New insights into the biology of cytokinin degradation

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.

Meta-analysis of transcriptomic studies of cytokinin-treated rice roots defines a core set of cytokinin response genes

Cytokinins regulate diverse aspects of plant growth and development, primarily through modulation of gene expression. The cytokinin-responsive transcriptome has been thoroughly described in dicots, especially Arabidopsis, but much less so in monocots. Here, we present a meta-analysis of five different transcriptomic analyses of rice (Oryza sativa) roots treated with cytokinin, including three previously unpublished experiments. We developed a treatment method in which hormone is added to the media of rice seedlings grown in sterile hydroponic culture under a continuous airflow, which resulted in minimal perturbation of the seedlings, thus greatly reducing changes in gene expression in the absence of exogenous hormone. We defined a core set of 205 upregulated and 86 downregulated genes that were differentially expressed in at least three of the transcriptomic datasets. This core set includes genes encoding the type-A response regulators (RRs) and cytokinin oxidases/dehydrogenases, which have been shown to be primary cytokinin response genes. GO analysis revealed that the upregulated genes were enriched for terms related to cytokinin/hormone signaling and metabolism, while the downregulated genes were significantly enriched for genes encoding transporters. Variations of type-B RR binding motifs were significantly enriched in the promoters of the upregulated genes, as were binding sites for other potential partner transcription factors. The promoters of the downregulated genes were generally enriched for distinct cis-acting motifs and did not include the type-B RR binding motif. This analysis provides insight into the molecular mechanisms underlying cytokinin action in a monocot and provides a useful foundation for future studies of this hormone in rice and other cereals.

Auxin and Cytokinin Interplay during Leaf Morphogenesis and Phyllotaxy

Auxins (IAA) and cytokinins (CKs) are the most influential phytohormones, having multifaceted roles in plants. They are key regulators of plant growth and developmental processes. Additionally, their interplay exerts tight control on plant development and differentiation. Although several reviews have been published detailing the auxin-cytokinin interplay in controlling root growth and differentiation, their roles in the shoot, particularly in leaf morphogenesis are largely unexplored. Recent reports have provided new insights on the roles of these two hormones and their interplay on leaf growth and development. In this review, we focus on the effect of auxins, CKs, and their interactions in regulating leaf morphogenesis. Additionally, the regulatory effects of the auxins and CKs interplay on the phyllotaxy of plants are discussed.

Effects of Epichloë sinensis Endophyte and Host Ecotype on Physiology of Festuca sinensis under Different Soil Moisture Conditions

This study explored the effects of the Epichloë sinensis endophyte on growth, photosynthesis, ionic content (K+ and Ca2+), phytohormones (abscisic acid-ABA, cytokinin-CTK, indolE-3-acetic acid-IAA, and gibberellin-GA), and elements-C, N, P (in the shoot and root) in two ecotypes of Festuca sinensis (ecotypes 111 and 141) under different soil water conditions (35% and 65% relative saturation moisture content (RSMC)). The results showed that 35% RSMC inhibited the plants' growth, and compared with 65% RSMC, there was a significant (p < 0.05) decrease in the growth and photosynthesis indices, the contents of CTK and GA, Ca2+ concentration, and the contents of C, N, and P (in both the aboveground and underground parts) under 35% RSMC. E. sinensis had beneficial effects on host growth and stress tolerance. Under both 35% and 65% RSMC, the presence of E. sinensis significantly (p < 0.05) increased host plant height, tiller number, root length, root volume, shoot dry weight, chlorophyll content, and the rate of photosynthesis of both ecotypes. Furthermore, the shoot C, N, and P contents in plants infected with E. sinensis (E+) from the two ecotypes, under both conditions of RSMC, were significantly (p < 0.05) higher than those in corresponding plants that were not infected with E. sinensis (E-). Under 35% RSMC, the contents of ABA, K+, Ca2+, and root P contents in E+ plants were significantly (p < 0.05) higher than those in corresponding E- plants in both ecotypes. However, under 65% RSMC, root C, N, and P contents in E+ plants of ecotype 111 and 141 were significantly (p < 0.05) higher than those in corresponding E- plants. In addition, the host ecotype also had effects on host growth and stress tolerance; the growth and photosynthetic indices of ecotype 141 were significantly (p < 0.05) higher than those of ecotype 111 under 35% RSMC, which suggested that ecotype 141 is more competitive than ecotype 111 under water deficiency conditions. These findings suggest that the endophyte improved the host plant resistance to water deficiency by maintaining the growth of the plant, improving photosynthesis, accumulating K+ and Ca2+, promoting nutrient absorption, and adjusting the metabolism of plant hormones.

Cytokinin signaling localized in phloem noncell-autonomously regulates cambial activity during secondary growth of Populus stems

The regulation of cytokinin on secondary vascular development has been uncovered by modulating cytokinin content. However, it remains unclear how cytokinin enriched in developing secondary phloem regulates cambium activity in poplar. Here, we visualized the gradient distribution of cytokinin with a peak in the secondary phloem of poplar stem via immunohistochemical imaging, and determined the role of phloem-located cytokinin signaling during wood formation. We generated transgenic poplar harboring cytokinin oxidase/dehydrogenase (CKX)2, a gene encoding a cytokinin degrading enzyme, driven by the phloem-specific CLE41b promoter, indicating that the disruption of the cytokinin gradient pattern restricts the cambial activity. The RNA interference-based knockdown of the histidine kinase (HK) genes encoding cytokinin receptors specifically in secondary phloem significantly compromised the division activity of cambial cells, whereas the phloem-specific expression of a type-B response regulator (RR) transcription factor stimulated cambial proliferation, providing evidence for the noncell-autonomous regulation of local cytokinin signaling on the cambial activity. Moreover, the cambium-specific knockdown of HKs also led to restricted cambial activity, and the defects were aggravated by the reduced cytokinin accumulation. Our results showed that local cytokinin signaling in secondary phloem regulates cambial activity noncell-autonomously, and coordinately with its local signaling in cambium.

Putrescine Depletion Affects Arabidopsis Root Meristem Size by Modulating Auxin and Cytokinin Signaling and ROS Accumulation

Polyamines (PAs) dramatically affect root architecture and development, mainly by unknown mechanisms; however, accumulating evidence points to hormone signaling and reactive oxygen species (ROS) as candidate mechanisms. To test this hypothesis, PA levels were modified by progressively reducing ADC1/2 activity and Put levels, and then changes in root meristematic zone (MZ) size, ROS, and auxin and cytokinin (CK) signaling were investigated. Decreasing putrescine resulted in an interesting inverted-U-trend in primary root growth and a similar trend in MZ size, and differential changes in putrescine (Put), spermidine (Spd), and combined spermine (Spm) plus thermospermine (Tspm) levels. At low Put concentrations, ROS accumulation increased coincidently with decreasing MZ size, and treatment with ROS scavenger KI partially rescued this phenotype. Analysis of double AtrbohD/F loss-of-function mutants indicated that NADPH oxidases were not involved in H2O2 accumulation and that elevated ROS levels were due to changes in PA back-conversion, terminal catabolism, PA ROS scavenging, or another pathway. Decreasing Put resulted in a non-linear trend in auxin signaling, whereas CK signaling decreased, re-balancing auxin and CK signaling. Different levels of Put modulated the expression of PIN1 and PIN2 auxin transporters, indicating changes to auxin distribution. These data strongly suggest that PAs modulate MZ size through both hormone signaling and ROS accumulation in Arabidopsis.

