CYTOKININ METABOLISM AND ACTION

Spatially distributed cytokinins: Metabolism, signaling, and transport

Cytokinins are mobile phytohormones that regulate plant growth, development, and environmental adaptability. The major cytokinin species include isopentenyl adenine (iP), trans-zeatin (tZ), cis-zeatin (cZ), and dihydrozeatin (DZ). The spatial distributions of different cytokinin species in different organelles, cells, tissues, and organs are primarily shaped by biosynthesis via isopentenyltransferases (IPT), cytochrome P450 monooxygenase, and 5'-ribonucleotide phosphohydrolase and by conjugation or catabolism via glycosyltransferase or cytokinin oxidase/dehydrogenase. Cytokinins bind to histidine receptor kinases in the endoplasmic reticulum or plasma membrane and relay signals to response regulators in the nucleus via shuttle proteins known as histidine phosphotransfer proteins. The movements of cytokinins from sites of biosynthesis to sites of signal perception usually require long-distance, intercellular, and intracellular transport. In the past decade, ATP-binding cassette (ABC) transporters, purine permeases (PUP), AZA-GUANINE RESISTANT (AZG) transporters, equilibrative nucleoside transporters (ENT), and Sugars Will Eventually Be Exported transporters (SWEET) have been characterized as involved in cytokinin transport processes. This review begins by introducing the spatial distributions of various cytokinins and the subcellular localizations of the proteins involved in their metabolism and signaling. Highlights focus on an inventory of the characterized transporters involved in cytokinin compartmentalization, including long-distance, intercellular, and intracellular transport, and the regulation of the spatial distributions of cytokinins by environmental cues. Future directions for cytokinin research are also discussed.

Editing of SlWRKY29 by CRISPR-activation promotes somatic embryogenesis in Solanum lycopersicum cv. Micro-Tom

At present, the development of plants with improved traits like superior quality, high yield, or stress resistance, are highly desirable in agriculture. Accelerated crop improvement, however, must capitalize on revolutionary new plant breeding technologies, like genetically modified and gene-edited crops, to heighten food crop traits. Genome editing still faces ineffective methods for the transformation and regeneration of different plant species and must surpass the genotype dependency of the transformation process. Tomato is considered an alternative plant model system to rice and Arabidopsis, and a model organism for fleshy-fruited plants. Furthermore, tomato cultivars like Micro-Tom are excellent models for tomato research due to its short life cycle, small size, and capacity to grow at high density. Therefore, we developed an indirect somatic embryo protocol from cotyledonary tomato explants and used this to generate epigenetically edited tomato plants for the SlWRKY29 gene via CRISPR-activation (CRISPRa). We found that epigenetic reprogramming for SlWRKY29 establishes a transcriptionally permissive chromatin state, as determined by an enrichment of the H3K4me3 mark. A whole transcriptome analysis of CRISPRa-edited pro-embryogenic masses and mature somatic embryos allowed us to characterize the mechanism driving somatic embryo induction in the edited tomato cv. Micro-Tom. Furthermore, we show that enhanced embryo induction and maturation are influenced by the transcriptional effector employed during CRISPRa, as well as by the medium composition and in vitro environmental conditions such as osmotic components, plant growth regulators, and light intensity.

Flavin mononucleotide regulated photochemical isomerization and degradation of zeatin

cis-Zeatin (cZ), a cytokinin often overlooked compared to trans-zeatin (tZ), can now be controlled in live cells and plants through a new biocompatible reaction. Using flavin photosensitizers, cZ can be isomerized to tZ or degraded, depending on the presence of a reducing reagent. This breakthrough offers a novel approach for regulating plant growth through chemical molecules.

Plant nucleoside N-ribohydrolases: riboside binding and role in nitrogen storage mobilization

Cells save their energy during nitrogen starvation by selective autophagy of ribosomes and degradation of RNA to ribonucleotides and nucleosides. Nucleosides are hydrolyzed by nucleoside N-ribohydrolases (nucleosidases, NRHs). Subclass I of NRHs preferentially hydrolyzes the purine ribosides while subclass II is more active towards uridine and xanthosine. Here, we performed a crystallographic and kinetic study to shed light on nucleoside preferences among plant NRHs followed by in vivo metabolomic and phenotyping analyses to reveal the consequences of enhanced nucleoside breakdown. We report the crystal structure of Zea mays NRH2b (subclass II) and NRH3 (subclass I) in complexes with the substrate analog forodesine. Purine and pyrimidine catabolism are inseparable because nucleobase binding in the active site of ZmNRH is mediated via a water network and is thus unspecific. Dexamethasone-inducible ZmNRH overexpressor lines of Arabidopsis thaliana, as well as double nrh knockout lines of moss Physcomitrium patents, reveal a fine control of adenosine in contrast to other ribosides. ZmNRH overexpressor lines display an accelerated early vegetative phase including faster root and rosette growth upon nitrogen starvation or osmotic stress. Moreover, the lines enter the bolting and flowering phase much earlier. We observe changes in the pathways related to nitrogen-containing compounds such as β-alanine and several polyamines, which allow plants to reprogram their metabolism to escape stress. Taken together, crop plant breeding targeting enhanced NRH-mediated nitrogen recycling could therefore be a strategy to enhance plant growth tolerance and productivity under adverse growth conditions.

The Long-Distance Transport of Some Plant Hormones and Possible Involvement of Lipid-Binding and Transfer Proteins in Hormonal Transport

Adaptation to changes in the environment depends, in part, on signaling between plant organs to integrate adaptive response at the level of the whole organism. Changes in the delivery of hormones from one organ to another through the vascular system strongly suggest that hormone transport is involved in the transmission of signals over long distances. However, there is evidence that, alternatively, systemic responses may be brought about by other kinds of signals (e.g., hydraulic or electrical) capable of inducing changes in hormone metabolism in distant organs. Long-distance transport of hormones is therefore a matter of debate. This review summarizes arguments for and against the involvement of the long-distance transport of cytokinins in signaling mineral nutrient availability from roots to the shoot. It also assesses the evidence for the role of abscisic acid (ABA) and jasmonates in long-distance signaling of water deficiency and the possibility that Lipid-Binding and Transfer Proteins (LBTPs) facilitate the long-distance transport of hormones. It is assumed that proteins of this type raise the solubility of hydrophobic substances such as ABA and jasmonates in hydrophilic spaces, thereby enabling their movement in solution throughout the plant. This review collates evidence that LBTPs bind to cytokinins, ABA, and jasmonates and that cytokinins, ABA, and LBTPs are present in xylem and phloem sap and co-localize at sites of loading into vascular tissues and at sites of unloading from the phloem. The available evidence indicates a functional interaction between LBTPs and these hormones.

