Dynamics of cytokinins in apical shoot meristems of a day-neutral tobacco during floral transition and flower formation

BSA-Seq and Transcriptomic Analysis Provide Candidate Genes Associated with Inflorescence Architecture and Kernel Orientation by Phytohormone Homeostasis in Maize

The developmental plasticity of the maize inflorescence depends on meristems, which directly affect reproductive potential and yield. However, the molecular roles of upper floral meristem (UFM) and lower floral meristem (LFM) in inflorescence and kernel development have not been fully elucidated. In this study, we characterized the reversed kernel1 (rk1) novel mutant, which contains kernels with giant embryos but shows normal vegetative growth like the wild type (WT). Total RNA was extracted from the inflorescence at three stages for transcriptomic analysis. A total of 250.16-Gb clean reads were generated, and 26,248 unigenes were assembled and annotated. Gene ontology analyses of differentially expressed genes (DEGs) detected in the sexual organ formation stage revealed that cell differentiation, organ development, phytohormonal responses and carbohydrate metabolism were enriched. The DEGs associated with the regulation of phytohormone levels and signaling were mainly expressed, including auxin (IAA), jasmonic acid (JA), gibberellins (GA), and abscisic acid (ABA). The transcriptome, hormone evaluation and immunohistochemistry observation revealed that phytohormone homeostasis were affected in rk1. BSA-Seq and transcriptomic analysis also provide candidate genes to regulate UFM and LFM development. These results provide novel insights for understanding the regulatory mechanism of UFM and LFM development in maize and other plants.

Dynamics of endogenous levels and subcellular localization of ABA and cytokinins during pollen germination in spruce and tobacco

We used the enzyme-linked immunosorbent assay (ELISA) to assess the level of endogenous hormones in spruce pollen, and immunolocalization and confocal microscopy to study hormone localization in spruce and tobacco pollen. During pollen activation, the levels of ABA, zeatin, and its riboside significantly decreased. After the initiation of polar growth, the levels of all cytokinins increased sharply; ABA level also increased. In dormant spruce pollen grains, zeatin and ABA were localized uniformly throughout the cytoplasm. Zeatin was not detected in the nuclei, and the antheridial cell showed higher levels than the vegetative cell; ABA signal was detected in the cytoplasm and the nuclei. In germinating pollen, both hormones were detected mainly in plastids. The similar pattern was found in growing pollen tubes; signal from ABA also had a noticeable level in the cytosol of the tube cell, and was weaker in the antheridial cell. Zeatin fluorescence, on the other hand, was more pronounced in the antheridial cell. In non-germinated grains of tobacco, zeatin was localized mainly in organelles. ABA in dormant pollen grains demonstrated uniform localization, including the nuclei and cytoplasm of both cells. After germination, zeatin was accumulated in the plasmalemma or cell wall. ABA signal in the cytoplasm decreased; in the nuclei, it remained high. In growing tubes, the strongest zeatin and ABA signals were observed at the plasma membrane. The differences in ABA and cytokinin localization between species and dynamic changes in their level in spruce pollen highlight the key spatial and temporal parameters of hormonal regulation of gymnosperm pollen germination.

The RhLOL1-RhILR3 module mediates cytokinin-induced petal abscission in rose

In many plant species, petal abscission can be considered the final step of petal senescence. Cytokinins (CKs) are powerful suppressors of petal senescence; however, their role in petal abscission is ambiguous. Here, we observed that, in rose (Rosa hybrida), biologically active CK is accumulated during petal abscission and acts as an accelerator of the abscission process. Using a combination of reverse genetics, and molecular and biochemical techniques, we explored the roles of a LESION SIMULATING DISEASE1 (LSD1) family member RhLOL1 interacting with a bHLH transcription factor RhILR3 in CK-induced petal abscission. Silencing RhLOL1 delays rose petal abscission, while the overexpression of its ortholog SlLOL1 in tomato (Solanum lycopersicum) promotes pedicel abscission, indicating the conserved function of LOL1 in activating plant floral organ abscission. In addition, we identify a bHLH transcription factor, RhILR3, that interacts with RhLOL1. We show that RhILR3 binds to the promoters of the auxin signaling repressor auxin/indole-3-acetic acid (Aux/IAA) genes to inhibit their expression; however, the interaction of RhLOL1 with RhILR3 activates the expression of the Aux/IAA genes including RhIAA4-1. Silencing RhIAA4-1 delays rose petal abscission. Our results thus reveal a RhLOL1-RhILR3 regulatory module involved in CK-induced petal abscission via the regulation of the expression of the Aux/IAA genes.

Up-regulation of LsKN1 promotes cytokinin and suppresses gibberellin biosynthesis to generate wavy leaves in lettuce

Lettuce (Lactuca sativa) is one of the most popular vegetables worldwide, and diverse leaf shapes, including wavy leaves, are important commercial traits. In this study, we examined the genetics of wavy leaves using an F2 segregating population, and identified a major QTL controlling wavy leaves. The candidate region contained LsKN1, which has previously been shown to be indispensable for leafy heads in lettuce. Complementation tests and knockout experiments verified the function of LsKN1 in producing wavy leaves. The LsKN1∇ allele, which has the insertion of a transposon and has previously been shown to control leafy heads, promoted wavy leaves in our population. Transposition of the CACTA transposon from LsKN1 compromised its function for wavy leaves. High expression of LsKN1 up-regulated several key genes associated with cytokinin (CK) to increase the content in the leaves, whereas it down-regulated the expression of genes in the gibberellin (GA) biosynthesis pathway to decrease the content. Application of CK to leaves enhanced the wavy phenotype, while application of GA dramatically flattened the leaves. We conclude that the changes in CK and GA contents that result from high expression of LsKN1 switch determinate cells to indeterminate, and consequently leads to the development of wavy leaves.

Azospirillum brasilense Sp245 triggers cytokinin signaling in root tips and improves biomass accumulation in Arabidopsis through canonical cytokinin receptors

The plant growth promoting rhizobacterium Azospirillum brasilense Sp245 enhances biomass production in cereals and horticultural species and is an interesting model to study the physiology of the phytostimulation program. Although auxin production by Azospirillum appears to be critical for root architectural readjustments, the role of cytokinins in the growth promoting effects of Azospirillum remains unclear. Here, Arabidopsis thaliana seedlings were co-cultivated in vitro with A. brasilense Sp245 to assess whether direct contact of roots with bacterial colonies or exposure to the bacterial volatiles using divided Petri plates would affect biomass production and root organogenesis. Both interaction types increased root and shoot fresh weight but had contrasting effects on primary root length, lateral root formation and root hair development. Cell proliferation in root meristems analyzed with the CYCB1;1::GUS reporter decreased over time with direct contact, but was augmented by plant exposure to volatiles. Noteworthy, the expression of the cytokinin-inducible reporters TCS::GFP and ARR5::GUS increased in root tips in response to bacterial contact, without being affected by the volatiles. In A. thaliana having single (cre1-12, ahk2-2, ahk3-3), double (cre1-12/ahk2-2, cre1-12/ahk3-3, ahk2-2/ahk3-3) or triple (cre1-12/ahk2-2/ahk3-3) mutations in canonical cytokinin receptors, only the triple mutant had a marked effect on plant growth in response to A. brasilense. These results show that different mechanisms are elicited by A. brasilense, which influence the cytokinin-signaling pathway.

