Molecular mechanisms of cytokinin action

Purines enrich root-associated Pseudomonas and improve wild soybean growth under salt stress

The root-associated microbiota plays an important role in the response to environmental stress. However, the underlying mechanisms controlling the interaction between salt-stressed plants and microbiota are poorly understood. Here, by focusing on a salt-tolerant plant wild soybean (Glycine soja), we demonstrate that highly conserved microbes dominated by Pseudomonas are enriched in the root and rhizosphere microbiota of salt-stressed plant. Two corresponding Pseudomonas isolates are confirmed to enhance the salt tolerance of wild soybean. Shotgun metagenomic and metatranscriptomic sequencing reveal that motility-associated genes, mainly chemotaxis and flagellar assembly, are significantly enriched and expressed in salt-treated samples. We further find that roots of salt stressed plants secreted purines, especially xanthine, which induce motility of the Pseudomonas isolates. Moreover, exogenous application for xanthine to non-stressed plants results in Pseudomonas enrichment, reproducing the microbiota shift in salt-stressed root. Finally, Pseudomonas mutant analysis shows that the motility related gene cheW is required for chemotaxis toward xanthine and for enhancing plant salt tolerance. Our study proposes that wild soybean recruits beneficial Pseudomonas species by exudating key metabolites (i.e., purine) against salt stress.

Screening of Differentially Expressed Genes and Localization Analysis of Female Gametophyte at the Free Nuclear Mitosis Stage in Pinus tabuliformis Carr

Female sterility is a common phenomenon in the plant world, and systematic research has not been carried out in gymnosperms. In this study, the ovules of No. 28 sterile line and No. 15 fertile line Pinus tabuliformis were used as materials, and a total of 18 cDNA libraries were sequenced by the HiSeqTM 4000 platform to analyze the differentially expressed genes (DEGs) and simple sequence repeats (SSRs) between the two lines. In addition, this study further analyzed the DEGs involved in the signal transduction of plant hormones, revealing that the signal pathways related to auxin, cytokinin, and gibberellin were blocked in the sterile ovule. Additionally, real-time fluorescent quantitative PCR verified that the expression trend of DEGs related to plant hormones was consistent with the results of high-throughput sequencing. Frozen sections and fluorescence in situ hybridization (FISH) were used to study the temporal and spatial expression patterns of PtRab in the ovules of P. tabuliformis. It was found that PtRab was significantly expressed in female gametophytes and rarely expressed in the surrounding diploid tissues. This study further explained the molecular regulation mechanism of female sterility in P. tabuliformis, preliminarily mining the key factors of ovule abortion in gymnosperms at the transcriptional level.

Harnessing Synergistic Biostimulatory Processes: A Plausible Approach for Enhanced Crop Growth and Resilience in Organic Farming

Simple Summary

Demand for organically grown crops has risen globally due to its healthier and safer food products. From a sustainability perspective, organic farming offers an eco-friendly cultivation system that minimizes agrochemicals and producing food with little or no environmental footprint. However, organic agriculture’s biggest drawback is the generally lower and variable yield in contrast to conventional farming. Compatible with organic farming, the selective use of biostimulants can close the apparent yield gap between organic and conventional cultivation systems. A biostimulant is defined as natural microorganisms (bacteria, fungi) or biologically active substances that are able to improve plant growth and yield through several processes. Biostimulants are derived from a range of natural resources including organic materials (composts, seaweeds), manures (earthworms, fish, insects) and extracts derived from microbes, plant, insect or animal origin. The current trend is indicative that a mixture of biostimulants is generally delivering better growth, yield and quality rather than applying biostimulant individually. When used correctly, biostimulants are known to help plants cope with stressful situations like drought, salinity, extreme temperatures and even certain diseases. More research is needed to understand the different biostimulants, key components, and also to adjust the formulations to improve their reliability in the field.

Abstract

Demand for organically grown food crops is rising substantially annually owing to their contributions to human health. However, organic farm production is still generally lower compared to conventional farming. Nutrient availability, content consistency, uptake, assimilation, and crop responses to various stresses were reported as critical yield-limiting factors in many organic farming systems. In recent years, plant biostimulants (BSs) have gained much interest from researchers and growers, and with the objective of integrating these products to enhance nutrient use efficiency (NUE), crop performance, and delivering better stress resilience in organic-related farming. This review gave an overview of direct and indirect mechanisms of microbial and non-microbial BSs in enhancing plant nutrient uptake, physiological status, productivity, resilience to various stressors, and soil-microbe-plant interactions. BSs offer a promising, innovative and sustainable strategy to supplement and replace agrochemicals in the near future. With greater mechanistic clarity, designing purposeful combinations of microbial and non-microbial BSs that would interact synergistically and deliver desired outcomes in terms of acceptable yield and high-quality products sustainably will be pivotal. Understanding these mechanisms will improve the next generation of novel and well-characterized BSs, combining microbial and non-microbial BSs strategically with specific desired synergistic bio-stimulatory action, to deliver enhanced plant growth, yield, quality, and resilience consistently in organic-related cultivation.