A peptidoglycan N-deacetylase specific for anhydroMurNAc chain termini in Agrobacterium tumefaciens

During growth, bacteria remodel and recycle their peptidoglycan (PG). A key family of PG-degrading enzymes is the lytic transglycosylases, which produce anhydromuropeptides, a modification that caps the PG chains and contributes to bacterial virulence. Previously, it was reported that the polar-growing Gram-negative plant pathogen Agrobacterium tumefaciens lacks anhydromuropeptides. Here, we report the identification of an enzyme, MdaA (MurNAc deacetylase A), which specifically removes the acetyl group from anhydromuropeptide chain termini in A. tumefaciens, resolving this apparent anomaly. A. tumefaciens lacking MdaA accumulates canonical anhydromuropeptides, whereas MdaA was able to deacetylate anhydro-N-acetyl muramic acid in purified sacculi that lack this modification. As for other PG deacetylases, MdaA belongs to the CE4 family of carbohydrate esterases but harbors an unusual Cys residue in its active site. MdaA is conserved in other polar-growing bacteria, suggesting a possible link between PG chain terminus deacetylation and polar growth.

miRNAs and genes as molecular regulators of rice grain morphology and yield

Rice is one of the most consumed crops worldwide and the genetic and molecular basis of its grain yield attributes are well understood. Various studies have identified different yield-related parameters in rice that are regulated by the microRNAs (miRNAs). MiRNAs are endogenous small non-coding RNAs that silence gene expression during or after transcription. They control a variety of biological or genetic activities in plants including growth, development and response to stress. In this review, we have summarized the available information on the genetic control of panicle architecture and grain yield (number and morphology) in rice. The miRNA nodes that are associated with their regulation are also described while focussing on the central role of miR156-SPL node to highlight the co-regulation of two master regulators that determine the fate of panicle development. Since abiotic stresses are known to negatively affect yield, the impact of abiotic stress induced alterations on the levels of these miRNAs are also discussed to highlight the potential of miRNAs for regulating crop yields.

Modification of Gene Expression of Tomato Plants through Foliar Flavonoid Application in Relation to Enhanced Growth

The exogenous application of phenolic compounds is increasingly recognized as a valuable strategy for promoting growth and mitigating the adverse effects of abiotic stress. However, the biostimulant effect under optimal conditions has not been thoroughly explored. In this study, we investigated the impact of foliar application of flavonoids, specifically CropBioLife (CBL), on tomato plants grown under controlled conditions. Our study focused on determining growth parameters, such as cell size, and assessing the concentration of hormones. Principal component analysis (PCA) from all physiological variables was determined. Additionally, we utilized high-throughput mRNA-sequencing technology and bioinformatic methodologies to robustly analyze the transcriptomes of tomato leaves regulated by flavonoids. The findings revealed that CBL primarily influenced cell enlargement by 60%, leading to increased growth. Furthermore, CBL-treated plants exhibited higher concentrations of the hormone zeatin, but lower concentrations of IAA (changes of 50%). Moreover, RNA-seq analysis indicated that CBL-treated plants required increased mineral transport and water uptake, as evidenced by gene expression patterns. Genes related to pathways such as fatty acid degradation, phenylpropanoid biosynthesis, and ABC transporters showed regulatory mechanisms governing internal flavonoid biosynthesis, transport, and tissue concentration, ultimately resulting in higher flavonoid concentrations in tomato leaves.

The interplay between cell wall integrity and cell cycle progression in plants

Plant cell walls are dynamic structures that play crucial roles in growth, development, and stress responses. Despite our growing understanding of cell wall biology, the connections between cell wall integrity (CWI) and cell cycle progression in plants remain poorly understood. This review aims to explore the intricate relationship between CWI and cell cycle progression in plants, drawing insights from studies in yeast and mammals. We provide an overview of the plant cell cycle, highlight the role of endoreplication in cell wall composition, and discuss recent findings on the molecular mechanisms linking CWI perception to cell wall biosynthesis and gene expression regulation. Furthermore, we address future perspectives and unanswered questions in the field, such as the identification of specific CWI sensing mechanisms and the role of CWI maintenance in the growth-defense trade-off. Elucidating these connections could have significant implications for crop improvement and sustainable agriculture.

The Cytokinins BAP and 2-iP Modulate Different Molecular Mechanisms on Shoot Proliferation and Root Development in Lemongrass (Cymbopogon citratus)

The known activities of cytokinins (CKs) are promoting shoot multiplication, root growth inhibition, and delaying senescence. 6-Benzylaminopurine (BAP) has been the most effective CK to induce shoot proliferation in cereal and grasses. Previously, we reported that in lemongrass (Cymbopogon citratus) micropropagation, BAP 10 µM induces high shoot proliferation, while the natural CK 6-(γ,γ-Dimethylallylamino)purine (2-iP) 10 µM shows less pronounced effects and developed rooting. To understand the molecular mechanisms involved, we perform a protein-protein interaction (PPI) network based on the genes of Brachypodium distachyon involved in shoot proliferation/repression, cell cycle, stem cell maintenance, auxin response factors, and CK signaling to analyze the molecular mechanisms in BAP versus 2-iP plants. A different pattern of gene expression was observed between BAP- versus 2-iP-treated plants. In shoots derived from BAP, we found upregulated genes that have already been demonstrated to be involved in de novo shoot proliferation development in several plant species; CK receptors (AHK3, ARR1), stem cell maintenance (STM, REV and CLV3), cell cycle regulation (CDKA-CYCD3 complex), as well as the auxin response factor (ARF5) and CK metabolism (CKX1). In contrast, in the 2-iP culture medium, there was an upregulation of genes involved in shoot repression (BRC1, MAX3), ARR4, a type A-response regulator (RR), and auxin metabolism (SHY2).

Improving citrus bud grafting efficiency

Commercial citrus trees are composed of a scion grafted onto a rootstock. Because grafting is one of the most expensive methods of plant propagation, grafting efficiency is of large practical importance. The purpose of this study was to improve citrus bud-grafting efficiency. The effects of six factors that included BA, Tween-20, DMSO, type of solvent (water or EtOH), cardinal orientation of grafted bud, and type of supplemental light (LED, metal halide, none) on forty-four bud-grafting measures were determined using a multifactor design of experiment approach. Four measures useful for identifying treatments of practical value included the number of rootstock axial buds that formed shoots, the percentage of grafted buds that formed shoots, the length of the longest shoot formed from the grafted buds, and the total leaf area of the grafted bud shoots. The factors that most affected these responses were no supplemental light to minimize the number of shoots from rootstock axial buds, a south orientation and 5 mM BA to maximize the percentage of grafted buds that formed shoots, a north orientation and 5 mM BA to maximize the length of the longest grafted bud shoot, and 5 mM BA to maximize the leaf area of the grafted bud shoots.

The Effect of Topophysis on the In Vitro Development of Handroanthus guayacan and on Its Metabolism of Meta-Topolin Riboside

An important factor affecting the uniformity of in vitro cultures is the topophysical position of the original explant. We investigated this phenomenon in Handroanthus guayacan, a tropical woody tree species. Shoots from a stock culture were separated into upper, middle and basal sections and transferred to a modified MS medium containing meta-topolin-riboside and indole-butyric acid. After 8 weeks, the middle section produced the most shoots, the longest shoots and the highest number of nodes per plant. Shoots derived from the upper section were elongated, but had the shortest internodes, while those from the basal section formed the largest callus. None of the three types of explants rooted during the proliferation phase. The topophysically dependent spatial distribution of endogenous cytokinins and auxins was determined. The topophysical effect observed could not be explained solely by analyzing the endogenous isoprenoid and auxin. However, the metabolism and distribution of the aromatic cytokinin could provide an explanation. The concentration of the meta hydroxy-substituted topolins was highest in shoots derived from the middle section. Aromatic N- and O-glucosides were much more concentrated in the leaves than in the stems. In conclusion, it is recommended to consider the explant's topophysis when developing a multiplication protocol to avoid heterogeneity in an in vitro culture.

