and structural features across a wide range of plant species, including location at plant exchange surfaces, rich in secondary wall ingrowths, facilitation of nutrient flow, and passage of select molecules. The fate of endosperm TCs is determined in the TC fate acquisition stage (TCF), before the structure features are formed in the TC differentiation stage (TCD). At present, the molecular basis of TC development in plants remains largely unknown. In this review, we summarize the important roles of the signaling molecules in different development phases, such as sugars in TCF and phytohormones in TCD, and discuss the genetic and epigenetic factors, including TC-specific genes and endogenous plant peptides, and their crosstalk with these signaling molecules as a complex regulatory network in regulation of TC development in plants.

The Putative O-Linked N-Acetylglucosamine Transferase SPINDLY Inhibits Class I TCP Proteolysis to Promote Sensitivity to Cytokinin

Arabidopsis (Arabidopsis thaliana) SPINDLY (SPY) is a putative serine and threonine O-linked N-acetylglucosamine transferase (OGT). While SPY has been shown to suppress gibberellin signaling and to promote cytokinin (CK) responses, its catalytic OGT activity was never demonstrated and its effect on protein fate is not known. We previously showed that SPY interacts physically and functionally with TCP14 and TCP15 to promote CK responses. Here, we aimed to identify how SPY regulates TCP14/15 activities and how these TCPs promote CK responses. We show that SPY activity is required for TCP14 stability. Mutation in the putative OGT domain of SPY (spy-3) stimulated TCP14 proteolysis by the 26S proteasome, which was reversed by mutation in CULLIN1 (CUL1), suggesting a role for SKP, CUL1, F-box E3 ubiquitin ligase in TCP14 proteolysis. TCP14 proteolysis in spy-3 suppressed all TCP14 misexpression phenotypes, including the enhanced CK responses. The increased CK activity in TCP14/15-overexpressing flowers resulted from increased sensitivity to the hormone and not from higher CK levels. TCP15 overexpression enhanced the response of the CK-induced synthetic promoter pTCS to CK, suggesting that TCP14/15 affect early steps in CK signaling. We propose that posttranslational modification of TCP14/15 by SPY inhibits their proteolysis and that the accumulated proteins promote the activity of the CK phosphorelay cascade in developing Arabidopsis leaves and flowers.

Ovary Apical Abortion under Water Deficit Is Caused by Changes in Sequential Development of Ovaries and in Silk Growth Rate in Maize

Grain abortion allows the production of at least a few viable seeds under water deficit but causes major yield loss. It is maximum for water deficits occurring during flowering in maize (Zea mays). We have tested the hypothesis that abortion is linked to the differential development of ovary cohorts along the ear and to the timing of silk emergence. Ovary volume and silk growth were followed over 25 to 30 d under four levels of water deficit and in four hybrids in two experiments. A position-time model allowed characterizing the development of ovary cohorts and their silk emergence. Silk growth rate decreased in water deficit and stopped 2 to 3 d after first silk emergence, simultaneously for all ovary cohorts, versus 7 to 8 d in well-watered plants. Abortion rate in different treatments and positions on the ear was not associated with ovary growth rate. It was accounted for by the superposition of (1) the sequential emergence of silks originating from ovaries of different cohorts along the ear with (2) one event occurring on a single day, the simultaneous silk growth arrest. Abortion occurred in the youngest ovaries whose silks did not emerge 2 d before silk arrest. This mechanism accounted for more than 90% of drought-related abortion in our experiments. It resembles the control of abortion in a large range of species and inflorescence architectures. This finding has large consequences for breeding drought-tolerant maize and for modeling grain yields in water deficit.

Cytokinin metabolism in maize: Novel evidence of cytokinin abundance, interconversions and formation of a new trans-zeatin metabolic product with a weak anticytokinin activity

Cytokinins (CKs) are an important group of phytohormones. Their tightly regulated and balanced levels are essential for proper cell division and plant organ development. Here we report precise quantification of CK metabolites and other phytohormones in maize reproductive organs in the course of pollination and kernel maturation. A novel enzymatic activity dependent on NADP(+) converting trans-zeatin (tZ) to 6-(3-methylpyrrol-1-yl)purine (MPP) was detected. MPP shows weak anticytokinin properties and inhibition of CK dehydrogenases due to their ability to bind to an active site in the opposite orientation than substrates. Although the physiological significance of tZ side-chain cyclization is not anticipated as the MPP occurrence in maize tissue is very low, properties of the novel CK metabolite indicate its potential for utilization in plant in vitro tissue culture. Furthermore, feeding experiments with different isoprenoid CKs revealed distinct preferences in glycosylation of tZ and cis-zeatin (cZ). While tZ is preferentially glucosylated at the N9 position, cZ forms mainly O-glucosides. Since O-glucosides, in contrast to N9-glucosides, are resistant to irreversible cleavage catalyzed by CK dehydrogenases, the observed preference of maize CK glycosyltransferases to O-glycosylate zeatin in the cis-position might be a reason why cZ derivatives are over-accumulated in different maize tissues and organs.