Cytokinin oxidase/dehydrogenase OsCKX11 coordinates source and sink relationship in rice by simultaneous regulation of leaf senescence and grain number

The flag leaf and grain belong to the source and sink, respectively, of cereals, and both have a bearing on final yield. Premature leaf senescence significantly reduces the photosynthetic rate and severely lowers crop yield. Cytokinins play important roles in leaf senescence and determine grain number. Here, we characterized the roles of the rice (Oryza sativa L.) cytokinin oxidase/dehydrogenase OsCKX11 in delaying leaf senescence, increasing grain number, and coordinately regulating source and sink. OsCKX11 was predominantly expressed in the roots, leaves, and panicles and was strongly induced by abscisic acid and leaf senescence. Recombinant OsCKX11 protein catalysed the degradation of various types of cytokinins but showed preference for trans-zeatin and cis-zeatin. Cytokinin levels were significantly increased in the flag leaves of osckx11 mutant compared to those of the wild type (WT). In the osckx11 mutant, the ABA-biosynthesizing genes were down-regulated and the ABA-degrading genes were up-regulated, thereby reducing the ABA levels relative to the WT. Thus, OsCKX11 functions antagonistically between cytokinins and ABA in leaf senescence. Moreover, osckx11 presented with significantly increased branch, tiller, and grain number compared with the WT. Collectively, our findings reveal that OsCKX11 simultaneously regulates photosynthesis and grain number, which may provide new insights into leaf senescence and crop molecular breeding.

Dynamic cytokinin signalling landscapes during lateral root formation in Arabidopsis

By forming lateral roots, plants expand their root systems to improve anchorage and absorb more water and nutrients from the soil. Each phase of this developmental process in Arabidopsis is tightly regulated by dynamic and continuous signalling of the phytohormones cytokinin and auxin. While the roles of auxin in lateral root organogenesis and spatial accommodation by overlying cell layers have been well studied, insights on the importance of cytokinin is still somewhat limited. Cytokinin is a negative regulator of lateral root formation with versatile modes of action being activated at different root developmental zones. Here, we review the latest progress made towards our understanding of these spatially separated mechanisms of cytokinin-mediated signalling that shape lateral root initiation, outgrowth and emergence and highlight some of the enticing open questions.

Genome-wide identification and characterization of cytokinin oxidase/dehydrogenase family genes in Medicago truncatula

Cytokinin oxidase/dehydrogenases (CKXs) play a key role in the irreversible degradation of phytohormone cytokinin that is necessary for various plant growth and development processes. However, thus far, detailed investigations of the CKX gene family in the model legume Medicago truncatula are limited. In this study, we identified 9 putative CKX homologues with conserved FAD- and cytokinin-binding domains in the M. truncatula genome. We analyzed their phylogenetic relationship, gene structure, conserved domain, expression pattern, protein subcellular locations and other properties. The tissue-specific expression profiles of the MtCKX genes are different among different members and these MtCKXs also displayed different patterns in response to synthetic cytokinin 6-benzylaminopurine (6-BA) and indole-3-acetic acid (IAA), suggesting their diverse roles in M. truncatula development. To further understand the biological function of MtCKXs, we identified and characterized mutants of each MtCKX by taking advantage of the Tnt1 mutant population in M. truncatula. Results indicated that M. truncatula plants harboring Tnt1 insertions in each single MtCKX genes showed no morphological changes in aerial parts, suggesting functional redundancy of MtCKXs in M. truncatula shoot development. However, disruption of Medtr4g126160, which is predominantly expressed in roots, leads to an obvious reduced primary root length and increased lateral root number, indicating the specific roles of cytokinin in regulating root architecture. We systematically analyzed the MtCKX gene family at the genome-wide level and revealed their possible roles in M. truncatula shoot and root development, which shed lights on understanding the biological function of CKX family genes in related legume plants.

The Cytokinin Status of the Epidermis Regulates Aspects of Vegetative and Reproductive Development in Arabidopsis thaliana

The epidermal cell layer of plants has important functions in regulating plant growth and development. We have studied the impact of an altered epidermal cytokinin metabolism on Arabidopsis shoot development. Increased epidermal cytokinin synthesis or breakdown was achieved through expression of the cytokinin synthesis gene LOG4 and the cytokinin-degrading CKX1 gene, respectively, under the control of the epidermis-specific AtML1 promoter. During vegetative growth, increased epidermal cytokinin production caused an increased size of the shoot apical meristem and promoted earlier flowering. Leaves became larger and the shoots showed an earlier juvenile-to-adult transition. An increased cytokinin breakdown had the opposite effect on these phenotypic traits indicating that epidermal cytokinin metabolism can be a factor regulating these aspects of shoot development. The phenotypic consequences of abbreviated cytokinin signaling in the epidermis achieved through expression of the ARR1-SRDX repressor were generally milder or even absent indicating that the epidermal cytokinin acts, at least in part, cell non-autonomously. Enhanced epidermal cytokinin synthesis delayed cell differentiation during leaf development leading to an increased cell proliferation and leaf growth. Genetic analysis showed that this cytokinin activity was mediated mainly by the AHK3 receptor and the transcription factor ARR1. We also demonstrate that epidermal cytokinin promotes leaf growth in a largely cell-autonomous fashion. Increased cytokinin synthesis in the outer layer of reproductive tissues and in the placenta enhanced ovule formation by the placenta and caused the formation of larger siliques. This led to a higher number of seeds in larger pods resulting in an increased seed yield per plant. Collectively, the results provide evidence that the cytokinin metabolism in the epidermis is a relevant parameter determining vegetative and reproductive plant growth and development.

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

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

Different sets of TaCKX genes affect yield-related traits in wheat plants grown in a controlled environment and in field conditions

BACKGROUND

TaCKX wheat gene family members (GFMs) encode the enzyme cytokinin oxidase/dehydrogenase (CKX), which irreversibly degrades cytokinins. The genes are important regulators of cytokinin content and take part in growth and development, with a major impact on yield-related traits. The goal of this research was to test whether these genes might be differentially expressed in the field compared to laboratory conditions and consequently differently affect plant development and yield.