Stem Cells: Engines of Plant Growth and Development

The development of both animals and plants relies on populations of pluripotent stem cells that provide the cellular raw materials for organ and tissue formation. Plant stem cell reservoirs are housed at the shoot and root tips in structures called meristems, with the shoot apical meristem (SAM) continuously producing aerial leaf, stem, and flower organs throughout the life cycle. Thus, the SAM acts as the engine of plant development and has unique structural and molecular features that allow it to balance self-renewal with differentiation and act as a constant source of new cells for organogenesis while simultaneously maintaining a stem cell reservoir for future organ formation. Studies have identified key roles for intercellular regulatory networks that establish and maintain meristem activity, including the KNOX transcription factor pathway and the CLV-WUS stem cell feedback loop. In addition, the plant hormones cytokinin and auxin act through their downstream signaling pathways in the SAM to integrate stem cell activity and organ initiation. This review discusses how the various regulatory pathways collectively orchestrate SAM function and touches on how their manipulation can alter stem cell activity to improve crop yield.

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.

Transcriptomic Profiling of Shoot Apical Meristem Aberrations in the Multi-Main-Stem Mutant (ms) of Brassica napus L

Rapeseed (Brassica napus L.) is a globally important oilseed crop with various uses, including the consumption of its succulent stems as a seasonal vegetable, but its uniaxial branching habit limits the stem yield. Therefore, developing a multi-stem rapeseed variety has become increasingly crucial. In this study, a natural mutant of the wild type (ZY511, Zhongyou511) with stable inheritance of the multi-stem trait (ms) was obtained, and it showed abnormal shoot apical meristem (SAM) development and an increased main stem number compared to the WT. Histological and scanning electron microscopy analyses revealed multiple SAMs in the ms mutant, whereas only a single SAM was found in the WT. Transcriptome analyses showed significant alterations in the expression of genes involved in cytokinin (CK) biosynthesis and metabolism pathways in the ms mutant. These findings provide insight into the mechanism of multi-main-stem formation in Brassica napus L. and lay a theoretical foundation for breeding multi-main-stem rapeseed vegetable varieties.

Signaling and crosstalk of rhizobacterial and plant hormones that mediate abiotic stress tolerance in plants

Agricultural areas exhibiting numerous abiotic stressors, such as elevated water stress, temperatures, and salinity, have grown as a result of climate change. As such, abiotic stresses are some of the most pressing issues in contemporary agricultural production. Understanding plant responses to abiotic stressors is important for global food security, climate change adaptation, and improving crop resilience for sustainable agriculture, Over the decades, explorations have been made concerning plant tolerance to these environmental stresses. Plant growth-promoting rhizobacteria (PGPR) and their phytohormones are some of the players involved in developing resistance to abiotic stress in plants. Several studies have investigated the part of phytohormones in the ability of plants to withstand and adapt to non-living environmental factors, but very few have focused on rhizobacterial hormonal signaling and crosstalk that mediate abiotic stress tolerance in plants. The main objective of this review is to evaluate the functions of PGPR phytohormones in plant abiotic stress tolerance and outline the current research on rhizobacterial hormonal communication and crosstalk that govern plant abiotic stress responses. The review also includes the gene networks and regulation under diverse abiotic stressors. The review is important for understanding plant responses to abiotic stresses using PGPR phytohormones and hormonal signaling. It is envisaged that PGPR offer a useful approach to increasing plant tolerance to various abiotic stresses. However, further studies can reveal the unclear patterns of hormonal interactions between plants and rhizobacteria that mediate abiotic stress tolerance.

Phytohormone Crosstalk of Cytokinin Biosynthesis and Signaling Family Genes in Moso Bamboo (Phyllostachys edulis)

Cytokinin is widely involved in the regulation of plant growth, but its pathway-related genes have not been reported in Moso bamboo. In this study, a total of 129 candidate sequences were identified by bioinformatic methods. These included 15 IPT family genes, 19 LOG family genes, 22 HK family genes, 11 HP family genes and 62 RR family genes. Phylogenetic analysis revealed that the cytokinin pathway was closely related to rice, and evolutionary pattern analysis found that most of the genes have syntenic relationship with rice-related genes. The Moso bamboo cytokinin pathway was evolutionarily conservative and mainly underwent purifying selection, and that gene family expansion was mainly due to whole-gene duplication events. Analysis of transcriptome data revealed a tissue-specific expression pattern of Moso bamboo cytokinin family genes, with auxin and gibberellin response patterns. Analysis of co-expression patterns at the developmental stages of Moso bamboo shoots revealed the existence of a phytohormone co-expression pattern centered on cytokinin signaling genes. The auxin signaling factor PheARF52 was identified by yeast one-hybrid assay as regulating the PheRR3 gene through a P-box element in the PheRR3 promoter region. Auxin and cytokinin signaling crosstalk to regulate Moso bamboo growth. Overall, we systematically identified and analyzed key gene families of the cytokinin pathway in Moso bamboo and obtained key factors for auxin and cytokinin crosstalk, laying the foundation for the study of hormone regulation in Moso bamboo.

A First Approach for the Micropropagation of the Edible and Medicinal Halophyte Inula crithmoides L

Inula crithmoides L. (golden samphire) is an edible aromatic halophyte species with confirmed nutritional and medicinal properties attributed to the presence of important metabolites, including proteins, carotenoids, vitamins, and minerals. Therefore, this study aimed at establishing a micropropagation protocol for golden samphire that can serve as a nursery approach to its standardized commercial cultivation. For that purpose, a complete regeneration protocol was developed by improving shoot multiplication from nodal explants, rooting, and acclimatization methodologies. The treatment with BAP alone induced the maximum shoot formation (7-7.8 shoots/explant), while IAA treatment increased the shoot height (9.26-9.5 cm). Furthermore, the treatment that coupled best shoot multiplication (7.8 shoots/explant) and highest shoot height (7.58 cm) was MS medium supplemented with 0.25 mg/L BAP. Moreover, all shoots produced roots (100% rooting), and multiplication treatments did not exert significant effect on root length (7.8-9.7 cm/plantlet). Moreover, by the end of the rooting phase, plantlets cultivated with 0.25 mg/L BAP had the highest shoot number (4.2 shoots/plantlet), and plantlets from 0.6 mg/L IAA + 1 mg/L BAP presented the highest shoot height (14.2 cm) similar to control plantlets (14.0 cm). The survival up to the ex-vitro acclimatization stage was increased from 9.8% (control) to 83.3%, when plants were treated with a paraffin solution. Nevertheless, the in vitro multiplication of golden samphire is a promising way for its rapid propagation and can be used as a nursery method, contributing to the development of this species as an alternative food and medicinal crop.

Nitrate, Auxin and Cytokinin-A Trio to Tango

Nitrogen is an important macronutrient required for plant growth and development, thus directly impacting agricultural productivity. In recent years, numerous studies have shown that nitrogen-driven growth depends on pathways that control nitrate/nitrogen homeostasis and hormonal networks that act both locally and systemically to coordinate growth and development of plant organs. In this review, we will focus on recent advances in understanding the role of the plant hormones auxin and cytokinin and their crosstalk in nitrate-regulated growth and discuss the significance of novel findings and possible missing links.

