Alteration of hormone levels in transgenic tobacco plants overexpressing the rice homeobox gene OSH1

Transcriptomic analysis reveals the mechanism of the alleviation of salt stress by salicylic acid in pepper (Capsicum annuum L.)

Background

The growth and yield of pepper (Capsicum annuum L.) is often affected by the critical salt stress. Salicylic acid (SA) can improve plants’ stress tolerance by promoting growth and regulating ion absorption and transportation.

Methods and results

To uncover the alleviated mechanism of salt stress by SA in pepper, we conducted morphological, physiological, cytological, and transcriptomic analyses under a single SA treatment and NaCl with and without SA pre-treatment for 9 days. Seedlings under NaCl treatment showed yellow shrunken leaves, this tatus were alleviated by NS treatment (NaCl with SA pre-treatment). Compared with plants under NaCl treatment, those in the NS treatment showed reduced lipid peroxidation, and significantly increased contents of chlorophyll and osmotic regulators (proline, soluble sugars). Treatment with SA balanced the Na+/K+ ratio. We conducted transcriptome sequencing and identified differentially expressed genes (DEGs) contributing to alleviation of salt stress by SA in pepper. Besides photosynthesis related genes, GO and KEGG analyses revealed that the DEGs were enriched in ‘sequence-specific DNA binding’, ‘transcription regulator activity’ and ‘DNA binding transcription factor activity’ by GO terms. And our results showed that TFs, such as MYB, bZIP, BBX, AP2/ERF, NAC, etc., probably make a great contribution in the alleviation of salt stress by SA.

Conclusions

These results reveal that SA can improve plants’ stress tolerance by balancing ion absorption, gene expression and transcriptional regulation, which provide new ideas and resources for subsequent research on the mechanism of salt tolerance in pepper.

Genome-wide identification of the KNOTTED HOMEOBOX gene family and their involvement in stalk development in flowering Chinese cabbage

Gibberellin and cytokinin synergistically regulate the stalk development in flowering Chinese cabbage. KNOX proteins were reported to function as important regulators of the shoot apex to promote meristem activity by synchronously inducing CTK and suppressing GA biosynthesis, while their regulatory mechanism in the bolting and flowering is unknown. In this study, 9 BcKNOX genes were identified and mapped unevenly on 6 out of 10 flowering Chinese cabbage chromosomes. The BcKNOXs were divided into three subfamilies on the basis of sequences and gene structure. The proteins contain four conserved domains except for BcKNATM. Three BcKNOX TFs (BcKNOX1, BcKNOX3, and BcKNOX5) displayed high transcription levels on tested tissues at various stages. The major part of BcKNOX genes showed preferential expression patterns in response to low-temperature, zeatin (ZT), and GA₃ treatment, indicating that they were involved in bud differentiation and bolting. BcKNOX1 and BcKNOX5 showed high correlation level with gibberellins synthetase, and CTK metabolic genes. BcKONX1 also showed high correlation coefficients within BcRGA1 and BcRGL1 which are negative regulators of GA signaling. In addition, BcKNOX1 interacted with BcRGA1 and BcRGL1, as confirmed by yeast two-hybrid (Y2H) and biomolecular fluorescence complementation assay (BiFC). This analysis has provided useful foundation for the future functional roles' analysis of flowering Chinese cabbage KNOX genes.

RLB (RICE LATERAL BRANCH) recruits PRC2-mediated H3K27 tri-methylation on OsCKX4 to regulate lateral branching

Lateral branches such as shoot and panicle are determining factors and target traits for rice (Oryza sativa L.) yield improvement. Cytokinin promotes rice lateral branching; however, the mechanism underlying the fine-tuning of cytokinin homeostasis in rice branching remains largely unknown. Here, we report the map-based cloning of RICE LATERAL BRANCH (RLB) encoding a nuclear-localized, KNOX-type homeobox protein from a rice cytokinin-deficient mutant showing more tillers, sparser panicles, defected floret morphology as well as attenuated shoot regeneration from callus. RLB directly binds to the promoter and represses the transcription of OsCKX4, a cytokinin oxidase gene with high abundance in panicle branch meristem. OsCKX4 over-expression lines phenocopied rlb, which showed upregulated OsCKX4 levels. Meanwhile, RLB physically binds to Polycomb repressive complex 2 (PRC2) components OsEMF2b and co-localized with H3K27me3, a suppressing histone modification mediated by PRC2, in the OsCKX4 promoter. We proposed that RLB recruits PRC2 to the OsCKX4 promoter to epigenetically repress its transcription, which suppresses the catabolism of cytokinin, thereby promoting rice lateral branching. Moreover, antisense inhibition of OsCKX4 under the LOG promoter successfully increased panicle size and spikelet number per plant without affecting other major agronomic traits. This study provides insight into cytokinin homeostasis, lateral branching in plants, and also promising target genes for rice genetic improvement.

At the Root of Nodule Organogenesis: Conserved Regulatory Pathways Recruited by Rhizobia

The interaction between legume plants and soil bacteria rhizobia results in the formation of new organs on the plant roots, symbiotic nodules, where rhizobia fix atmospheric nitrogen. Symbiotic nodules represent a perfect model to trace how the pre-existing regulatory pathways have been recruited and modified to control the development of evolutionary "new" organs. In particular, genes involved in the early stages of lateral root development have been co-opted to regulate nodule development. Other regulatory pathways, including the players of the KNOX-cytokinin module, the homologues of the miR172-AP2 module, and the players of the systemic response to nutrient availability, have also been recruited to a unique regulatory program effectively governing symbiotic nodule development. The role of the NIN transcription factor in the recruitment of such regulatory modules to nodulation is discussed in more details.

Identification and Application of BhAPRR2 Controlling Peel Colour in Wax Gourd (Benincasa hispida)

Peel color is an important factor affecting commodity quality in vegetables; however, the genes controlling this trait remain unclear in wax gourd. Here, we used two F₂ genetic segregation populations to explore the inheritance patterns and to clone the genes associated with green and white skin in wax gourd. The F₂ and BC₁ trait segregation ratios were 3:1 and 1:1, respectively, and the trait was controlled by nuclear genes. Bulked segregant analysis of both F₂ plants revealed peaks on Chr5 exceeding the confidence interval. Additionally, 6,244 F₂ plants were used to compress the candidate interval into a region of 179 Kb; one candidate gene, Bch05G003950 (BhAPRR2), encoding two-component response regulator-like protein Arabidopsis pseudo-response regulator2 (APRR2), which is involved in the regulation of peel color, was present in this interval. Two bases (GA) present in the coding sequence of BhAPRR2 in green-skinned wax gourd were absent from white-skinned wax gourd. The latter contained a frameshift mutation, a premature stop codon, and lacked 335 residues required for the protein functional region. The chlorophyll content and BhAPRR2 expression were significantly higher in green-skinned than in white-skinned wax gourd. Thus, BhAPRR2 may regulate the peel color of wax gourd. This study provides a theoretical foundation for further studies of the mechanism of gene regulation for the fruit peel color of wax gourd.

