Ethylene stimulates endoreduplication but inhibits cytokinesis in cucumber hypocotyl epidermis

New insights into light spectral quality inhibits the plasticity elongation of maize mesocotyl and coleoptile during seed germination

The plastic elongation of mesocotyl (MES) and coleoptile (COL), which can be repressed by light exposure, plays a vital role in maize seedling emergence and establishment under adverse environmental conditions. Understanding the molecular mechanisms of light-mediated repression of MES and COL elongation in maize will allow us to develop new strategies for genetic improvement of these two crucial traits in maize. A maize variety, Zheng58, was used to monitor the transcriptome and physiological changes in MES and COL in response to darkness, as well as red, blue, and white light. The elongation of MES and COL was significantly inhibited by light spectral quality in this order: blue light > red light > white light. Physiological analyses revealed that light-mediated inhibition of maize MES and COL elongation was closely related to the dynamics of phytohormones accumulation and lignin deposition in these tissues. In response to light exposure, the levels of indole-3-acetic acid, trans-zeatin, gibberellin 3, and abscisic acid levels significantly decreased in MES and COL; by contrast, the levels of jasmonic acid, salicylic acid, lignin, phenylalanine ammonia-lyase, and peroxidase enzyme activity significantly increased. Transcriptome analysis revealed multiple differentially expressed genes (DEGs) involved in circadian rhythm, phytohormone biosynthesis and signal transduction, cytoskeleton and cell wall organization, lignin biosynthesis, and starch and sucrose metabolism. These DEGs exhibited synergistic and antagonistic interactions, forming a complex network that regulated the light-mediated inhibition of MES and COL elongation. Additionally, gene co-expression network analysis revealed that 49 hub genes in one and 19 hub genes in two modules were significantly associated with the elongation plasticity of COL and MES, respectively. These findings enhance our knowledge of the light-regulated elongation mechanisms of MES and COL, and provide a theoretical foundation for developing elite maize varieties with improved abiotic stress resistance.

Exogenous 6-BA inhibited hypocotyl elongation under darkness in Picea crassifolia Kom revealed by transcriptome profiling

Hypocotyl elongation is an important process in plant growth and development, and is under hormonal regulatory signaling pathways. In our study, exogenous 6-BA significantly inhibited Picea crassifolia hypocotyl elongation more than ethylene in the dark, indicating the existence of different regulatory strategies in conifers, therefore, the P. crassifolia transcriptome was studied to explore the responsive genes and their regulatory pathways for exogenous N⁶-benzyladenine (6-BA) inhibition of hypocotyl elongation using RNA-Sequencing approach. We present the first transcriptome assembly of P. crassifolia obtained from 24.38 Gb clean data. With lowly-expressed and short contigs excluded, the assembly contains roughly 130,612 unigenes with an N50 length of 1,278 bp. Differential expression analysis found 3,629 differentially expressed genes (DEGs) and found that the differential expression fold of genes was mainly concentrated between 2 and 8 (1 ≤ log2FoldChange ≤ 3). Functional annotation showed that the GO term with the highest number of enriched genes (83 unigenes) was the shoot system development (GO: 0048367) and the KEGG category, plant hormone signal transduction (ko04075), was enriched 30 unigenes. Further analysis revealed that several cytokinin dehydrogenase genes (PcCTD1, PcCTD3 and PcCTD6) catabolized cytokinins, while xyloglucan endotransglucosylase hydrolase gene (PcXTH31), WALLS ARE THIN 1-like gene (PcWAT1-1) and Small auxin-induced gene (PcSAUR15) were strongly repressed thus synergistically completing the inhibition of hypocotyl elongation in P. crassifolia. Besides, PcbHLH149, PcMYB44 and PcERF14 were predicted to be potential core TFs that may form a multi-layered regulatory network with the above proteins for the regulation of hypocotyl growth.