The Localization of Phytohormones within the Gall-inducing Insect Eurosta solidaginis (Diptera: Tephritidae)

The phytohormone production hypothesis suggests that organisms, including insects, induce galls by producing and secreting plant growth hormones. Auxins and cytokinins are classes of phytohormones that induce cell growth and cell division, which could contribute to the plant tissue proliferation which constitutes the covering gall. Bacteria, symbiotic with insects, may also play a part in gall induction by insects through the synthesis of phytohormones or other effectors. Past studies have shown that concentrations of cytokinins and auxins in gall-inducing insects are higher than in their host plants. However, these analyses have involved whole-body extractions. Using immunolocalization of cytokinin and auxin, in the gall inducing stage of Eurosta solidaginis, we found both phytohormones to localize almost exclusively to the salivary glands. Co-localization of phytohormone label with a nucleic acid stain in the salivary glands revealed the absence of Wolbachia sp., the bacterial symbiont of E. solidaginis, which suggests that phytohormone production is symbiont independent. Our findings are consistent with the hypothesis that phytohormones are synthesized in and secreted from the salivary glands of E. solidaginis into host-plant tissues for the purpose of manipulating the host plant.

Comparative Study between Exogenously Applied Plant Growth Hormones versus Metabolites of Microbial Endophytes as Plant Growth-Promoting for Phaseolus vulgaris L

Microbial endophytes organize symbiotic relationships with the host plant, and their excretions contain diverse plant beneficial matter such as phytohormones and bioactive compounds. In the present investigation, six bacterial and four fungal strains were isolated from the common bean (Phaseolus vulgaris L.) root plant, identified using molecular techniques, and their growth-promoting properties were reviewed. All microbial isolates showed varying activities to produce indole-3-acetic acid (IAA) and different hydrolytic enzymes such as amylase, cellulase, protease, pectinase, and xylanase. Six bacterial endophytic isolates displayed phosphate-solubilizing capacity and ammonia production. We conducted a field experiment to evaluate the promotion activity of the metabolites of the most potent endophytic bacterial (Bacillus thuringiensis PB2 and Brevibacillus agri PB5) and fungal (Alternaria sorghi PF2 and, Penicillium commune PF3) strains in comparison to two exogenously applied hormone, IAA, and benzyl adenine (BA), on the growth and biochemical characteristics of the P. vulgaris L. Interestingly, our investigations showed that bacterial and fungal endophytic metabolites surpassed the exogenously applied hormones in increasing the plant biomass, photosynthetic pigments, carbohydrate and protein contents, antioxidant enzyme activity, endogenous hormones and yield traits. Our findings illustrate that the endophyte Brevibacillus agri (PB5) provides high potential as a stimulator for the growth and productivity of common bean plants.

Effects of Phosphate Shortage on Root Growth and Hormone Content of Barley Depend on Capacity of the Roots to Accumulate ABA

Although changes in root architecture in response to the environment can optimize mineral and water nutrient uptake, mechanisms regulating these changes are not well-understood. We investigated whether P deprivation effects on root development are mediated by abscisic acid (ABA) and its interactions with other hormones. The ABA-deficient barley mutant Az34 and its wild-type (WT) were grown in P-deprived and P-replete conditions, and hormones were measured in whole roots and root tips. Although P deprivation decreased growth in shoot mass similarly in both genotypes, only the WT increased primary root length and number of lateral roots. The effect was accompanied by ABA accumulation in root tips, a response not seen in Az34. Increased ABA in P-deprived WT was accompanied by decreased concentrations of cytokinin, an inhibitor of root extension. Furthermore, P-deficiency in the WT increased auxin concentration in whole root systems in association with increased root branching. In the ABA-deficient mutant, P-starvation failed to stimulate root elongation or promote branching, and there was no decline in cytokinin and no increase in auxin. The results demonstrate ABA’s ability to mediate in root growth responses to P starvation in barley, an effect linked to its effects on cytokinin and auxin concentrations.

Effect of Pruning Young Branches on Fruit and Seed Set in Cassava

Flowering in cassava is closely linked with branching. Early-flowering genotypes branch low and abundantly. Although farmers prefer late flowering genotypes because of their erect plant architecture, their usefulness as progenitors in breeding is limited by their low seed production. In general, the first inflorescence aborts in cassava. Preventing this abortion would result in early production of seeds and make cassava breeding more efficient. The objective of this study was to assess if pruning young branches prevents the abortion of first inflorescences and promotes early fruit and seed set. Four genotypes with early, late, very late, and no flowering habits were grown under an extended photoperiod (EP) or normal dark night conditions (DN). Additional treatments included pruning young branches at the first or second flowering event and spraying (or not) benzyladenine (BA) after pruning. One genotype failed to flower and was not considered further. For the remaining genotypes, EP proved crucial to induce an earlier flowering, which is a pre-requisite for pruning. Total production of seeds in EP plots was 2,971 versus 150 in DN plots. For plants grown under EP, the average number of seeds per plant without pruning was 3.88, whereas those pruned produced 17.60 seeds per plant. Pruning at the first branching event led to higher number of seeds per plant (26.25) than pruning at the second flowering event (8.95). In general, applying BA was beneficial (38.52 and 13.98 seeds/plant with or without spraying it, respectively). The best combination of treatments was different for each genotype. Pruning young branches and applying BA in the first flowering event not only prevented the abortion of inflorescences but also induced the feminization of male flowers into hermaphrodite or female-only flowers. The procedures suggested from this study (combining EP, pruning young branches, and spraying BA), allowed the production of a high number of seeds from erect cassava genotypes in a short period. The implementation of these procedures will improve the breeding efficiency in cassava.

Enzyme-mediated metabolism in nutritive tissues of galls induced by Ditylenchus gallaeformans (Nematoda: Anguinidae)

The galls induced by Ditylenchus gallaeformans (Nematoda) on leaves of Miconia albicans have unique features when compared to other galls. The nematode colonies are surrounded by nutritive tissues with promeristematic cells, capable of originating new emergences facing the larval chamber, and providing indeterminate growth to these galls. Considering enzyme activity as essential for the translocation of energetic molecules from the common storage tissue (CST) to the typical nutritive tissue (TNT), and the major occurrence of carbohydrates in nematode galls, it was expected that hormones would mediate sink strength relationships by activating enzymes in indeterminate growth regions of the galls. Histochemical, immunocytochemical and quantitative analyses were made in order to demonstrate sites of enzyme activity and hormones, and comparative levels of total soluble sugars, water soluble polysaccharides and starch. The source-sink status, via carbohydrate metabolism, is controlled by the major accumulation of cytokinins in totipotent nutritive cells and new emergences. Thus, reducing sugars, such as glucose and fructose, accumulate in the TNT, where they supply the energy for successive cycles of cell division and for nematode feeding. The histochemical detection of phosphorylase and invertase activities indicates the occurrence of starch catabolism and sucrose transformation into reducing sugars, respectively, in the establishment of a gradient from the CST towards the TNT. Reducing sugars in the TNT are important for the production of new cell walls during the indeterminate growth of the galls, which have increased levels of water-soluble polysaccharides that corroborate such a hypothesis. Functional relationship between plant hormone accumulation, carbohydrate metabolism and cell differentiation in D. gallaeformans-induced galls is attested, providing new insights on cell development and plant metabolism.

