New insights into the biology of cytokinin degradation

Nitric oxide and cytokinin cross-talk and their role in plant hypoxia response

Nitric oxide (NO) and cytokinins (CKs) are known for their crucial contributions to plant development, growth, senescence, and stress response. Despite the importance of both signals in stress responses, their interaction remains largely unexplored. The interplay between NO and CKs emerges as particularly significant not only regarding plant growth and development but also in addressing plant stress response, particularly in the context of extreme weather events leading to yield loss. In this review, we summarize NO and CKs metabolism and signaling. Additionally, we emphasize the crosstalk between NO and CKs, underscoring its potential impact on stress response, with a focus on hypoxia tolerance. Finally, we address the most urgent questions that demand answers and offer recommendations for future research endeavors.

A predictive model for ethylene-mediated auxin and cytokinin patterning in the Arabidopsis root

The interaction between auxin and cytokinin is important in many aspects of plant development. Experimental measurements of both auxin and cytokinin concentration and reporter gene expression clearly show the coexistence of auxin and cytokinin concentration patterning in Arabidopsis root development. However, in the context of crosstalk among auxin, cytokinin, and ethylene, little is known about how auxin and cytokinin concentration patterns simultaneously emerge and how they regulate each other in the Arabidopsis root. This work utilizes a wide range of experimental observations to propose a mechanism for simultaneous patterning of auxin and cytokinin concentrations. In addition to revealing the regulatory relationships between auxin and cytokinin, this mechanism shows that ethylene signaling is an important factor in achieving simultaneous auxin and cytokinin patterning, while also predicting other experimental observations. Combining the mechanism with a realistic in silico root model reproduces experimental observations of both auxin and cytokinin patterning. Predictions made by the mechanism can be compared with a variety of experimental observations, including those obtained by our group and other independent experiments reported by other groups. Examples of these predictions include patterning of auxin biosynthesis rate, changes in PIN1 and PIN2 patterns in pin3,4,7 mutants, changes in cytokinin patterning in the pls mutant, PLS patterning, and various trends in different mutants. This research reveals a plausible mechanism for simultaneous patterning of auxin and cytokinin concentrations in Arabidopsis root development and suggests a key role for ethylene pattern integration.

Abscisic acid induces PpeKIL1 to terminate fruit growth and promote fruit abortion in peach (Prunus persica)

Abnormal pollination from chance events or hybridization between species leads to unusual embryo development, resulting in fruit abortion. To elucidate the mechanism underlying fruit abortion, we conducted a comprehensive analysis of the transcriptome and hormone profiles in aborting fruits (AF) derived from an interspecific cross between the peach cultivar 'Huangjinmi 3' and the Prunus mume cultivar 'Jiangmei', as well as in normal-seeded fruits (NF) resulting from an intraspecific cross of 'Huangjinmi 3' with the 'Manyuanhong' peach cultivars. Growth of AF was inhibited during the exponential growth phase, with up-regulation of oxidative stress related genes and down-regulation of DNA replication and cell cycle genes. Accumulation of the tissue growth-related hormones auxin and cytokinin was reduced in AF, while levels of the growth inhibiting hormone abscisic acid (ABA) were higher compared to NF. The increased ABA concentration aligned with down-regulation of the ABA catabolism gene CYP707A2, which encodes abscisic acid 8'-hydroxylase. Correlation analysis showed ABA could explain the maximum proportion of differently expressed genes between NF and AF. We also showed that expression of KIRA1-LIKE1 (PpeKIL1), a peach ortholog of the Arabidopsis KIRA1 gene, was up-regulated in AF. PpeKIL1 promotes senescence or delays normal growth in tobacco and Arabidopsis, and its promoter activity increases with exogenous ABA treatment. Our study demonstrates a candidate mechanism where ABA induces expression of PpeKIL1, which further blocks normal fruit growth and triggers fruit abscission.

Insights into dynamic coenocytic endosperm development: Unraveling molecular, cellular, and growth complexity

The endosperm, a product of double fertilization, is one of the keys to the evolution and success of angiosperms in conquering the land. While there are differences in endosperm development among flowering plants, the most common form is coenocytic growth, where the endosperm initially undergoes nuclear division without cytokinesis and eventually becomes cellularized. This complex process requires interplay among networks of transcription factors such as MADS-box, auxin response factors (ARFs), and phytohormones. The role of cytoskeletal elements in shaping the coenocytic endosperm and influencing seed growth also becomes evident. This review offers a recent understanding of the molecular and cellular dynamics in coenocytic endosperm development and their contributions to the final seed size.

miR156-PvSPL2 controls culm development by transcriptional repression of switchgrass CYTOKININ OXIDASE/DEHYDROGENASE4

Culm development in grasses can be controlled by both miR156 and cytokinin. However, the crosstalk between the miR156-SPL module and the cytokinin metabolic pathway remains largely unknown. Here, we found CYTOKININ OXIDASE/DEHYDROGENASE4 (PvCKX4) plays a negative regulatory role in culm development of the bioenergy grass Panicum virgatum (switchgrass). Overexpression of PvCKX4 in switchgrass reduced the internode diameter and length without affecting tiller number. Interestingly, we also found that PvCKX4 was always upregulated in miR156 overexpressing (miR156OE) transgenic switchgrass lines. Additionally, upregulation of either miR156 or PvCKX4 in switchgrass reduced the content of isopentenyl adenine (iP) without affecting trans-zeatin (tZ) accumulation. It is consistent with the evidence that the recombinant PvCKX4 protein exhibited much higher catalytic activity against iP than tZ in vitro. Furthermore, our results showed that miR156-targeted SPL2 bound directly to the promoter of PvCKX4 to repress its expression. Thus, alleviating the SPL2-mediated transcriptional repression of PvCKX4 through miR156 overexpression resulted in a significant increase in cytokinin degradation and impaired culm development in switchgrass. On the contrary, suppressing PvCKX4 in miR156OE transgenic plants restored iP content, internode diameter, and length to wild-type levels. Most strikingly, the double transgenic lines retained the same increased tiller numbers as the miR156OE transgenic line, which yielded more biomass than the wild type. These findings indicate that the miR156-SPL module can control culm development through transcriptional repression of PvCKX4 in switchgrass, which provides a promising target for precise design of shoot architecture to yield more biomass from grasses.