Natural Surfactant Saponin from Tissue of Litsea glutinosa and Its Alternative Sustainable Production

In this research, we assessed the detergency properties along with chemical characteristic of the surfactant extracts from the most frequently cited detergent plants in Northern Thailand, namely, Sapindus rarak, Acacia concinna, and Litsea glutinosa. Moreover, as to provide the sustainable option for production of such valuable ingredients, plant tissue culture (PTC) as alternative method for industrial metabolite cultivation was also proposed herein. The results illustrated that detergent plant extracts showed moderate in foaming and detergency abilities compared with those of synthetic surfactant. The phytochemical analysis illustrated the positive detection of saponins in L. glutinosa plant extracts. The highest callus formation was found in L. glutinosa explant cultured with MS medium supplemented with 2.0 mg/L Indole-3-acetic acid (IAA). The callus extract was chemical elucidated using chromatography, which illustrated the presence of saponin similar to those from the crude leaf and Quillaja saponin extracts. Compact mass spectrometry confirmed that the surfactant was of the steroidal diagnostic type.

Impact of magnetically treated water on the growth and development of tobacco (Nicotiana tabacum var. Turkish)

Magnetism is one of the physical methods affecting water properties. It is considered as an environmental factor that plays a role in the physiological and biochemical reactions. A hydroponic experiment was conducted using four types of treated water (distilled water, magnetically treated distilled water, magnetically treated tap water, and tap water). Tobacco plants (Nicotiana tabacum var. Turkish) were placed in a growth chamber for three weeks. Plants irrigated with magnetically treated distilled water had a significant increase in the physiological parameters including shoot height and root length (P < 0.0001). The same pattern was seen in the photosynthetic rate and protein content, but no significant differences in the stomatal conductance and transpiration rate (P < 0.5601). In contrast, a significant increase of total carbohydrate content was exhibited in plant irrigated with tap water (P < 0.0064). Electron micrographs showed deformed chloroplasts with damaged thylakoid membranes associated with plastoglobules in plants irrigated with tap water and magnetically treated tap water. Lastly, this study suggests that magnetically treated water is an excellent option to improve irrigation methods and thus obtains agricultural production with high efficiency.

Effects of light intensity and plant growth regulators on callus proliferation and shoot regeneration in the ornamental succulent Haworthia

BACKGROUND

Haworthia are desert succulents belonging to the Asphodelaceae family. Haworthia species are cultivated commercially as ornamentals and some rare species are quite valuable at retail market but growth slowly and difficult to propagation. However, an efficient micropropagation protocol was remained insufficient.

RESULTS

The organogenic cultures obtained from inflorescence explants were cultured on Murashige and Skoog (MS) medium supplemented with various combinations of 6-benzylaminopurine (BA) and α-naphthalene acetic acid (NAA) under a light intensity of 10 μmol m^-2 s^-1 or 45 μmol m^-2 s^-1. The highest callus proliferation index (93.15%) with 1.0 mg L^-1 BA + 0.1 mg L^-1 NAA under a light intensity of 10 μmol m^-2 s^-1. The best shoot proliferation rates were on media with either 1 mg L^-1 BA + 0-0.4 mg L^-1 NAA (65.57-81.01%) under a light intensity of 45 μmol m^-2 s^-1. The highest root length (15.57 mm) and the highest rooting frequency (17 roots per shoot) were obtained when adventitious shoots were inoculated on MS medium with 0.4 mg L^-1 NAA + 0.4 mg L^-1 IBA. The survival rate of the transplanted plantlets was about 100%. The efficient micropropagation protocol proliferated Haworthia regenerate plants from inflorescence within 11 weeks.

CONCLUSIONS

The present study determined the best combination of light intensity and plant growth regulators (PGRs) for improved organogenesis of Haworthia during propagation by tissue culture. This optimized protocol showed light intensity is an important factor for efficient callus or shoot regeneration. These results indicate that it will be useful to optimize the light conditions for future commercial cultivation, germplasm conservation, genetic engineering and molecular biology research of this ornamental plant.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Analysis of basic leucine zipper genes and their expression during bud dormancy in peach (Prunus persica)