Genomic Assessment of the Contribution of the Wolbachia Endosymbiont of Eurosta solidaginis to Gall Induction

We explored the genome of the Wolbachia strain, wEsol, symbiotic with the plant-gall-inducing fly Eurosta solidaginis with the goal of determining if wEsol contributes to gall induction by its insect host. Gall induction by insects has been hypothesized to involve the secretion of the phytohormones cytokinin and auxin and/or proteinaceous effectors to stimulate cell division and growth in the host plant. We sequenced the metagenome of E. solidaginis and wEsol and assembled and annotated the genome of wEsol. The wEsol genome has an assembled length of 1.66 Mbp and contains 1878 protein-coding genes. The wEsol genome is replete with proteins encoded by mobile genetic elements and shows evidence of seven different prophages. We also detected evidence of multiple small insertions of wEsol genes into the genome of the host insect. Our characterization of the genome of wEsol indicates that it is compromised in the synthesis of dimethylallyl pyrophosphate (DMAPP) and S-adenosyl L-methionine (SAM), which are precursors required for the synthesis of cytokinins and methylthiolated cytokinins. wEsol is also incapable of synthesizing tryptophan, and its genome contains no enzymes in any of the known pathways for the synthesis of indole-3-acetic acid (IAA) from tryptophan. wEsol must steal DMAPP and L-methionine from its host and therefore is unlikely to provide cytokinin and auxin to its insect host for use in gall induction. Furthermore, in spite of its large repertoire of predicted Type IV secreted effector proteins, these effectors are more likely to contribute to the acquisition of nutrients and the manipulation of the host's cellular environment to contribute to growth and reproduction of wEsol than to aid E. solidaginis in manipulating its host plant. Combined with earlier work that shows that wEsol is absent from the salivary glands of E. solidaginis, our results suggest that wEsol does not contribute to gall induction by its host.

Cytokinin signaling promotes salt tolerance by modulating shoot chloride exclusion in maize

Excessive accumulation of chloride (Cl^-) in the aboveground tissues under saline conditions is harmful to crops. Increasing the exclusion of Cl^- from shoots promotes salt tolerance in various crops. However, the underlying molecular mechanisms remain largely unknown. In this study, we demonstrated that a type A response regulator (ZmRR1) modulates Cl^- exclusion from shoots and underlies natural variation of salt tolerance in maize. ZmRR1 negatively regulates cytokinin signaling and salt tolerance, likely by interacting with and inhibiting His phosphotransfer (HP) proteins that are key mediators of cytokinin signaling. A naturally occurring non-synonymous SNP variant enhances the interaction between ZmRR1 and ZmHP2, conferring maize plants with a salt-hypersensitive phenotype. We found that ZmRR1 undergoes degradation under saline conditions, leading to the release of ZmHP2 from ZmRR1 inhibition, and subsequently ZmHP2-mediated signaling improves salt tolerance primarily by promoting Cl^- exclusion from shoots. Furthermore, we showed that ZmMATE29 is transcriptionally upregulated by ZmHP2-mediated signaling under highly saline conditions and encodes a tonoplast-located Cl^- transporter that promotes Cl^- exclusion from shoots by compartmentalizing Cl^- into the vacuoles of root cortex cells. Collectively, our study provides an important mechanistic understanding of the cytokinin signaling-mediated promotion of Cl^- exclusion from shoots and salt tolerance and suggests that genetic modification to promote Cl^- exclusion from shoots is a promising route for developing salt-tolerant maize.

Photo- and Tetrazine-Responsive Modulation of trans-Zeatin

Photoirradiation and small organic molecule triggering of appropriately designed caged hormones enable the control and manipulation of the corresponding biological processes with high spatial and temporal resolution. Caged trans-zeatin substituted with nitrobenzene carbonates as photoremovable protecting groups and trans-cyclooctene as the tetrazine-responsive motif have been synthesized. A smooth release of the trapped trans-zeatin molecule has been achieved, permitting targeted perturbation of biological processes, including degradation, glucosylation, and recognition by appropriate enzymes.

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.

Cytokinin Response Factor 9 Represses Cytokinin Responses in Flower Development

A multi-step phosphorelay system is the main conduit of cytokinin signal transduction. However, several groups of additional factors that also play a role in this signaling pathway have been found-among them the Cytokinin Response Factors (CRFs). In a genetic screen, CRF9 was identified as a regulator of the transcriptional cytokinin response. It is mainly expressed in flowers. Mutational analysis indicates that CRF9 plays a role in the transition from vegetative to reproductive growth and silique development. The CRF9 protein is localized in the nucleus and functions as a transcriptional repressor of Arabidopsis Response Regulator 6 (ARR6)-a primary response gene for cytokinin signaling. The experimental data suggest that CRF9 functions as a repressor of cytokinin during reproductive development.

The Role of a Cytokinin Antagonist in the Progression of Clubroot Disease

Plasmodiophora brassicae is an obligate biotrophic pathogen causing clubroot disease in cruciferous plants. Infected plant organs are subject to profound morphological changes, the roots form characteristic galls, and the leaves are chlorotic and abscise. The process of gall formation is governed by timely changes in the levels of endogenous plant hormones that occur throughout the entire life cycle of the clubroot pathogen. The homeostasis of two plant hormones, cytokinin and auxin, appears to be crucial for club development. To investigate the role of cytokinin and auxin in gall formation, we used metabolomic and transcriptomic profiling of Arabidopsis thaliana infected with clubroot, focusing on the late stages of the disease, where symptoms were more pronounced. Loss-of-function mutants of three cytokinin receptors, AHK2, AHK3, and CRE1/AHK4, were employed to further study the homeostasis of cytokinin in response to disease progression; ahk double mutants developed characteristic symptoms of the disease, albeit with varying intensity. The most susceptible to clubroot disease was the ahk3 ahk4 double mutant, as revealed by measuring its photosynthetic performance. Quantification of phytohormone levels and pharmacological treatment with the cytokinin antagonist PI-55 showed significant changes in the levels of endogenous cytokinin and auxin, which was manifested by both enhanced and reduced development of disease symptoms in different genotypes.

Establishment of an efficient regeneration and Agrobacterium transformation system in mature embryos of calla lily (Zantedeschia spp.)

Calla lily (Zantedeschia spp.) have great aesthetic value due to their spathe-like appearance and richness of coloration. However, embryonic callus regeneration is absent from its current regeneration mechanism. As a result, constructing an adequate and stable genetic transformation system is hampered, severely hindering breeding efforts. In this research, the callus induction effectiveness of calla lily seed embryos of various maturities was evaluated. The findings indicated that mature seed embryos were more suitable for in vitro regeneration. Using orthogonal design experiments, the primary elements influencing in vitro regeneration, such as plant growth regulators, genotypes, and nanoscale materials, which was emergent uses for in vitro regeneration, were investigated. The findings indicated that MS supplemented with 6-BA 2 mg/L and NAA 0.1 mg/L was the optimal medium for callus induction (CIM); the germination medium (GM) was MS supplemented with 6-BA 2 mg/L NAA 0.2 mg/L and 1 mg/L CNTs, and the rooting medium (RM) was MS supplemented with 6-BA 2 mg/L NAA 0.7 mg/L and 2 mg/L CNTs. This allowed us to verify, in principle, that the Agrobacterium tumefaciens-mediated genetic transformation system operates under optimal circumstances using the GUS reporter gene. Here, we developed a seed embryo-based genetic transformation regeneration system, which set the stage for future attempts to create new calla lily varieties.

Callus induction and regeneration in high-altitude Himalayan rice genotype SR4 via seed explant

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In the present study, an efficient protocol for callusing and regeneration via direct and indirect means was established using mature seeds as source of explants as they are accessible round the year and receptive to transformation via callus.

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Highest frequency (96.6%) of callus induction was obtained in MS media containing 3.0 mg/L 2, 4-D.