RESULTS

We compared expression and crosstalk of the TaCKX GFMs and TaNAC2-5A gene in modern varieties grown in a growth chamber (GC) and in the field and looked for differences in their impact on yield-related traits. The TaNAC2-5A gene was included in the research since it was expected to play an important role in co-regulation of these genes. The range of relative expression levels of TaCKX GFMs and TaNAC2-5A gene among tested cultivars was from 5 for TaCKX8 to more than 100 for TaCKX9 in the GC and from 6 for TaCKX8 to 275 for TaCKX10 in the field. The range was similar for four of them in the GC, but was much higher for seven others and TaNAC2-5A in the field. The TaCKX GFMs and TaNAC2-5A form co-expression groups, which differ depending on growth conditions. Consequently, the genes also differently regulate yield-related traits in the GC and in the field. TaNAC2-5A took part in negative regulation of tiller number and CKX activity in seedling roots only in controlled GC conditions. Grain number and grain yield were negatively regulated by TaCKX10 in the GC but positively by TaCKX8 and others in the field. Some of the genes, which were expressed in seedling roots, negatively influenced tiller number and positively regulated seedling root weight, CKX activity in the spikes, thousand grain weight (TGW) as well as formation of semi-empty spikes.

CONCLUSIONS

We have documented that: 1) natural variation in expression levels of tested genes in both environments is very high, indicating the possibility of selection of beneficial genotypes for breeding purposes, 2) to create a model of an ideotype for breeding, we need to take into consideration the natural environment.

Connections in the cambium, receptors in the ring

In plants, pluripotent cells in meristems divide to provide cells for the formation of postembryonic tissues. The cambium is the meristem from which the vascular tissue is derived and is the main driver for secondary (radial) growth in dicots. Xylem and phloem are specified on opposing sides of the cambium, and tightly regulated cell divisions ensure their spatial separation. Peptide ligands, phytohormones, and their receptors are central to maintaining this patterning and regulating proliferation. Here, we describe recent advances in our understanding of how these signals are integrated to control vascular development and secondary growth.

Research Progress on the Roles of Cytokinin in Plant Response to Stress

Cytokinins promote plant growth and development under normal plant growth conditions and also play an important role in plant resistance to stress. Understanding the working mechanisms of cytokinins under adverse conditions will help to make full use of cytokinins in agriculture to increase production and efficiency of land use. In this article, we review the progress that has been made in cytokinin research in plant response to stress and propose its future application prospects.

Response of cytokinins and nitrogen metabolism in the fronds of Pteris sp. under arsenic stress

Given the close relationship between cytokinins (CKs), photosynthesis and nitrogen metabolism, this study assessed the effect of arsenic (As) contamination on these metabolic components in the As-hyperaccumulators Pteris cretica L. var. Albo-lineata (Pc-A) and var. Parkerii (Pc-P) as well as the As-non-hyperaccumulator Pteris straminea Mett. ex Baker (Ps). The ferns were cultivated in a pot experiment for 23 weeks in soil spiked with As at the levels 20 and 100 mg·kg-1. For the purpose of this study, the CKs were placed into five functionally different groups according to their structure and physiological roles: bioactive forms (bCKs; CK free bases); inactive or weakly active forms (dCKs; CK N-glucosides); transport forms (tCKs; CK ribosides); storage forms (sCKs; O-glucosides); and primary products of CK biosynthesis (ppbCKs; CK nucleotides). An important finding was higher CKs total content, accumulation of sCKs and reduction of dCKs in As-hyperaccumulators in contrast to non-hyperaccumulator ferns. A significant depletion of C resources was confirmed in ferns, especially Ps, which was determined by measuring the photosynthetic rate and chlorophyll fluorescence. A fluorescence decrease signified a reduction in the C/N ratio, inducing an increase of bioactive CKs forms in Pc-P and Ps. The impact of As on N utilization was significant in As-hyperaccumulators. The glutamic acid/glutamine ratio, an indicator of primary N assimilation, diminished in all ferns with increased As level in the soil. In conclusion, the results indicate a large phenotypic diversity of Pteris species to As and suggest that the CKs composition and the glutamic acid/glutamine ratio can be used as a tool to diagnose As stress in plants.

Cytokinin dehydrogenase: a genetic target for yield improvement in wheat

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

Is naphthylphthalamic acid a specific phytotropin? It elevates ethylene and alters metabolic homeostasis in tomato

In higher plants, phytohormone indole-3-acetic acid is characteristically transported from the apex towards the base of the plant, termed as polar auxin transport (PAT). Among the inhibitors blocking PAT, N-1-naphthylphthalamic acid (NPA) that targets ABCB transporters is most commonly used. NPA-treated light-grown Arabidopsis seedlings show severe inhibition of hypocotyl and root elongation. In light-grown tomato seedlings, NPA inhibited root growth, but contrary to Arabidopsis stimulated hypocotyl elongation. The NPA-stimulation of hypocotyl elongation was milder in blue, red, and far-red light-grown seedlings. The NPA-treatment stimulated emission of ethylene from the seedlings. The scrubbing of ethylene by mercuric perchlorate reduced NPA-stimulated hypocotyl elongation. NPA action on hypocotyl elongation was antagonized by 1-methylcyclopropene, an inhibitor of ethylene action. NPA-treated seedlings had reduced levels of indole-3-butyric acid and higher levels of zeatin in the shoots. NPA did not alter indole-3-acetic levels in shoots. The analysis of metabolic networks indicated that NPA-treatment induced moderate shifts in the networks compared to exogenous ethylene that induced a drastic shift in metabolic networks. Our results indicate that in addition to ethylene, NPA-stimulated hypocotyl elongation in tomato may also involve zeatin and indole-3- butyric acid. Our results indicate that NPA-mediated physiological responses may vary in a species-specific fashion.

Hormonal and transcriptional analyses of fruit development and ripening in different varieties of black pepper (Piper nigrum)

Black pepper (Piper nigrum L.) is one of the most popular and oldest spices in the world with culinary uses and various pharmacological properties. In order to satisfy the growing worldwide demand for black pepper, improved productivity of pepper is highly desirable. A primary constraint in black pepper production is the non-synchronous nature of flower development and non-uniform fruit ripening within a spike. The uneven ripening of pepper berries results in a high labour requirement for selective harvesting contributes to low productivity and affects the quality of the pepper products. In Malaysia, there are a few recommended varieties for black pepper planting, each having some limitations in addition to the useful characteristics. Therefore, a comparative study of different black pepper varieties will provide a better understanding of the mechanisms regulates fruit development and ripening. Plant hormones are known to influence the fruit development process and their roles in black pepper flower and fruit development were inferred based on the probe-based gene expression analysis and the quantification of the multiple plant hormones using high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS). In this study, jasmonic acid and salicylic acid were found to play roles in flowering and fruit setting, whereas auxin, gibberellin and cytokinins are important for fruit growth. Abscisic acid has positive role in fruit maturation and ripening in the development process. Distinct pattern of plant hormones related gene expression profiles with the hormones accumulation profiles suggested a complex network of regulation is involved in the signaling process and crosstalk between plant hormones was another layer of regulation in the black pepper fruit development mechanisms. The current study provides clues to help in elucidating the timing of the action of each specific plant hormone during fruit development and ripening which could be applied to enhance our ability to control the ripening process, leading to improving procedures for the production and post-harvest handling of pepper fruits.