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.

Study of cytokinin transport from shoots to roots of wheat plants is informed by a novel method of differential localization of free cytokinin bases or their ribosylated forms by means of their specific fixation

The aim of the present report was to demonstrate how a novel approach for immunohistochemical localization of cytokinins in the leaf and particularly in the phloem may complement to the study of their long-distance transport. Different procedures of fixation were used to conjugate either cytokinin bases or their ribosides to proteins of cytoplasm to enable visualization and differential localization of these cytokinins in the leaf cells of wheat plants. In parallel to immunolocalization of cytokinins in the leaf cells, we immunoassayed distribution of free bases of cytokinins, their nucleotides and ribosides between roots and shoots of wheat plants as well as their presence in phloem sap after incubation of leaves in a solution supplemented with either trans-zeatin or isopentenyladenine. The obtained data show ribosylation of the zeatin applied to the leaves and its elevated level in the phloem sap supported by in vivo localization showing the presence of ribosylated forms of zeatin in leaf vessels. This suggests that conversion of zeatin to its riboside is important for the shoot-to-root transport of zeatin-type cytokinins in wheat. Exogenous isopentenyladenine was not modified, but diffused from the leaves as free base. These metabolic differences may not be universal and may depend on the plant species and age. Although the measurements of cytokinins in the phloem sap and root tissue is the most defining for determining cytokinin transport, study of immunolocalization of either free cytokinin bases or their ribosylated forms may be a valuable source of information for predicting their transport in the phloem and to the roots.

Zooming In on Plant Hormone Analysis: Tissue- and Cell-Specific Approaches

Plant hormones are a group of naturally occurring, low-abundance organic compounds that influence physiological processes in plants. Our knowledge of the distribution profiles of phytohormones in plant organs, tissues, and cells is still incomplete, but advances in mass spectrometry have enabled significant progress in tissue- and cell-type-specific analyses of phytohormones over the last decade. Mass spectrometry is able to simultaneously identify and quantify hormones and their related substances. Biosensors, on the other hand, offer continuous monitoring; can visualize local distributions and real-time quantification; and, in the case of genetically encoded biosensors, are noninvasive. Thus, biosensors offer additional, complementary technologies for determining temporal and spatial changes in phytohormone concentrations. In this review, we focus on recent advances in mass spectrometry-based quantification, describe monitoring systems based on biosensors, and discuss validations of the various methods before looking ahead at future developments for both approaches.

GA4 and IAA were involved in the morphogenesis and development of flowers in Agapanthus praecox ssp. orientalis

The transition from vegetative to reproductive growth represents a major phase change in angiosperms. Hormones play important roles in this process. In this study, gibberellic acid (GA), cytokinins (CKs), indoleacetic acid (IAA), and abscisic acid (ABA) were analyzed during the flowering in Agapanthus praecox ssp. orientalis. Eleven types of endogenous gibberellins in addition to GA1 were detected in various organs. GA9 was detected with the highest concentrations, followed by GA5, GA8, and GA19. However, GA4 was the main bioactive GA that was involved in the regulation of flowering. Eight types of endogenous cytokinins were detected in A. praecox ssp. orientalis, and zeatin, zeatin riboside, zeatin-O-glucoside, and N(6)-isopentenyladenosine-5-monophosphate were present at higher levels throughout the study, of which zeatin plays an important role in the development of various organs. IAA increased by 581% in the shoot tips from the vegetative to inflorescence bud stages and had the most significant changes during flowering. Phytohormone immunolocalization analysis suggested that IAA involved in differentiation and development of each floral organs, GA and zeatin play important roles in floret primordia differentiation and ovule development. Using exogenous plant growth regulators proved that GA signaling regulate the scape elongation and stimulate early-flowering, and IAA signaling is involved in the pedicel and corolla elongation and delay flowering slightly.

Quo vadis plant hormone analysis?

Plant hormones act as chemical messengers in the regulation of myriads of physiological processes that occur in plants. To date, nine groups of plant hormones have been identified and more will probably be discovered. Furthermore, members of each group may participate in the regulation of physiological responses in planta both alone and in concert with members of either the same group or other groups. The ideal way to study biochemical processes involving these signalling molecules is 'hormone profiling', i.e. quantification of not only the hormones themselves, but also their biosynthetic precursors and metabolites in plant tissues. However, this is highly challenging since trace amounts of all of these substances are present in highly complex plant matrices. Here, we review advances, current trends and future perspectives in the analysis of all currently known plant hormones and the associated problems of extracting them from plant tissues and separating them from the numerous potentially interfering compounds.

Relationship between endogenous hormonal content and somatic organogenesis in callus of peach (Prunus persica L. Batsch) cultivars and Prunus persica×Prunus dulcis rootstocks

The relationship between endogenous hormones content and the induction of somatic peach plant was studied. To induce multiple shoots from callus derived from the base of stem explants of the scion cultivars 'UFO-3', 'Flariba' and 'Alice Bigi', and the peach×almond rootstocks 'Garnem' and 'GF677', propagated plants were cultured on Murashige and Skoog salts augmented with 0.1mgL(-1) of indolebutyric acid, 1mgL(-1) of 6-benzylaminopurine and 3% sucrose. The highest regeneration rate was obtained with the peach×almond rootstocks. Endogenous levels of abscisic acid (ABA), indole-3-acetic acid (IAA), zeatin (Z), zeatin riboside (ZR), ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), salicylic acid (SA), and jasmonic acid (JA) were analyzed in the organogenic callus. Lower levels of several hormones, namely Z, ZR, ABA, and ACC were found in the peach×almond rootstock compared to peach cultivars, while IAA and SA presented inconclusive returns. These results suggest that the difference in somatic organogenesis capacity observed in peach and peach×almond hybrids is markedly affected by the endogenous hormonal content of the studied genotypes.

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.

The Arabidopsis homeobox gene SHOOT MERISTEMLESS has cellular and meristem-organisational roles with differential requirements for cytokinin and CYCD3 activity

The Arabidopsis class-1 KNOX gene SHOOT MERISTEMLESS (STM) encodes a homeodomain transcription factor essential for shoot apical meristem (SAM) formation and sustained activity. STM activates cytokinin (CK) biosynthesis in the SAM, but the extent to which STM function is mediated through CK is unclear. Here we show that STM inhibits cellular differentiation and endoreduplication, acting through CK and the CK-inducible CYCD3 cell cycle regulators, establishing a mechanistic link to cell cycle control which provides sustained mitotic activity to maintain a pool of undifferentiated cells in the SAM. Equivalent functions are revealed for the related KNOX genes KNAT1/BP and KNAT2 through ectopic expression. STM is also required for proper meristem organisation and can induce de novo meristem formation when expressed ectopically, even when CK levels are reduced or CK signaling is impaired. This function in meristem establishment and organisation can be replaced by KNAT1/BP, but not KNAT2, despite its activation of CK responses, suggesting that promotion of CK responses alone is insufficient for SAM organisation. We propose that STM has dual cellular and meristem-organisational functions that are differentially represented in the class-1 KNOX gene family and have differing requirements for CK and CYCD3.