Arbuscular mycorrhizal fungi modulates dynamics tolerance expression to mitigate drought stress in Ephedra foliata Boiss

Arbuscular mycorrhizal fungi (AMF) are one of the most important drivers of soil ecosystem dynamics. AMF have the potential to improve plant growth and development by modulating key hormonal pathways, which result in decreasing the adverse impact of abiotic stress, such as drought. Pot experiments were conducted in this study to investigate the ability of AMF to ameliorate the adverse impact of drought in Ephedra foliate. Non-inoculated AMF E. foliate (Ef) plants, exhibited reduced growth in response to drought stress with a concomitant lowering of chlorophyll pigments, relative to non-stressed and AMF inoculated plant. AMF inoculated E. foliate showed improved nitrogen metabolism by positively regulating nitrate and nitrite reductase activity which results in greater ammonium availability for the synthesis of amino acids. Inoculation with AMF also increased antioxidant enzyme activity, ascorbic acid contents, and reduction in glutathione level. This resulted in significant amelioration of oxidative damage to plant membranes by restricting the excess generation of reactive oxygen species (ROS), such as hydrogen peroxide. Greater content of proline, glucose, and total soluble protein in AMF-inoculated plants provided further benefit to E. foliate plants and their ability to withstand drought stress, and also evident by a greater level of sucrose phosphate synthase activity. AMF significantly enhanced the uptake of essential nutrients like K, Mg, and Ca. Importantly, higher concentrations of plant hormones, including indole acetic acid (IAA), indole butyric acid (IBA), gibberellic acid (GA), and abscisic acid (ABA), were maintained in AMF-inoculated Ef plants. AMF inoculation also boosted phosphorous metabolism by increasing alkaline and acid phosphatase enzyme activity. In summary, AMF-inoculation of Ef plants significantly reduced the deleterious effect of drought stress by up-regulating the antioxidant defense system, synthesis of osmolytes, and maintaining phytohormone levels.

Comparing symbiotic performance and physiological responses of two soybean cultivars to arbuscular mycorrhizal fungi under salt stress

The presented experiments evaluated the symbiotic performance of soybean genotypes with contrasting salt stress tolerance to arbuscular mycorrhizal fungi (AMF) inoculation. In addition, the physiological stress tolerance mechanisms in plants derived from mutualistic interactions between AMF and the host plants were evaluated. Plant growth, nodulation, nitrogenase activity and levels of endogenous growth hormones, such as indole acetic acid and indole butyric acid, of salt-tolerant and salt-sensitive soybean genotypes significantly decreased at 200 mM NaCl. The inoculation of soybean with AMF improved the symbiotic performance of both soybean genotypes by improving nodule formation, leghemoglobin content, nitrogenase activity and auxin synthesis. AMF colonization also protected soybean genotypes from salt-induced membrane damage and reduced the production of hydrogen peroxide, subsequently reducing the production of TBARS and reducing lipid peroxidation. In conclusion, the results of the present investigation indicate that AMF improve the symbiotic performance of soybean genotypes regardless of their salt stress tolerance ability by mitigating the negative effect of salt stress and stimulating endogenous level of auxins that contribute to an improved root system and nutrient acquisition under salt stress.

Cytokinin-induced expression of OSH1 in a shoot-regenerating rice callus

The expression of a KNOX class 1 gene OSH1 is induced by cytokinin during regeneration of shoots from callus in Oryza sativa L. (rice). This cytokinin-induced expression was enhanced by overexpression of homologues of cytokinin-signalling phosphorelay genes such as a histidine kinase gene OHK3, a phosphotransmitter gene OHP2 and a response regulator gene ORR1 in cultured cells. Regionally overlapped expression of these genes and OSH1 was observed in shoot apex. These results suggest that these cytokinin-signalling genes are positive regulators of the expression of OSH1, and mediate the OSH expression upon shoot regeneration from callus in rice.

Arbuscular mycorrhizal fungi regulate the oxidative system, hormones and ionic equilibrium to trigger salt stress tolerance in Cucumis sativus L

Arbuscular mycorrhizal fungi (AMF) association increases plant stress tolerance. This study aimed to determine the mitigation effect of AMF on the growth and metabolic changes of cucumbers under adverse impact of salt stress. Salinity reduced the water content and synthesis of pigments. However, AMF inoculation ameliorated negative effects by enhancing the biomass, synthesis of pigments, activity of antioxidant enzymes, including superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase, and the content of ascorbic acid, which might be the result of lower level lipid peroxidation and electrolyte leakage. An accumulation of phenols and proline in AMF-inoculated plants also mediated the elimination of superoxide radicals. In addition, jasmonic acid, salicylic acid and several important mineral elements (K, Ca, Mg, Zn, Fe, Mn and Cu) were enhanced with significant reductions in the uptake of deleterious ions like Na+. These results suggested that AMF can protect cucumber growth from salt stress.

NbGIS regulates glandular trichome initiation through GA signaling in tobacco

KEY MESSAGE

A novel gene NbGIS positively regulates glandular trichome initiation through GA Signaling in tobacco. NbMYB123-like regulates glandular trichome initiation by acting downstream of NbGIS in tobacco. Glandular trichome is a specialized multicellular structure which has capability to synthesize and secrete secondary metabolites and protects plants from biotic and abiotic stresses. Our previous results revealed that a C2H2 zinc-finger transcription factor GIS and its sub-family genes act upstream of GL3/EGL3-GL1-TTG1 transcriptional activator complex to regulate trichome initiation in Arabidopsis. In this present study, we found that NbGIS could positively regulate glandular trichome development in Nicotiana benthamiana (tobacco). Our result demonstrated that 35S:NbGIS lines exhibited much higher densities of trichome on leaves, main stems, lateral branches and sepals than WT plants, while NbGIS:RNAi lines had the opposite phenotypes. Furthermore, our results also showed that NbGIS was required in response to GA signal to control glandular trichome initiation in Nicotiana benthamiana. In addition, our results also showed that NbGIS significantly influenced GA accumulation and expressions of marker genes of the GA biosynthesis, might result in the changes of growth and maturation in tobacco. Lastly, our results also showed that NbMYB123-like regulated glandular trichome initiation in tobacco by acting downstream of NbGIS. These findings provide new insights to discover the molecular mechanism by which C2H2 transcriptional factors regulates glandular trichome initiation through GA signaling pathway in tobacco.

Is auxin involved in the induction of genetic instability in barley homeotic double mutants?

The triggers of genetic instability in barley homeotic double mutants are tweaky spike -type mutations associated with an auxin imbalance in separate spike phytomeres. Barley homeotic tweaky spike;Hooded (tw;Hd) double mutants are characterized by an inherited instability of spike and flower development, which is absent in the single parental constituents. The aim of the present study was to show that the trigger of genetic instability in the double mutants is the tw mutations, which are associated with an auxin imbalance in the developing spikes. Their pleiotropic effects on genes related to spike/flower development may cause the genetic instability of double mutants. The study of four double-mutant groups composed of different mutant alleles showed that the instability arose only if the mutant allele tw was a constituent of the double mutants. Application of auxin inhibitors and 2,4-dichlorophenoxyacetic acid (2,4-D) demonstrated the relationship of the instability of the double mutants and the phenotype of the tw mutants to auxin imbalance. 2,4-D induced phenocopies of the tw mutation in wild-type plants and rescued the phenotypes of three allelic tw mutants. The differential display (dd-PCR) method allowed the identification of several putative candidate genes in tw that may be responsible for the initiation of instability in the double mutants by pleiotropic variations of their expression in the tw mutant associated with auxin imbalance in the developing spikes. The results of the present study linked the genetic instability of homeotic double mutants with an auxin imbalance caused by one of the constituents (tw). The genetic instability of the double mutants in relation to auxin imbalance was studied for the first time. A matrocliny on instability expression was also observed.