Cytokinin: From autoclaved DNA to two-component signaling

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

TOR coordinates cytokinin and gibberellin signals mediating development and defense

Plants constantly perceive and process environmental signals and balance between the energetic demands of growth and defense. Growth arrest upon pathogen attack was previously suggested to result from a redirection of the plants' metabolic resources towards the activation of plant defense. The energy sensor Target of Rapamycin (TOR) kinase is a conserved master coordinator of growth and development in all eukaryotes. Although TOR is positioned at the interface between development and defense, little is known about the mechanisms by which TOR may potentially regulate the relationship between these two modalities. The plant hormones cytokinin (CK) and gibberellin (GA) execute various aspects of plant development and defense. The ratio between CK and GA was reported to determine the outcome of developmental programmes. Here, investigating the interplay between TOR-mediated development and TOR-mediated defense in tomato, we found that TOR silencing resulted in rescue of several different aberrant developmental phenotypes, demonstrating that TOR is required for the execution of developmental cues. In parallel, TOR inhibition enhanced immunity in genotypes with a low CK/GA ratio but not in genotypes with a high CK/GA ratio. TOR-inhibition mediated disease resistance was found to depend on developmental status, and was abolished in strongly morphogenetic leaves, while being strongest in mature, differentiated leaves. CK repressed TOR activity, suggesting that CK-mediated immunity may rely on TOR downregulation. At the same time, TOR activity was promoted by GA, and TOR silencing reduced GA sensitivity, indicating that GA signalling requires normal TOR activity. Our results demonstrate that TOR likely acts in concert with CK and GA signalling, executing signalling cues in both defense and development. Thus, differential regulation of TOR or TOR-mediated processes could regulate the required outcome of development-defense prioritisation.

The cytokinin receptor OHK4/OsHK4 regulates inflorescence architecture in rice via an IDEAL PLANT ARCHITECTURE1/WEALTHY FARMER'S PANICLE-mediated positive feedback circuit

Inflorescence architecture is important for rice (Oryza sativa) grain yield. The phytohormone cytokinin (CK) has been shown to regulate rice inflorescence development; however, the underlying mechanism mediated by CK perception is still unclear. Employing a forward genetic approach, we isolated an inactive variant of the CK receptor OHK4/OsHK4 gene named panicle length1, which shows decreased panicle size due to reduced inflorescence meristem (IM) activity. A 2-amino acid deletion in the long α-helix stalk of the sensory module of OHK4 impairs the homodimerization and ligand-binding capacity of the receptor, even though the residues do not touch the ligand-binding domain or the dimerization interface. This deletion impairs CK signaling that occurs through the type-B response regulator OsRR21, which acts downstream of OHK4 in controlling inflorescence size. Meanwhile, we found that IDEAL PLANT ARCHITECTURE1(IPA1)/WEALTHY FARMER'S PANICLE (WFP), encoding a positive regulator of IM development, acts downstream of CK signaling and is directly activated by OsRR21. Additionally, we revealed that IPA1/WFP directly binds to the OHK4 promoter and upregulates its expression through interactions with 2 TCP transcription factors, forming a positive feedback circuit. Altogether, we identified the OHK4-OsRR21-IPA1 regulatory module, providing important insights into the role of CK signaling in regulating rice inflorescence architecture.

Transcriptomic Analyses Reveal the Role of Cytokinin and the Nodal Stem in Microtuber Sprouting in Potato (Solanum tuberosum L.)

In potatoes, tuber secondary growth, especially sprouting, deforms the tubers and severely lowers their commercial value. Tuber sprouting is induced by signal substances, such as gibberellin (GA), which are transported to the tuber from the plant body. The molecular mechanism underlying GA-induced sprouting remains ambiguous. Here, we tried to recreate tuber secondary growth using in vitro stemmed microtubers (MTs) (with the nodal stem attached) and MT halves (with the nodal stem entirely removed). Our experiments showed that GA alone could initiate the sprouting of stemmed microtubers; however, GA failed to initiate MT halves unless 6-benzyladenine, a synthetic cytokinin CK, was co-applied. Here, we analyzed the transcriptional profiles of sprouting buds using these in vitro MTs. RNA-seq analysis revealed a downregulation of cytokinin-activated signaling but an upregulation of the "Zeatin biosynthesis" pathway, as shown by increased expression of CYP735A, CISZOG, and UGT85A1 in sprouting buds; additionally, the upregulation of genes, such as IAA15, IAA22, and SAUR50, associated with auxin-activated signaling and one abscisic acid (ABA) negative regulator, PLY4, plays a vital role during sprouting growth. Our findings indicate that the role of the nodal stem is synonymous with CK in sprouting growth, suggesting that CK signaling and homeostasis are critical to supporting GA-induced sprouting. To effectively control tuber sprouting, more effort is required to be devoted to these critical genes.

SlTCP24 and SlTCP29 synergistically regulate compound leaf development through interacting with SlAS2 and activating transcription of SlCKX2 in tomato

The complexity of compound leaves results primarily from the leaflet initiation and arrangement during leaf development. However, the molecular mechanism underlying compound leaf development remains a central research question. SlTCP24 and SlTCP29, two plant-specific transcription factors with the conserved TCP motif, are shown here to synergistically regulate compound leaf development in tomato. When both of them were knocked out simultaneously, the number of leaflets significantly increased, and the shape of the leaves became more complex. SlTCP24 and SlTCP29 could form both homodimers and heterodimers, and such dimerization was impeded by the leaf polarity regulator SlAS2, which interacted with SlTCP24 and SlTCP29. SlTCP24 and SlTCP29 could bind to the TCP-binding cis-element of the SlCKX2 promoter and activate its transcription. Transgenic plants with SlTCP24 and SlTCP29 double-gene knockout had a lowered transcript level of SlCKX2 and an elevated level of cytokinin. This work led to the identification of two key regulators of tomato compound leaf development and their targeted genes involved in cytokinin metabolic pathway. A model of regulation of compound leaf development was proposed based on observations of this study.

The transcription factor BnaWRKY10 regulates cytokinin dehydrogenase BnaCKX2 to control cytokinin distribution and seed size in Brassica napus

Cytokinins (CKs) are phytohormones that promote cell division and differentiation. However, the regulation of CK distribution and homeostasis in Brassica napus is poorly understood. Here, the endogenous CKs were first quantified by LC-ESI-MS/MS in rapeseed tissues and visualized by TCSn::GUS reporter lines. Interestingly, the cytokinin oxidase/dehydrogenase BnaCKX2 homologs were mainly expressed in reproductive organs. Subsequently, the quadruple mutants of the four BnaCKX2 homologs were generated. Endogenous CKs were increased in the seeds of the BnaCKX2 quadruple mutants, resulting in a significantly reduced seed size. In contrast, overexpression of BnaA9.CKX2 resulted in larger seeds, probably by delaying endosperm cellularization. Furthermore, the transcription factor BnaC6.WRKY10b, but not BnaC6.WRKY10a, positively regulated BnaA9.CKX2 expression by binding directly to its promoter region. Overexpression of BnaC6.WRKY10b rather than BnaC6.WRKY10a resulted in lower concentration of CKs and larger seeds by activating BnaA9.CKX2 expression, indicating that the functional differentiation of BnaWRKY10 homologs might have occurred during B. napus evolution or domestication. Notably, the haploid types of BnaA9.CKX2 were associated with 1000-seed weight in the natural B. napus population. Overall, the study reveals the distribution of CKs in B. napus tissues, and shows that BnaWRKY10-mediated BnaCKX2 expression is essential for seed size regulation, providing promising targets for oil crop improvement.