Differential expression and modification of proteins during ontogenesis in Malus domestica

Many morphological and physiological changes have been widely reported during ontogeny in higher plants. In order for the better understanding of the proteomic differences between ontogenetic phases, protein compositions between leaves of juvenile, adult vegetative and reproductive phases were compared in an apple (Malus domestica Borkh., Jonathan × Golden Delicious) seedling. Totally, 122 differentially expressed or modified protein spots were separated by DIGE. Of the 122 protein spots, 44, 17 and 29 were abundant in the leaf samples from the juvenile, adult vegetative and reproductive phases, respectively, two spots showed a lower level in the adult vegetative tissue, while the amount of protein increased in 21 spots during ontogeny and declined in nine spots. One hundred and fifteen spots were successfully picked and 95 spots were identified by MALDI-TOF-TOF high-resolution tandem mass spectrometry. Twenty-three juvenile phase abundant or down-regulated spots were photosynthesis-associated proteins, implying a juvenile phase-related photosynthesis enhancement. The expression of 10 enzymes and coenzymes involved in protein synthesis and catabolism was elevated in the adult reproductive phase or up-regulated during ontogeny, contributing a phase change-related activation in protein metabolism. Six proteins generated 30 differential gel spots via post-translational modifications. The differential expression of NADP-dependent D-sorbitol-6-phosphate dehydrogenase was confirmed by Western blotting in six seedlings derived from two hybrid populations. The results of semi-quantitative PCR indicate that some but not all of these proteomic changes were transcriptionally regulated.

Differential spatial expression of A- and B-type CDKs, and distribution of auxins and cytokinins in the open transverse root apical meristem of Cucurbita maxima

BACKGROUND AND AIMS

Aside from those on Arabidopsis, very few studies have focused on spatial expression of cyclin-dependent kinases (CDKs) in root apical meristems (RAMs), and, indeed, none has been undertaken for open meristems. The extent of interfacing between cell cycle genes and plant growth regulators is also an increasingly important issue in plant cell cycle studies. Here spatial expression/localization of an A-type and B-type CDK, auxin and cytokinins are reported in relation to the hitherto unexplored anatomy of RAMs of Cucurbita maxima.

METHODS

Median longitudinal sections were cut from 1-cm-long primary root tips of C. maxima. Full-length A-type CDKs and a B-type CDK were cloned from C. maxima using degenerate primers, probes of which were localized on sections of RAMs using in situ hybridization. Isopentenyladenine (iPA), trans-zeatin (t-Z) and indole-3yl-acetic acid (IAA) were identified on sections by immunolocalization.

KEY RESULTS

The C. cucurbita RAM conformed to an open transverse (OT) meristem typified by an absence of a clear boundary between the eumeristem and root cap columella, but with a distinctive longitudinally thickened epidermis. Cucma;CDKA;1 expression was detected strongly in the longitudinally thickened epidermis, a tissue with mitotic competence that contributes cells radially to the root cap of OT meristems. Cucma;CDKB2 was expressed mainly in proliferative regions of the RAM and in lateral root primordia. iPA and t-Z were mainly distributed in differentiated cells whilst IAA was distributed more uniformly in all tissues of the RAM.

CONCLUSIONS

Cucma;CDKA;1 was expressed most strongly in cells that have proliferative competence whereas Cucma;CDKB2 was confined mainly to mitotic cells. iPA and t-Z marked differentiated cells in the RAM, consistent with the known effect of cytokinins in promoting differentiation in root systems. iPA/t-Z were distributed in a converse pattern to Cucma;CDKB2 expression whereas IAA was detected in most cells in the RAM regardless of their proliferative potential.

Spatial and temporal profiles of cytokinin biosynthesis and accumulation in developing caryopses of maize

BACKGROUND AND AIMS

Cytokinins are a major group of plant hormones and are associated with various developmental processes. Developing caryopses of maize have high levels of cytokinins, but little is known about their spatial and temporal distribution. The localization and quantification of cytokinins was investigated in maize (Zea mays) caryopsis from 0 to 28 d after pollination together with the expression and localization of isopentenyltransferase ZmIPT1 involved in cytokinin biosynthesis and ZmCNGT, the gene putatively involved in N9-glucosylation.

METHODS

Biochemical, cellular and molecular approaches resolved the overall cytokinin profiles, and several gene expression assays were used for two critical genes to assess cytokinin cell-specific biosynthesis and conversion to the biologically inactive form. Cytokinins were immunolocalized for the first time in maize caryopses.

KEY RESULTS

During the period 0-28 d after pollination (DAP): (1) large quantities of cytokinins were detected in the maternal pedicel region relative to the filial tissues during the early stages after fertilization; (2) unpollinated ovules did not accumulate cytokinins; (3) the maternal nucellar region showed little or no cytokinin signal; (4) the highest cytokinin concentrations in filial endosperm and embryo were detected at 12 DAP, predominantly zeatin riboside and zeatin-9-glucoside, respectively; and (5) a strong cytokinin immuno-signal was detected in specific cell types in the pedicel, endosperm and embryo.

CONCLUSIONS

The cytokinins of developing maize caryopsis may originate from both local syntheses as well as by transport. High levels of fertilization-dependent cytokinins in the pedicel suggest filial control on metabolism in the maternal tissue; they may also trigger developmental programmed cell death in the pedicel.

Epigenetic and physiological effects of gibberellin inhibitors and chemical pruners on the floral transition of azalea

The ability to control the timing of flowering is a key strategy in planning the production of ornamental species such as azaleas; however, it requires a thorough understanding of floral transition. DNA methylation is involved in controlling the functional state of chromatin and gene expression during floral induction pathways in response to environmental and developmental signals. Plant hormone signalling is also known to regulate suites of morphogenic processes in plants and its role in flowering-time control is starting to emerge as a key controlling step. This work investigates if the gibberellin (GA) inhibitors and chemical pinching applied in improvement of azalea flowering alter the dynamics of DNA methylation or the levels of polyamines (PAs), GAs and cytokinins (CKs) during floral transition, and whether these changes could be related to the effects observed on flowering ability. DNA methylation during floral transition and endogenous content of PAs, GAs and CKs were analysed after the application of GA synthesis inhibitors (daminozide, paclobutrazol and chlormequat chloride) and a chemical pruner (fatty acids). The application of GA biosynthesis inhibitors caused alterations in levels of PAs, GAs and CKs and in global DNA methylation levels during floral transition; also, these changes in plant growth regulators and DNA methylation were correlated with flower development. DNA methylation, PA, GA and CK levels can be used as predictive markers of plant floral capacity in azalea.