Ethylene-regulated asymmetric growth of the petal base promotes flower opening in rose (Rosa hybrida)

Flowers are the core reproductive structures and key distinguishing features of angiosperms. Flower opening to expose stamens and gynoecia is important in cases where pollinators much be attracted to promote cross-pollination, which can enhance reproductive success and species preservation. The floral opening process is accompanied by the coordinated movement of various floral organs, particularly petals. However, the mechanisms underlying petal movement and flower opening are not well understood. Here, we integrated anatomical, physiological, and molecular approaches to determine the petal movement regulatory network using rose (Rosa hybrida) as a model. We found that PETAL MOVEMENT-RELATED PROTEIN1 (RhPMP1), a homeodomain transcription factor (TF) gene, is a direct target of ETHYLENE INSENSITIVE3, a TF that functions downstream of ethylene signaling. RhPMP1 expression was upregulated by ethylene and specifically activated endoreduplication of parenchyma cells on the adaxial side of the petal (ADSP) base by inducing the expression of RhAPC3b, a gene encoding the core subunit of the Anaphase-Promoting Complex. Cell expansion of the parenchyma on the ADSP base was subsequently enhanced, thus resulting in asymmetric growth of the petal base, leading to the typical epinastic movement of petals and flower opening. These findings provide insights into the pathway regulating petal movement and associated flower-opening mechanisms.�.

Phytochrome interacting factor proteins regulate cytokinesis in Arabidopsis

Dicotyledonous plants form an apical hook to protect the fragile apical meristem during upward protrusion from the soil. Etiolated pifq (pif1 pif3 pif4 pif5) seedlings display constitutive apical hook opening. Here, we show that PIF proteins control apical hook opening by regulating the expression of Budding Uninhibited by Benzimidazole 3.1 (BUB3.1) and affecting cytokinesis. Consistent with the major function of BUB3.1 in the organization of phragmoplasts during cytokinesis, the phragmoplasts are well formed in dark-grown pifq but not in wild type. DNA staining and flow cytometry analysis further demonstrate that cellular endoreduplication levels are dramatically reduced in pifq. Chemical treatment with caffeine, an inhibitor of phragmoplast-based cytokinesis, shows that cytokinesis is involved in the apical hook opening. Genetically, BUB3.1 is epistatic to PIFq in the regulation of cytokinesis. Our findings reveal an organ-specific role of PIF proteins in regulating cytokinesis by BUB3.1 during apical hook development.

Host-produced ethylene is required for marked cell expansion and endoreduplication in dodder search hyphae

The genus Cuscuta comprises stem holoparasitic plant species with wide geographic distribution. Cuscuta spp. obtain water, nutrients, proteins, and mRNA from their host plants via a parasitic organ called the haustorium. As the haustorium penetrates into the host tissue, search hyphae elongate within the host tissue and finally connect with the host's vascular system. Invasion by Cuscuta spp. evokes various reactions within the host plant's tissues. Here, we show that, when Arabidopsis (Arabidopsis thaliana) is invaded by Cuscuta campestris, ethylene biosynthesis by the host plant promotes elongation of the parasite's search hyphae. The expression of genes encoding 1-aminocylclopropane-1-carboxylic acid (ACC) synthases, ACC SYNTHASE2 (AtACS2) and ACC SYNTHASE6 (AtACS6), was activated in the stem of Arabidopsis plants upon invasion by C. campestris. When the ethylene-deficient Arabidopsis acs octuple mutant was invaded by C. campestris, cell elongation and endoreduplication of the search hyphae were significantly reduced, and the inhibition of search hyphae growth was complemented by exogenous application of ACC. In contrast, in the C. campestris-infected Arabidopsis ethylene-insensitive mutant etr1-3, no growth inhibition of search hyphae was observed, indicating that ETHYLENE RESPONSE1-mediated ethylene signaling in the host plant is not essential for parasitism by C. campestris. Overall, our results suggest that C. campestris recognizes host-produced ethylene as a stimulatory signal for successful invasion.

Isolation of Lineage Specific Nuclei Based on Distinct Endoreduplication Levels and Tissue-Specific Markers to Study Chromatin Accessibility Landscapes

The capacity for achieving immense specificity and resolution in science increases day to day. Fluorescence-activated nuclear sorting (FANS) offers this great precision, enabling one to count and separate distinct types of nuclei from specific cells of heterogeneous mixtures. We developed a workflow to collect nuclei from Arabidopsis thaliana by FANS according to cell lineage and endopolyploidy level with high efficiency. We sorted GFP-labeled nuclei with different ploidy levels from the epidermal tissue layer of three-day, dark-grown hypocotyls followed by a shift to light for one day and compared them to plants left in the dark. We then accessed early chromatin accessibility patterns associated with skotomorphogenesis and photomorphogenesis by the assay for transposase-accessible chromatin using sequencing (ATAC-seq) within primarily stomatal 2C and fully endoreduplicated 16C nuclei. Our quantitative analysis shows that dark- and light-treated samples in 2C nuclei do not exhibit any different chromatin accessibility landscapes, whereas changes in 16C can be linked to transcriptional changes involved in light response.