Cytokinin Signaling Downstream of the His-Asp Phosphorelay Network: Cytokinin-Regulated Genes and Their Functions

The plant hormone cytokinin, existing in several molecular forms, is perceived by membrane-localized histidine kinases. The signal is transduced to transcription factors of the type-B response regulator family localized in the nucleus by a multi-step histidine-aspartate phosphorelay network employing histidine phosphotransmitters as shuttle proteins across the nuclear envelope. The type-B response regulators activate a number of primary response genes, some of which trigger in turn further signaling events and the expression of secondary response genes. Most genes activated in both rounds of transcription were identified with high confidence using different transcriptomic toolkits and meta analyses of multiple individual published datasets. In this review, we attempt to summarize the existing knowledge about the primary and secondary cytokinin response genes in order to try connecting gene expression with the multitude of effects that cytokinin exerts within the plant body and throughout the lifespan of a plant.

The Mode of Cytokinin Functions Assisting Plant Adaptations to Osmotic Stresses

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

The Lonely Guy (LOG) Homologue SiRe_0427 from the Thermophilic Archaeon Sulfolobus islandicus REY15A Is a Phosphoribohydrolase Representing a Novel Group

Lonely Guy (LOG) proteins are important enzymes in cellular organisms, which catalyze the final step in the production of biologically active cytokinins via dephosphoribosylation. LOG proteins are vital enzymes in plants for the activation of cytokinin precursors, which is crucial for plant growth and development. In fungi and bacteria, LOGs are implicated in pathogenic or nonpathogenic interactions with their plant hosts. However, LOGs have also been identified in the human pathogen Mycobacterium tuberculosis, and the accumulation of cytokinin-degraded products, aldehydes, within bacterial cells is lethal to the bacterium in the presence of nitric oxide, suggesting diverse roles of LOGs in various species. In this study, we conducted biochemical and genetic analysis of a LOG homologue, SiRe_0427, from the hyperthermophilic archaeon Sulfolobus islandicus REY15A. The protein possessed the LOG motif GGGxGTxxE and exhibited phosphoribohydrolase activity on adenosine-5-monophosphate (AMP), similar to LOGs from eukaryotes and bacteria. Alanine mutants at either catalytic residues or substrate binding sites lost their activity, resembling other known LOGs. SiRe_0427 is probably a homotetramer, as revealed by size exclusion chromatography and chemical cross-linking. We found that the gene encoding SiRe_0427 could be knocked out; however, the Δsire_0427 strain exhibited no apparent difference in growth compared to the wild type, nor did it show any difference in sensitivity to UV irradiation under our laboratory growth conditions. Overall, these findings indicate that archaeal LOG homologue is active as a phosphoribohydrolase.IMPORTANCE Lonely Guy (LOG) is an essential enzyme for the final biosynthesis of cytokinins, which regulate almost every aspect of growth and development in plants. LOG protein was originally discovered 12 years ago in a strain of Oryza sativa with a distinct floral phenotype of a single stamen. Recently, the presence of LOG homologues has been reported in Mycobacterium tuberculosis, an obligate human pathogen. To date, active LOG proteins have been reported in plants, pathogenic and nonpathogenic fungi, and bacteria, but there have been no experimental reports of LOG protein from archaea. In the current work, we report the identification of a LOG homologue active on AMP from Sulfolobus islandicus REY15A, a thermophilic archaeon. The protein likely forms a tetramer in solution and represents a novel evolutionary lineage. The results presented here expand our knowledge regarding proteins with phosphoribohydrolase activities and open an avenue for studying signal transduction networks of archaea and potential applications of LOG enzymes in agriculture and industry.

Gibberellic acid inhibition of tillering in tall fescue involving crosstalks with cytokinins and transcriptional regulation of genes controlling axillary bud outgrowth

Tiller production in grass species is controlled by both axillary bud initiation and bud outgrowth, which may be regulated by plant hormones. However, how gibberellic acid (GA) affects tillering in perennial grass species is still unclear. This study aims to elucidate the roles and the underlying mechanisms of GA in regulating tiller development. Tall fescue seedlings were treated with different concentrations of GA3 by foliar application, dose-dependent inhibitory effects of GA on tiller production were observed. GA3 (25 μM) slowed down the transition from axillary buds to tillers by specifically inhibiting the outgrowth of axillary buds. GA-inhibition of tillering were not related to endogenous content for auxin or strigolactone, but was mainly due to the antagonistic interaction with cytokinins (CK), as shown by the decreased CK content and up-regulation expression of CK degradation genes in GA3-treated plants. Furthermore, GA could act through regulating the expression of FaTB1 specifically expressed in axillary buds to repress bud outgrowth. These results provide insights for the regulatory mechanisms of GA for tiller bud outgrowth through crosstalks with CK and signaling of FaTB1 expression.

Graphene oxide and ABA cotreatment regulates root growth of Brassica napus L. by regulating IAA/ABA

Previous studies have proven that graphene oxide (GO) regulates abscisic acid (ABA) and indole-3-acetic acid (IAA) contents and modulates plant root growth. To better understand the mechanism of plant growth and development regulated by GO and crosstalk between ABA and GO, Zhongshuang No. 9 seedlings were treated with GO and ABA. The results indicated that GO and ABA significantly affected the morphological properties and endogenous phytohormone contents in seedlings, and there was significant crosstalk between GO and ABA. ABA treatments combined with GO led to a rapid decrease in triphenyltetrazolium chloride (TTC) reduction intensity, and the inhibitory effect was enhanced with increasing ABA concentration. The treatments significantly affected the transcriptional levels of some key genes involved in the ABA, IAA, cytokinin (CTK), salicylic acid (SA), and ethane (ETH) pathways and increased the ABA and gibberellin (GA) contents in rapeseed seedlings. The effects of the treatments on the IAA and CTK contents were complex, but, importantly, the treatments suppressed root elongation. Correlation analysis also indicated that the relationship between root length and IAA/ABA could be described by a polynomial function: y = 88.11x² - 25.15x + 4.813(R² = 0.912). The treatments increased the ACS2 transcript abundance for ETH biosynthesis and the ICS1 transcriptional level of the key genes involved in salicylic acid (SA) biosynthesis, as well as the downstream signaling genes CBP60 and SARD1. This finding indicated that ABA is an important factor regulating the effects of GO on the growth and development of Brassica napus L., and that ETH and SA pathways may be potential pathways involved in the response of rape seedlings to GO treatment.