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Highest regeneration frequency (100%) from callus was obtained from MS media supplemented with 5.0 mg/L BAP and 0.5 mg/L NAA with highest number of shoots having an average shoot length 9.14 ± 0.204 (cm) after four weeks of culture.

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Direct multiple shoot regeneration from seed explants was obtained using various concentrations of TDZ and BAP with highest regeneration frequency was observed in MS media supplemented with 6 mg/L of TDZ with maximum number of shoots.

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The present study is attempt to develop highly efficient regeneration protocol via both direct and indirect ways utilizing mature seeds as source explant and thus can be utilized for molecular studies for genetic advancement of Himalayan rice genotype SR4 through transformation.

SR4 genotype of rice is high altitude Himalayan rice prone to various abiotic stresses such as cold stress and therefore gives a poor yield. An efficient protocol for callusing and regeneration via direct and indirect means was established using mature seeds as an explant which can be utilized for molecular studies for genetic advancement of Himalayan rice genotype SR4 through transformation. Highest frequency (96.6%) of callus induction was obtained on MS media 3.0 mg/L 2, 4-D. While maximum regeneration frequency (100%), number of shoots with maximum length 9.14 ± 0.204 (cm) from callus was recovered from MS media amended with 5.0 mg/L BAP in combination with 0.5 mg/L NAA with highest number of shoots having an average shoot length 9.14 ± 0.204 (cm) after four weeks of culture. Direct multiple shoot regeneration from seed explants was obtained using various concentrations of TDZ and BAP with highest regeneration frequency was observed on MS media fortified with 6 mg/L of TDZ with maximum number of shoots. The shoots developed roots on MS media supplemented with 0.6 mg/L IBA.

Role of cytokinins in seed development in pulses and oilseed crops: Current status and future perspective

Cytokinins constitutes a vital group of plant hormones regulating several developmental processes, including growth and cell division, and have a strong influence on grain yield. Chemically, they are the derivatives of adenine and are the most complex and diverse group of hormones affecting plant physiology. In this review, we have provided a molecular understanding of the role of cytokinins in developing seeds, with special emphasis on pulses and oilseed crops. The importance of cytokinin-responsive genes including cytokinin oxidases and dehydrogenases (CKX), isopentenyl transferase (IPT), and cytokinin-mediated genetic regulation of seed size are described in detail. In addition, cytokinin expression in germinating seeds, its biosynthesis, source-sink dynamics, cytokinin signaling, and spatial expression of cytokinin family genes in oilseeds and pulses have been discussed in context to its impact on increasing economy yields. Recently, it has been shown that manipulation of the cytokinin-responsive genes by mutation, RNA interference, or genome editing has a significant effect on seed number and/or weight in several crops. Nevertheless, the usage of cytokinins in improving crop quality and yield remains significantly underutilized. This is primarily due to the multigene control of cytokinin expression. The information summarized in this review will help the researchers in innovating newer and more efficient ways of manipulating cytokinin expression including CKX genes with the aim to improve crop production, specifically of pulses and oilseed crops.

Plant Hormone and Inorganic Ion Concentrations in the Xylem Exudate of Grafted Plants Depend on the Scion-Rootstock Combination

In grafted plants, inorganic ions and plant hormones in the xylem exudate transported from the rootstock to the scion directly or indirectly affect the scion, thereby improving the traits. Therefore, the concentration of these components in the xylem exudate of grafted plants may be an indicator for rootstock selection. On the other hand, few reports have presented a comprehensive analysis of substances transferred from the rootstock to the scion in plants grafted onto different rootstocks, primarily commercial cultivars. In this study, we measured inorganic ions and plant hormones in the xylem exudate from the rootstock to the scion in various grafted plants of tomato and eggplant. The results revealed that the concentrations of inorganic ions and plant hormones in the xylem exudate significantly differed depending on the type of rootstock. In addition, we confirmed the concentration of the inorganic ions and plant hormones in the xylem exudate of plants grafted onto the same tomato rootstock cultivars as rootstock with tomato or eggplant as the scions. As a result, the concentrations of inorganic ions and plant hormones in the xylem exudate were significantly different in the grafted plants with eggplant compared with tomato as the scion. These results suggest that signals from the scion (shoot) control the inorganic ions and plant hormones transported from the rootstock (root).

Quantification of six types of cytokinins: Integration of an ultra-performance liquid chromatographic-electrospray tandem mass spectrometric method with antibody based immunoaffinity columns equally recognizing cytokinins in free base and nucleoside forms

Cytokinins (CTKs) exist in various types in plants. The accurate quantification of free base and nucleoside types of cytokinins are helpful for better understanding their physiological role. In the present study, antibodies against trans-zeatin riboside (tZR) and N⁶-isopentenyladenine riboside (iPR) antibodies with equal recognition to free base and nucleoside cytokinins were developed. The cross-reactivity of tZR mAb 3G101G7 with tZR, trans-zeatin (tZ), dihydrozeatin riboside (DHZR), dihydrozeatin (DHZ), iPR, and N⁶-isopentenyladenine (iP) was 100.0%, 95.7%, 19.1%, 18.0%, 1.1%, and 0.7%, and that of iPR mAb 5C82F1 with above-mentioned 6 types of cytokinins was 1.5%, 1.4%, 5.7%, 3.1%, 100.0% and 92.6%, respectively. The obtained antibodies were used to prepare two immunoaffinity columns (IAC). The elution efficiencies of tZR 3G101G7-IAC for tZ and tZR, DHZ and DHZR and of iPR 5C82F1-IAC for iP and iPR were almost no difference with the same loading amount on their corresponding IACs. Subsequently, six types of cytokinins in mepiquat chloride (MC)-treated cotton (Gossypium hirsutum L.) roots were determined by IACs combined with ultra-performance liquid chromatography-electrospray tandem mass spectrometry (UPLC-ESI-MS/MS). The contents of tZR, iPR and DHZR were increased by 9.3∼38.5%, 6.6∼23.5%, and 30.1∼110.0%, respectively, whereas those of tZ and iP were reduced by 5.3∼20.0% and 27.7∼32.1%, respectively. The decreased tZ and iP levels led to the ratio of auxin-to-active cytokinins increase to promote lateral root initiation in MC-treated cotton seeding. Integration of the IACs equally recognizing cytokinins in their free base and nucleoside forms with UPLC-ESI-MS/MS can accurately quantify different cytokinins in plant tissues.

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.

Structure and character analysis of cotton response regulator genes family reveals that GhRR7 responses to draught stress

Background

Cytokinin signal transduction is mediated by a two-component system (TCS). Two-component systems are utilized in plant responses to hormones as well as to biotic and abiotic environmental stimuli. In plants, response regulatory genes (RRs) are one of the main members of the two-component system (TCS).

Method

From the aspects of gene structure, evolution mode, expression type, regulatory network and gene function, the evolution process and role of RR genes in the evolution of the cotton genome were analyzed.

Result

A total of 284 RR genes in four cotton species were identified. Including 1049 orthologous/paralogous gene pairs were identified, most of which were whole genome duplication (WGD). The RR genes promoter elements contain phytohormone responses and abiotic or biotic stress-related cis-elements. Expression analysis showed that RR genes family may be negatively regulate and involved in salt stress and drought stress in plants. Protein regulatory network analysis showed that RR family proteins are involved in regulating the DNA-binding transcription factor activity (COG5641) pathway and HP kinase pathways. VIGS analysis showed that the GhRR7 gene may be in the same regulatory pathway as GhAHP5 and GhPHYB, ultimately negatively regulating cotton drought stress by regulating POD, SOD, CAT, H₂O₂ and other reactive oxygen removal systems.