Coupling of Cell Division and Differentiation in Arabidopsis thaliana Cultured Cells with Interaction of Ethylene and ABA Signaling Pathways

Recent studies indicate direct links between molecular cell cycle and cell differentiation machineries. Ethylene and abscisic acid (ABA) are known to affect cell division and differentiation, but the mechanisms of such effects are poorly understood. As ethylene and ABA signaling routes may interact, we examined their involvement in cell division and differentiation in cell tissue cultures derived from several Arabidopsis thaliana plants: wild type (Col-0), and ethylene-insensitive mutants etr1-1, ctr1-1, and ein2-1. We designed an experimental setup to analyze the growth-related parameters and molecular mechanisms in proliferating cells upon short exposure to ABA. Here, we provide evidence for the ethylene-ABA signaling pathways' interaction in the regulation of cell division and differentiation as follows: (1) when the ethylene signal transduction pathway is functionally active (Col-0), the cells actively proliferate, and exogenous ABA performs its function as an inhibitor of DNA synthesis and division; (2) if the ethylene signal is not perceived (etr1-1), then, in addition to cell differentiation (tracheary elements formation), cell death can occur. The addition of exogenous ABA can rescue the cells via increasing proliferation; (3) if the ethylene signal is perceived, but not transduced (ein2-1), then cell differentiation takes place-the latter is enhanced by exogenous ABA while cell proliferation is reduced; (4) when the signal transduction pathway is constitutively active, the cells begin to exit the cell cycle and proceed to endo-reduplication (ctr1-1). In this case, the addition of exogenous ABA promotes reactivation of cell division.

The distribution pattern of endopolyploidy in maize

We discovered that endopolyploidization is common in various organs and tissues of maize at different development stages. Endopolyploidy is not specific in maize germplasm populations. Endopolyploidy is caused by DNA endoreplication, a special type of mitosis with normal DNA synthesis and a lack of cell division; it is a common phenomenon and plays an important role in plant development. To systematically study the distribution pattern of endopolyploidy in maize, flow cytometry was used to determine the ploidy by measuring the cycle (C) value in various organs at different developmental stages, in embryos and endosperm during grain development, in roots under stress conditions, and in the roots of 119 inbred lines from two heterotic groups, Shaan A and Shaan B. Endopolyploidy was observed in most organs at various developmental stages except in expanded leaves and filaments. The endosperm showed the highest C value among all organs. During tissue development, the ploidy increased in all organs except the leaves. In addition, the endopolyploidization of the roots was significantly affected by drought stress. Multiple comparisons of the C values of seven subgroups revealed that the distribution of endopolyploidization was not correlated with the population structure. A correlation analysis at the seedling stage showed a positive relationship between the C value and both the length of the whole plant and the length of main root. A genome-wide association study (GWAS) identified a total of 9 significant SNPs associated with endopolyploidy (C value) in maize, and 8 candidate genes that participate in cell cycle regulation and DNA replication were uncovered in 119 maize inbred lines.

Spontaneous polyploidization in cucumber

This is the first quantitative estimation of spontaneous polyploidy in cucumber and we detected 2.2% polyploids in a greenhouse study. We provide evidence that polyploidization is consistent with endoreduplication and is an on-going process during plant growth. Cucumber occasionally produces polyploid plants, which are problematic for growers because these plants produce misshaped fruits with non-viable seeds. In this study, we undertook the first quantitative study to estimate the relative frequency of spontaneous polyploids in cucumber. Seeds of recombinant inbred lines were produced in different environments, plants were grown in the field and greenhouse, and flow cytometry was used to establish ploidies. From 1422 greenhouse-grown plants, the overall relative frequency of spontaneous polyploidy was 2.2%. Plants possessed nuclei of different ploidies in the same leaves (mosaic) and on different parts of the same plant (chimeric). Our results provide evidence of endoreduplication and polysomaty in cucumber, and that it is an on-going and dynamic process. There was a significant effect (p = 0.018) of seed production environment on the occurrence of polyploid plants. Seed and seedling traits were not accurate predictors of eventual polyploids, and we recommend that cucumber producers rogue plants based on stature and leaf serration to remove potential polyploids.