The Roles of Plant Hormones and Their Interactions with Regulatory Genes in Determining Meristem Activity

Plants, unlike animals, have developed a unique system in which they continue to form organs throughout their entire life cycle, even after embryonic development. This is possible because plants possess a small group of pluripotent stem cells in their meristems. The shoot apical meristem (SAM) plays a key role in forming all of the aerial structures of plants, including floral meristems (FMs). The FMs subsequently give rise to the floral organs containing reproductive structures. Studies in the past few decades have revealed the importance of transcription factors and secreted peptides in meristem activity using the model plant Arabidopsis thaliana. Recent advances in genomic, transcriptomic, imaging, and modeling technologies have allowed us to explore the interplay between transcription factors, secreted peptides, and plant hormones. Two different classes of plant hormones, cytokinins and auxins, and their interaction are particularly important for controlling SAM and FM development. This review focuses on the current issues surrounding the crosstalk between the hormonal and genetic regulatory network during meristem self-renewal and organogenesis.

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

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

A Genome-Wide Association Study of Highly Heritable Agronomic Traits in Durum Wheat

Uncovering the genetic basis of key agronomic traits, and particularly of drought tolerance, addresses an important priority for durum wheat improvement. Here, a genome-wide association study (GWAS) in 493 durum wheat accessions representing a worldwide collection was employed to address the genetic basis of 17 agronomically important traits and a drought wilting score. Using a linear mixed model with 4 inferred subpopulations and a kinship matrix, we identified 90 marker-trait-associations (MTAs) defined by 78 markers. These markers could be merged into 44 genomic loci by linkage disequilibrium (r 2 > 0.2). Based on sequence alignment of the markers to the reference genome of bread wheat, we identified 14 putative candidate genes involved in enzymes, hormone-response, and transcription factors. The GWAS in durum wheat and a previous quantitative trait locus (QTL) analysis in bread wheat identified a consensus QTL locus.4B.1 conferring drought tolerance, which was further scanned for the presence of potential candidate genes. A haplotype analysis of this region revealed that two minor haplotypes were associated with both drought tolerance and reduced plant stature, thought to be the effect of linkage with the semi-dwarfing gene Rht-B1. Haplotype variants in the key chromosome 4B region were informative regarding evolutionary divergence among durum, emmer and bread wheat. Over all, the data are relevant in the context of durum wheat improvement and the isolation of genes underlying variation in some important quantitative traits.

Specificity of expression of TaCKX family genes in developing plants of wheat and their co-operation within and among organs

Multigene families of CKX genes encode cytokinin oxidase/dehydrogenase proteins (CKX), which regulate cytokinin content in organs of developing plants. It has already been documented that some of them play important roles in plant productivity. The presented research is the first step of comprehensive characterization of the bread wheat TaCKX gene family with the goal to select genes determining yield-related traits. The specificity of expression patterns of fifteen formerly annotated members of the TaCKX family was tested in different organs during wheat development. Based on this, the genes were assigned to four groups: TaCKX10, TaCKX5 and TaCKX4, highly specific to leaves; TaCKX3, TaCKX6 and TaCKX11, expressed in various levels through all the organs tested; TaCKX1, TaCKX2.3, TaCKX2.2, TaCKX2.1, TaCKX2.4 and TaCKX2.5 specific to developing spikes and inflorescences; TaCKX9, TaCKX8 and TaCKX7, highly specific to roots. Amplification products of tested genes were mapped to the chromosomes of the A, B or D genome using T. aestivum Ensembl Plants. Based on analysis of TaCKX transcripts as well as encoded amino acids in T. aestivum and Hordeum vulgare the number of CKX genes in wheat was limited to 11 and new numbering of selected TaCKX genes was proposed. Moreover, we found that there were developmental differences in expression of TaCKX in the first and the second spike and expression of some of the genes was daily time dependent. A very high and significant correlation was found between expression levels of TaCKX7 and TaCKX9, genes specific to seedling roots, TaCKX1, TaCKX2.1 and TaCKX2.2, specific to developing spikes, and the group of TaCKX3, 4, 5, 6, 10 and 11, highly expressed in leaves and other organs. The genes also co-operated among organs and were included in two groups representing younger or maturating stages of developing plants. The first group was represented by seedling roots, leaves from 4-week old plants, inflorescences and 0 DAP spikes; the second by developing spikes, 0 DAP, 7 DAP and 14 DAP. The key genes which might determine yield-related traits are indicated and their possible roles in breeding strategies are discussed.

The Dynamics of Cytokinin Changes after Grafting of Vegetative Apices on Flowering Rapeseed Plants

Despite numerous studies, the role of hormones in the induction of shoot apical meristem leading to reproductive development, especially regarding thermoperiodic plants, is still not fully understood. The key problem is separating the effects of the low temperature required for vernalization from those responsible for low temperature stress. An earlier experiment demonstrated the correlation between an increase of cytokinin level in the apical parts of winter rapeseed and the transition time into their reproductive phase during vernalization, i.e., low temperature treatment. From data obtained from the presented experiments, this study aims to contribute to the understanding the role of cytokinins in the induction of flowering based on the grafting of vegetative apical parts of winter rapeseed (scion) on the reproductive (stock) winter and spring genotypes. On the basis of analyses carried out using ultra-high-performance liquid chromatography coupled with tandem mass spectrometry in combination with microscopic observation of changes at the apical meristem, it was indicated that the increase in the amount of trans-zeatin and trans- and cis-zeatin-O-glucoside derivatives appeared in the early stages of apex floral differentiation. During further development, the content of all investigated cytokinins passed through the maximum level followed by their decrease. The final level in reproductive apices was found to be higher than that in vegetative ones.

Root enhancement in cytokinin-deficient oilseed rape causes leaf mineral enrichment, increases the chlorophyll concentration under nutrient limitation and enhances the phytoremediation capacity

BACKGROUND

Cytokinin is a negative regulator of root growth, and a reduction of the cytokinin content or signalling causes the formation a larger root system in model plants, improves their growth under drought and nutrient limitation and causes increased accumulation of elements in the shoot. Roots are an important but understudied target of plant breeding. Here we have therefore explored whether root enhancement by lowering the cytokinin content can also be achieved in oilseed rape (Brassica napus L.) plants.

RESULTS

Transgenic plants overexpressing the CKX2 gene of Arabidopsis thaliana encoding a cytokinin-degrading cytokinin oxidase/dehydrogenase showed higher CKX activity and a strongly reduced cytokinin content. Cytokinin deficiency led to the formation of a larger root system under different growth conditions, which was mainly due to an increased number of lateral and adventitious roots. In contrast, shoot growth was comparable to wild type, which caused an enhanced root-to-shoot ratio. Transgenic plants accumulated in their leaves higher concentrations of macro- and microelements including P, Ca, Mg, S, Zn, Cu, Mo and Mn. They formed more chlorophyll under Mg- and S-deficiency and accumulated a larger amount of Cd and Zn from contaminated medium and soil.