Mitigation of NaCl Stress by Arbuscular Mycorrhizal Fungi through the Modulation of Osmolytes, Antioxidants and Secondary Metabolites in Mustard (Brassica juncea L.) Plants

Present work was carried out to investigate the possible role of arbuscular mycorrhizal fungi (AMF) in mitigating salinity-induced alterations in Brassica juncea L. Exposure to NaCl stress altered the morphological, physio-biochemical attributes, antioxidant activity, secondary metabolites and phytohormones in the mustard seedlings. The growth and biomass yield, leaf water content, and total chlorophyll content were decreased with NaCl stress. However, AMF-inoculated plants exhibited enhanced shoot and root length, elevated relative water content, enhanced chlorophyll content, and ultimately biomass yield. Lipid peroxidation and proline content were increased by 54.53 and 63.47%, respectively with 200 mM NaCl concentration. Further increase in proline content and decrease in lipid peroxidation was observed in NaCl-treated plants inoculated with AMF. The antioxidants, superoxide dismutase, ascorbate peroxidase, glutathione reductase, and reduced glutathione were increased by 48.35, 54.86, 43.85, and 44.44%, respectively, with 200 mM NaCl concentration. Further increase in these antioxidants has been observed in AMF-colonized plants indicating the alleviating role of AMF to salinity stress through antioxidant modulation. The total phenol, flavonoids, and phytohormones increase with NaCl treatment. However, NaCl-treated plants colonized with AMF showed further increase in the above parameters except ABA, which was reduced with NaCl+AMF treatment over the plants treated with NaCl alone. Our results demonstrated that NaCl caused negative effect on B. juncea seedlings; however, colonization with AMF enhances the NaCl tolerance by reforming the physio-biochemical attributes, activities of antioxidant enzymes, and production of secondary metabolites and phytohormones.

Involvement of an ent-copalyl diphosphate synthase in tissue-specific accumulation of specialized diterpenes in Andrographis paniculata

Ent-labdane-related diterpene (ent-LRD) specialized (i.e. secondary) metabolites of the medicinal plant kalmegh (Andrographis paniculata) have long been known for several pharmacological activities. However, our understanding of the ent-LRD biosynthetic pathway has remained largely incomplete. Since ent-LRDs accumulate in leaves, we carried out a comparative transcriptional analysis using leaf and root tissues, and identified 389 differentially expressed transcripts, including 223 transcripts that were preferentially expressed in leaf tissue. Analysis of the transcripts revealed various specialized metabolic pathways, including transcripts of the ent-LRD biosynthetic pathway. Two class II diterpene synthases (ApCPS1 and ApCPS2) along with one (ApCPS1') and two (ApCPS2' and ApCPS2″) transcriptional variants that were the outcomes of alternative splicing of the precursor mRNA and alternative transcriptional termination, respectively, were identified. ApCPS1 and ApCPS2 encode for 832- and 817-amino acids proteins, respectively, and are phylogenetically related to the dicotyledons ent-copalyl diphosphate synthases (ent-CPSs). The spatio-temporal patterns of ent-LRD metabolites accumulation and gene expression suggested a likely role for ApCPS1 in general (i.e. primary) metabolism, perhaps by providing precursor for the biosynthesis of phytohormone gibberellin (GA). However, ApCPS2 is potentially involved in tissue-specific accumulation of ent-LRD specialized metabolites. Bacterially expressed recombinant ApCPS2 catalyzed the conversion of (E,E,E)-geranylgeranyl diphosphate (GGPP), the general precursor of diterpenes to ent-copalyl diphosphate (ent-CPP), the precursor of ent-LRDs. Taken together, these results advance our understanding of the tissue-specific accumulation of specialized ent-LRDs of medicinal importance.

Time-Course RNA-Seq Analysis Reveals Transcriptional Changes in Rice Plants Triggered by Rice stripe virus Infection

Rice stripe virus (RSV) has become a major pathogen of rice. To determine how the rice transcriptome is modified in response to RSV infection, we used RNA-Seq to perform a genome-wide gene expression analysis of a susceptible rice cultivar. The transcriptomes of RSV-infected samples were compared to those of mock-treated samples at 3, 7, and 15 days post-infection (dpi). From 8 to 11% of the genes were differentially expressed (>2-fold difference in expression) in RSV-infected vs. noninfected rice. Among them, 532 genes were differentially expressed at all three time points. Surprisingly, 37.6% of the 532 genes are related to transposons. Gene ontology enrichment analysis revealed that many chloroplast genes were down-regulated in infected plants at 3 and 15 dpi. Expression of genes associated with cell differentiation and flowering was significantly down-regulated in infected plants at 15 dpi. In contrast, most of the up-regulated genes in infected plants concern the cell wall, plasma membrane, and vacuole and are known to function in various metabolic pathways and stress responses. In addition, transcripts of diverse transcription factors gradually accumulated in infected plants with increasing infection time. We also confirmed that the expression of gene subsets (including NBS-LRR domain-containing genes, receptor-like kinase genes, and genes involving RNA silencing) was changed by RSV infection. Taken together, we demonstrated that down-regulation of genes related to photosynthesis and flowering was strongly associated with disease symptoms caused by RSV and that up-regulation of genes involved in metabolic pathways, stress responses, and transcription was related to host defense mechanisms.

Enhancing growth performance and systemic acquired resistance of medicinal plant Sesbania sesban (L.) Merr using arbuscular mycorrhizal fungi under salt stress

Pot experiments were conducted to evaluate the damaging effects of salinity on Sesbania sesban plants in the presence and absence of arbuscular mycorrhizal fungi (AMF). The selected morphological, physiological and biochemical parameters of S. sesban were measured. Salinity reduced growth and chlorophyll content drastically while as AMF inoculated plants improved growth. A decrease in the number of nodules, nodule weight and nitrogenase activity was also evident due to salinity stress causing reduction in nitrogen fixation and assimilation potential. AMF inoculation increased these parameters and also ameliorated the salinity stress to some extent. Antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) as well as non enzymatic antioxidants (ascorbic acid and glutathione) also exhibited great variation with salinity treatment. Salinity caused great alterations in the endogenous levels of growth hormones with abscisic acid showing increment. AMF inoculated plants maintained higher levels of growth hormones and also allayed the negative impact of salinity.

Increased gibberellin contents contribute to accelerated growth and development of transgenic tobacco overexpressing a wheat ubiquitin gene

KEY MESSAGE

Overexpressing TaUb2 promoted stem growth and resulted in early flowering in transgenic tobacco plants. Ubiquitin are involved in the production, metabolism and proper function of gibberellin. The ubiquitin-26S proteasome system (UPS), in which ubiquitin (Ub) functions as a marker, is a post-translational regulatory system that plays a prominent role in various biological processes. To investigate the impact of different Ub levels on plant growth and development, transgenic tobacco (Nicotiana tabacum L.) plants were engineered to express an Ub gene (TaUb2) from wheat (Triticum aestivum L.) under the control of cauliflower mosaic virus 35S promoter. Transgenic tobacco plants overexpressing TaUb2 demonstrated an accelerated growth rate at early stage and an early flowering phenotype in development. The preceding expression of MADS-box genes also corresponded to the accelerated developmental phenotypes of the transgenic tobacco plants compared to that of wild-type (WT). Total gibberellin (GA) and active GA contents in transgenic tobacco plants were higher than those in WT at the corresponding developmental stages, and some GA metabolism genes were upregulated. Treatment with GA(3) conferred a similarly accelerated grown rate in WT plants to that of transgenic tobacco plants, while growth was inhibited when transgenic tobacco plants were treated with a GA biosynthesis inhibitor. Thus, the results suggest that Ub are involved in the production, metabolism and proper function of GA, which is important in the regulation of plant growth and development.

KNOX1 is expressed and epigenetically regulated during in vitro conditions in Agave spp

BACKGROUND

The micropropagation is a powerful tool to scale up plants of economical and agronomical importance, enhancing crop productivity. However, a small but growing body of evidence suggests that epigenetic mechanisms, such as DNA methylation and histone modifications, can be affected under the in vitro conditions characteristic of micropropagation. Here, we tested whether the adaptation to different in vitro systems (Magenta boxes and Bioreactors) modified epigenetically different clones of Agave fourcroydes and A. angustifolia. Furthermore, we assessed whether these epigenetic changes affect the regulatory expression of KNOTTED1-like HOMEOBOX (KNOX) transcription factors.