Heavy Metal-Associated Isoprenylated Plant Proteins (HIPPs) at Plasmodesmata: Exploring the Link between Localization and Function

Heavy metal-associated isoprenylated plant proteins (HIPPs) are a metallochaperone-like protein family comprising a combination of structural features unique to vascular plants. HIPPs possess both one or two heavy metal-binding domains and an isoprenylation site, facilitating a posttranslational protein lipid modification. Recent work has characterized individual HIPPs across numerous different species and provided evidence for varied functionalities. Interestingly, a significant number of HIPPs have been identified in proteomes of plasmodesmata (PD)-nanochannels mediating symplastic connectivity within plant tissues that play pivotal roles in intercellular communication during plant development as well as responses to biotic and abiotic stress. As characterized functions of many HIPPs are linked to stress responses, plasmodesmal HIPP proteins are potentially interesting candidate components of signaling events at or for the regulation of PD. Here, we review what is known about PD-localized HIPP proteins specifically, and how the structure and function of HIPPs more generally could link to known properties and regulation of PD.

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.

The origin and early evolution of cytokinin signaling

Angiosperms, especially Arabidopsis and rice, have long been at the center of plant research. However, technological advances in sequencing have led to a dramatic increase in genome and transcriptome data availability across land plants and, more recently, among green algae. These data allowed for an in-depth study of the evolution of different protein families - including those involved in the metabolism and signaling of phytohormones. While most early studies on phytohormone evolution were phylogenetic, those studies have started to be complemented by genetic and biochemical studies in recent years. Examples of such functional analyses focused on ethylene, jasmonic acid, abscisic acid, and auxin. These data have been summarized recently. In this review, we will focus on the progress in our understanding of cytokinin biology. We will use these data to synthesize key points about the evolution of cytokinin metabolism and signaling, which might apply to the evolution of other phytohormones as well.

Halostachys caspica pathogenesis-related protein 10 acts as a cytokinin reservoir to regulate plant growth and development

Pathogenesis-related class 10 (PR-10) proteins play a role in plant growth and development, but the underlying molecular mechanisms are unclear. Here, we isolated a salt-induced PR-10 gene from the halophyte Halostachys caspica and named it HcPR10. HcPR10 was constitutively expressed during development and HcPR10 localized to the nucleus and cytoplasm. HcPR10-mediated phenotypes including bolting, earlier flowering, increased branch number and siliques per plant are highly correlated with increased cytokinin levels in transgenic Arabidopsis. Meanwhile, increased levels of cytokinin in plants is temporally correlated with HcPR10 expression patterns. Although the expression of cytokinin biosynthesis genes validated was not upregulated, cytokinin-related genes including chloroplast-related genes, cytokinin metabolism and cytokinin responses genes and flowering-related genes were significantly upregulated in the transgenic Arabidopsis compared to the wild type by transcriptome deep sequencing. Analysis of the crystal structure of HcPR10 revealed a trans-zeatin riboside (a type of cytokinin) located deep in its cavity, with a conserved conformation and protein-ligand interactions, supporting HcRP10 acts as a cytokinin reservoir. Moreover, HcPR10 in Halostachys caspica predominantly accumulated in vascular tissue, the site of long-distance translocation of plant hormones. Collectively, we draw that HcPR10 as a cytokinin reservoir induces cytokinin-related signal transduction in plants, thereby promoting plant growth and development. These findings could provide intriguing insights into the role of HcPR10 proteins in phytohormone regulation in plants and advance our understanding of cytokinin-mediated plant development and could facilitate the breeding of transgenic crops with earlier mature, higher yielding agronomic traits.

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.

Plant Hormonomics: A Key Tool for Deep Physiological Phenotyping to Improve Crop Productivity

Agriculture is particularly vulnerable to climate change. To cope with the risks posed by climate-related stressors to agricultural production, global population growth, and changes in food preferences, it is imperative to develop new climate-smart crop varieties with increased yield and environmental resilience. Molecular genetics and genomic analyses have revealed that allelic variations in genes involved in phytohormone-mediated growth regulation have greatly improved productivity in major crops. Plant science has remarkably advanced our understanding of the molecular basis of various phytohormone-mediated events in plant life. These findings provide essential information for improving the productivity of crops growing in changing climates. In this review, we highlight the recent advances in plant hormonomics (multiple phytohormone profiling) and discuss its application to crop improvement. We present plant hormonomics as a key tool for deep physiological phenotyping, focusing on representative plant growth regulators associated with the improvement of crop productivity. Specifically, we review advanced methodologies in plant hormonomics, highlighting mass spectrometry- and nanosensor-based plant hormone profiling techniques. We also discuss the applications of plant hormonomics in crop improvement through breeding and agricultural management practices.

Rose WRKY13 promotes disease protection to Botrytis by enhancing cytokinin content and reducing abscisic acid signaling

The plant hormones cytokinin (CK) and abscisic acid (ABA) play critical and often opposite roles during plant growth, development, and responses to abiotic and biotic stresses. Rose (Rosa sp.) is an economically important ornamental crop sold as cut flowers. Rose petals are extremely susceptible to gray mold disease caused by the necrotrophic fungal pathogen Botrytis cinerea. The infection of rose petals by B. cinerea leads to tissue collapse and rot, causing severe economic losses. In this study, we showed that CK and ABA play opposite roles in the susceptibility of rose to B. cinerea. Treatment with CK enhanced the disease protection of rose petals to B. cinerea, while ABA promoted disease progression. We further demonstrated that rose flowers activate CK-mediated disease protection via a B. cinerea-induced rose transcriptional repressor, Rosa hybrida (Rh)WRKY13, which is an ortholog of Arabidopsis (Arabidopsis thaliana), AtWRKY40. RhWRKY13 binds to promoter regions of the CK degradation gene CKX3 (RhCKX3) and the ABA-response gene ABA insensitive4 (RhABI4), leading to simultaneous inhibition of their expression in rose petals. The increased CK content and reduced ABA responses result in enhanced protection from B. cinerea. Collectively, these data reveal opposite roles for CK and ABA in the susceptibility of rose petals against B. cinerea infection, which is mediated by B. cinerea-induced RhWRKY13 expression.

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.