CONCLUSIONS

These findings demonstrate the usefulness of ectopic CKX gene expression to achieve root enhancement in oilseed rape and underpin the functional relevance of a larger root system. Furthermore, the lack of major developmental consequences on shoot growth in cytokinin-deficient oilseed rape indicates species-specific differences of CKX gene and/or cytokinin action.

Structural and functional insights into the modulation of the activity of a flax cytokinin oxidase by flax rust effector AvrL567-A

During infection, plant pathogens secrete effector proteins to facilitate colonization. In comparison with our knowledge of bacterial effectors, the current understanding of how fungal effectors function is limited. In this study, we show that the effector AvrL567-A from the flax rust fungus Melampsora lini interacts with a flax cytosolic cytokinin oxidase, LuCKX1.1, using both yeast two-hybrid and in planta bimolecular fluorescence assays. Purified LuCKX1.1 protein shows catalytic activity against both N6-(Δ2-isopentenyl)-adenine (2iP) and trans-zeatin (tZ) substrates. Incubation of LuCKX1.1 with AvrL567-A results in increased catalytic activity against both substrates. The crystal structure of LuCKX1.1 and docking studies with AvrL567-A indicate that the AvrL567 binding site involves a flexible surface-exposed region that surrounds the cytokinin substrate access site, which may explain its effect in modulating LuCKX1.1 activity. Expression of AvrL567-A in transgenic flax plants gave rise to an epinastic leaf phenotype consistent with hormonal effects, although no difference in overall cytokinin levels was observed. We propose that, during infection, plant pathogens may differentially modify the levels of extracellular and intracellular cytokinins.

Natural variation in cytokinin maintenance improves salt tolerance in apple rootstocks

Plants experiencing salt-induced stress often reduce cytokinin levels during the early phases of stress-response. Interestingly, we found that the cytokinin content in the apple rootstock "robusta" was maintained at a high level under salt stress. Through screening genes involved in cytokinin biosynthesis and catabolism, we found that the high expression levels of IPT5b in robusta roots were involved in maintaining the high cytokinin content. We identified a 42 bp deletion in the promoter region of IPT5b, which elevated IPT5b expression levels, and this deletion was linked to salt tolerance in robusta×M.9 segregating population. The 42 bp deletion resulted in the deletion of a Proline Response Element (ProRE), and our results suggest that ProRE negatively regulates IPT5b expression in response to proline. Under salt stress, the robusta cultivar maintains high cytokinin levels as IPT5b expression cannot be inhibited by proline due to the deletion of ProRE, leading to improve salt tolerance.

Interaction of glucose and phytohormone signaling in plants

Energy acts as a primary prerequisite for plant growth like all the other organisms. Soluble sugars function in providing enough supply of nutrients which further helps in building macromolecules and energy to carry out specific and coordinated development. Sugars functions as nutrient as well as signaling molecule to promote cell division and differentiation in plants. Intriguingly, glucose has emerged as a crucial signaling molecule where hexokinase1 acts as the conserved glucose sensor. On the molecular scale, an extensive crosstalk between glucose and phytohormone signaling has been observed where glucose signals trigger multiple hexokinase1-dependent as well as hexokinase1-independent pathways to mediate diverse developmental, physiological and molecular mechanisms. Taken together, these findings this review focused on the glucose crosstalk with several classical plant hormonal-signaling pathways and the crucial role of hexokinase1 in modulating plant physiological processes.

Interplay of Auxin and Cytokinin in Lateral Root Development

The spacing and distribution of lateral roots are critical determinants of plant root system architecture. In addition to providing anchorage, lateral roots explore the soil to acquire water and nutrients. Over the past several decades, we have deepened our understanding of the regulatory mechanisms governing lateral root formation and development. In this review, we summarize these recent advances and provide an overview of how auxin and cytokinin coordinate the regulation of lateral root formation and development.

Production and Role of Hormones During Interaction of Fusarium Species With Maize (Zea mays L.) Seedlings

It has long been known that hormones affect the interaction of a phytopathogen with its host plant. The pathogen can cause changes in plant hormone homeostasis directly by affecting biosynthesis or metabolism in the plant or by synthesizing and secreting the hormone itself. We previously demonstrated that pathogenic fungi of the Fusarium species complex are able to produce three major types of hormones: auxins, cytokinins, and gibberellins. In this work, we explore changes in the levels of these hormones in maize and mango plant tissues infected with Fusarium. The ability to produce individual phytohormones varies significantly across Fusarium species and such differences likely impact host specificity inducing the unique responses noted in planta during infection. For example, the production of gibberellins by F. fujikuroi leads to elongated rice stalks and the suppression of gibberellin biosynthesis in plant tissue. Although all Fusarium species are able to synthesize auxin, sometimes by multiple pathways, the ratio of its free form and conjugates in infected tissue is affected more than the total amount produced. The recently characterized unique pathway for cytokinin de novo synthesis in Fusarium appears silenced or non-functional in all studied species during plant infection. Despite this, a large increase in cytokinin levels was detected in F. mangiferae infected plants, caused likely by the up-regulation of plant genes responsible for their biosynthesis. Thus, the accumulation of active cytokinins may contribute to mango malformation of the reproductive organs upon infection of mango trees. Together, our findings provide insight into the complex role fungal and plant derived hormones play in the fungal-plant interactions.

Unraveling the Initial Plant Hormone Signaling, Metabolic Mechanisms and Plant Defense Triggering the Endomycorrhizal Symbiosis Behavior

Arbuscular mycorrhizal (AM) fungi establish probably one of the oldest mutualistic relationships with the roots of most plants on earth. The wide distribution of these fungi in almost all soil ecotypes and the broad range of host plant species demonstrate their strong plasticity to cope with various environmental conditions. AM fungi elaborate fine-tuned molecular interactions with plants that determine their spread within root cortical tissues. Interactions with endomycorrhizal fungi can bring various benefits to plants, such as improved nutritional status, higher photosynthesis, protection against biotic and abiotic stresses based on regulation of many physiological processes which participate in promoting plant performances. In turn, host plants provide a specific habitat as physical support and a favorable metabolic frame, allowing uptake and assimilation of compounds required for the life cycle completion of these obligate biotrophic fungi. The search for formal and direct evidences of fungal energetic needs raised strong motivated projects since decades, but the impossibility to produce AM fungi under axenic conditions remains a deep enigma and still feeds numerous debates. Here, we review and discuss the initial favorable and non-favorable metabolic plant context that may fate the mycorrhizal behavior, with a focus on hormone interplays and their links with mitochondrial respiration, carbon partitioning and plant defense system, structured according to the action of phosphorus as a main limiting factor for mycorrhizal symbiosis. Then, we provide with models and discuss their significances to propose metabolic targets that could allow to develop innovations for the production and application of AM fungal inocula.