RESULTS

To gain a better understanding of epigenetic changes during in vitro and ex vitro conditions in Agave fourcroydes and A. angustifolia, we analyzed global DNA methylation, as well as different histone modification marks, in two different systems: semisolid in Magenta boxes (M) and temporary immersion in modular Bioreactors (B). No significant difference was found in DNA methylation in A. fourcroydes grown in either M or B. However, when A. fourcroydes was compared with A. angustifolia, there was a two-fold difference in DNA methylation between the species, independent of the in vitro system used. Furthermore, we detected an absence or a low amount of the repressive mark H3K9me2 in ex vitro conditions in plants that were cultured earlier either in M or B. Moreover, the expression of AtqKNOX1 and AtqKNOX2, on A. fourcroydes and A. angustifolia clones, is affected during in vitro conditions. Therefore, we used Chromatin ImmunoPrecipitation (ChIP) to know whether these genes were epigenetically regulated. In the case of AtqKNOX1, the H3K4me3 and H3K9me2 were affected during in vitro conditions in comparison with AtqKNOX2.

CONCLUSIONS

Agave clones plants with higher DNA methylation during in vitro conditions were better adapted to ex vitro conditions. In addition, A. fourcroydes and A. angustifolia clones displayed differential expression of the KNOX1 gene during in vitro conditions, which is epigenetically regulated by the H3K4me3 and H3K9me2 marks. The finding of an epigenetic regulation in key developmental genes will make it important in future studies to identify factors that help to find climate-resistant micropropagated plants.

Proper gibberellin localization in vascular tissue is required to control auxin-dependent leaf development and bud outgrowth in hybrid aspen

Bioactive gibberellins (GAs) are involved in many developmental aspects in the life cycle of plants, acting either directly or through interaction with other hormones. One way to study the role of GA in specific mechanisms is to modify the levels of bioactive GA in specific tissues. We increased GA catabolism in different parts of the vascular tissue by overexpressing two different GA 2-oxidase genes that encode oxidases with affinity for C₂₀- or C₁₉-GA. We show that, irrespective of their localization in the vascular tissue, the expression of different members of this gene family leads to similar modifications in the primary and secondary growth of the stem of hybrid aspen. We also show that the precise localization of bioactive GA downregulation is important for the proper control of other developmental aspects, namely leaf shape and bud dormancy. Expression under the control of one of the studied promoters significantly affected both the shape of the leaves and the number of sylleptic branches. These phenotypic defects were correlated with alterations in the levels and repartitioning of auxins. We conclude that a precise localization of bioactive GA in the vasculature of the apex is necessary for the normal development of the plant through the effect of GAs on auxin transport.

Regulation of meristem size by cytokinin signaling

The plant meristems possess unique features that involve maintaining the stem cell populations while providing cells for continued development. Although both the primary shoot apical meristem (SAM) and the root apical meristem (RAM) are specified during embryogenesis, post-embryonic tissue proliferation is required for their full establishment and maintenance throughout a plants' life. The phytohormone cytokinin (CK) interacts with other systemic signals and is a key regulator of meristem size and functions. The SAM and the RAM respond to CK stimulations in different manners: CK promotes tissue proliferation in the SAM through pathways dominated by homeobox transcription factors, including the class I KNOX genes, STIP, and WUS; and curiously, it favors proliferation at low levels and differentiation at a slightly higher concentration in the RAM instead. Here we review the current understanding of the molecular mechanisms underlying CK actions in regulating meristematic tissue proliferation.

Ectopic expression of class 1 KNOX genes induce adventitious shoot regeneration and alter growth and development of tobacco (Nicotiana tabacum L) and European plum (Prunus domestica L)

Transgenic plants of tobacco (Nicotiana tabacum L) and European plum (Prunus domestica L) were produced by transforming with the apple class 1 KNOX genes (MdKN1 and MdKN2) or corn KNOX1 gene. Transgenic tobacco plants were regenerated in vitro from transformed leaf discs cultured in a medium lacking cytokinin. Ectopic expression of KNOX genes retarded shoot growth by suppressing elongation of internodes in transgenic tobacco plants. Expression of each of the three KNOX1 genes induced malformation and extensive lobbing in tobacco leaves. In situ regeneration of adventitious shoots was observed from leaves and roots of transgenic tobacco plants expressing each of the three KNOX genes. In vitro culture of leaf explants and internode sections excised from in vitro grown MdKN1 expressing tobacco shoots regenerated adventitious shoots on MS (Murashige and Skoog 1962) basal medium in the absence of exogenous cytokinin. Transgenic plum plants that expressed the MdKN2 or corn KNOX1 gene grew normally but MdKN1 caused a significant reduction in plant height, leaf shape and size and produced malformed curly leaves. A high frequency of adventitious shoot regeneration (96%) was observed in cultures of leaf explants excised from corn KNOX1-expressing transgenic plum shoots. In contrast to KNOX1-expressing tobacco, leaf and internode explants of corn KNOX1-expressing plum required synthetic cytokinin (thidiazuron) in the culture medium to induce adventitious shoot regeneration. The induction of high-frequency regeneration of adventitious shoots in vitro from leaves and stem internodal sections of plum through the ectopic expression of a KNOX1 gene is the first such report for a woody perennial fruit trees.

Identification of rhizome-specific genes by genome-wide differential expression analysis in Oryza longistaminata

BACKGROUND

Rhizomatousness is a key component of perenniality of many grasses that contribute to competitiveness and invasiveness of many noxious grass weeds, but can potentially be used to develop perennial cereal crops for sustainable farmers in hilly areas of tropical Asia. Oryza longistaminata, a perennial wild rice with strong rhizomes, has been used as the model species for genetic and molecular dissection of rhizome development and in breeding efforts to transfer rhizome-related traits into annual rice species. In this study, an effort was taken to get insights into the genes and molecular mechanisms underlying the rhizomatous trait in O. longistaminata by comparative analysis of the genome-wide tissue-specific gene expression patterns of five different tissues of O. longistaminata using the Affymetrix GeneChip Rice Genome Array.

RESULTS

A total of 2,566 tissue-specific genes were identified in five different tissues of O. longistaminata, including 58 and 61 unique genes that were specifically expressed in the rhizome tips (RT) and internodes (RI), respectively. In addition, 162 genes were up-regulated and 261 genes were down-regulated in RT compared to the shoot tips. Six distinct cis-regulatory elements (CGACG, GCCGCC, GAGAC, AACGG, CATGCA, and TAAAG) were found to be significantly more abundant in the promoter regions of genes differentially expressed in RT than in the promoter regions of genes uniformly expressed in all other tissues. Many of the RT and/or RI specifically or differentially expressed genes were located in the QTL regions associated with rhizome expression, rhizome abundance and rhizome growth-related traits in O. longistaminata and thus are good candidate genes for these QTLs.

CONCLUSION

The initiation and development of the rhizomatous trait in O. longistaminata are controlled by very complex gene networks involving several plant hormones and regulatory genes, different members of gene families showing tissue specificity and their regulated pathways. Auxin/IAA appears to act as a negative regulator in rhizome development, while GA acts as the activator in rhizome development. Co-localization of the genes specifically expressed in rhizome tips and rhizome internodes with the QTLs for rhizome traits identified a large set of candidate genes for rhizome initiation and development in rice for further confirmation.