Dormancy is a biological characteristic developed to resist the cold conditions in winter. The bZIP transcription factors are present exclusively in eukaryotes and have been identified and classified in many species. bZIP proteins are known to regulate numerous biological processes, however, the role of bZIP in bud dodormancy has not been studied extensively. In total, 50 PpbZIP transcription factor-encoding genes were identified and categorized them into 10 groups (A-I and S). Similar intron/exon structures, additional conserved motifs, and DNA-binding site specificity supported our classification scheme. Additionally, chromosomal distribution and collinearity analyses suggested that expansion of the PpbZIP transcription factor family was due to segment/chromosomal duplications. We also predicted the dimerization properties based on characteristic features of the leucine zipper and classified PpbZIP proteins into 23 subfamilies. Furthermore, qRT-PCR results indicated that PpbZIPs genes may be involved in regulating dormancy. The same gene of different species might participate in different regulating networks through interactions with specific partners. Our expression profiling results complemented the microarray data, suggesting that co-expression patterns of bZIP transcription factors during dormancy differed among deciduous fruit trees. Our findings further clarify the molecular characteristics of the PpbZIP transcription factor family, including potential gene functions during dormancy. This information may facilitate further research on the evolutionary history and biological functions of bZIP proteins in peach and other rosaceae plants.

Role of AINTEGUMENTA-like gene NtANTL in the regulation of tobacco organ growth

The Nicotiana tabacum AINTEGUMENTA-like gene (NtANTL), encoding one of AP2/ERF transcription factors, is a putative ortholog of the AtANT gene from Arabidopsis thaliana. In wild-type tobacco plants, the NtANTL gene was expressed in the actively dividing young flowers, shoot apices, and calluses, while the level of its mRNA increased considerably after treatment with exogenous 6-benzylaminopurine, indoleacetic acid and 24-epibrassinolide. We found a positive correlation among the expression levels of NtANTL, cyclin NtCYCD3;1 and cyclin-dependent kinase NtCDKB1-1 genes, suggesting possible molecular links between AINTEGUMENTA and cell cycle regulators in tobacco plants. However, no correlation was observed between NtANTL, NtCYCD3;1 and NtCDKB1-1 expression levels in response to NaCl and ABA. These observations indicate that the transcription factor NtANTL was not involved in the regulation of the cellular response to salinity nor did it affect the expression of NtCYCD3;1 and NtCDKB1-1 when tobacco plants were exposed to salt stress and ABA. In addition, we generated transgenic tobacco plants with both up-regulated and down-regulated expression of the NtANTL gene. Constitutive expression of the NtANTL gene contributed to an increase in the size of leaves and corolla of transgenic plants. Transgenic plants with reduced expression of the NtANTL gene had smaller leaves, flowers and stems, but showed a compensatory increase in the cell size of leaves and flowers. The results show the significance of the NtANTL gene for the control of organ growth by both cell division and expansion in tobacco plants.

Functional roles of three cutin biosynthetic acyltransferases in cytokinin responses and skotomorphogenesis

Cytokinins (CKs) regulate plant development and growth via a two-component signaling pathway. By forward genetic screening, we isolated an Arabidopsis mutant named grow fast on cytokinins 1 (gfc1), whose seedlings grew larger aerial parts on MS medium with CK. gfc1 is allelic to a previously reported cutin mutant defective in cuticular ridges (dcr). GFC1/DCR encodes a soluble BAHD acyltransferase (a name based on the first four enzymes characterized in this family: Benzylalcohol O-acetyltransferase, Anthocyanin O-hydroxycinnamoyltransferase, anthranilate N-hydroxycinnamoyl/benzoyltransferase and Deacetylvindoline 4-O-acetyltransferase) with diacylglycerol acyltransferase (DGAT) activity in vitro and is necessary for normal cuticle formation on epidermis in vivo. Here we show that gfc1 was a CK-insensitive mutant, as revealed by its low regeneration frequency in vitro and resistance to CK in adventitious root formation and dark-grown hypocotyl inhibition assays. In addition, gfc1 had de-etiolated phenotypes in darkness and was therefore defective in skotomorphogenesis. The background expression levels of most type-A Arabidopsis Response Regulator (ARR) genes were higher in the gfc1 mutant. The gfc1-associated phenotypes were also observed in the cutin-deficient glycerol-3-phosphate acyltransferase 4/8 (gpat4/8) double mutant [defective in glycerol-3-phosphate (G3P) acyltransferase enzymes GPAT4 and GPAT8, which redundantly catalyze the acylation of G3P by hydroxyl fatty acid (OH-FA)], but not in the cutin-deficient mutant cytochrome p450, family 86, subfamily A, polypeptide 2/aberrant induction of type three 1 (cyp86A2/att1), which affects the biosynthesis of some OH-FAs. Our results indicate that some acyltransferases associated with cutin formation are involved in CK responses and skotomorphogenesis in Arabidopsis.