Isopentenyl transferase gene (ipt) downstream transcriptionally fused with gene expression improves the growth of transgenic plants

This research reports a promising approach to increase a plant's physiological cytokinin content. This approach also enables the increase to play a role in plant growth and development by introducing the ipt gene to downstream transcriptionally fuse with other genes under the control of a CaMV35S promoter, in which the ipt gene is far from the 35S promoter. According to Kozak's ribosome screening model, expression of the ipt gene is reduced by the terminal codon of the first gene and the internal untranslated nucleotides between the fused genes. In the transgenic plants pVKH35S-GUS-ipt, pVKH35S-AOC-ipt, and pVKH35S-AtGolS2-ipt, cytokinins were increased only two to threefold, and the plants grew more vigorously than the pVKH35S-AOC or pVKH35S-AtGolS2 transgenic plants lacking the ipt gene. The vigorous growth was reflected in rapid plant growth, a longer flowering period, a greater number of flowers, more seed product, and increased chlorophyll synthesis. The AOC and AtGolS2 genes play a role in a plant's tolerance of salt or cold, respectively. When the ipt gene transcriptionally fuses with AOC or AtGolS2 in the frame of AOC-ipt and AtGolS2-ipt, slight cytokinin increases were obtained in their transgenic plants; furthermore, those increases played a positive role in improvements of plant growth. Notably, an increased cytokinin volume at the physiological level, in concert with AtGolS2 expression, enhances a plant's tolerance to cold.

Enhancement of flowering and branching phenotype in chrysanthemum by expression of ipt under the control of a 0.821 kb fragment of the LEACO1 gene promoter

The cytokinin biosynthesis gene, isopentenyl transferase (ipt), under the control of an 821 bp fragment of the LEACO1 gene promoter (from Lycopersicon esculentum) was introduced into Dendranthema x grandiflorium 'Iridon' (chrysanthemum). LEACO1(0.821kb)-ipt transgenic lines grown in the vegetative state, exhibited a range of phenotypic changes including increased branching and reduced internode lengths. LEACO1(0.821kb)-ipt transgenic lines grown in the generative state, exhibited increased flower bud count that ranged from 3.8- to 6.7-times the number produced by wild-type plants. Dramatic increases in flower number were associated with a delay of flower bud development and a decrease in flower bud diameter. RT-PCR analysis indicated differences in ipt gene expression between individual transgenic lines that exhibited a range of phenotypes. Within an individual transgenic line, RT-PCR analysis revealed changes in ipt gene expression at different stages of generative shoot development. Expression of ipt in transgenic lines correlated well with high concentrations of the sum total to bioactive cytokinins plus the glucosides and phosphate derivatives of these species, under both vegetative and generative growth conditions. In general, transgenic lines accumulated higher concentrations of both storage-form cytokinins (O-glucosides) and deactivated-form cytokinins (N-glucosides) in generative shoots of than in vegetative shoots. Based on the range of phenotypes observed in various transgenic chrysanthemum lines, we conclude that the LEACO1 (0.821kb) -ipt gene appears to have great potential for use in ornamental crop improvement.

Histological and immunohistochemical studies on flower induction in the olive tree (Olea europaea L.)

The aim of this research was to study flower bud differentiation processes in two oil olive cultivars from Tuscan germplasm (Leccino and Puntino). The effect of fruit-set was studied using 'ON' (with fruits) and 'OFF' (without fruits) shoots. Axillary buds were periodically collected at different phenological stages, from endocarp sclerification (July) until budbreak in the following spring. Thin sections were analysed using histology (apex size), histochemistry (RNA, starch and soluble carbohydrates) and cytokinin immunocytochemistry (zeatin localisation). The micromorphological observations and histochemical procedures did not allow us to distinguish axillary buds sampled from 'ON' and 'OFF' shoots. Cytokinin immunocytochemistry revealed early different localisation patterns between 'ON' and 'OFF' samples. Zeatin accumulated only in 'OFF' axillary bud meristems, particularly in July, when endocarp sclerification of fruits from the previous flowering is taking place. At this time, a strong RNA signal was also observed. Both these signals were correlated with floral evocation, and their coincidence with a phenological stage of development provided a useful tool to determine the time when axillary buds switch from the vegetative to the reproductive phase.

Induction of in vitro flowering in Dendrobium Madame Thong-In (Orchidaceae) seedlings is associated with increase in endogenous N(6)-(Delta (2)-isopentenyl)-adenine (iP) and N (6)-(Delta (2)-isopentenyl)-adenosine (iPA) levels

We analysed the endogenous cytokinin levels of Dendrobium Madame Thong-In seedlings grown in vitro during vegetative and flowering-inductive periods. HPLC was used to fractionate the extracts and radioimmunoassay (RIA) was used for assay of zeatin (Z), dihydrozeatin (DZ), N(6)-(Delta(2)-isopentenyl)-adenine (iP) and their derivatives. Coconut water used in experiments was found to contain high level (>136 pmol ml(-1)) of zeatin riboside (ZR). Protocorms and seedlings cultured in medium with coconut water were found to contain 0.5-3.9 pmol g(-1) FW of the cytokinins analysed. Seedlings (1.0-1.5 cm) cultured in flowering-inductive liquid medium containing 6-benzyladenine (BA, 4.4 muM) and coconut water (CW, 15%) contained up to 200 and 133 pmol g(-1) FW of iP and iPA, respectively. These levels were significantly higher than all other cytokinins analysed in seedlings of the same stage and were about 80- to 150-folds higher than seedlings cultured in non-inductive medium. During the transitional (vegetative to reproductive) stage, the endogenous levels of iP (178 pmol g(-1) FW) and iPA (63 pmol g(-1) FW) were also significantly higher than cytokinins in the zeatine (Z) and dihydrozeatin (DZ) families in the same seedlings. Seedlings that grew on inductive medium but remained vegetative contained lower levels of iPA. The importance of the profiles of iP and its derivatives in induction of in vitro flowering of D. Madame Thong-In is discussed.

Characterization of KNOX genes in Medicago truncatula

We isolated three class I and three class II KNOX genes in Medicago truncatula. The predicted amino acid sequences suggested a possible orthology to the Arabidopsis homeodomain proteins STM, KNAT1/BP, KNAT3 and KNAT7 that was confirmed by phylogenetic and conserved structural domain analyses. Moreover, the STM-like MtKNOX1 and MtKNOX6 proteins were shown to retain the capability to interact with the Arabidopsis BELL protein partners of STM and KNAT1/BP. Amino acid residues that characterize the different classes of KNOX proteins were identified. Gene expression studies revealed organ-specificity, possible cytokinin-dependent transcriptional activation of two MtKNOXs and expression of one STM-like and a BP/KNAT1-like MtKNOX in roots. Interestingly, mRNA localization studies carried out on class I MtKNOX genes revealed important differences with previously characterised legume KNOXs. M. truncatula transcripts were not down-regulated in leaf primordia and early stages of leaf development, features shared with the more distant compound-leaved species Solanum lycopersicum.

Two tobacco proline dehydrogenases are differentially regulated and play a role in early plant development

Proline dehydrogenase is the rate-limiting enzyme in proline degradation and serves important functions in the stress responses and development of plants. We isolated two tobacco proline dehydrogenases, NtPDH1 and NtPDH2, in the course of screening for genes upregulated in stressed tobacco (Nicotiana tabacum) microspores. Expression analysis revealed that the two genes are differentially regulated. Under unstressed conditions, their steady-state transcript levels were similar in mature pollen and apical meristems, whereas NtPDH2 was expressed predominantly in vegetative organs, styles, and ovules. The expression of NtPDH1 was maintained at a constant low level during 24 h of dehydration, whereas NtPDH2 was upregulated within 1 h after the onset of stress and subsequently downregulated to undetectable levels. Differential and sustained expression was also found for the two enzymatic isoforms of Arabidopsis thaliana AtPDH. Silencing of the NtPDH genes by RNA interference using the CaMV 35S promoter led to increased proline contents, decreased seed set, delayed seed germination and retarded seedling development pointing towards an important function of at least one of the two NtPDH genes during plant reproductive development.