Cross talk between the KNOX and ethylene pathways is mediated by intron-binding transcription factors in barley

In the barley (Hordeum vulgare) Hooded (Kap) mutant, the duplication of a 305-bp intron sequence leads to the overexpression of the Barley knox3 (Bkn3) gene, resulting in the development of an extra flower in the spikelet. We used a one-hybrid screen to identify four proteins that bind the intron-located regulatory element (Kap intron-binding proteins). Three of these, Barley Ethylene Response Factor1 (BERF1), Barley Ethylene Insensitive Like1 (BEIL1), and Barley Growth Regulating Factor1 (BGRF1), were characterized and their in vitro DNA-binding capacities verified. Given the homology of BERF1 and BEIL1 to ethylene signaling proteins, we investigated if these factors might play a dual role in intron-mediated regulation and ethylene response. In transgenic rice (Oryza sativa), constitutive expression of the corresponding genes produced phenotypic alterations consistent with perturbations in ethylene levels and variations in the expression of a key gene of ethylene biosynthesis. In barley, ethylene treatment results in partial suppression of the Kap phenotype, accompanied by up-regulation of BERF1 and BEIL1 expression, followed by down-regulation of Bkn3 mRNA levels. In rice protoplasts, BEIL1 activates the expression of a reporter gene driven by the 305-bp intron element, while BERF1 can counteract this activation. Thus, BEIL1 and BERF1, likely in association with other Kap intron-binding proteins, should mediate the fine-tuning of Bkn3 expression by ethylene. We propose a hypothesis for the cross talk between the KNOX and ethylene pathways.

A branched-chain aminotransferase may regulate hormone levels by affecting KNOX genes in plants

Branched-chain amino acid transaminases (BCATs) play a crucial role in the metabolic pathway of leucine, isoleucine and valine by catalyzing the last step of synthesis and/or the initial step of degradation of these amino acids. In this study, we characterized a new BCAT from Nicotiana benthamiana (NbBCAT, GeneBank accession No. EU194916), the deduced amino acid sequence of which exhibits a very high percentage of identity to the homologous enzymes from Solanum tuberosum (StBCAT-2, 91.5%) and Arabidopsis thaliana (AtBCAT1-6, 56.4-68.6%). Complementation experiment using a Deltabat1/Deltabat2 double knockout yeast strain system demonstrated enzymatic activities for NbBCAT. Ectopically expressed NbBCAT::green fluorescence fusion protein was targeted predominantly to the chloroplasts in tobacco protoplasts. The highest levels of NbBCAT transcripts were found in open flowers as well as in young leaves. Virus-induced gene silencing of NbBCAT resulted in abnormal leaf development and loss of apical dominance. In NbBCAT-silenced plants, two KNOTTED1-type genes, NTH15 and NTH23, were upregulated. This was accompanied by various hormone changes, as a result of transcriptional regulation of gibberellin 20-oxidase (Ntc12) and adenosine phosphate isopentenyltransferase. The transcript levels of NbBCAT could also be repressed by hormone treatment. These results suggest that NbBCAT, an enzyme in the branched-chain amino acid metabolic pathway, may be involved in the regulation of endogenous hormones by its effect on KNOX genes.

Hormone mediated regulation of the shoot apical meristem

Recent work on hormone mediated regulation of the SAM is reviewed, emphasizing how combinations of genetic, molecular and modelling approaches have refined models based on classic experimental and physiological work. Special emphasis is given to newly described mechanisms that modulate the responsiveness of specific tissues to hormones and their potential to direct position dependent determination processes.

Phosphate starvation responses and gibberellic acid biosynthesis are regulated by the MYB62 transcription factor in Arabidopsis

The limited availability of phosphate (Pi) in most soils results in the manifestation of Pi starvation responses in plants. To dissect the transcriptional regulation of Pi stress-response mechanisms, we have characterized the biological role of MYB62, an R2R3-type MYB transcription factor that is induced in response to Pi deficiency. The induction of MYB62 is a specific response in the leaves during Pi deprivation. The MYB62 protein localizes to the nucleus. The overexpression of MYB62 resulted in altered root architecture, Pi uptake, and acid phosphatase activity, leading to decreased total Pi content in the shoots. The expression of several Pi starvation-induced (PSI) genes was also suppressed in the MYB62 overexpressing plants. Overexpression of MYB62 resulted in a characteristic gibberellic acid (GA)-deficient phenotype that could be partially reversed by exogenous application of GA. In addition, the expression of SOC1 and SUPERMAN, molecular regulators of flowering, was suppressed in the MYB62 overexpressing plants. Interestingly, the expression of these genes was also reduced during Pi deprivation in wild-type plants, suggesting a role for GA biosynthetic and floral regulatory genes in Pi starvation responses. Thus, this study highlights the role of MYB62 in the regulation of phosphate starvation responses via changes in GA metabolism and signaling. Such cross-talk between Pi homeostasis and GA might have broader implications on flowering, root development and adaptive mechanisms during nutrient stress.

Expression of class I knotted1-like homeobox genes in the storage roots of sweetpotato (Ipomoea batatas)

As a first step in clarifying the involvement of class I knotted1-like homeobox (KNOXI) genes in the storage root development of sweetpotato (Ipomoea batatas), we isolated three KNOXI genes, named Ibkn1, Ibkn2 and Ibkn3, expressed in the storage roots. Phylogenetic analysis showed that Ibkn1 was homologous to the SHOOT MERISTEMLESS (STM) gene of Arabidopsis, while Ibkn2 and Ibkn3 were homologous to the BREVIPEDICELLUS (BP) gene. Of these, expression of Ibkn1 and Ibkn2 were upregulated in developing and mature storage roots compared with fibrous roots. Ibkn1 and Ibkn2 showed different expression patterns in the storage roots. Ibkn1 was preferentially expressed at the proximal end and around the primary vascular cambium, while Ibkn2 expression was highest in the thickest part and lower in both the proximal and distal ends. In contrast to Ibkn1 and Ibkn2, expression of Ibkn3 in roots was not consistent among sweetpotato cultivars. The distribution of endogenous trans-zeatin riboside (t-ZR) in sweetpotato roots showed a similarity to the expression pattern of KNOXI genes, supporting the idea that KNOXI genes control cytokinin levels in the storage roots. The physiological functions of these KNOXI genes in storage root development are discussed.

Gibberellin metabolism and its regulation

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.

A BELL1-like gene of potato is light activated and wound inducible

BELL1-like transcription factors interact with their protein partners from the KNOTTED1 family to bind to target genes and regulate numerous developmental and metabolic processes. In potato (Solanum tuberosum), the BELL1 transcription factor StBEL5 and its protein partner POTH1 regulate tuber formation by affecting hormone levels. Overexpression of StBEL5 in transgenic lines produces plants that consistently exhibit enhanced tuber formation, and the mRNA of this gene moves through phloem cells in a long-distance signaling pathway regulated by photoperiod. Whereas photoperiod mediates the movement of StBEL5 RNA, activation of transcription of the StBEL5 gene in leaves is regulated by white light, regardless of photoperiod or light intensity. Illumination with either red or blue light induces the StBEL5 promoter, whereas far-red light had no effect. As expected, the StBEL5 promoter harbors numerous conventional light-responsive cis-acting elements like GT1, GATA, and AT1 motifs. Deletion constructs were analyzed to determine what sequences are involved in light activation. Transcriptional activity was also mediated by wounding on stems, insect predation on leaves, and photoperiod in stolons. These results demonstrate that StBEL5 gene activity in the leaf is correlated with wavelengths optimal for photosynthesis. The number of factors that affect the StBEL5 promoter supports the premise that the BELL1-like genes play a role in a wide range of functions.