Cytokinins

Cytokinins are N (6) substituted adenine derivatives that affect many aspects of plant growth and development, including cell division, shoot initiation and growth, leaf senescence, apical dominance, sink/source relationships, nutrient uptake, phyllotaxis, and vascular, gametophyte, and embryonic development, as well as the response to biotic and abiotic factors. Molecular genetic studies in Arabidopsis have helped elucidate the mechanisms underlying the function of this phytohormone in plants. Here, we review our current understanding of cytokinin biosynthesis and signaling in Arabidopsis, the latter of which is similar to bacterial two-component phosphorelays. We discuss the perception of cytokinin by the ER-localized histidine kinase receptors, the role of the AHPs in mediating the transfer of the phosphoryl group from the receptors to the response regulators (ARRs), and finally the role of the large ARR family in cytokinin function. The identification and genetic manipulation of the genes involved in cytokinin metabolism and signaling have helped illuminate the roles of cytokinins in Arabidopsis. We discuss these diverse roles, and how other signaling pathways influence cytokinin levels and sensitivity though modulation of the expression of cytokinin signaling and metabolic genes.

Cytokinin receptor-dependent and receptor-independent pathways in the dehydration response of Arabidopsis thaliana

Cytokinin signaling in Arabidopsis thaliana utilizes a multi-step two-component signaling (TCS) system comprised of sensor histidine kinases (AHKs), histidine phosphotransfer proteins (AHPs), and response regulators (ARRs). Recent studies have suggested that the cytokinin TCS system is involved in a variety of other signaling and metabolic pathways. To further explore a potential function of the cytokinin TCS in the Arabidopsis dehydration stress response, we investigated the expression of all type-A ARR genes and a type-C ARR, ARR22, in both wild type and ahk single, double, and triple mutants in response to dehydration compared to cytokinin as well as dehydration tolerance of ahk mutants. We found that drought significantly induced the expression of a subset of ARR genes, ARR5, ARR7, ARR15, and ARR22. The results of expression analyses in ahk single, double, and triple mutants demonstrated that the cytokinin receptors AHK2 and AHK3 are redundantly involved in dehydration-inducible expression of ARR7, but not that of ARR5, ARR15, or ARR22. Dehydration tolerance assays showed that ahk2 and ahk3 single mutants exhibited enhanced dehydration tolerance compared with that of wild-type plants and ahk4 mutants, and that ahk2 ahk3 double mutants exhibited stronger drought tolerance than that of ahk3 ahk4, which exhibited more enhanced drought tolerance than that of wild-type plants and ahk single mutants. Taken together, these results demonstrate that while the cytokinin receptors AHK2 and AHK3 are critically involved in the dehydration tolerance response, both cytokinin receptor-dependent pathway and receptor-independent pathway occur in the dehydration response regulating ARR gene expression. In addition, preincubating ahk2, ahk3, ahk4, and the wild-type plants with cytokinin induced enhanced dehydration stress tolerance in these plants, demonstrating that cytokinins are involved in regulating plant response to dehydration stress.

Deletion of the initial 45 residues of ARR18 induces cytokinin response in Arabidopsis

The plant hormone cytokinin plays important roles in various aspects of plant growth and development. Cytokinin signaling is mediated by a multistep phosphorelay similar to bacterial two-component system. Type-B ARRs lie at the end of the cytokinin signaling, typically mediating the output response. However, it is still unclear how type-B ARRs are regulated in response to cytokinin. Typical type-B ARR contains an N-terminal receiver domain and a C-terminal effector domain. In this study, we performed a genome-wild comparative analysis by overexpressing full length and C-terminal effector domain of seven representative type-B ARRs. Our results indicated that overexpression of C-terminal effector domain causes short primary roots and short hypocotyls without the addition of cytokinin, suggesting that the inhibitory role of the receiver domain in the activity of the effector domain is a common mechanism in type-B ARRs. To investigate how the receiver domain inhibits the activity of the effector domain, we performed a deletion analysis. We found that deletion of the initial 45 residues of ARR18 (the 45 residues from N-terminus) causes pleiotropic growth defects by directly inducing cytokinin responsive genes. Together, our results suggest that the initial 45 residues are critical for the inhibitory role of the receiver domain to the effector domain in ARR18.

cis-Cytokinins that predominate inPisum sativumduring early embryogenesis will accelerate embryo growth in vitro

Correlative data from monocots suggest that cytokinin (CK) regulates seed development. The involvement of CKs in seed growth was investigated using pea, a eudicot with an unknown CK profile, as a model system. CK profiles were measured by liquid chromatography – tandem mass spectrometry against major stages of embryogenesis, which were documented histologically. Like other grain legumes, CK levels of developing pea seeds fluctuated through development and had mainly nucleotide and riboside forms. Among the 11 CKs detected, cis-isomers (cis-[9R]Z (zeatin riboside), and cis-[9RMP]Z (zeatin riboside 5′ monophosphate)), along with their isopentenyl precursors, were the major forms during pea embryogenesis, whereas corresponding trans-isomers appeared as minor constituents. Highest CK concentrations occurred at the heart-shape stage, when there are high rates of cell division and sugar metabolism. To assess the significance of high CK concentrations observed at the heart-shape stage, a bioassay was developed wherein heart-shaped embryos were excised and cultured on medium containing either cis-[9R]Z, trans-[9R]Z, or kinetin. Growth of cultured heart-shaped embryo explants was significantly augmented by all exogenous CKs relative to controls that were not supplemented with CK. Moreover, at concentrations equivalent to those experienced by an embryo in vivo, cis-[9R]Z was active in enhancing the growth of cultured pea embryos to an extent equal to that of trans-[9R]Z. Overall, the results endorse a growth-promoting role for cis-CKs during seed development.