Methyl recycling activities are co-ordinately regulated during plant development

A large number of compounds including lignin, phospholipids, pectin, DNA, mRNA, and proteins require methyl groups for their functionality. A detailed study of the expression and activities of two enzymes, adenosine kinase (ADK) and S-adenosylhomocysteine hydrolase (SAHH), which are both required for the maintenance and recycling of S-adenosylmethionine-dependent methylation in plants, was carried out. The abundance and tissue localization of ADK and SAHH transcripts and protein were monitored along with their enzyme activities in leaves, stems, buds, siliques, and roots of Arabidopsis. In all but roots and seed coats, the transcript abundance of ADK and SAHH fluctuated co-ordinately, matching changes in their protein and enzyme activities. To evaluate whether this expression pattern was associated with methyl recycling, the protein content and distribution of S-adenosylmethionine synthetase and phosphoethanolamine N-methyltransferase, a key methyltransferase involved in phospholipid synthesis, were investigated. These were found to accumulate in a pattern similar to ADK and SAHH. ADK and SAHH protein and transcript amounts were shown to fluctuate similarly in tissues accumulating lignin. Additionally, the amounts of ADK and SAHH mRNAs were also found at high levels in inflorescence meristems likely to support their higher rates of cell division. Thus, the results point to a co-ordinated and probably transcriptional regulation of these genes in most organs of Arabidopsis; SAHH abundance is distinctly higher in seeds and roots which suggests it may have a non-methyl-related role in these organs.

Thidiazuron influences the endogenous levels of cytokinins and IAA during the flowering of isolated shoots of Dendrobium

This study reports the effects of thidiazuron (TDZ) on the endogenous levels of indoleacetic acid (IAA), zeatin, zeatin riboside ([9R]Z), isopentenyladenine and isopentenyladenosine ([9R]iP) as well as structural changes in the apical meristem of Dendrobium Second Love shoots during flower induction and initial development in vitro. The results revealed that the presence of 1.8microM TDZ had a profound effect on the endogenous cytokinins (CKs) and IAA levels of the explants, when compared to those grown on a TDZ-free medium. A significant increase in CKs (especially [9R]iP and [9R]Z) and IAA in the first samples (taken at day 5) grown on TDZ-enriched medium was associated with flower induction, while a second increase in the level of these hormones after 25d of culture was related to flower development. The histological changes detected in the shoot apical meristem of explants grown in the presence of 1.8microM TDZ during 30d of culture are also described. Based on these findings, it is suggested that both auxin and CKs seem to be involved with the floral transition of Dendrobium Second Love in vitro. However, a possible direct effect of TDZ on flower formation is not discarded.

Auxin immunolocalization implicates vesicular neurotransmitter-like mode of polar auxin transport in root apices

Immunolocalization of auxin using a new specific antibody revealed, besides the expected diffuse cytoplasmic signal, enrichments of auxin at end-poles (cross-walls), within endosomes and within nuclei of those root apex cells which accumulate abundant F-actin at their end-poles. In Brefeldin A (BFA) treated roots, a strong auxin signal was scored within BFA-induced compartments of cells having abundant actin and auxin at their end-poles, as well as within adjacent endosomes, but not in other root cells. Importantly, several types of polar auxin transport (PAT) inhibitors exert similar inhibitory effects on endocytosis, vesicle recycling, and on the enrichments of F-actin at the end-poles. These findings indicate that auxin is transported across F-actin-enriched end-poles (synapses) via neurotransmitter-like secretion. This new concept finds genetic support from the semaphore1, rum1 and rum1/lrt1 mutants of maize which are impaired in PAT, endocytosis and vesicle recycling, as well as in recruitment of F-actin and auxin to the auxin transporting end-poles. Although PIN1 localizes abundantly to the end-poles, and they also fail to support the formation of in these mutants affected in PAT, auxin and F-actin are depleted from their end-poles which also fail to support formation of the large BFA-induced compartments.

Crosstalk between auxin, cytokinins, and sugars in the plant cell cycle

Plant meristems are utilization sinks, in which cell division activity governs sink strength. However, the molecular mechanisms by which cell division activity and sink strength are adjusted to a plant's developmental program in its environmental setting are not well understood. Mitogenic hormonal as well as metabolic signals drive and modulate the cell cycle, but a coherent idea of how this is accomplished, is still missing. Auxin and cytokinins are known as endogenous mitogens whose concentrations and timing, however, can be externally affected. Although the sites and mechanisms of signal interaction in cell cycle control have not yet been unravelled, crosstalk of sugar and phytohormone signals could be localized to several biochemical levels. At the expression level of cell cycle control genes, like cyclins, Cdks, and others, synergistic but also antagonistic interactions could be demonstrated. Another level of crosstalk is that of signal generation or modulation. Cytokinins affect the activity of extracellular invertases and hexose-uptake carriers and thus impinge on an intracellular sugar signal. With tobacco BY-2 cells, a coordinated control of cell cycle activity at both regulatory levels could be shown. Comparison of the results obtained with the root cell-representing BY-2 cells with literature data from shoot tissues or green cell cultures of Arabidopsis and Chenopodium suggests opposed and tissue-specific regulatory patterns of mitogenic signals and signal crosstalk in root and shoot meristems.

Zeatin accumulation and misexpression of a class I knox gene are intimately linked in the epiphyllous response of the interspecific hybrid EMB-2 (Helianthus annuus x H. tuberosus)

Epiphylly, occurring in a somaclonal variant (EMB-2) of the interspecific hybrid Helianthus annuus x H. tuberosus, was used to investigate molecular and cyto-physiological mechanisms that underlie cellular fate change. EMB-2 plants are characterized by profuse proliferation of shoot- and embryo-like structures on some leaves. We addressed the putative relationship between cytokinins and knox genes in EMB-2 plants. A class I knox gene, HtKNOT1, was isolated from H. tuberosus. A high level of HtKNOT1 transcripts was detected in EMB-2 epiphyllous leaves compared to non-epiphyllous (NEP) ones. In addition, epiphylly was related to a localized increases in zeatin and N-glycosylated cytokinins. As ectopic morphogenesis proceeded, HtKNOT1 transcripts and zeatin co-localized and showed different patterns in ectopic shoot compared with embryo-like structures, consistent with the differential role of both cytokinin and knox genes in the two morphogenetic events. Notably, a massive shoot/embryo regeneration was induced in EMB-2 NEP leaves by in vitro zeatin treatment. These results clearly indicate that localized cytokinin accumulation and ectopic expression of HtKNOT1 are closely linked in the epiphylly of EMB-2 plants.