Conclusion

This study is the first to gain insight into RR gene members in cotton. Our research lays the foundation for discovering the genes related to drought and salt tolerance and creating new cotton germplasm materials for drought and salt tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40659-022-00394-2.

Cytokinin and abiotic stress tolerance -What has been accomplished and the way forward?

More than a half-century has passed since it was discovered that phytohormone cytokinin (CK) is essential to drive cytokinesis and proliferation in plant tissue culture. Thereafter, cytokinin has emerged as the primary regulator of the plant cell cycle and numerous developmental processes. Lately, a growing body of evidence suggests that cytokinin has a role in mitigating both abiotic and biotic stress. Cytokinin is essential to defend plants against excessive light exposure and a unique kind of abiotic stress generated by an altered photoperiod. Secondly, cytokinin also exhibits multi-stress resilience under changing environments. Furthermore, cytokinin homeostasis is also affected by several forms of stress. Therefore, the diverse roles of cytokinin in reaction to stress, as well as its interactions with other hormones, are discussed in detail. When it comes to agriculture, understanding the functioning processes of cytokinins under changing environmental conditions can assist in utilizing the phytohormone, to increase productivity. Through this review, we briefly describe the biological role of cytokinin in enhancing the performance of plants growth under abiotic challenges as well as the probable mechanisms underpinning cytokinin-induced stress tolerance. In addition, the article lays forth a strategy for using biotechnological tools to modify genes in the cytokinin pathway to engineer abiotic stress tolerance in plants. The information presented here will assist in better understanding the function of cytokinin in plants and their effective investigation in the cropping system.

Improving bread wheat yield through modulating an unselected AP2/ERF gene

Crop breeding heavily relies on natural genetic variation. However, additional new variations are desired to meet the increasing human demand. Inflorescence architecture determines grain number per spike, a major determinant of bread wheat (Triticum aestivum L.) yield. Here, using Brachypodium distachyon as a wheat proxy, we identified DUO-B1, encoding an APETALA2/ethylene response factor (AP2/ERF) transcription factor, regulating spike inflorescence architecture in bread wheat. Mutations of DUO-B1 lead to mild supernumerary spikelets, increased grain number per spike and, importantly, increased yield under field conditions without affecting other major agronomic traits. DUO-B1 suppresses cell division and promotes the expression of BH^t/WFZP, whose mutations could lead to branched 'miracle-wheat'. Pan-genome analysis indicated that DUO-B1 has not been utilized in breeding, and holds promise to increase wheat yield further.

Delayed Leaf Senescence by Upregulation of Cytokinin Biosynthesis Specifically in Tomato Roots

Cytokinins (CKs) regulate numerous plant developmental processes, including photosynthesis and leaf senescence. Isopentenyltransferase (IPT) is a rate-limiting enzyme in the CK-biosynthesis pathway. We overexpressed ipt under tissue-specific promoters to study the long-range effect of CK on the functioning of tomato source leaves. Photosynthetic activity over time provided the measure for leaf aging. Significantly delayed leaf senescence was observed in plants expressing ipt under a root-specific promoter, but not in those expressing the gene under a source leaf-specific promoter. The root-derived influence on leaf aging was further confirmed by grafting experiments. CK concentration in source leaves of both transgenic lines increased significantly, with different proportions of its various derivatives. On the other hand, root CK concentration was only slightly elevated. Nevertheless, the significant change in the proportion of CK derivatives in the root indicated that CK biosynthesis and metabolism were altered. Partial leaf defoliation upregulates photosynthetic rate in the remaining leaf; however, overexpression of ipt in either tissues eliminated this response. Interestingly, stem girdling also eliminated the photosynthetic response. Taken together, our findings suggest that leaf senescence is regulated by a CK-mediated root-shoot communication network. We propose that CK-mediated signal is translocated to the leaf via the xylem where it alters CK biosynthesis, resulting in delayed senescence.

Genes determining panicle morphology and grain quality in rice (Oryza sativa)

The world's increase in rice (Oryza sativa L.) production is not keeping up with the increase in its population. To boost the introduction of new high-yielding cultivars, knowledge is being gained on the genes and quantitative trait loci (QTLs) determining the panicle phenotype. The important are those determining yield of the crop, such as grain numbers per panicle and size and weight of the grains. Biochemical and molecular functions of many of them are understood in some details. Among these, OsCKX2 and OsSPL14 have been shown to increase panicle branching and grain numbers when overexpressed. Furthermore, miRNAs appear to play an important role in determining the panicle morphology by regulating the expressions of the genes like OsSPL14 and GRF4 involved in panicle branching and grain numbers and length. Mutations also greatly influence the grain shape and size. However, the information gained so far on the genetic regulation of grain filling and panicle morphology has not been successfully put into commercial application. Furthermore, the identification of the gene(s)/QTLs regulating panicle compactness is still lacking, which may enable the researchers to convert a compact-panicle cultivar into a lax/open one, and thereby increasing the chances of enhancing the yield of a desired compact-panicle cultivar obtained by the breeding effort.

Genetic and transcriptomic dissection of an artificially induced paired spikelets mutant of wheat (Triticum aestivum L.)

Morphological, genetic and transcriptomic characterizations of an EMS-induced wheat paired spikelets (PS) mutant were performed. A novel qualitative locus WPS1 on chromosome 1D was identified. Grain yield of wheat is significantly associated with inflorescence or spike architecture. However, few genes related to wheat spike development have been identified and their underlying mechanisms are largely unknown. In this study, we characterized an ethyl methanesulfonate (EMS)-induced wheat mutant, wheat paired spikelets 1 (wps1). Unlike a single spikelet that usually develops at each node of rachis, a secondary spikelet appeared below the primary spikelet at most of the rachis nodes of wps1. The microscope observation showed that the secondary spikelet initiated later than the primary spikelet. Genetic analysis suggested that the PS of wps1 is controlled by a single dominant nuclear gene, designated WHEAT PAIRED SPIKELETS 1 (WPS1). Further RNA-seq based bulked segregant analysis and molecular marker mapping localized WPS1 in an interval of 208.18-220.92 Mb on the chromosome arm 1DL, which is different to known genes related to spike development in wheat. By using wheat omics data, TraesCS1D02G155200 encoding a HD-ZIP III transcription factor was considered as a strong candidate gene for WPS1. Transcriptomic analysis indicated that PS formation in wps1 is associated with auxin-related pathways and may be regulated by networks involving TB1, Ppd1, FT1, VRN1, etc. This study laid the solid foundation for further validation of the causal gene of WPS1 and explored its regulatory mechanism in PS formation and inflorescence development, which may benefit to kernel yield improvement of wheat based on optimization or design of spike architecture in the future.

Review: Isoprenoid and aromatic cytokinins in shoot branching

Shoot branching is an important event of plant development that defines growth and reproduction. The BRANCHED1 gene (BRC1/TB1/FC1) is crucial for this process. Within the phytohormones, cytokinins directly activate axillary buds to promote shoot branching. In addition, strigolactones and auxins inhibit bud outgrowth. This review addresses the involvement of aromatic and isoprenoid cytokinins in shoot branching. And how auxins and strigolactones contribute to regulating this process also. The results obtained by others and our working group with lemongrass (Cymbopogon citratus) show that cytokinins affect both shoot and root apical meristem development, consistent with other plant species. However, many questions remain about how cytokinins and strigolactones antagonistically regulate BRC1 gene expression. Additionally, many details of the interaction among cytokinins, auxins, and strigolactones need to be clarified. We will gain a more comprehensive scheme of bud outgrowth with these details.