Is auxin enough? Cytokinins and margin patterning in simple leaves

The interplay between auxin and cytokinins affects facets of plant development as different as ovule formation and lateral root initiation. Moreover, cytokinins favor complexity in the development of Solanum lycopersicum and Cardamine hirsuta compound leaves. Nevertheless, no role has been proposed for cytokinins in patterning the margins of the simple leaves of Arabidopsis thaliana, a process that is assumed to be sufficiently explained by auxin localization. Here, we discuss evidence supporting the hypothesis that cytokinins play a role in simple leaf margin morphogenesis via crosstalk with auxin, as occurs in other plant developmental events. Indeed, mutant or transgenic arabidopsis plants defective in cytokinin biosynthesis or signaling, or with increased cytokinin degradation have leaf margins less serrated than the wild type.

The B-type response regulator GmRR11d mediates systemic inhibition of symbiotic nodulation

Key to the success of legumes is the ability to form and maintain optimal symbiotic nodules that enable them to balance the trade-off between symbiosis and plant growth. Cytokinin is essential for homeostatic regulation of nodulation, but the mechanism remains incompletely understood. Here, we show that a B-type response regulator GmRR11d mediates systemic inhibition of nodulation. GmRR11d is induced by rhizobia and low level cytokinin, and GmRR11d can suppress the transcriptional activity of GmNSP1 on GmNIN1a to inhibit soybean nodulation. GmRR11d positively regulates cytokinin response and its binding on the GmNIN1a promoter is enhanced by cytokinin. Intriguingly, rhizobial induction of GmRR11d and its function are dependent upon GmNARK that is a CLV1-like receptor kinase and inhibits nodule number in shoots. Thus, GmRR11d governs a transcriptional program associated with nodulation attenuation and cytokinin response activation essential for systemic regulation of nodulation.

The S40 family members delay leaf senescence by promoting cytokinin synthesis

Leaf senescence is regulated by both endogenous hormones and environmental stimuli in a programmed and concerted way. The members of the S40 family have been reported to play roles in leaf senescence. Here we identified an S40 family member, CiS40-11, from Caragana intermedia. Phylogenetic analysis revealed that the CiS40-11 protein had the highest identity with AtS40-5 (AT1G11700) and AtS40-6 (AT1G61930) of Arabidopsis thaliana. CiS40-11 was highly expressed in leaves and was down-regulated after dark treatment. The subcellular localization analysis showed that CiS40-11 was a cytoplasm-nucleus dual-localized protein. Leaf senescence was delayed in both the CiS40-11 overexpressed A. thaliana and its transiently expressed C. intermedia. Transcriptomic analysis and endogenous hormones assay revealed that CiS40-11 inhibited leaf senescence via promoting the biosynthesis of cytokinins by blocking AtMYB2 expression in the CiS40-11 overexpression lines. Furthermore, overexpression of either AtS40-5 or AtS40-6 showed similar phenotype as the CiS40-11 overexpressing lines, while in the ats40-5a or ats40-6a mutants, the AtMYB2 expression was increased and their leaves exhibited a premature senescence phenotype. These results provide a new molecular mechanism of the S40 family in leaf senescence regulation of plants.

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.

Genome-wide identification and analysis of the cytokinin oxidase/dehydrogenase (ckx) gene family in finger millet (Eleusine coracana)

Cytokinin dehydrogenase/oxidase (CKX) enzymes play a key role in regulating cytokinin (CK) levels in plants by degrading the excess of this phytohormone. CKX genes have proven an attractive target for genetic engineering, as their silencing boosts cytokinin accumulation in various tissues, thereby contributing to a rapid increase in biomass and overall plant productivity. We previously reported a similar effect in finger millet (Eleusine coracana) somaclonal lines, caused by downregulation of EcCKX1 and EcCKX2. However, the CKX gene family has numerous representatives, especially in allopolyploid crop species, such as E. coracana. To date, the entire CKX gene family of E. coracana and its related species has not been characterized. We offer here, for the first time, a comprehensive genome-wide identification and analysis of a panel of CKX genes in finger millet. The functional genes identified in the E. coracana genome are compared with the previously-identified genes, EcCKX1 and EcCKX2. Exon-intron structural analysis and motif analysis of FAD- and CK-binding domains are performed. The phylogeny of the EcCKX genes suggests that CKX genes are divided into several distinct groups, corresponding to certain isotypes. Finally, the phenotypic effect of EcCKX1 and EcCKX2 in partially silencing the SE7 somaclonal line is investigated, showing that lines deficient in CKX-expression demonstrate increased grain yield and greater bushiness, enhanced biomass accumulation, and a shorter vegetation cycle.

Abscisic acid: Metabolism, transport, crosstalk with other plant growth regulators, and its role in heavy metal stress mitigation

Heavy metal (HM) stress is threatening agricultural crops, ecological systems, and human health worldwide. HM toxicity adversely affects plant growth, physiological processes, and crop productivity by disturbing cellular ionic balance, metabolic balance, cell membrane integrity, and protein and enzyme activities. Plants under HM stress intrinsically develop mechanisms to counter the adversities of HM but not prevent them. However, the exogenous application of abscisic acid (ABA) is a strategy for boosting the tolerance capacity of plants against HM toxicity by improving osmolyte accumulation and antioxidant machinery. ABA is an essential plant growth regulator that modulates various plant growth and metabolic processes, including seed development and germination, vegetative growth, stomatal regulation, flowering, and leaf senescence under diverse environmental conditions. This review summarizes ABA biosynthesis, signaling, transport, and catabolism in plant tissues and the adverse effects of HM stress on crop plants. Moreover, we describe the role of ABA in mitigating HM stress and elucidating the interplay of ABA with other plant growth regulators.

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.

Effect of Different Cytokinins on Shoot Outgrowth and Bioactive Compounds Profile of Lemograss Essential Oil

Lemongrass (Cymbopogon citratus) essential oil (EO) is a major source of bioactive compounds (BC) with anticancer activity such as α-citral, limonene, geraniol, geranyl acetate, and β-caryophyllene. Comparative studies about cytokinin effects on BC profiles in lemongrass are missing. Here, we evaluated four cytokinins (2iP, tZ, BAP, and KIN) in two different osmotic media, MS-N (3% sucrose, 3 g L−1 Gelrite™) and MS-S (5% sucrose, 5 g L−1 Gelrite™). It results in a higher multiplication rate in BAP containing medium compared to tZ, KIN, and 2iP (p ≤ 0.05). While shoots grown on MS-N/BAP, tZ, and KIN exhibited a highly branching morphology, MS-N/2iP produced a less branching architecture. BC profile analysis of established plants in pots revealed that their maxima production depends on the in vitro shoot growth conditions: i.e., highest content (80%) of α-citral in plants that were cultured in MS-S/BAP (p ≤ 0.05), limonene (41%) in MS-N/2iP, or geranyl acetate (25.79%) in MS-S/2iP. These results indicate that it is possible to increase or address the production of BC in lemongrass by manipulating the cytokinin type and osmotic pressure in culture media. The culture protocol described here is currently successfully applied for somatic embryogenesis induction and genetic transformation in lemongrass.