Soundwave stimulation triggers the content change of the endogenous hormone of the Chrysanthemum mature callus

Hormones have been suggested to play a prominent role in the control of callus growth. In this paper, with the method of indirect enzyme-linked immunosorbent assays (ELISA), we investigated the induction effect of soundwave on the dynamic change of endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) in vitro during the differentiation process of Chrysanthemum synchronized mature Callus. These experiments showed that groups treated by optimal soundwave (1.4 kHz, 0.095 kdb) had significantly higher IAA levels and lower ABA than that of the control, which had been implicated activation of endogenous IAA and inhibition of ABA. Through the biochemical analysis, it revealed that the increased level of IAA as well as decreased levels of ABA correlated with soundwave stimulus. High rate of IAA/ABA was favorable to development of the callus and differentiation of mature callus. We conclude that soundwave contributes to endogenous hormone as well as the control of callus growth.

The Rg-1 encoded regeneration capacity of tomato is not related to an altered cytokinin homeostasis

•  Cytokinin (CK) metabolism was analyzed in tomato (Lycopersicon esculentum) Rg-1 hybrids during in vitro shoot organogenesis from root explants. •  Data were obtained by combining physicochemical analysis with quantification and in situ detection methods. •  Although exogenous zeatin is added in all classical regeneration protocols, we show here that regenerating (Rg⁺ ) tomato explants did not require an exogenous CK source for regeneration. Irrespective of the presence or absence of exogenous zeatin, the endogenous CK levels were not affected by Rg-1 in the initial explants or in the early callus phase. In a later stage, and related to the presence of numerous shoots, the Rg⁺ explants showed much lower endogenous CK concentrations than the nonregenerating (rg^- ) explants. Cells of rg^- explants were not able to differentiate, despite their high endogenous CK content, and did not respond to exogenously applied CKs. •  We show that the insensitivity of rg^- explants to a hormonal signal, normally initiating regeneration, is not related to an altered endogenous CK metabolism. We therefore postulate that Rg-1 action involves a regeneration-specific CK receptor or a regeneration-specific CK signal transduction pathway.

Levels and immunolocalization of endogenous cytokinins in thidiazuron-induced shoot organogenesis in carnation

We evaluated the capacity of the plant growth regulator thidiazuron (TDZ), a substituted phenylurea with high cytokinin-like activity, to promote organogenesis in petals and leaves of several carnation cultivars (Dianthus spp.), combined with 1-naphthaleneacetic acid (NAA). The involvement of the endogenous auxin indole-3-acetic acid (IAA) and purine-type cytokinins was also studied. Shoot differentiation was found to depend on the explant, cultivar and balance of growth regulators. TDZ alone (0.5 and 5.0 micromol/L) as well as synergistically with NAA (0.5 and 5.0 micromol/L) promoted shoot organogenesis in petals, and was more active than N6-benzyladenine. In petals of the White Sim cultivar, TDZ induced cell proliferation in a concentration-dependent manner and, on day 7 of culture, the proportion of meristematic regions in those petals allowed the prediction of shoot regeneration capacity after 30 days of culture. Immunolocalization of CK ribosides, N6-(delta2-isopentenyl)adenosine, zeatin riboside (ZR) and dihydrozeatin riboside (DHZR), in organogenic petals showed them to be highly concentrated in the tips of bud primordia and in the regions with proliferation capacity. All of them may play a role in cell proliferation, and possibly in differentiation, during the organogenic process. After seven days of culture of White Sim petals, NAA may account for the changes found in the levels of IAA and DHZR, whereas TDZ may be responsible for the remarkable increases in N6-(delta2-isopentenyl)adenine (iP) and ZR. ZR is induced by low TDZ concentrations (0.0-0.005 micromol/L), whereas iP, that correlates with massive cell proliferation and the onset of shoot differentiation, is associated with high TDZ levels (0.5 micromol/L). In addition to the changes observed in quantification and in situ localization of endogenous phytohormones during TDZ-induced shoot organogenesis, we propose that TDZ also promotes growth directly, through its own biological activity. To our knowledge, this study is the first to evaluate the effect of TDZ on endogenous phytohormones in an organogenic process.

Biosynthesis of cytokinins

Cytokinins are adenine derivatives with an isoprenoid side chain and play an essential role in plant development. Plant isopentenyltransferases that catalyze the first and rate-limiting steps of cytokinin biosynthesis have recently been identified. Unlike bacterial enzymes, which catalyze the transfer of the isopentenyl moiety from dimethylallyldiphosphate (DMAPP) to the N(6) position of adenosine 5'-monophosphate (AMP), plant enzymes catalyze the transfer of the isopentenyl moiety from DMAPP preferentially to ATP and to ADP. The isopentenylated side chain is hydroxylated to form zeatin-type cytokinins. An alternative pathway, in which a hydroxylated side chain is directly added to the N(6) position of the adenine moiety, has also been suggested.

Identification of new aromatic cytokinins in Arabidopsis thaliana and Populus x canadensis leaves by LC-(+)ESI-MS and capillary liquid chromatography/frit-fast atom bombardment mass spectrometry

A search for naturally occurring aromatic cytokinins (ARCKs) in Arabidopsis thaliana plants and Populus x canadensis leaves led to the discovery of four new plant hormone substances: 6-(2-methoxybenzylamino)purine (ortho-methoxytopolin, MeoT), 6-(3-methoxybenzylamino)purine (meta-methoxytopolin, MemT) (Fig. 1) and their 9-beta-D-ribofuranosyl derivatives. These substances were identified by liquid chromatography electrospray ionization mass spectrometry [LC (+)ESI-MS] and capillary-liquid chromatography/frit-fast atom bombardment-mass spectrometry [CapLC/frit-FAB-MS] after pre-column derivatization. The chemical structures were subsequently confirmed by chemical synthesis. Because of lack of heavy labelled internal standards, the endogenous levels of methoxytopolins in A. thaliana plants, Populus x canadensis leaves and samples derived from cultures of Agrobacterium tumefaciens strain GV3101 were determined by enzyme-linked immunosorbent assay (ELISA) of HPLC-fractionated extracts. While the levels of MeoT, MemT and their ribosides in A. thaliana shoots and Populus x canadensis leaves were relatively low (approximately 0.25-10 pmol g-1 FW for MeoT and MemT, respectively), the A. tumefaciens strain produced up to 600 times more of the newly identified substances. Cytokinin activity of methoxytopolines was demonstrated in three bioassays testing their ability to stimulate tobacco callus growth, to delay chlorophyll degradation in excised wheat leaves, and to induce betacyanin synthesis in Amaranthus caudatus var. atropurpurea cotyledons. Notably, their anti-senescing activity in the wheat leaf assay exceeded that of BAP and Z by almost 200%. Methoxytopolins are proposed to be new members of the biologically active aromatic cytokinin family, which might have specific physiological functions.

Cytokinin affinity purification and identification of a tobacco BY-2 adenosine kinase

Adenosine kinase is one of the enzymes potentially responsible for the formation of cytokinin nucleotides in plants. Using a zeatin affinity column a 40 kDa protein was isolated from tobacco Bright Yellow 2 (TBY-2) and identified by mass spectrometry as adenosine kinase. The ligand interaction reported here can be disrupted by several other adenine- but not guanine-based purine derivatives. The observed interaction with cytokinins is discussed in view of a putative role for adenosine kinase in TBY-2 cytokinin metabolism. The presented results show for the first time a plant adenosine kinase affinity-purified to homogeneity that was identified by primary structure analysis.