Arabidopsis Ovate Family Protein 1 is a transcriptional repressor that suppresses cell elongation

Transcription factors regulate multiple aspects of plant growth and development. Here we report the identification and functional analysis of a plant-specific, novel transcription factor in Arabidopsis. We isolated a dominant, gain-of-function mutant that displays reduced lengths in all aerial organs including hypocotyl, rosette leaf, cauline leaf, inflorescence stem, floral organs and silique. Molecular cloning revealed that these phenotypes are caused by elevated expression of the Arabidopsis thaliana Ovate Family Protein 1 (AtOFP1). This mutant was designated as Atofp1-1D. We show that the altered morphology of Atofp1-1D mutant is caused by reduced cell length resulting from reduced cell elongation, and demonstrate that a mutant harboring a transposon insertion that disrupts the OVATE domain of AtOFP1 is indistinguishable from wild-type plants. Plants overexpressing other closely related AtOFP genes phenocopy plants overexpressing AtOFP1, implying a possible overlapping function among members of the AtOFP gene family. We found that AtOFP1 localizes in the nucleus, and that AtOFP1 functions as an active transcriptional repressor. Chromatin immunoprecipitation results indicated that AtGA20ox1, a gene encoding the key enzyme in GA biosynthesis, is a target gene regulated by AtOFP1. Consistent with this, exogenous gibberellic acid can partially restore defects in cell elongation in plants overexpressing AtOFP1, suggesting that such a reduced cell elongation is caused, in part, by the deficiency in gibberellin biosynthesis. Taken together, our results indicate that AtOFP1 is an active transcriptional repressor that has a role in regulating cell elongation in plants.

The rice OsLOL2 gene encodes a zinc finger protein involved in rice growth and disease resistance

Arabidopsis LSD1-related proteins that contain LSD1-like zinc finger domains have been identified to be involved in disease resistance and programmed cell death. To investigate the potential role of LSD1-related gene in rice (Oryza sativa L.), we cloned an LSD1 ortholog, OsLOL2, from the rice cDNA plasmid library. The OsLOL2 gene is predicted to encode a polypeptide of 163 amino acids with two LSD1-like zinc finger domains with 74.5% identity to those of LSD1. Southern blot analysis indicated that OsLOL2 was a single-copy gene in the rice genome. Transgenic rice lines carrying the antisense strand of OsLOL2 with decreased expression of OsLOL2 had dwarf phenotypes, and the dwarfism could be restored by exogenous GA(3) treatment, suggesting that the dwarfism was the result of a deficiency in bioactive gibberellin (GA). In agreement with this possibility, the content of endogenous bioactive GA(1) decreased in the antisense transgenic lines. Expression of OsKS1, one of the genes encoding for GA biosynthetic enzymes, was suppressed in the antisense transgenic lines. Sense transgenic lines with increased expression of OsLOL2 were more resistant to rice bacterial blight, while antisense transgenic lines were less resistant to rice bacterial blight. The OsLOL2-GFP (green fluorescence protein) fusion protein was localized in the nucleus of cells of transgenic BY2 tobacco (Nicotiana tabacum L.). These data suggest that OsLOL2 is involved in rice growth and disease resistance.

A comparative analysis of the Arabidopsis mutant amp1-1 and a novel weak amp1 allele reveals new functions of the AMP1 protein

Ethylene and gibberellins have a synergistic stimulatory effect on hypocotyl elongation of light-grown Arabidopsis thaliana (L.) Heynh. seedlings. A screen for mutants with decreased response to these hormones led to the isolation of a novel allele (ampl-7) of the ALTERED MERISTEM PROGRAM (AMP) 1 locus. The amp1-7 allele contains a missense mutation causing a phenotype, which is weaker than that of the amp1-1 mutant that carries a nonsense mutation. The mutant phenotype prompted the hypothesis that AMP1 is involved in ethylene and GA signalling pathways or in a parallel pathway-controlling cell and hypocotyl elongation and cellular organization. Amp1 mutants contain higher zeatin concentrations causing enlargement of the apical meristem, which was confirmed by cytokinin application to wild type seedlings. Light grown amp1 seedlings have shorter hypocotyls than wild type; however, application of cytokinins promotes hypocotyl elongation of both Col-0 and amp1. We suggest that in amp1 mutants either zeatin overproduction or its action is strictly localized.

Antisense expression of a gene encoding a calcium-binding protein in transgenic tobacco leads to altered morphology and enhanced chlorophyll

Entamoeba histolytica contains a novel calcium-binding protein like calmodulin,which was discovered earlier,and we have reported the presence of its homologue(s)and a dependent protein kinase in plants.To understand the functions of these in plants,a cDNA encoding a calcium-binding protein isolated from Entamoeba histolytica (EhCaBP)was cloned into vector pBI121 in antisense orientation and transgenic tobacco plants were raised.These plants showed variation in several phenotypic characters,of which two distinct features,more greenness and leaf thickness,were inherited in subsequent generations.The increase in the level of total chlorophyll in different plants ranged from 60% to 70%.There was no major change in chloroplast structure and in the protein level of D1,D2,LHCP and RuBP carboxylase.These morphological changes were not seen in antisense calmodulin transgenic tobacco plants,nor was the calmodulin level altered in EhCaBP antisense plants.

Involvement of hormones and KNOXI genes in early Arabidopsis seedling development

Plant hormones control plant development by modulating the expression of regulatory genes, including homeobox-containing KNOXI genes. However, much remains to be elucidated about the interactions involved. Therefore, hormonal regulation of KNOXI gene expression was investigated using hormone applications and an inducible transgenic ipt expression system to increase endogenous cytokinin (CK) levels. Treatments with auxin, abscisic acid (ABA), cytokinins, ethylene, and gibberellin (GA) did not result in ectopic expression of the BP (BREVIPEDICELLUS) gene. However, BP expression was strongly reduced by ABA, increased by auxin treatment (correlating with the initiation of lateral root meristems, which strongly express BP), and did not significantly respond to short-term treatments with the other hormones in whole seedlings. Following short-term ipt activation, organ-specific differential regulation of KNOXI gene expression was observed. While several KNOXI genes were transiently up-regulated to low levels, STM was selectively repressed (especially at low light) in hypocotyls. In cotyledons, activation of CK-responsive genes preceded ipt induction, suggesting that CKs are transported more rapidly than the inducing agent (dexamethasone). Long-term increases in CK levels induced raised levels of several KNOXI transcripts in hypocotyls, correlating with the radial expansion of vascular tissues, the main domains of KNOXI gene expression, suggesting that CKs had little effect on KNOXI promoter activity. No alterations in hormone sensitivity were observed in a bp null mutant. Constitutive BP overexpression caused reductions in the length and number of lateral roots, while the primary root remained unaffected. The transgenic seedlings displayed weak, but significant, alterations in sensitivity to ABA, CK, and 1-amino-cyclopropane-1-carboxylic acid.

Ectopic expression of KNOTTED1-like homeobox protein induces expression of cytokinin biosynthesis genes in rice

Some phytohormones such as gibberellins (GAs) and cytokinins (CKs) are potential targets of the KNOTTED1-like homeobox (KNOX) protein. To enhance our understanding of KNOX protein function in plant development, we identified rice (Oryza sativa) genes for adenosine phosphate isopentenyltransferase (IPT), which catalyzes the rate-limiting step of CK biosynthesis. Molecular and biochemical studies revealed that there are eight IPT genes, OsIPT1 to OsIPT8, in the rice genome, including a pseudogene, OsIPT6. Overexpression of OsIPTs in transgenic rice inhibited root development and promoted axillary bud growth, indicating that OsIPTs are functional in vivo. Phenotypes of OsIPT overexpressers resembled those of KNOX-overproducing transgenic rice, although OsIPT overexpressers did not form roots or ectopic meristems, both of which are observed in KNOX overproducers. Expression of two OsIPT genes, OsIPT2 and OsIPT3, was up-regulated in response to the induction of KNOX protein function with similar kinetics to those of down-regulation of GA 20-oxidase genes, target genes of KNOX proteins in dicots. However, expression of these two OsIPT genes was not regulated in a feedback manner. These results suggest that OsIPT2 and OsIPT3 have unique roles in the developmental process, which is controlled by KNOX proteins, rather than in the maintenance of bioactive CK levels in rice. On the basis of these findings, we concluded that KNOX protein simultaneously decreases GA biosynthesis and increases de novo CK biosynthesis through the induction of OsIPT2 and OsIPT3 expression, and the resulting high-CK and low-GA condition is required for formation and maintenance of the meristem.