Signaling through MAP kinase networks in plants

Protein phosphorylation is the most important mechanism for controlling many fundamental cellular processes in all living organisms including plants. A specific class of serine/threonine protein kinases, the mitogen-activated protein kinases (MAP kinases) play a central role in the transduction of various extra- and intracellular signals and are conserved throughout eukaryotes. These generally function via a cascade of networks, where MAP kinase (MAPK) is phosphorylated and activated by MAPK kinase (MAPKK), which itself is activated by MAPKK kinase (MAPKKK). Signaling through MAP kinase cascade can lead to cellular responses including cell division, differentiation as well as response to various stresses. In plants, MAP kinases are represented by multigene families and are organized into a complex network for efficient transmission of specific stimuli. Putative plant MAP kinase cascades have been postulated based on experimental analysis of in vitro interactions between specific MAP kinase components. These cascades have been tested in planta following expression of epitope-tagged kinases in protoplasts. It is known that signaling for cell division and stress responses in plants are mediated through MAP kinases and even auxin, ABA and possibly ethylene and cytokinin also utilize a MAP kinase pathway. Most of the biotic (pathogens and pathogen-derived elicitors) including wounding and abiotic stresses (salinity, cold, drought, and oxidative) can induce defense responses in plants through MAP kinase pathways. In this article we have covered the historical background, biochemical assay, activation/inactivation, and targets of MAP kinases with emphasis on plant MAP kinases and the responses regulated by them. The cross-talk between plant MAP kinases is also discussed to bring out the complexity within this three-component module.

2D autocorrelation modelling of the inhibitory activity of cytokinin-derived cyclin-dependent kinase inhibitors

The inhibitory activity towards p34(cdc2)/cyclin b kinase (CBK) enzyme of 30 cytokinin-derived compounds has been successfully modelled using 2D spatial autocorrelation vectors. Predictive linear and non-linear models were obtained by forward stepwise multi-linear regression analysis (MRA) and artificial neural network (ANN) approaches respectively. A variable selection routine that selected relevant non-linear information from the data set was employed prior to networks training. The best ANN with three input variables was able to explain about 87% data variance in comparison with 80% by the linear equation using the same number of descriptors. Similarly, the neural network had higher predictive power. The MRA model showed a linear dependence between the inhibitory activities and the spatial distributions of masses, electronegativities and van der Waals volumes on the inhibitors molecules. Meanwhile, ANN model evidenced the occurrence of non-linear relationships between the inhibitory activity and the mass distribution at different topological distance on the cytokinin-derived compounds. Furthermore, inhibitors were well distributed regarding its activity levels in a Kohonen self-organizing map (SOM) built using the input variables of the best neural network.

hca: an Arabidopsis mutant exhibiting unusual cambial activity and altered vascular patterning

By screening a T-DNA population of Arabidopsis mutants for alterations in inflorescence stem vasculature, we have isolated a mutant with a dramatic increase in vascular tissue development, characterized by a continuous ring of xylem/phloem. This phenotype is the consequence of premature and numerous cambial cell divisions in both the fascicular and interfascicular regions that result in the loss of the alternate vascular bundle/fiber organization typically observed in Arabidopsis stems. The mutant was therefore designated high cambial activity (hca). The hca mutation also resulted in pleiotropic effects including stunting and a delay in developmental events such as flowering and senescence. The physiological characterization of hca seedlings in vitro revealed an altered auxin and cytokinin response and, most strikingly, an enhanced sensitivity to cytokinin. These results were substantiated by comparative microarray analysis between hca and wild-type plants. The genetic analysis of hca indicated that the mutant phenotype was not tagged by the T-DNA and that the hca mutation segregated as a single recessive locus, mapping to the long arm of chromosome 4. We propose that hca is involved in mechanisms controlling the arrangement of vascular bundles throughout the plant by regulating the auxin-cytokinin sensitivity of vascular cambial cells. Thus, the hca mutant is a useful model for examining the genetic and hormonal control of cambial growth and differentiation.