Perception and signal transduction of cytokinins

Cytokinins are plant hormones implicated in diverse and essential processes in plant growth and development, and key genes for the metabolism and actions of cytokinins have recently been identified. Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. The final destination of the transferred phosphoryl groups is response regulators. The type-B Arabidopsis response regulators (ARRs) are DNA-binding transcriptional activators that are required for cytokinin responses. On the other hand, the type-A ARRs act as repressors of cytokinin-activated transcription. How phosphorelay regulate response regulators and how response regulators control downstream events are open questions and discussed in this review.

The plant cell cycle

Cell division in plants is controlled by the activity of cyclin-dependent kinase (CDK) complexes. Although this basic mechanism is conserved with all other eukaryotes, plants show novel features of cell-cycle control in the molecules involved and their regulation, including novel CDKs showing strong transcriptional regulation in mitosis. Plant development is characterized by indeterminate growth and reiteration of organogenesis and is therefore intimately associated with cell division. This may explain why plants have a large number of cell-cycle regulators that appear to have overlapping and distinct functions. Here we review the recent considerable progress in understanding how core cell-cycle regulators are involved in integrating and coordinating cell division at the molecular level.

Pasticcino2 is a protein tyrosine phosphatase-like involved in cell proliferation and differentiation in Arabidopsis

The pasticcino2 (pas2) mutant shows impaired embryo and seedling development associated with cell de-differentiation and proliferation. This process is specifically enhanced in presence of cytokinins leading to callus-like structure of the apical part of the seedling. Cell proliferation concerns localized and stochastic nodules of dividing cells. In absence of cytokinins, cell proliferation leads to small calli on stems but, most often, cell proliferation is associated with post-genital organ fusion. The PAS2 gene was identified by positional cloning. PAS2 expression was found in every plant organ and was not regulated by PAS1 and PAS3 genes. PAS2 encodes the Arabidopsis member of the protein tyrosine phosphatase-like (Ptpl) family, a new PTP family originally described in mice and humans and characterized by a mutated PTP active site. This family of proteins has a yeast homolog that is essential for cell viability. The absence of yeast PAS2 homolog can be functionally replaced by the Arabidopsis PAS2 protein, demonstrating that PAS2 function is conserved between higher and lower eukaryotes.

Local expression of the ipt gene in transgenic tobacco (Nicotiana tabacum L. cv. SR1) axillary buds establishes a role for cytokinins in tuberization and sink formation

The developmental characteristics of a transgenic tobacco line (BIK62) expressing the ipt cytokinin-biosynthetic gene under the control of a tagged promoter were analysed. In situ hybridization and cytokinin immunocytochemistry revealed that the ipt gene was mainly expressed in the axillary buds after the floral transition. The ipt-expressing axillary buds presented morphological alterations such as short and narrow scale-leaflets, and swollen internodes filled with starch grains, giving rise to short and tuberized lateral branches. In addition, the modification of the endogenous cytokinin balance in the axillary meristems resulted in a fast rate of leaf initiation and cytokinins accumulated mostly in the lateral zones of the reactivated axillary meristems, suggesting a role in leaf organogenesis. Cell cycle analysis revealed that the reactivated axillary meristems were characterized by predominant S+G2 nuclei. Terminal internodes displayed low levels of hexose and sucrose concomitant with starch accumulation. Extracellular invertases (EC 3.1.26) were also present in higher amounts in the tuberizing internodes compared to the axillary buds of wild-type tobacco. These results underline the role of cytokinins in cell cycle regulation and in the creation of a sink--source effect. They also provide new information about cytokinin involvement in the process of tuberization and their overproduction in axillary buds giving rise to tuberized lateral branches in a naturally non-tuberizing species.

Induction of early bolting in Arabidopsis thaliana by triacontanol, cerium and lanthanum is correlated with increased endogenous concentration of isopentenyl adenosine (iPAdos)

The effects of triacontanol (TRIA), applied singly or in combination with cerium nitrate and lanthanum nitrate, on bolting of Arabidopsis thaliana were studied. Triacontanol (0.1 to 0.6 microM) added to the culture medium induced early bolting. TRIA (0.3 microM) applied with low concentrations of cerium and lanthanum caused a synergistic stimulation of bolting. In medium containing 0.3 microM TRIA, 0.1 microM cerium nitrate and 0.1 mM lanthanum nitrate, 82% of the plants bolted 20 d after seed sowing compared to only 8.6% in basal medium and 47.8% in medium with TRIA only. The changes in the endogenous concentrations of total cytokinins of the isopentenyl adenine (IP) subfamily in the leaf and root tissues were correlated with TRIA-induced early bolting. The combined treatment of TRIA (0.3 microM), cerium nitrate (0.1 microM) and lanthanum nitrate (0.1 mM) resulted in a significant increase in the endogenous concentrations of total cytokinins of the IP subfamily in the root and leaf tissues compared to plants growing in the basal medium and medium with TRIA. The exogenous application of six natural cytokinins to the plants revealed that only isopentenyl adenosine (iPAdos) was as effective as TRIA on floral bud formation. iPAdos was also found to have similar effects as TRIA on root growth and reproductive growth. These results suggest a correlation between the early bolting induced by TRIA, cerium and lanthanum and the production of higher concentrations of endogenous iPAdos.

Apical dominance in Pssu-ipt-transformed tobacco

In Pssu-ipt-transformed tobacco, apical dominance was released by defoliation of the upper nodes, while the apex remained intact. After defoliation, the concentration of cytokinins (CKs) increased whereas IAA remained constant, evoking an increase in the CK/IAA ratio in the buds. Moreover, defoliation resulted in a tremendous increase in the concentrations of aromatic amines (AAs): tyramine (TYR), phenethylamine (PEA) and an as yet unidentified compound. Although the total aliphatic monoamine and polyamine (PA) concentration remained constant, putrescine (PUT) and spermidine (SPD) concentrations in the axillary buds decreased, whereas the concentration of spermine (SPM) increased. Similar changes in PAs and AAs could be observed in the buds of untransformed SR1 plants after decapitation, whereas defoliation without removal of the apex had no effect. This is the first report on the possible involvement of PAs and AAs in apical dominance.

Regulation of plant growth by cytokinin

Cytokinins are a class of plant-specific hormones that play a central role during the cell cycle and influence numerous developmental programs. Because of the lack of biosynthetic and signaling mutants, the regulatory roles of cytokinins are not well understood. We genetically engineered cytokinin oxidase expression in transgenic tobacco plants to reduce their endogenous cytokinin content. Cytokinin-deficient plants developed stunted shoots with smaller apical meristems. The plastochrone was prolonged, and leaf cell production was only 3-4% that of wild type, indicating an absolute requirement of cytokinins for leaf growth. In contrast, root meristems of transgenic plants were enlarged and gave rise to faster growing and more branched roots. These results suggest that cytokinins are an important regulatory factor of plant meristem activity and morphogenesis, with opposing roles in shoots and roots.