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.

Spatio-temporal transcriptome profiling and subgenome analysis in Brassica napus

Brassica napus is an important oil crop and an allotetraploid species. However, the detailed analysis of gene function and homoeologous gene expression in all tissues at different developmental stages was not explored. In this study, we performed a global transcriptome analysis of 24 vegetative and reproductive tissues at six developmental stages (totally 111 tissues). These samples were clustered into eight groups. The gene functions of silique pericarp were similar to roots, stems and leaves. In particular, glucosinolate metabolic process was associated with root and silique pericarp. Genes involved in protein phosphorylation were often associated with stamen, anther and the early developmental stage of seeds. Transcription factor (TF) genes were more specific than structural genes. A total of 17 100 genes that were preferentially expressed in one tissue (tissue-preferred genes, TPGs), including 889 TFs (5.2%), were identified in the 24 tissues. Some TPGs were identified as hub genes in the co-expression network analysis, and some TPGs in different tissues were involved in different hormone pathways. About 67.0% of the homoeologs showed balanced expression, whereas biased expression of homoeologs was associated with structural divergence. In addition, the spatiotemporal expression of homoeologs was related to the presence of transposable elements (TEs) and regulatory elements (REs); more TEs and fewer REs in the promoters resulted in divergent expression in different tissues. This study provides a valuable transcriptional map for understanding the growth and development of B. napus, for identifying important genes for future crop improvement, and for exploring gene expression patterns in the B. napus.

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.

Roles of auxin in the inhibition of shoot branching in 'Dugan' fir

Shoot branching substantially impacts vegetative and reproductive growth as well as wood characteristics in perennial woody species by shaping the shoot system architecture. Although plant hormones have been shown to play a fundamental role in shoot branching in annual species, their corresponding actions in perennial woody plants are largely unknown, in part due to the lack of branching mutants. Here, we demonstrated the role of plant hormones in bud dormancy transition toward activation and outgrowth in woody plants by comparing the physiological and molecular changes in the apical shoot stems of 'Yangkou' 020 fir and 'Dugan' fir, two Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) clones with normal and completely abolished branching phenotypes, respectively. Our studies showed that the defect in bud outgrowth was the cause of failed shoot branching in 'Dugan' fir whereas apically derived signals acted as triggers of this ectopic bud activity. Further studies indicated that auxin played a key role in inhibiting bud outgrowth in 'Dugan' fir. During bud dormancy release, the differential auxin resistant 1/Like AUX1 (AUX1/LAX) and PIN-formed (PIN) activity resulted in an ectopic auxin/indole-3-acetic acid (IAA) accumulation in the apical shoot stem of 'Dugan' fir, which could inhibit the cell cycle in the axillary meristem by decreasing cytokinin (CK) biosynthesis but increasing abscisic acid (ABA) production and response through the signaling pathway. In contrast, during bud activation and outgrowth, the striking increase in auxin biosynthesis and PIN activity in the shoot tip of 'Dugan' fir may trigger the correlative inhibition of axillary buds by modulating the polar auxin transport stream (PATS) and connective auxin transport (CAT) in shoots, and by influencing the biosynthesis of secondary messengers, including CK, gibberellin (GA) and ABA, thereby inducing the paradormancy of axillary buds in 'Dugan' fir by apical dominance under favorable conditions. The findings of this study provide important insights into the roles of plant hormones in bud outgrowth control in perennial woody plants.

Genes Impacting Grain Weight and Number in Wheat (Triticum aestivum L. ssp. aestivum)

The primary goal of common wheat (T. aestivum) breeding is increasing yield without negatively impacting the agronomic traits or product quality. Genetic approaches to improve the yield increasingly target genes that impact the grain weight and number. An energetic trade-off exists between the grain weight and grain number, the result of which is that most genes that increase the grain weight also decrease the grain number. QTL associated with grain weight and number have been identified throughout the hexaploid wheat genome, leading to the discovery of numerous genes that impact these traits. Genes that have been shown to impact these traits will be discussed in this review, including TaGNI, TaGW2, TaCKX6, TaGS5, TaDA1, WAPO1, and TaRht1. As more genes impacting the grain weight and number are characterized, the opportunity is increasingly available to improve common wheat agronomic yield by stacking the beneficial alleles. This review provides a synopsis of the genes that impact grain weight and number, and the most beneficial alleles of those genes with respect to increasing the yield in dryland and irrigated conditions. It also provides insight into some of the genetic mechanisms underpinning the trade-off between grain weight and number and their relationship to the source-to-sink pathway. These mechanisms include the plant size, the water soluble carbohydrate levels in plant tissue, the size and number of pericarp cells, the cytokinin and expansin levels in developing reproductive tissue, floral architecture and floral fertility.

Cytokinin and Its Key Role to Enrich the Plant Nutrients and Growth Under Adverse Conditions-An Update

Among the field crops, wheat is regarded as one of the most paramount cereal crops because it is widely grown, consumed as food across the world, and also known as the staple food for nearly 35 per cent of the world population. However, it is threatened by spot blotch disease causing considerable yield loss, with existing genotypes lacking the resistance and the necessary nutrients. Cytokinins (CKs) are key phytohormones that not only regulate the plant growth/development but also play an important role during stress and in the nutrient metabolic pathway of crop plants. Deficiency of important nutrients like zinc, iron, and vitamin A causes irreparable damage to the body, pressing the need to increase the accumulation of such micronutrients in the edible parts of the plant. Crop bio-fortification is one of the emerging approaches through which the quantities of these nutrients could be increased to an advisable amount. Cytokinin is observed to have a pivotal role in managing environmental stress/climate change and defense systems of plants, and apart from this, it is also found that it has an impact over Zn accumulation in cereal crops. Manipulation of the cytokine dehydrogenase (CKX) enzyme that degrades cytokinin could affect the yield, root growth, and important nutrients. Several instances revealed that an increment in the contents of Zn, S, Fe, and Mn in the seeds of cereals is a reflection of increasing the activity of CKX enzyme resulting the enhancement of the root system which not only helps in the absorption of water in a drought prone area but is also beneficial for scavenging nutrients to the deeper ends of the soil. Exploring micronutrients from the lithosphere via the root system helps in the uptake of the micronutrients and transporting them via the vascular system to the sink of crop plants, therefore, identification and incorporation of CKs/CKX linked gene(s) into targeted crop plants, exploring a bio-fortification approach including CRISPR-Cas9 through conventional and molecular breeding approaches could be the most paramount job for improving the important traits and stress management in order to enhance the plant growth, productivity, and nutritional value of the wheat crops, which would be useful for mankind.