Histidine-containing phosphotransfer domain extinction by RNA interference turns off a cytokinin signalling circuitry in Catharanthus roseus suspension cells

We previously reported that cytokinins (CK) induce the fast and specific transcription of CrRR1, a gene encoding a type A response regulator in Catharanthus roseus cell cultures. Here, we characterized the CrHPt1 gene that encodes a histidine-containing phosphotransfer domain. CrHPt1 was silenced through RNA interference (RNAi) to test its possible implication in the CK signalling pathway. In transgenic lines stably transformed with an intron-spliced construct, the degradation of CrHPt1 transcripts abolishes the CK inductive effect on CrRR1 transcription. These result give a new in vivo functional argument for the crucial role of HPt proteins in the CK signalling pathway leading to the expression of the genes encoding type A response regulators. They also show that RNAi is a powerful strategy to turn off the CK signalling circuitry.

Plant Growth and the TOR Pathway

In mammalian, insect, and yeast cells, TOR proteins are essential regulators of cell growth in response to environmental signals including nutrients, mitogens, and stresses. Although many aspects of the TOR-dependent signalling pathway are conserved between animals and fungi, important differences have also been found and are likely to be related to the ecophysiological adaptations of these organisms. The TOR protein also exists in plants. This review will first discuss specific aspects of plants concerning the contribution of cell growth to overall growth, as well as their responses to nutrient starvation, with emphasis on recent results obtained through genetic analysis in the model plant Arabidopsis thaliana. This is followed by the current status of the genetic analysis of the TOR gene in this plant and the search for potential members of a TOR pathway in the Arabidopsis genome.

Knowing when to grow: signals regulating bud dormancy

Dormancy regulation in vegetative buds is a complex process necessary for plant survival, development and architecture. Our understanding of and ability to manipulate these processes are crucial for increasing the yield and availability of much of the world's food. In many cases, release of dormancy results in increased cell division and changes in developmental programs. Much can be learned about dormancy regulation by identifying interactions of signals in these crucial processes. Internal signals such as hormones and sugar, and external signals such as light act through specific, overlapping signal transduction pathways to regulate endo-, eco- and paradormancy. Epigenetic-like regulation of endodormancy suggests a possible role for chromatin remodeling similar to that known for the vernalization responses during flowering.

Expression of CycD3 is transiently increased by pollination and N-(2-chloro-4-pyridyl)-N'-phenylurea in ovaries of Lagenaria leucantha

Lagenaria leucantha is an important vegetable crop and a potential model for the study of fruit development. To study the function of D cyclins in fruit development, full-length cDNA clones for two D cyclin genes were isolated from young ovaries of Lagenaria leucantha. They were classified as D3 cyclins by sequence similarities and phylogenetic analysis, and nominated LlCycD3;1 and LlCycD3;2, respectively. The deduced amino acid sequence of both LlCycD3 genes contained a retinoblastoma-binding motif and a PEST-destruction motif. Unpollinated ovaries failed to develop and eventually aborted. N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) induced parthenocarpic fruit significantly larger than pollinated ones. In unpollinated ovaries, the expression of both LlCycD3 genes was abundant at anthesis and then suddenly decreased, concomitant with the cessation of cell division. Pollination/fertilization induced an activation of the cell cycle accompanied by a large increase in the transcript levels of LlCycD3;1 and LlCycD3;2 in young fruits. Treating ovaries with CPPU also reactivated cell division and transcription of CycD3 genes and the effect was more rapid and pronounced than after pollination/fertilization.

Purine and pyrimidine biosynthesis in higher plants

Purine and pyrimidine nucleotides have important functions in a multitude of biochemical and developmental processes during the life cycle of a plant. In higher plants the processes of nucleotide metabolism are poorly understood, but it is in principle accepted that nucleotides are essential constituents of fundamental biological functions. Despite of its significance, higher plant nucleotide metabolism has been poorly explored during the last 10-20 years (Suzuki and Takahashi 1977, Schubert 1986, Wagner and Backer 1992). But considerable progress was made on purine biosynthesis in nodules of ureide producing tropical legumes, where IMP-synthesis plays a dominant role in primary nitrogen metabolism (Atkins and Smith 2000, Smith and Atkins 2002). Besides these studies on tropical legumes, this review emphasises on progress made in analysing the function in planta of genes involved in purine and pyrimidine biosynthesis and their impact on metabolism and development.

A rapid cytokinin response assay in Arabidopsis indicates a role for phospholipase D in cytokinin signalling

Seedlings of Arabidopsis thaliana harboring a fusion of the cytokinin-responsive ARR5 gene promoter and the GUS reporter gene were used for a pharmacological approach to study cytokinin signal transduction. The assay was shown to be rapid, sensitive, dose-dependent and highly specific for cytokinins, both adenine and phenylurea derivatives. Numerous inhibitors of known signalling pathways were tested and some were shown to suppress reporter gene induction. Particularly, primary alcohols that specifically inhibit phospholipase D (PLD) partially prevented cytokinin-induced GUS activity and reduced the accumulation of ARR5 gene transcripts. This indicates a role for PLD early during cytokinin signalling.