Plant-specific small peptide AtZSP1 interacts with ROCK1 to regulate organ size in Arabidopsis

Organ size is an important agronomic trait. Small peptides function in various stages of plant growth, but their regulatory mechanisms in organ growth remain poorly understood. Here, we characterize a novel small peptide, AtZSP1, which positively regulates organ size in Arabidopsis. Loss-of-function mutant atzsp1-1 exhibited small organs, whereas AtZSP1 overexpression plants (p35S:AtZSP1#1) produced larger organs. Differentially expressed genes in the shoots of atzsp1-1 and p35S:AtZSP1#1 were enriched in the cytokinin pathway. Further analysis on shoots of atzsp1-1 showed that endogenous cytokinin levels were significantly reduced, consistent with reduced expression of the cytokinin response genes ARR5/6/7 and a decrease in pARR5:GUS activity. By contrast, cytokinin levels were elevated in p35S:AtZSP1#1. These results indicate that AtZSP1 affects shoot size via changes in cytokinin levels. AtZSP1 is ubiquitously expressed and encodes a 57-amino acid endomembrane-associated protein that is highly conserved among plant species. AtZSP1 interacts with ROCK1 at the endomembrane. Genetic analysis confirmed that the small organs and low cytokinin levels in atzsp1-1 shoots are partially suppressed by the rock1-4 mutation, suggesting that AtZSP1 may function in a common pathway with ROCK1 to antagonistically regulate organ growth. Our study identified an unknown small peptide, AtZSP1, and defined its function in regulating organ size in Arabidopsis.

Transcriptomic and Metabolic Profiling of Kenaf Stems under Salinity Stress

Kenaf (Hibiscus cannabinus L.) is an indispensable fiber crop that faces increasing salinity stress. In previous studies regarding the molecular mechanisms of how kenaf may respond to salt stress, no metabolic evidences have been reported. Meanwhile, studies regarding kenaf stems under adverse growth conditions have not been conducted. In the present study, multiple-layer evidences including physiological, transcriptomic, and metabolic data regarding how kenaf stems were affected by the salt stress are provided, wherein the stem growth, especially the lignification process, is retarded. Meanwhile, the transcriptomic data indicated genes involved in the photosynthesis are significantly repressed while the multiple flavonoid metabolism genes are enriched. As to the metabolic data, the content variation for the growth-promotion phytohormones such as IAA and the stress-responding ones including ABA are within or without expectations, implying these phytohormones played complicated roles when the kenaf stems encounter salt stress. However, the metabolite variations did not always agree with the expression levels of corresponding key pathway genes, possibly because the metabolite could be biosynthesized or catabolized in multiple pathways. Collectively, our data may enlighten, more specifically, downstream studies on kenaf responses against salinity and other adverse conditions.

Cytokinin Confers Brown Planthopper Resistance by Elevating Jasmonic Acid Pathway in Rice

Plants have evolved a sophisticated defense system that employs various hormone pathways to defend against attacks by insect pests. Cytokinin (CK) plays an important role in plant growth and stress tolerance, but the role of CKs in plant-insect interaction remains largely unclear. Here, we report that CKs act as a positive regulator in rice resistance against brown planthopper (BPH), a devastating insect pest of rice. We found that BPH feeding promotes CK biosynthesis and signaling in rice. Exogenous application of CKs significantly increased the rice resistance to BPH. Increasing endogenous CKs by knocking out cytokinin oxidase/dehydrogenase (OsCKXs) led to enhanced resistance to BPH. Moreover, the levels of the plant hormone jasmonic acid (JA) and the expression of JA-responsive genes were elevated by CK treatment and in OsCKXs knockout plants. Furthermore, JA-deficient mutant og1 was more susceptible to BPH, and CK-induced BPH resistance was suppressed in og1. These results indicate that CK-mediated BPH resistance is JA-dependent. Our findings provide the direct evidence for the novel role of CK in promoting insect resistance, and demonstrate that CK-induced insect resistance is JA-dependent. These results provide important guidance for effective pest management strategies in the future.

Response Mechanism of Endogenous Hormones of Potential Storage Root to Phosphorus and Its Relationship With Yield and Appearance Quality of Sweetpotato

Field and pot experiments were conducted to explore the response mechanism of endogenous hormones of potential storage root to phosphorus and its relationship with yield and appearance quality of sweetpotato using five different rates of phosphorus addition. Application of adequate amounts of phosphorus (P₂ treatment, 112 kg of P₂O₅ ha^-1 in field experiment or 0.04 g of P₂O₅ kg^-1 in pot experiment) improved the yield and the appearance quality of sweetpotato when compared to the control treatment. This observation can be attributed to the fact that P₂ treatment significantly increased the expression of Ibkn1 and APRT genes and the concentration of ZR from 20 to 40 days after planting, but the results were the opposite at 10 days after planting. In addition, an increase in the expression of SRD1, NIT4, IbMADS1, and OPR3 and the concentrations of IAA and JA from day 10 to day 40 after planting were observed. Furthermore, the expression of GA3oX4 and the concentration of GA₃ decreased significantly from 20 to 30 days of planting and significantly increased after 40 days of planting. Moreover, a significant decrease in the expression of AAO and concentration of ABA was observed from 10 to 30 days after planting, and a significant increase was observed after 40 days of planting. The results show that P₂ treatment promoted root development, particularly significantly increased the number of roots and potential storage roots. P₂ treatment significantly increased the diameter, weight, and number of storage roots at 40 days after planting. Finally, proper phosphorus application (112 kg of P₂O₅ ha^-1) increased the yield (enhanced from 18.99 to 25.93%) by increasing the number of storage roots per plant and improving the appearance quality by increasing the length/diameter ratio and uniformity of storage root weight.

Research Progress in the Field of Microbial Mitigation of Drought Stress in Plants

Plants defend themselves against ecological stresses including drought. Therefore, they adopt various strategies to cope with stress, such as seepage and drought tolerance mechanisms, which allow plant development under drought conditions. There is evidence that microbes play a role in plant drought tolerance. In this study, we presented a review of the literature describing the initiation of drought tolerance mediated by plant inoculation with fungi, bacteria, viruses, and several bacterial elements, as well as the plant transduction pathways identified via archetypal functional or morphological annotations and contemporary "omics" technologies. Overall, microbial associations play a potential role in mediating plant protection responses to drought, which is an important factor for agricultural manufacturing systems that are affected by fluctuating climate.

Increased branching independent of strigolactone in cytokinin oxidase 2-overexpressing tomato is mediated by reduced auxin transport

Tomato production is influenced by shoot branching, which is controlled by different hormones. Here we produced tomato plants overexpressing the cytokinin-deactivating gene CYTOKININ OXYDASE 2 (CKX2). CKX2-overexpressing (CKX2-OE) plants showed an excessive growth of axillary shoots, the opposite phenotype expected for plants with reduced cytokinin content, as evidenced by LC-MS analysis and ARR5-GUS staining. The TCP transcription factor SlBRC1b was downregulated in the axillary buds of CKX2-OE and its excessive branching was dependent on a functional version of the GRAS-family gene LATERAL SUPPRESSOR (LS). Grafting experiments indicated that increased branching in CKX2-OE plants is unlikely to be mediated by root-derived signals. Crossing CKX2-OE plants with transgenic antisense plants for the strigolactone biosynthesis gene CAROTENOID CLEAVAGE DIOXYGENASE (CCD7-AS) produced an additive phenotype, indicating independent effects of cytokinin and strigolactones on increased branching. On the other hand, CKX2-OE plants showed reduced polar auxin transport and their bud outgrowth was reduced when combined with auxin mutants. Accordingly, CKX2-OE basal buds did not respond to auxin applied in the decapitated apex. Our results suggest that tomato shoot branching depends on a fine-tuning of different hormonal balances and that perturbations in the auxin status could compensate for the reduced cytokinin levels in CKX2-OE plants.