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.

Chickpea Biofortification for Cytokinin Dehydrogenase via Genome Editing to Enhance Abiotic-Biotic Stress Tolerance and Food Security

Globally more than two billion people suffer from micronutrient malnutrition (also known as "hidden hunger"). Further, the pregnant women and children in developing nations are mainly affected by micronutrient deficiencies. One of the most important factors is food insecurity which can be mitigated by improving the nutritional values through biofortification using selective breeding and genetic enhancement techniques. Chickpea is the second most important legume with numerous economic and nutraceutical properties. Therefore, chickpea production needs to be increased from the current level. However, various kind of biotic and abiotic stresses hamper global chickpea production. The emerging popular targets for biofortification in agronomic crops include targeting cytokinin dehydrogenase (CKX). The CKXs play essential roles in both physiological and developmental processes and directly impact several agronomic parameters i.e., growth, development, and yield. Manipulation of CKX genes using genome editing tools in several crop plants reveal that CKXs are involved in regulation yield, shoot and root growth, and minerals nutrition. Therefore, CKXs have become popular targets for yield improvement, their overexpression and mutants can be directly correlated with the increased yield and tolerance to various stresses. Here, we provide detailed information on the different roles of CKX genes in chickpea. In the end, we discuss the utilization of genome editing tool clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9 (CRISPR/Cas9) to engineer CKX genes that can facilitate trait improvement. Overall, recent advancements in CKX and their role in plant growth, stresses and nutrient accumulation are highlighted, which could be used for chickpea improvement.

ADA2b and GCN5 Affect Cytokinin Signaling by Modulating Histone Acetylation and Gene Expression during Root Growth of Arabidopsis thaliana

In Arabidopsis thaliana, the histone acetyltransferase GCN5 and the associated coactivator ADA2b regulate root growth and affect gene expression. The cytokinin signaling reporter TCS::GFP was introduced into gcn5-1, ada2b-1, and ada2a-2, as well as the ada2a-2ada2b-1 mutants. The early root growth (4 to 7 days post-germination) was analyzed using cellular and molecular approaches. TCS signal accumulated from the fourth to seventh days of root growth in the wild-type columella cells. In contrast, ada2b-1 and gcn5-1 and ada2a-2ada2b-1 double mutants displayed reduced TCS expression relative to wild type. Gene expression analysis showed that genes associated with cytokinin homeostasis were downregulated in the roots of gcn5-1 and ada2b-1 mutants compared to wild-type plants. H3K14 acetylation was affected in the promoters of cytokinin synthesis and catabolism genes during root growth of Arabidopsis. Therefore, GCN5 and ADA2b are positive regulators of cytokinin signaling during root growth by modulating histone acetylation and the expression of genes involved in cytokinin synthesis and catabolism. Auxin application in the roots of wild-type seedlings increased TCS::GFP expression. In contrast, ada2b and ada2ada2b mutant plants do not show the auxin-induced TCS signal, suggesting that GCN5 and ADA2b are required for the auxin-induced cytokinin signaling in early root growth.

Development of Whole Genome SNP-CAPS Markers and Preliminary QTL Mapping of Fruit Pedicel Traits in Watermelon

Fruit pedicel (FP) is an important determinant of premium fruit quality that directly affects commercial market value. However, in-depth molecular and genetic basis of pedicel-related traits has not been identified in watermelon. Herein, a quantitative trait locus (QTL) mapping strategy was used to identify the potential genetic regions controlling FP traits based on newly derived whole-genome single nucleotide polymorphism based cleaved amplified polymorphism sequence (SNP-CAPS) markers. Next-generation sequencing based whole-genome re-sequencing of two watermelon parent lines revealed 98.30 and 98.40% of average coverage, 4,989,869 SNP variants, and 182,949 CAPS loci pairs across the reference genome, respectively. A total of 221 sets of codominant markers exhibited 46.42% polymorphism rate and were effectively genotyped within 100-F_2:3 derived mapping population. The developed linkage map covered a total of 2,630.49 cM genetic length with averaged 11.90 cM, and depicted a valid marker-trait association. In total, 6 QTLs (qFPL4.1, qFPW4.1, qFPD2.1, qFPD2.2, qFPD8.1, qFPD10.1) were mapped with five major effects and one minor effect between the whole genome adjacent markers positioned over distinct chromosomes (02, 04, 08, 10), based on the ICIM-ADD mapping approach. These significant QTLs were similarly mapped in delimited flanking regions of 675.10, 751.38, 859.24, 948.39, and 947.51 kb, which collectively explained 8.64-13.60% PVE, respectively. A highly significant and positive correlation was found among the observed variables. To our knowledge, we first time reported the mapped QTLs/genes affecting FP traits of watermelon, and our illustrated outcomes will deliver the potential insights for fine genetic mapping as well as functional gene analysis through MAS-based breeding approaches.

Cytokinins: A Genetic Target for Increasing Yield Potential in the CRISPR Era

Over the last decade, remarkable progress has been made in our understanding the phytohormones, cytokinin's (CKs) biosynthesis, perception, and signalling pathways. Additionally, it became apparent that interfering with any of these steps has a significant effect on all stages of plant growth and development. As a result of their complex regulatory and cross-talk interactions with other hormones and signalling networks, they influence and control a wide range of biological activities, from cellular to organismal levels. In agriculture, CKs are extensively used for yield improvement and management because of their wide-ranging effects on plant growth, development and physiology. One of the primary targets in this regard is cytokinin oxidase/dehydrogenase (CKO/CKX), which is encoded by CKX gene, which catalyses the irreversible degradation of cytokinin. The previous studies on various agronomically important crops indicated that plant breeders have targeted CKX directly. In recent years, prokaryotic clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system has been increasingly used in editing the CKO/CKX gene and phenomenal results have been achieved. This review provides an updated information on the applications of CRISPR-based gene-editing tools in manipulating cytokinin metabolism at the genetic level for yield improvement. Furthermore, we summarized the current developments of RNP-mediated DNA/transgene-free genomic editing of plants which would broaden the application of this technology. The current review will advance our understanding of cytokinins and their role in sustainably increase crop production through CRISPR/Cas genome editing tool.