Local and long-range signaling pathways regulating plant responses to nitrate

Nitrate is the major source of nitrogen (N) for plants growing in aerobic soils. However, the NO3- ion is also used by plants as a signal to reprogram plant metabolism and to trigger changes in plant architecture. A striking example is the way that a root system can react to a localized source of NO3- by activating the NO3- uptake system and proliferating lateral roots preferentially within the NO3(-)-rich zone. That roots are able to respond autonomously in this fashion implies the existence of local signaling pathways that are sensitive to local changes in the external NO3- concentration. On the other hand, long-range signaling pathways are also needed to modulate these responses according to the plant's N status and to coordinate the allocation of resources between the root and the shoot. This review examines these signaling mechanisms and their interactions with sugar-sensing and hormonal response pathways.

Vascular tissue differentiation and pattern formation in plants

Vascular tissues, xylem and phloem, are differentiated from meristematic cells, procambium, and vascular cambium. Auxin and cytokinin have been considered essential for vascular tissue differentiation; this is supported by recent molecular and genetic analyses. Xylogenesis has long been used as a model for study of cell differentiation, and many genes involved in late stages of tracheary element formation have been characterized. A number of mutants affecting vascular differentiation and pattern formation have been isolated in Arabidopsis. Studies of some of these mutants have suggested that vascular tissue organization within the bundles and vascular pattern formation at the organ level are regulated by positional information.

Steroid signaling in plants: from the cell surface to the nucleus

Steroid hormones are signaling molecules important for normal growth, development and differentiation of multicellular organisms. Brassinosteroids (BRs) are a class of polyhydroxylated steroids that are necessary for plant development. Molecular genetic studies in Arabidopsis thaliana have led to the cloning and characterization of the BR receptor, BRI1, which is a transmembrane receptor serine/threonine kinase. The extracellular domain of BRI1, which is composed mainly of leucine-rich repeats, can confer BR responsivity to heterologous cells and is required for BR binding. Although downstream components of BR action are mostly unknown, multiple genes whose expression are regulated by BRs have been identified and suggest mechanisms by which BRs affect cell elongation and division.

CYTOKININ METABOLISM AND ACTION

Cytokinins are structurally diverse and biologically versatile. The chemistry and physiology of cytokinin have been studied extensively, but the regulation of cytokinin biosynthesis, metabolism, and signal transduction is still largely undefined. Recent advances in cloning metabolic genes and identifying putative receptors portend more rapid progress based on molecular techniques. This review centers on cytokinin metabolism with connecting discussions on biosynthesis and signal transduction. Important findings are summarized with emphasis on metabolic enzymes and genes. Based on the information generated to date, implications and future research directions are presented.

Promotive effect of brassinosteroids on cell division involves a distinct CycD3-induction pathway in Arabidopsis

Brassinosteroids (BRs) are steroid hormones that play an essential role in plant growth and development. However, the contradictory results of previous studies make their role in cell division unclear. Using a cDNA array, we identified genes that respond to BR in the det2 suspension culture of Arabidopsis, and found that epi-brassinolide upregulated transcription of the CycD3, a D-type plant cyclin gene through which cytokinin activates cell division. RNA gel-blot analysis and cell culturing showed that epi-brassinolide may promote cell division through CycD3, and can substitute cytokinin in culturing of Arabidopsis callus and suspension cells. The CycD3 induction by epi-brassinolide was further shown to involve de novo protein synthesis, but no protein phosphorylation or dephosphorylation. Induction was also found to occur in cells of a BR-insensitive mutant, bri1, suggesting that BR induces CycD3 transcription through a previously unknown signal pathway in plants.

Effects of elevated [CO(2)] and nitrogen nutrition on cytokinins in the xylem sap and leaves of cotton

We measured the level of xylem-derived cytokinins (CKs) entering a cotton leaf, and the CK levels in the same leaf, thus enabling xylem sap and foliar CKs to be compared concurrently. Although zeatin was the dominant CK in xylem sap, zeatin, dihydrozeatin, and N(6)-(2-isopentenyl) adenine were present in approximately equimolar levels in leaves. Elevated [CO(2)] (EC) has an effect on the levels of cytokinins in sap and leaf tissues. This effect was modulated by the two levels of root nitrogen nutrition (2 and 12 mM nitrate). Growth enhancement (70%) in EC over plants in ambient [CO(2)] (AC) was observed for both nitrogen nutrition treatments. Low-nitrogen leaves growing in EC exhibited photosynthetic acclimation, whereas there was no sign of photosynthetic acclimation in high-nitrogen grown leaves. Under these prevailing conditions, xylem sap and leaf tissues were obtained for CK analysis. Higher nitrogen nutrition increased the delivery per unit leaf area of CKs to the leaf at AC. EC caused a greater increase in CK delivery to the leaf at low nitrogen conditions (106%) than at high nitrogen conditions (17%). EC induced a significant increase in CK content in low-nitrogen leaves, whereas CK content in leaf tissues was similar for high-nitrogen leaves growing in AC and EC.