Ethylene Inhibits Cell Proliferation of the Arabidopsis Root Meristem

The root system of plants plays a critical role in plant growth and survival, with root growth being dependent on both cell proliferation and cell elongation. Multiple phytohormones interact to control root growth, including ethylene, which is primarily known for its role in controlling root cell elongation. We find that ethylene also negatively regulates cell proliferation at the root meristem of Arabidopsis (Arabidopsis thaliana). Genetic analysis indicates that the inhibition of cell proliferation involves two pathways operating downstream of the ethylene receptors. The major pathway is the canonical ethylene signal transduction pathway that incorporates CONSTITUTIVE TRIPLE RESPONSE1, ETHYLENE INSENSITIVE2, and the ETHYLENE INSENSITIVE3 family of transcription factors. The secondary pathway is a phosphorelay based on genetic analysis of receptor histidine kinase activity and mutants involving the type B response regulators. Analysis of ethylene-dependent gene expression and genetic analysis supports SHORT HYPOCOTYL2, a repressor of auxin signaling, as one mediator of the ethylene response and furthermore, indicates that SHORT HYPOCOTYL2 is a point of convergence for both ethylene and cytokinin in negatively regulating cell proliferation. Additional analysis indicates that ethylene signaling contributes but is not required for cytokinin to inhibit activity of the root meristem. These results identify key elements, along with points of cross talk with cytokinin and auxin, by which ethylene negatively regulates cell proliferation at the root apical meristem.

Ethylene-Mediated Regulation of A2-Type CYCLINs Modulates Hyponastic Growth in Arabidopsis

Upward leaf movement (hyponastic growth) is frequently observed in response to changing environmental conditions and can be induced by the phytohormone ethylene. Hyponasty results from differential growth (i.e. enhanced cell elongation at the proximal abaxial side of the petiole relative to the adaxial side). Here, we characterize Enhanced Hyponasty-d, an activation-tagged Arabidopsis (Arabidopsis thaliana) line with exaggerated hyponasty. This phenotype is associated with overexpression of the mitotic cyclin CYCLINA2;1 (CYCA2;1), which hints at a role for cell divisions in regulating hyponasty. Indeed, mathematical analysis suggested that the observed changes in abaxial cell elongation rates during ethylene treatment should result in a larger hyponastic amplitude than observed, unless a decrease in cell proliferation rate at the proximal abaxial side of the petiole relative to the adaxial side was implemented. Our model predicts that when this differential proliferation mechanism is disrupted by either ectopic overexpression or mutation of CYCA2;1, the hyponastic growth response becomes exaggerated. This is in accordance with experimental observations on CYCA2;1 overexpression lines and cyca2;1 knockouts. We therefore propose a bipartite mechanism controlling leaf movement: ethylene induces longitudinal cell expansion in the abaxial petiole epidermis to induce hyponasty and simultaneously affects its amplitude by controlling cell proliferation through CYCA2;1. Further corroborating the model, we found that ethylene treatment results in transcriptional down-regulation of A2-type CYCLINs and propose that this, and possibly other regulatory mechanisms affecting CYCA2;1, may contribute to this attenuation of hyponastic growth.