Transcriptome analysis of oil palm pistil during pollination and fertilization to unravel the role of phytohormone biosynthesis and signaling genes

Phytohormones play an important role in the pollination and fertilization of crops, but the regulatory mechanisms of oil palm pollination and fertilization are unclear. The purpose of this study is to explore the hormonal changes of oil palm pistils during flowering. We used RNA sequencing to evaluate differentially expressed genes (DEGs) in oil palm pistils at the pollination and non-pollination stages. In this study, we found that the hormone contents of oil palm pistil changed drastically after pollination. The transcriptome of the oil palm pistil without pollination and at 2 h, 4 h, 12 h, 24 h, and 48 h after pollination was comprehensively analyzed, and a large number of differential genes and metabolic pathways were explored. Based on the transcriptome data, it could be recognized that the changes of indoleacetic acid (IAA), zeatin riboside (ZR), and abscisic acid (ABA) during pollination were consistent with the changes in the corresponding gene transcripts. Differentially expressed genes during pollination and fertilization of oil palm were mainly related to energy metabolism and hormone signal transduction. It provides new insights to elucidate the interaction and regulation mechanisms of plant hormones before and after oil palm pollination, providing a theoretical basis and reference for the research on sexual reproduction of oil palm.

Cytokinins is involved in regulation of tomato pericarp thickness and fruit size

Although cytokinins (CKs) regulate fruit development, no direct genetic evidence supports the role of endogenous CKs in pericarp growth or development or fruit size. Here, we report that the reduction in endogenous active CKs level via overexpression of a CKs-inactivating enzyme gene AtCKX2 specifically in fruit tissues resulted in reduced pericarp thickness and smaller fruit size, compared to wild-type control fruits. The pericarp thickness and single fruit weight in transgenic plants were significantly reduced. Analysis of paraffin sections showed that the reduced pericarp thickness was due largely to a decreased number of cells, and thus decreased cell division. Transcriptome profiling showed that the expression of cell division- and expansion-related genes was reduced in AtCKX2-overexpressing fruits. In addition, the expression of auxin-signaling and gibberellin-biosynthetic genes was repressed, whereas that of gibberellin-inactivating genes was enhanced, in AtCKX2-overexpressing fruits. These results demonstrate that endogenous CKs regulate pericarp cell division and, subsequently, fruit size. They also suggest that CKs interact with auxin and gibberellins in regulating tomato pericarp thickness and fruit size.

A New RING Finger Protein, PLANT ARCHITECTURE and GRAIN NUMBER 1, Affects Plant Architecture and Grain Yield in Rice

Developing methods for increasing the biomass and improving the plant architecture is important for crop improvement. We herein describe a gene belonging to the RING_Ubox (RING (Really Interesting New Gene) finger domain and U-box domain) superfamily, PLANT ARCHITECTURE and GRAIN NUMBER 1 (PAGN1), which regulates the number of grains per panicle, the plant height, and the number of tillers. We used the CRISPR/Cas9 system to introduce loss-of-function mutations to OsPAGN1. Compared with the control plants, the resulting pagn1 mutant plants had a higher grain yield because of increases in the plant height and in the number of tillers and grains per panicle. Thus, OsPAGN1 may be useful for the genetic improvement of plant architecture and yield. An examination of evolutionary relationships revealed that OsPAGN1 is highly conserved in rice. We demonstrated that OsPAGN1 can interact directly with OsCNR10 (CELL NUMBER REGULATOR10), which negatively regulates the number of rice grains per panicle. A transcriptome analysis indicated that silencing OsPAGN1 affects the levels of active cytokinins in rice. Therefore, our findings have clarified the OsPAGN1 functions related to rice growth and grain development.

Identification and Functional Characterization of Apple MdCKX5.2 in Root Development and Abiotic Stress Tolerance

Cytokinin oxidase/dehydrogenases (CKXs) are the key enzymes in cytokinin degradation and have been widely studied in model plants. Little is known about apple’s (Malus×domestica) CKX genes. Here, using genome-wide analysis, we identified 10 MdCKX genes in apple. The phylogenetics, chromosome locations, and genome structures were then tested. Expression analysis showed that MdCKX genes had different expression profiles in apple, pointing to the different roles. Meanwhile, relative expression analysis showed that these genes have different expression patterns in response to several exogenous cytokinin factors, including trans-zeatin (ZT), thidiazuron (TDZ), and N6-furfuryladenine (KT). Finally, we introduced the MdCKX5.2 gene into Arabidopsis to evaluate its functions, and the results suggested the transgenic Arabidopsis displayed phenotypes related to promoting primary root and lateral root development, response to exogenous ZT, and conferring to drought and salt tolerant. Taken together, our results provide insights on the possible application of the MdCKX5.2 gene for molecular breeding in apples.

Effects of nitrogen topdressing and paclobutrazol at different stages on spike differentiation and yield of winter wheat

Background

Optimal nitrogen (N) application and plant growth regulators can improve wheat productivity. This can help to improve yield level and ensure food security with limited resources in the Huang-Huai-Hai Plain of China (HPC).

Methods

A 2-year field experiment was conducted using a randomized block design with four treatments (TS-N topdressing at pseudostem erection stage ; TPS-N topdressing combined with paclobutrazol application at pseudostem erection stage; TJ-N topdressing at jointing stage; TPJ-N topdressing at combined with paclobutrazol application at jointing stage) in 2011-2013.

Results

The grain number per ear, thousand kernel weight and yield for the TJ and TPJ treatments were higher than those of the TS and TPS treatments. Grain number per ear, yield, and thousands kernel weigh for the TPJ treatment were significantly higher than for the TS and TPS in 2011-2012 (9.82% and 7.27%, 10.23% and 8.99%, 6.12% and 5.58%) and in 2012-2013 (10.21% and 11.55%, 8.00% and 6.58%, 0.00 and 0.00), respectively. Thousands kernel weight under TJ were significantly higher than those under TS and TPS by 13.21% and 14.03%, respectively in 2012-2013. The floret number, significantly correlated with cytokinin content, was also significantly increased under TJ and TPJ at connectivum differentiation stage. For TPJ treatment, the floret number was significantly higher than for the TS, TPS, and TJ by 19.92%, 10.21%, 6.10% in 2011-2012; it was higher than for the TS and TPS by 28.06% and 29.61% in 2012-2013, respectively. The relative expression level of cytokinin oxidase/dehydrogenase gene (TaCKX2.2) was improved during flowering, when cytokinin content was at high level and was also inhibited by paclobutrazol with different degrees.

Conclusions

Therefore, nitrogen topdressing at jointing stage had increased grain number per ear, thousand kernel weight, and grain yield of wheat. Paclobutrazol could delay spike differentiation and promote cytokinin accumulation that induced expression of TaCKX2.2, maintaining hormonal balance and affecting wheat spike morphogenesis.

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.