The Cytokinin-hypersensitive genes of Arabidopsis negatively regulate the cytokinin-signaling pathway for cell division and chloroplast development

We isolated Arabidopsis thaliana mutants that respond more sensitively than the wild type to cytokinins. The calli produced from the mutants exhibit typical cytokinin responses, including rapid proliferation and chloroplast development in response to lower levels of cytokinins than in the wild type. The mutations are recessive and belong to two complementation groups designated ckh1 and ckh2 for cytokinin-hypersensitive. CKH1 and CKH2 were mapped to the top of chromosome I and the middle of chromosome II, respectively. The cytokinin levels in these mutants were not increased. We speculate that the CKH1 and CKH2 gene products negatively regulate the signaling pathway leading from cytokinin perception to cell proliferation and chloroplast development.

Short root mutant of Lotus japonicus with a dramatically altered symbiotic phenotype

Legume plants carefully control the extent of nodulation in response to rhizobial infection. To examine the mechanism underlying this process we conducted a detailed analysis of the Lotus japonicus hypernodulating mutants, har1-1, 2 and 3 that define a new locus, HYPERNODULATION ABERRANT ROOT FORMATION (Har1), involved in root and symbiotic development. Mutations in the Har1 locus alter root architecture by inhibiting root elongation, diminishing root diameter and stimulating lateral root initiation. At the cellular level these developmental alterations are associated with changes in the position and duration of root cell growth and result in a premature differentiation of har1-1 mutant root. No significant differences between har1-1 mutant and wild-type plants were detected with respect to root growth responses to 1-aminocyclopropane1-carboxylic acid, the immediate precursor of ethylene, and auxin; however, cytokinin in the presence of AVG (aminoetoxyvinylglycine) was found to stimulate root elongation of the har1-1 mutant but not the wild-type. After inoculation with Mesorhizobium loti, the har1 mutant lines display an unusual hypernodulation (HNR) response, characterized by unrestricted nodulation (hypernodulation), and a concomitant drastic inhibition of root and shoot growth. These observations implicate a role for the Har1 locus in both symbiotic and non-symbiotic development of L. japonicus, and suggest that regulatory processes controlling nodule organogenesis and nodule number are integrated in an overall mechanism governing root growth and development.

A critical role of sterols in embryonic patterning and meristem programming revealed by the fackel mutants of Arabidopsis thaliana

Here we report a novel Arabidopsis dwarf mutant,fackel-J79, whose adult morphology resembles that of brassinosteroid-deficient mutants but also displays distorted embryos, supernumerary cotyledons, multiple shoot meristems, and stunted roots. We cloned the FACKEL gene and found that it encodes a protein with sequence similarity to both the human sterol reductase family and yeast C-14 sterol reductase and is preferentially expressed in actively growing cells. Biochemical analysis indicates that the fk-J79mutation results in deficient C-14 sterol reductase activity, abnormal sterol composition, and reduction of brassinosteroids (BRs). Unlike other BR-deficient mutants, the defect of hypocotyl elongation infk-J79 cannot be corrected by exogenous BRs. The unique phenotypes and sterol composition in fk-J79 indicate crucial roles of sterol regulation and signaling in cell division and cell expansion in embryonic and post-embryonic development in plants.

Triggering the cell cycle in plants

In essence, the mitotic cell cycle in eukaryotes involves the duplication and separation of chromosomes, coupled to the process of dividing one cell into two. Cytokinesis is therefore the culmination of a series of events that were triggered during G1 phase, and brings the daughter cells back to the starting position in G1 for another possible round of division. In all eukaryotes, progression through the cell cycle is controlled by cyclin-dependent kinases that bind to positive regulators called cyclins. This review explores some of the pathways that trigger the plant cell cycle, with emphasis on the G1 phase. Examples include signalling pathways involving glutathione and cellular redox potential, the possible existence of a G1 DNA-damage checkpoint, and the plant hormones auxin and cytokinin. Progress in understanding the link between cell proliferation, cell differentiation and the cell-cycle machinery in a developmental context is discussed.

Two-component and phosphorelay signal transduction

Two-component and phosphorelay signal transduction systems are the major means by which bacteria recognize and respond to a variety of environmental stimuli. Recent results have implicated these systems in the regulation of a variety of essential processes including cell-cycle progression, pathogenicity, and developmental pathways. Elucidation of the structures of the interacting domains is leading to an understanding of the mechanisms of molecular recognition and phosphotransfer in these systems.