Ethylene Inhibits Root Elongation during Alkaline Stress through AUXIN1 and Associated Changes in Auxin Accumulation

Soil alkalinity causes major reductions in yield and quality of crops worldwide. The plant root is the first organ sensing soil alkalinity, which results in shorter primary roots. However, the mechanism underlying alkaline stress-mediated inhibition of root elongation remains to be further elucidated. Here, we report that alkaline conditions inhibit primary root elongation of Arabidopsis (Arabidopsis thaliana) seedlings by reducing cell division potential in the meristem zones and that ethylene signaling affects this process. The ethylene perception antagonist silver (Ag(+)) alleviated the inhibition of root elongation by alkaline stress. Moreover, the ethylene signaling mutants ethylene response1-3 (etr1-3), ethylene insensitive2 (ein2), and ein3-1 showed less reduction in root length under alkaline conditions, indicating a reduced sensitivity to alkalinity. Ethylene biosynthesis also was found to play a role in alkaline stress-mediated root inhibition; the ethylene overproducer1-1 mutant, which overproduces ethylene because of increased stability of 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE5, was hypersensitive to alkaline stress. In addition, the ethylene biosynthesis inhibitor cobalt (Co(2+)) suppressed alkaline stress-mediated inhibition of root elongation. We further found that alkaline stress caused an increase in auxin levels by promoting expression of auxin biosynthesis-related genes, but the increase in auxin levels was reduced in the roots of the etr1-3 and ein3-1 mutants and in Ag(+)/Co(2+)-treated wild-type plants. Additional genetic and physiological data showed that AUXIN1 (AUX1) was involved in alkaline stress-mediated inhibition of root elongation. Taken together, our results reveal that ethylene modulates alkaline stress-mediated inhibition of root growth by increasing auxin accumulation by stimulating the expression of AUX1 and auxin biosynthesis-related genes.

Synergistic action of auxin and ethylene on root elongation inhibition is caused by a reduction of epidermal cell length

Auxin and ethylene have been largely reported to reduce root elongation in maize primary root. However the effects of auxin are greater than those caused by ethylene. Although auxin stimulates ethylene biosynthesis through the specific increase of ACC synthase, the auxin inhibitory effect on root elongation is not mediated by the auxin-induced increase of ethylene production. Recently it has been demonstrated that root inhibition by the application of the synthetic auxin NAA (1-naphtalenacetic acid) is increased if combined with the ethylene precursor ACC (1-aminocyclopropane-1-carboxilic acid) when both compounds are applied at very low concentrations.   Root elongation is basically the result of two processes: a) cell divisions in the meristem where meristematic cells continuously generate new cells and b) subsequently polarized growth by elongation along the root axis as cells leave the meristem and enter the root elongation zone. Our results indicate that exogenous auxin reduced both root elongation and epidermal cell length. In a different way, ethylene at very low concentrations only inhibited root elongation without affecting significantly epidermal cell length. However, these concentrations of ethylene increased the inhibitory effect of auxin on root elongation and cell length. Consequently the results support the hypothesis that ethylene acts synergistically with auxin in the regulation of root elongation and that inhibition by both hormones is due, at least partially, to the reduction of cell length in the epidermal layer.

Nitrogen modulation of legume root architecture signaling pathways involves phytohormones and small regulatory molecules

Nitrogen, particularly nitrate is an important yield determinant for crops. However, current agricultural practice with excessive fertilizer usage has detrimental effects on the environment. Therefore, legumes have been suggested as a sustainable alternative for replenishing soil nitrogen. Legumes can uniquely form nitrogen-fixing nodules through symbiotic interaction with specialized soil bacteria. Legumes possess a highly plastic root system which modulates its architecture according to the nitrogen availability in the soil. Understanding how legumes regulate root development in response to nitrogen availability is an important step to improving root architecture. The nitrogen-mediated root development pathway starts with sensing soil nitrogen level followed by subsequent signal transduction pathways involving phytohormones, microRNAs and regulatory peptides that collectively modulate the growth and shape of the root system. This review focuses on the current understanding of nitrogen-mediated legume root architecture including local and systemic regulations by different N-sources and the modulations by phytohormones and small regulatory molecules.

Endoreduplication preferentially occurs at the proximal side of the abscission zone during abscission of tomato leaf

Endoreduplication is a cell cycle variant in which multiple rounds of DNA replication occur without subsequent mitosis, resulting in polyploid cells. Although cells with endoreduplicated nuclei were ubiquitously distributed throughout the abscission zone (AZ) of tomato leaf before abscission induction by ethylene, endoreduplication was detected mostly on the proximal side of the AZ after induction. The possible association between endoreduplication and intensive membrane trafficking in cells at the proximal side of the AZ is discussed.