Arabidopsis HIPP proteins regulate endoplasmic reticulum-associated degradation of CKX proteins and cytokinin responses

Eukaryotic organisms are equipped with quality-control mechanisms that survey protein folding in the endoplasmic reticulum (ER) and remove non-native proteins by ER-associated degradation (ERAD). Recent research has shown that cytokinin-degrading CKX proteins are subjected to ERAD during plant development. The mechanisms of plant ERAD, including the export of substrate proteins from the ER, are not fully understood, and the molecular components involved in the ERAD of CKX are unknown. Here, we show that heavy metal-associated isoprenylated plant proteins (HIPPs) interact specifically with CKX proteins synthesized in the ER and processed by ERAD. CKX-HIPP protein complexes were detected at the ER as well as in the cytosol, suggesting that the complexes involve retrotranslocated CKX protein species. Altered CKX levels in HIPP-overexpressing and higher-order hipp mutant plants suggest that the studied HIPPs control the ERAD of CKX. Deregulation of CKX proteins caused corresponding changes in the cytokinin signaling activity and triggered typical morphological cytokinin responses. Notably, transcriptional repression of HIPP genes by cytokinin indicates a feedback regulatory mechanism of cytokinin homeostasis and signaling responses. Moreover, loss of function of HIPP genes constitutively activates the unfolded protein response and compromises the ER stress tolerance. Collectively, these results suggests that HIPPs represent novel functional components of plant ERAD.

Regulation of rose petal dehydration tolerance and senescence by RhNAP transcription factor via the modulation of cytokinin catabolism

Petals and leaves share common evolutionary origins but have different phenotypic characteristics, such as the absence of stomata in the petals of most angiosperm species. Plant NAC transcription factor, NAP, is involved in ABA responses and regulates senescence-associated genes, and especially those that affect stomatal movement. However, the regulatory mechanisms and significance of NAP action in senescing astomatous petals is unclear. A major limiting factor is failure of flower opening and accelerated senescence. Our goal is to understand the finely regulatory mechanism of dehydration tolerance and aging in rose flowers. We functionally characterized RhNAP, an AtNAP-like transcription factor gene that is induced by dehydration and aging in astomatous rose petals. Cytokinins (CKs) are known to delay petal senescence and we found that a cytokinin oxidase/dehydrogenase gene 6 (RhCKX6) shares similar expression patterns with RhNAP. Silencing of RhNAP or RhCKX6 expression in rose petals by virus induced gene silencing markedly reduced petal dehydration tolerance and delayed petal senescence. Endogenous CK levels in RhNAP- or RhCKX6-silenced petals were significantly higher than those of the control. Moreover, RhCKX6 expression was reduced in RhNAP-silenced petals. This suggests that the expression of RhCKX6 is regulated by RhNAP. Yeast one-hybrid experiments and electrophoresis mobility shift assays showed that RhNAP binds to the RhCKX6 promoter in heterologous in vivo system and in vitro, respectively. Furthermore, the expression of putative signal transduction and downstream genes of ABA-signaling pathways were also reduced due to the repression of PP2C homolog genes by RhNAP in rose petals. Taken together, our study indicates that the RhNAP/RhCKX6 interaction represents a regulatory step enhancing dehydration tolerance in young rose petals and accelerating senescence in mature petals in a stomata-independent manner.

Cytokinin regulates vegetative phase change in Arabidopsis thaliana through the miR172/TOE1-TOE2 module

During vegetative growth plants pass from a juvenile to an adult phase causing changes in shoot morphology. This vegetative phase change is primarily regulated by the opposite actions of two microRNAs, the inhibitory miR156 and the promoting miR172 as well as their respective target genes, constituting the age pathway. Here we show that the phytohormone cytokinin promotes the juvenile-to-adult phase transition through regulating components of the age pathway. Reduction of cytokinin signalling substantially delayed the transition to the adult stage. tZ-type cytokinin was particularly important as compared to iP- and the inactive cZ-type cytokinin, and root-derived tZ influenced the phase transition significantly. Genetic and transcriptional analyses indicated the requirement of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors and miR172 for cytokinin activity. Two miR172 targets, TARGET OF EAT1 (TOE1) and TOE2 encoding transcriptional repressors were necessary and sufficient to mediate the influence of cytokinin on vegetative phase change. This cytokinin pathway regulating plant aging adds to the complexity of the regulatory network controlling the juvenile-to-adult phase transition and links cytokinin to miRNA action.

Auxin Interactions with Other Hormones in Plant Development

Auxin is a crucial growth regulator that governs plant development and responses to environmental perturbations. It functions at the heart of many developmental processes, from embryogenesis to organ senescence, and is key to plant interactions with the environment, including responses to biotic and abiotic stimuli. As remarkable as auxin is, it does not act alone, but rather solicits the help of, or is solicited by, other endogenous signals, including the plant hormones abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellic acid, jasmonates, salicylic acid, and strigolactones. The interactions between auxin and other hormones occur at multiple levels: hormones regulate one another's synthesis, transport, and/or response; hormone-specific transcriptional regulators for different pathways physically interact and/or converge on common target genes; etc. However, our understanding of this crosstalk is still fragmentary, with only a few pieces of the gigantic puzzle firmly established. In this review, we provide a glimpse into the complexity of hormone interactions that involve auxin, underscoring how patchy our current understanding is.

Morphological Characterization and Transcriptional Regulation of Corolla Closure in Ipomoea purpurea

Corolla closure protects pollen from high-temperature stress during pollen germination and fertilization in the ornamental plant morning glory (Ipomoea purpurea). However, the morphological nature of this process and the molecular events underpinning it remain largely unclear. Here, we examined the cellular and gene expression changes that occur during corolla closure in the I. purpurea. We divided the corolla closure process into eight stages (S0-S7) based on corolla morphology. During flower opening, bulliform cells appear papillate, with pigments in the adaxial epidermis of the corolla. These cells have distinct morphology from the smaller, flat cells in the abaxial epidermis in the corolla limb and intermediate of the corolla. During corolla closure, the bulliform cells of the adaxial epidermis severely collapse compared to cells on the abaxial side. Analysis of transparent tissue and cross sections revealed that acuminate veins in the corolla are composed of spiral vessels that begin to curve during corolla closure. When the acuminate veins were compromised, the corolla failed to close normally. We performed transcriptome analysis to obtain a time-course profile of gene expression during the process from the open corolla stage (S0) to semi-closure (S3). Genes that were upregulated from S0 to S1 were enriched in the polysaccharide degradation pathway, which positively regulates cell wall reorganization. Senescence-related transcription factor genes were expressed beginning at S1, leading to the activation of downstream autophagy-related genes at S2. Genes associated with peroxisomes and ubiquitin-mediated proteolysis were upregulated at S3 to enhance reactive oxygen species scavenging and protein degradation. Therefore, bulliform cells and acuminate veins play essential roles in corolla closure. Our findings provide a global understanding of the gene regulatory processes that occur during corolla closure in I. purpurea.