L1 division and differentiation patterns influence shoot apical meristem maintenance

Plant development requires regulation of both cell division and differentiation. The class 1 KNOTTED1-like homeobox (KNOX) genes such as knotted1 (kn1) in maize (Zea mays) and SHOOTMERISTEMLESS in Arabidopsis (Arabidopsis thaliana) play a role in maintaining shoot apical meristem indeterminacy, and their misexpression is sufficient to induce cell division and meristem formation. KNOX overexpression experiments have shown that these genes interact with the cytokinin, auxin, and gibberellin pathways. The L1 layer has been shown to be necessary for the maintenance of indeterminacy in the underlying meristem layers. This work explores the possibility that the L1 affects meristem function by disrupting hormone transport pathways. The semidominant Extra cell layers1 (Xcl1) mutation in maize leads to the production of multiple epidermal layers by overproduction of a normal gene product. Meristem size is reduced in mutant plants and more cells are incorporated into the incipient leaf primordium. Thus, Xcl1 may provide a link between L1 division patterns, hormonal pathways, and meristem maintenance. We used double mutants between Xcl1 and dominant KNOX mutants and showed that Xcl1 suppresses the Kn1 phenotype but has a synergistic interaction with gnarley1 and rough sheath1, possibly correlated with changes in gibberellin and auxin signaling. In addition, double mutants between Xcl1 and crinkly4 had defects in shoot meristem maintenance. Thus, proper L1 development is essential for meristem function, and XCL1 may act to coordinate hormonal effects with KNOX gene function at the shoot apex.

Cell differentiation and organ initiation at the shoot apical meristem

Plants continuously generate organs at the flanks of their shoot apical meristems (SAMs). The patterns in which these organs are initiated, also called patterns of phyllotaxis, are highly stereotypic and characteristic for a particular species or developmental stage. This stable, predictable behaviour of the meristem has led to the idea that organ initiation must be based on simple and robust mechanisms. This conclusion is less evident, however, if we consider the very dynamic behaviour of the individual cells. How dynamic cellular events are coordinated and how they are linked to the regular patterns of organ initiation is a major issue in plant developmental biology.

Arabidopsis KNOXI proteins activate cytokinin biosynthesis

Plant architecture is shaped through the continuous formation of organs by meristems. Class I KNOTTED1-like homeobox (KNOXI) genes are expressed in the shoot apical meristem (SAM) and are required for SAM maintenance. KNOXI proteins and cytokinin, a plant hormone intimately associated with the regulation of cell division, share overlapping roles, such as meristem maintenance and repression of senescence, but their mechanistic and hierarchical relationship have yet to be defined. Here, we show that activation of three different KNOXI proteins using an inducible system resulted in a rapid increase in mRNA levels of the cytokinin biosynthesis gene isopentenyl transferase 7 (AtIPT7) and in the activation of ARR5, a cytokinin response factor. We further demonstrate a rapid and dramatic increase in cytokinin levels following activation of the KNOXI protein SHOOT MERISTEMLESS (STM). Application of exogenous cytokinin or expression of a cytokinin biosynthesis gene through the STM promoter partially rescued the stm mutant. We conclude that activation of cytokinin biosynthesis mediates KNOXI function in meristem maintenance. KNOXI proteins emerge as central regulators of hormone levels in plant meristems.

KNOX action in Arabidopsis is mediated by coordinate regulation of cytokinin and gibberellin activities

The shoot apical meristem (SAM) is a pluripotent group of cells that gives rise to the aerial parts of higher plants. Class-I KNOTTED1-like homeobox (KNOX) transcription factors promote meristem function partly through repression of biosynthesis of the growth regulator gibberellin (GA). However, regulation of GA activity cannot fully account for KNOX action. Here, we show that KNOX function is also mediated by cytokinin (CK), a growth regulator that promotes cell division and meristem function. We demonstrate that KNOX activity is sufficient to rapidly activate both CK biosynthetic gene expression and a SAM-localized CK-response regulator. We also show that CK signaling is necessary for SAM function in a weak hypomorphic allele of the KNOX gene SHOOTMERISTEMLESS (STM). Additionally, we provide evidence that a combination of constitutive GA signaling and reduced CK levels is detrimental to SAM function. Our results indicate that CK activity is both necessary and sufficient for stimulating GA catabolic gene expression, thus reinforcing the low-GA regime established by KNOX proteins in the SAM. We propose that KNOX proteins may act as general orchestrators of growth-regulator homeostasis at the shoot apex of Arabidopsis by simultaneously activating CK and repressing GA biosynthesis, thus promoting meristem activity.

Integrating hormone signaling and patterning mechanisms in plant development

Plant growth and development are driven by the bustling integration of a vast number of signals, among which plant hormones dominate. Understanding the role of hormones in particular developmental events requires their integration with developmental regulators known to be specific to those events. Using the increasing number of tools that can be utilized to probe hormone biosynthesis, transport and response, several recent studies have taken such an integrative approach, and in so doing have contributed to a clearer picture of precisely how hormones control plant development.

Overexpression in transgenic tobacco reveals different roles for the rice homeodomain gene OsBIHD1 in biotic and abiotic stress responses

The rice OsBIHD1 gene encodes a transcriptional factor belonging to the homeodomain class. It had previously been shown to be activated by treatment with benzothiadiazole, a chemical inducer of disease resistance, and in an incompatible interaction between rice and the blast fungus. To allow a better understanding of the function of OsBIHD1 in plant disease resistance response, the OsBIHD1 gene in tobacco was overexpressed by Agrobacterium-mediated leaf disc transformation with a construct containing the OsBIHD1 ORF under control of the 35S promoter. Overexpression of the rice OsBIHD1 gene in some of the transgenic tobacco lines led to some morphological abnormalities in the top buds and roots. The transgenic tobacco plants showed an elevated level of defence-related PR-1 gene expression and enhanced disease resistance against infection by tomato mosaic virus, tobacco mosaic virus, and Phytophthora parasitica var. nicotianae. However, the transgenic tobacco plants overexpressing OsBIHD1 showed enhanced sensitivity to salt and oxidative stress as compared with the wild-type plants. The results suggested that the OsBIHD1 protein may be positively involved in activating expression of the defence-related genes in disease resistance responses, and is also important in rice development and abiotic stress tolerance.