Role of the type-B authentic response regulator gene family in fragrant rice under alkaline salt stress

Globally, rice is being consumed as a main staple food and faces different kinds of biotic and abiotic stresses such drought, salinity, and pest attacks. Through the cytokinin signaling, Type-B authentic response regulators (ARR-Bs) respond positively towards the environmental stimuli. ARR-Bs are involved in abiotic stress tolerance and plant development but their molecular mechanisms in fragrant rice are still not fully explored. The current study showed the genome-wide characterization of OsARR-B genes under alkaline salt stress. Results showed that in total, 24 OsARR-B genes were found and divided into four subgroups on the basis of a phylogenetic analysis. These genes were located on all rice chromosomes except 8 and 10. Analysis of gene duplications, gene structure, cis-elements, protein-protein interactions, and miRNA were performed. Gene ontology analysis showed that OsARR-B genes are involved in plant development through the regulation of molecular functions, biological processes, and cellular components. Furthermore, 117 and 192 RNA editing sites were detected in chloroplast and mitochondrial genes, respectively, encoding proteins of OsARR-B. In chloroplast and mitochondrial genes, six and nine types of amino acid changes, respectively, were caused by RNA editing, showing that RNA editing has a role in the alkaline salt stress tolerance in fragrant rice. We also used a comparative transcriptome approach to study the gene expression changes in alkaline tolerant and susceptible genotypes. Under alkaline salt stress, OsARR-B5, OsARR-B7, OsARR-B9, OsARR-B10, OsARR-B16, OsARR-B22, and OsARR-B23 showed higher transcript levels in alkaline salt tolerant genotypes as compared to susceptible ones. Quantitative RT-PCR showed upregulation of gene expression in the alkaline tolerant genotypes under alkaline stress. Our study explored the gene expression profiling and RESs of two rice contrasting genotypes, which will help to understand the molecular mechanisms of alkaline salt tolerance in fragrant rice.

Soaking Maize Seeds in Zeatin-Type Cytokinin Biostimulators Improves Salt Tolerance by Enhancing the Antioxidant System and Photosynthetic Efficiency

There is an urgent need for innovative strategies to raise the performance of environmentally stressed plants. The seeds of single-cross yellow Zea mays (L.) hybrid Giza-168 were soaked in Cis-(c-Z-Ck) or trans-zeatin-type cytokinin (t-Z-Ck) solutions at a concentration of 50 or 40 µM, respectively. Salinity stress was imposed at 0, 75 or 150 mM NaCl in the Hoagland nutrient solution (full strength) used for irrigation. The total carotenoids content was negatively affected by only 150 mM NaCl, while both 75 and 150 mM NaCl negatively affected the growth and yield components, relative water content, membrane stability index, photochemical activity, gas exchange, K⁺ and chlorophyll contents, K⁺/Na⁺ ratio, and photosynthetic efficiency. However, all of these traits were significantly improved by c-Z-Ck pretreatment and further enhanced by t-Z-Ck pretreatment compared with the corresponding controls. Furthermore, the contents of proline, soluble sugars, ascorbate, and glutathione, as well as enzymatic antioxidant activities, were significantly elevated by both salt stress concentrations and increased more by both biostimulators compared to the control. Compared to c-Z-Ck, t-Z-Ck was superior in mitigating the harmful effects of the high H₂O₂ levels caused by salt stress on the levels of malondialdehyde and ion leakage compared to the control. Under normal or stress conditions, t-Z-Ck pretreatment was better than c-Z-Ck pretreatment, while both positively affected maize hormonal contents. As a result, t-Z-Ck is recommended to enhance the growth and productivity of maize plants by suppressing the effects of oxidative stress caused by saline water irrigation.

Profiles of Cytokinins Metabolic Genes and Endogenous Cytokinins Dynamics during Shoot Multiplication In Vitro of Phalaenopsis

Shoot multiplication induced by exogenous cytokinins (CKs) has been commonly used in Phalaenopsis micropropagation for commercial production. Despite this, mechanisms of CKs action on shoot multiplication remain unclear in Phalaenopsis. In this study, we first identified key CKs metabolic genes, including six isopentenyltransferase (PaIPTs), six cytokinin riboside 5′ monophosphate phosphoribohydrolase (PaLOGs), and six cytokinin dehydrogenase (PaCKXs), from the Phalaenopsis genome. Then, we investigated expression profiles of these CKs metabolic genes and endogenous CKs dynamics in shoot proliferation by thidiazuron (TDZ) treatments (an artificial plant growth regulator with strong cytokinin-like activity). Our data showed that these CKs metabolic genes have organ-specific expression patterns. The shoot proliferation in vitro was effectively promoted with increased TDZ concentrations. Following TDZ treatments, the highly expressed CKs metabolic genes in micropropagated shoots were PaIPT1, PaLOG2, and PaCKX4. By 30 days of culture, TDZ treatments significantly induced CK-ribosides levels in micropropagated shoots, such as tZR and iPR (2000-fold and 200-fold, respectively) as compared to the controls, whereas cZR showed only a 10-fold increase. Overexpression of PaIPT1 and PaLOG2 by agroinfiltration assays resulted in increased CK-ribosides levels in tobacco leaves, while overexpression of PaCKX4 resulted in decreased CK-ribosides levels. These findings suggest de novo biosynthesis of CKs induced by TDZ, primarily in elevation of tZR and iPR levels. Our results provide a better understanding of CKs metabolism in Phalaenopsis micropropagation.

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.

Involvement of cytokinins in STOP1-mediated resistance to proton toxicity

STOP1 (sensitive to proton rhizotoxicity1) is a master transcription factor that governs the expression of a set of regulatory and structural genes involved in resistance to aluminum and low pH (i.e., proton) stresses in Arabidopsis. However, the mechanisms and regulatory networks underlying STOP1-mediated resistance to proton stresses are largely unclear. Here, we report that low-pH stresses severely inhibited root growth of the stop1 plants by suppressing root meristem activities. Interestingly, the stop1 plants were less sensitive to exogenous cytokinins at normal and low pHs than the wild type. Significantly, low concentrations of cytokinins promoted root growth of the stop1 mutant under low-pH stresses. Moreover, lateral and adventitious root formation was stimulated in stop1 and by low-pH stresses but suppressed by cytokinins. Further studies of the expression patterns of a cytokinin signaling reporter suggest that both the loss-of-function mutation of STOP1 and low-pH stresses suppressed cytokinin signaling outputs in the root. Furthermore, the expression of critical genes involved in cytokinin biosynthesis, biodegradation, and signaling is altered in the stop1 mutant in response to low-pH stresses. In conclusion, our results reveal a complex network of resistance to low-pH stresses, which involves coordinated actions of STOP1, cytokinins, and an additional low-pH-resistant mechanism for controlling root meristem activities and root growth upon proton stresses.