A Functional Connection between the Circadian Clock and Hormonal Timing in Arabidopsis

The rotation of the Earth entails changes in environmental conditions that pervasively influence an organism's physiology and metabolism. An internal cellular mechanism known as the circadian clock acts as an internal timekeeper that is able to perceive the changes in environmental cues to generate 24-h rhythms in synchronization with daily and seasonal fluctuations. In plants, the circadian clock function is particularly important and regulates nearly every aspect of plant growth and development as well as proper responses to stresses. The circadian clock does not function in isolation but rather interconnects with an intricate network of different pathways, including those of phytohormones. Here, we describe the interplay of the circadian clock with a subset of hormones in Arabidopsis. The molecular components directly connecting the circadian and hormone pathways are described, highlighting the biological significance of such connections in the control of growth, development, fitness, and survival. We focus on the overlapping as well as contrasting circadian and hormonal functions that together provide a glimpse on how the Arabidopsis circadian system regulates hormone function in response to endogenous and exogenous cues. Examples of feedback regulation from hormone signaling to the clock are also discussed.

Auxins and Cytokinins-The Role of Subcellular Organization on Homeostasis

Plant hormones are master regulators of plant growth and development. Better knowledge of their spatial signaling and homeostasis (transport and metabolism) on the lowest structural levels (cellular and subcellular) is therefore crucial to a better understanding of developmental processes in plants. Recent progress in phytohormone analysis at the cellular and subcellular levels has greatly improved the effectiveness of isolation protocols and the sensitivity of analytical methods. This review is mainly focused on homeostasis of two plant hormone groups, auxins and cytokinins. It will summarize and discuss their tissue- and cell-type specific distributions at the cellular and subcellular levels.

The interplay between cytokinins and light during senescence in detached Arabidopsis leaves

Light and cytokinins are known to be the key players in the regulation of plant senescence. In detached leaves, the retarding effect of light on senescence is well described; however, it is not clear to what extent is this effect connected with changes in endogenous cytokinin levels. We have performed a detailed analysis of changes in endogenous content of 29 cytokinin forms in detached leaves of Arabidopsis thaliana (wild-type and 3 cytokinin receptor double mutants). Leaves were kept under different light conditions, and changes in cytokinin content were correlated with changes in chlorophyll content, efficiency of photosystem II photochemistry, and lipid peroxidation. In leaves kept in darkness, we have observed decreased content of the most abundant cytokinin free bases and ribosides, but the content of cis-zeatin increased, which indicates the role of this cytokinin in the maintenance of basal leaf viability. Our findings underscore the importance of light conditions on the content of specific cytokinins, especially N⁶ -(Δ² -isopentenyl)adenine. On the basis of our results, we present a scheme summarizing the contribution of the main active forms of cytokinins, cytokinin receptors, and light to senescence regulation. We conclude that light can compensate the disrupted cytokinin signalling in detached leaves.

A B-ARR-mediated cytokinin transcriptional network directs hormone cross-regulation and shoot development

Cytokinin fulfills its diverse roles in planta through a series of transcriptional responses. We identify the in vivo DNA binding site profiles for three genetically redundant type-B ARABIDOPSIS RESPONSE REGULATORS (B-ARRs): ARR1, ARR10, and ARR12. The expression and genome-wide DNA binding locations of the three B-ARRs extensively overlap. Constructing a primary cytokinin response transcriptional network reveals a recurring theme of widespread cross-regulation between the components of the cytokinin pathway and other plant hormone pathways. The B-ARRs are found to have similar DNA binding motifs, though sequences flanking the core motif were degenerate. Cytokinin treatments amalgamate the three different B-ARRs motifs to identical DNA binding signatures (AGATHY, H(a/t/c), Y(t/c)) which suggests cytokinin may regulate binding activity of B-ARR family members. Furthermore, we find that WUSCHEL, a key gene required for apical meristem maintenance, is a cytokinin-dependent B-ARR target gene, demonstrating the importance of the cytokinin transcription factor network in shoot development.

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

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

Cytokinin signaling in plant development

The phytohormone cytokinin plays diverse roles in plant development, influencing many agriculturally important processes, including growth, nutrient responses and the response to biotic and abiotic stresses. Cytokinin levels in plants are regulated by biosynthesis and inactivation pathways. Cytokinins are perceived by membrane-localized histidine-kinase receptors and are transduced through a His-Asp phosphorelay to activate a family of transcription factors in the nucleus. Here, and in the accompanying poster, we summarize the current understanding of cytokinin metabolism, transport and signaling, and discuss how this phytohormone regulates changes in gene expression to mediate its pleiotropic effects.

Identification and expression analysis of the IPT and CKX gene families during axillary bud outgrowth in apple (Malus domestica Borkh.)

Cytokinins (CKs) play a crucial role in promoting axillary bud outgrowth and targeting the control of CK metabolism can be used to enhance branching in plants. CK levels are maintained mainly by CK biosynthesis (isopentenyl transferase, IPT) and degradation (dehydrogenase, CKX) genes in plants. A systematic study of the IPT and CKX gene families in apple, however, has not been conducted. In the present study, 12 MdIPTs and 12 MdCKXs were identified in the apple genome. Systematic phylogenetic, structural, and synteny analyses were performed. Expression analysis of these genes in different tissues was also assessed. MdIPT and MdCKX genes exhibit distinct expression patterns in different tissues. The response of MdIPT, MdCKX, and MdPIN1 genes to various treatments (6-BA, decapitation and Lovastatin, an inhibitor of CKs synthesis) that impact branching were also investigated. Results indicated that most of the MdIPT and MdCKX, and MdPIN1 genes were upregulated by 6-BA and decapitation treatment, but inhibited by Lovastatin, a compound that effectively suppresses axillary bud outgrowth induced by decapitation. These findings suggest that cytokinin biosynthesis is required for the activation of bud break and the export of auxin from buds in apple tree with intact primary shoot apex or decapitated apple tree. MdCKX8 and MdCKX10, however, exhibited little response to decapitation, but were significantly up-regulated by 6-BA and Lovastatin, a finding that warrants further investigation in order to understand their function in bud-outgrowth.