Auxin-cytokinin and auxin-gibberellin interactions during morphogenesis of the compound leaves of pea (Pisum sativum)

A number of mutations that alter the form of the compound leaf in pea (Pisum sativum) has proven useful in elucidating the role that auxin might play in pea leaf development. The goals of this study were to determine if auxin application can rescue any of the pea leaf mutants and if gibberellic acid (GA) plays a role in leaf morphogenesis in pea. A tissue culture system was used to determine the effects of various auxins, GA or a GA biosynethesis inhibitor (paclobutrazol) on leaf development. The GA mutant, nana1 (na1) was analyzed. The uni-tac mutant was rescued by auxin and GA and rescue involved both a conversion of the terminal leaflet into a tendril and an addition of a pair of lateral tendrils. This rescue required the presence of cytokinin. The auxins tested varied in their effectiveness, although methyl-IAA worked best. The terminal tendrils of wildtype plantlets grown on paclobutrazol were converted into leaflets, stubs or were aborted. The number of lateral pinna pairs produced was reduced and leaf initiation was impaired. These abnormalities resembled those caused by auxin transport inhibitors and phenocopy the uni mutants. The na1 mutant shared some morphological features with the uni mutants; including, flowering late and producing leaves with fewer lateral pinna pairs. These results show that both auxin and GA play similar and significant roles in pea leaf development. Pea leaf morphogenesis might involve auxin regulation of GA biosynthesis and GA regulation of Uni expression.

Evolution of leaf developmental mechanisms

Leaves are determinate organs produced by the shoot apical meristem. Land plants demonstrate a large range of variation in leaf form. Here we discuss evolution of leaf form in the context of our current understanding of leaf development, as this has emerged from molecular genetic studies in model organisms. We also discuss specific examples where parallel studies of development in different species have helped understanding how diversification of leaf form may occur in nature.

Up-regulation of OsBIHD1, a rice gene encoding BELL homeodomain transcriptional factor, in disease resistance responses

In the present study, we cloned and identified a full-length cDNA of a rice gene, OsBIHD1, encoding a homeodomain type transcriptional factor. OsBIHD1 is predicted to encode a 642 amino acid protein and the deduced protein sequence of OsBIHD1 contains all conserved domains, a homeodomain, a BELL domain, a SKY box, and a VSLTLGL box, which are characteristics of the BELL type homedomain proteins. The recombinant OsBIHD1 protein expressed in Escherichia coli bound to the TGTCA motif that is the characteristic cis-element DNA sequence of the homeodomain transcriptional factors. Subcellular localization analysis revealed that the OsBIHD1 protein localized in the nucleus of the plant cells. The OsBIHD1 gene was mapped to chromosome 3 of the rice genome and is a single-copy gene with four exons and three introns. Northern blot analysis showed that expression of OsBIHD1 was activated upon treatment with benzothiadiazole (BTH), which is capable of inducing disease resistance. Expression of OsBIHD1 was also up-regulated rapidly during the first 6 h after inoculation with Magnaporthe grisea in BTH-treated rice seedlings and during the incompatible interaction between M. grisea and a resistant genotype. These results suggest that OsBIHD1 is a BELL type of homeodomain transcription factor present in the nucleus, whose induction is associated with resistance response in rice.

CORKSCREW1 defines a novel mechanism of domain specification in the maize shoot

In higher plants, determinate leaf primordia arise in regular patterns on the flanks of the indeterminate shoot apical meristem (SAM). The acquisition of leaf form is then a gradual process, involving the specification and growth of distinct domains within the three leaf axes. The recessive corkscrew1 (cks1) mutation of maize (Zea mays) disrupts both leaf initiation patterns in the SAM and domain specification within the mediolateral and proximodistal leaf axes. Specifically, cks1 mutant leaves exhibit multiple midribs and leaf sheath tissue differentiates in the blade domain. Such perturbations are a common feature of maize mutants that ectopically accumulate KNOTTED1-like homeobox (KNOX) proteins in leaf tissue. Consistent with this observation, at least two knox genes are ectopically expressed in cks1 mutant leaves. However, ectopic KNOX proteins cannot be detected. We therefore propose that CKS1 primarily functions within the SAM to establish boundaries between meristematic and leaf zones. Loss of gene function disrupts boundary formation, impacts phyllotactic patterns, and leads to aspects of indeterminate growth within leaf primordia. Because these perturbations arise independently of ectopic KNOX activity, the cks1 mutation defines a novel component of the developmental machinery that facilitates leaf-versus-shoot development in maize.

New pOp/LhG4 vectors for stringent glucocorticoid-dependent transgene expression in Arabidopsis

To facilitate glucocorticoid-inducible transgene expression from the pOp promoter in Arabidopsis the ligand-binding domain of a rat glucocorticoid receptor (GR LBD) was fused to the amino terminus of the synthetic transcription factor LhG4 to generate LhGR-N. Fusions bearing the GR LBD at other positions in LhG4 exhibited incomplete repression or inefficient induction. LhGR-N was stringently repressed in the absence of exogenous glucocorticoid but was fully activated by addition of 2 microm dexamethasone which resulted in 1000-fold increase in GUS reporter activity. Half maximal induction was achieved with 0.2 microm dexamethasone. Reporter transcripts were detectable within 2 h of dexamethasone application and peaked 4-10 h later. Neither LhGR-N nor dexamethasone affected seedling development although ethanol retarded development when used as a solvent for dexamethasone. The efficiency of the pOp target promoter was improved 10- to 20-fold by incorporating six copies of the ideal lac operator with sufficient inter-operator spacing to allow simultaneous occupancy. Introduction of the TMV Omega sequence into the 5'UTR resulted in a further 10-fold increase in dexamethasone-inducible reporter activity and an increase in the induction factor to 10(4). Although promoters containing the TMV Omega sequence exhibited slightly increased basal expression levels in the absence of dexamethasone, stringent regulation of the cytokinin biosynthetic gene ipt was achieved with all promoters. Despite the severity of the induced ipt phenotypes, transcripts for the KNOX homoeodomain transcription factors BREVIPEDICELLUS and SHOOTMERISTEMLESS were not significantly increased within 48 h of dexamethasone application to seedlings.

Engineered sorbitol accumulation induces dwarfism in Japanese persimmon

A cDNA encoding sorbitol-6-phosphate dehydrogenase (S6PDH), which is a key enzyme in sorbitol biosynthesis in Rosaceae, was introduced into the Japanese persimmon (Diospyros kaki) to increase the environmental stress tolerance. Resultant transformants exhibited salt-tolerance with dwarfing phenotypes. Therefore, we studied two transgenic lines to understand the physiological mechanism of this dwarfism: lines PS1 and PS6 accumulated high and moderate levels of sorbitol, respectively. The average length of shoots was significantly shorter as compared with the wild-type in line PS1, while no such decrease was observed in line PS6. The myo-inositol and glucose 6-phosphate (G6P) contents were measured in the transgenic lines because previous work with tobacco transformed with S6PDH had suggested that growth inhibition was due to depletion of these metabolites. Although the myo-inositol content was decreased in PS1 plants, the decrease was much smaller than that observed in transgenic tobacco that accumulates sorbitol. The G6P contents were the same in PS1 plants and phenotypically normal PS6 plants. The level of indole-3-acetic acid (IAA), which affects stem elongation, in line PS1 was similar to the levels in the other lines. A decrease in gibberellin (GA) content generally induces dwarfism in plants. However, GA was not decreased in PS1 plants compared with wild-type or control plants. Therefore, we focused on sorbitol accumulation as the most remarkable feature of PS1 plants. As one possibility, the observed growth inhibition was likely caused by an osmotic imbalance between the cytosol and vacuole.

Compound leaves: equal to the sum of their parts?

The leaves of seed plants can be classified as being either simple or compound according to their shape. Two hypotheses address the homology between simple and compound leaves, which equate either individual leaflets of compound leaves with simple leaves or the entire compound leaf with a simple leaf. Here we discuss the genes that function in simple and compound leaf development, such as KNOX1 genes, including how they interact with growth hormones to link growth regulation and development to cause changes in leaf complexity. Studies of transcription factors that control leaf development, their downstream targets, and how these targets are regulated are areas of inquiry that should increase our understanding of how leaf complexity is regulated and how it evolved through time.