Antibody-mediated modulation of cytokinins in tobacco: organ-specific changes in cytokinin homeostasis

Cytokinins comprise a group of phytohormones with an organ-specific mode of action. Although the mechanisms controlling the complex networks of cytokinin metabolism are partially known, the role of individual cytokinin types in the maintenance of cytokinin homeostasis remains unclear. Utilizing the overproduction of single-chain Fv antibodies selected for their ability to bind trans-zeatin riboside and targeted to the endoplasmic reticulum, we post-synthetically modulated cytokinin ribosides, the proposed transport forms of cytokinins. We observed asymmetric activity of cytokinin biosynthetic genes and cytokinin distribution in wild-type tobacco seedlings with higher cytokinin abundance in the root than in the shoot. Antibody-mediated modulation of cytokinin ribosides further enhanced the relative cytokinin abundance in the roots and induced cytokinin-related phenotypes in an organ-specific manner. The activity of cytokinin oxidase/dehydrogenase in the roots was strongly up-regulated in response to antibody-mediated formation of the cytokinin pool in the endoplasmic reticulum. However, we only detected a slight decrease in the root cytokinin levels. In contrast, a significant decrease of cytokinins occurred in the shoot. We suggest the roots as the main site of cytokinin biosynthesis in tobacco seedlings. Conversely, cytokinin levels in the shoot seem to depend largely on long-range transport of cytokinin ribosides from the root and their subsequent metabolic activation.

Cytokinin signalling regulates organ identity via AHK4 receptor in Arabidopsis

Mutual interactions of the phytohormones cytokinins and auxin determine root or shoot identity during postembryonic de novo organogenesis in plants. However, our understanding to the role of hormonal metabolism and perception during early stages of cell fate reprograming is still elusive.

In the hypocotyl explant assay, auxin activated root formation while cytokinins mediated early loss of the root identity, primordia disorganization and initiation of shoot development. Exogenous but also endogenous cytokinins influenced the initiation of newly formed organs as well as the pace of organ developmental sequence. The process of de novo shoot apical meristem establishment was accompanied by accumulation of endogenous cytokinins, differential regulation of genes for individual cytokinin receptors, strong activation of AHK4-mediated signalling and induction of shoot-specific homeodomain regulator WUSCHEL. The latter associated with upregulation of isopentenyladenine-type cytokinins, revealing higher shoot-forming potential when compared with trans-zeatin. Moreover, AHK4-controlled cytokinin signalling negatively regulated root stem cell organizer WUSCHEL RELATED HOMEOBOX 5 in the root quiescent centre. We propose an important role of endogenous cytokinin biosynthesis and AHK4-mediated cytokinin signalling in the control of de novo induced organ identity.

A sensitive synthetic reporter for visualizing cytokinin signaling output in rice

BACKGROUND

Cytokinins play many essential roles in plant growth and development, mainly through signal transduction pathways. Although the cytokinin signaling pathway in rice has been clarified, no synthetic reporter for cytokinin signaling output has been reported for rice. The sensitive synthetic reporter two-component signaling sensor (TCSn) is used in the model plant Arabidopsis; however, whether the reporter reflects the cytokinin signaling output pattern in rice remains unclear.

RESULTS

Early-cytokinin-responsive type-A OsRR-binding element (A/G)GAT(C/T) was more clustered in the 15 type-A OsRRs than in the 13 control genes. Quantitative polymerase chain reaction analysis showed that the relative expression of seven type-A OsRRs in roots and shoots was significantly induced by exogenous cytokinin application, and that of seven OsRRs, mainly in roots, was inhibited by exogenous auxin application. We constructed a transgenic rice plant harboring a beta-glucuronidase (GUS) driven by the synthetic promoter TCSn. TCSn::GUS was expressed in the meristem of germinated rice seed and rice seedlings. Furthermore, TCSn::GUS expression in rice seedlings was induced specifically by exogenous cytokinin application and decreased by exogenous auxin application. Moreover, no obvious reduction in GUS levels was observed after three generations of selfing of transgenic plants, indicating that TCSn::GUS is not subject to transgene silencing.

CONCLUSIONS

We report here a robust and sensitive synthetic sensor for monitoring the transcriptional output of the cytokinin signaling network in rice.

Transcriptional changes of rice in response to rice black-streaked dwarf virus

Rice black-streaked dwarf virus (RBSDV), a member of the genus Fijivirus in the family Reoviridae, causes significant economic losses in rice production in China and many other Asian countries. Although a great deal of effort has been made to elucidate the interactions among the virus, insect vectors, host and environmental conditions, few RBSDV proteins involved in pathogenesis have been identified, and the biological basis of disease development in rice remains largely unknown. Transcriptomic information associated with the disease development in rice would be helpful to unravel the biological mechanism. To determine how the rice transcriptome changes in response to RBSDV infection, we carried out RNA-Seq to perform a genome-wide gene expression analysis of a susceptible rice cultivar KTWYJ3. The transcriptomes of RBSDV-infected samples were compared to those of RBSDV-free (healthy) at two time points (time points are represented by group I and II). The results derived from the differential expression analysis in RBSDV-infected libraries vs. healthy ones in group I revealed that 102 out of a total of 281 significant differentially expressed genes (DEGs) were up-regulated and 179 DEGs were down-regulated. Of the 2592 identified DEGs in group II, 1588 DEGs were up-regulated and 1004 DEGs were down-regulated. A total of 66 DEGs were commonly identified in both groups. Of these 66 DEGs, expression patterns for 36 DEGs were similar in both groups. Our analysis demonstrated that some genes related to disease defense and stress resistance were up-regulated while genes associated with chloroplast were down-regulated in response to RBSDV infection. In addition, some genes associated with plant-height were differentially expressed. This result indicates those genes might be involved in dwarf symptoms caused by RBSDV. Taken together, our results provide a genome-wide transcriptome analysis for rice plants in response to RBSDV infection which may contribute to the understanding of the regulatory mechanisms involved in rice-RBSDV interaction and the biological basis of rice black-streaked dwarf disease development in rice.

Cold interferes with male meiotic cytokinesis in Arabidopsis thaliana independently of the AHK2/3-AHP2/3/5 cytokinin signaling module

Previously we have shown that low temperature stress in Arabidopsis causes defects in microtubule organization and cytokinesis in male meiocytes, which leads to the formation of diploid pollen. Because cytokinin (CK) mediates multiple physiological responses to cold stress, we investigated whether CK signaling is involved in cold-induced diploid pollen formation. To this end, we monitored male sporogenesis in a series of mutants defective in CK metabolism and signalling. Arabidopsis plants with altered CK homeostasis, that is, the ahk2-2 ahk3-3 double and the ahp2-1 ahp3 ahp5-2 triple mutant, were cold sensitive and displayed similar defective male meiotic cytokinesis as wild type plants upon cold stress. These findings demonstrate that the AHK2/3-AHP2/3/5 CK-signaling module is not required for cold-induced ploidy stability of male gamete in Arabidopsis. Cytological analysis further revealed that the cold-induced cytokinesis defects in the ahk2-2 ahk3-3 mutant correlated with irregular organization of the radial microtubule array (RMA) in tetrad microspores at the end of male meiosis. Contrary to the ahk and ahp mutants, Arabidopsis plants defective for ARR1, a downstream target of ahk and ahp mediated CK signalling, displayed higher cold-tolerance of male meiotic cytokinesis program. We here suggest that the transcription regulator ARR1 may act independently from the CK AHK2/3-AHP2/3/5 signaling module in conveying the cold response to male meiocytes.