Ethylene in vegetative development: a tale with a riddle

The vegetative development of plants is strongly dependent on the action of phytohormones. For over a century, the effects of ethylene on plants have been studied, illustrating the profound impact of this gaseous hormone on plant growth, development and stress responses. Ethylene signaling is under tight self-control at various levels. Feedback regulation occurs on both biosynthesis and signaling. For its role in developmental processes, ethylene has a close and reciprocal relation with auxin, another major determinant of plant architecture. Here, we discuss, in view of novel findings mainly in the reference plant Arabidopsis, how ethylene is distributed and perceived throughout the plant at the organ, tissue and cellular levels, and reflect on how plants benefit from the complex interaction of ethylene and auxin, determining their shape. Furthermore, we elaborate on the implications of recent discoveries on the control of ethylene signaling.

Ethylene-induced differential petiole growth in Arabidopsis thaliana involves local microtubule reorientation and cell expansion

• Hyponastic growth is an upward petiole movement induced by plants in response to various external stimuli. It is caused by unequal growth rates between adaxial and abaxial sides of the petiole, which bring rosette leaves to a more vertical position. The volatile hormone ethylene is a key regulator inducing hyponasty in Arabidopsis thaliana. Here, we studied whether ethylene-mediated hyponasty occurs through local stimulation of cell expansion and whether this involves the reorientation of cortical microtubules (CMTs). • To study cell size differences between the two sides of a petiole in ethylene and control conditions, we analyzed epidermal imprints. We studied the involvement of CMT orientation in epidermal cells using the tubulin marker line as well as genetic and pharmacological means of CMT manipulation. • Our results demonstrate that ethylene induces cell expansion at the abaxial side of the- petiole and that this can account for the observed differential growth. At the abaxial side, ethylene induces CMT reorientation from longitudinal to transverse, whereas, at the adaxial side, it has an opposite effect. The inhibition of CMTs disturbed ethylene-induced hyponastic growth. • This work provides evidence that ethylene stimulates cell expansion in a tissue-specific manner and that it is associated with tissue-specific changes in the arrangement of CMTs along the petiole.

Hormonal control of endoreduplication in sugar beet (Beta vulgaris L.) seedlings growing in vitro

The effect on endoreduplication in sugar beet (Beta vulgaris L.) seedlings of five plant hormones in MS medium, ethylene, 24-epibrassinolide (EBR), gibberellic acid (GA(3) ), kinetin and 1-naphthaleneacetic acid (NAA), as well as a combination of kinetin and NAA at two different concentrations, was studied using flow cytometry. Analyses of DNA content in nuclei of the root, hypocotyl and cotyledons of seedlings growing in vitro were performed during their early development, starting from when the root was 0.5-1.0 cm long until expansion of the first pair of leaves. The proportions of nuclei with different DNA contents were established and the mean C-value calculated. The presence of exogenous plant hormones changed endoreduplication intensity, although to different extents, depending on the organ and developmental stage. Ethylene and NAA stimulated the process, while EBR and kinetin suppressed it and GA did not clearly affect it.

Petiole hyponasty: an ethylene-driven, adaptive response to changes in the environment

BACKGROUND

Many plant species can actively reorient their organs in response to dynamic environmental conditions. Organ movement can be an integral part of plant development or can occur in response to unfavourable external circumstances. These active reactions take place with or without a directional stimulus and can be driven either by changes in turgor pressure or by asymmetric growth. Petiole hyponasty is upward movement driven by a higher rate of cell expansion on the lower (abaxial) compared with the upper (adaxial) side. Hyponasty is common among rosette species facing environmental stresses such as flooding, proximity of neighbours or elevated ambient temperature. The complex regulatory mechanism of hyponasty involves activation of pathways at molecular and developmental levels, with ethylene playing a crucial role.

SCOPE

We present current knowledge on the mechanisms that promote hyponasty in the context of other organ movements, including tropic and nastic reactions together with circumnutation. We describe major environmental cues resulting in hyponasty and briefly discuss their perception and signal transduction. Since ethylene is a central agent triggering hyponasty, we focus on ethylene in controlling different stages during plant development and summarize current knowledge on the relationship between ethylene and cell growth.

Continuous UV-B irradiation induces endoreduplication and peroxidase activity in epidermal cells surrounding trichomes on cucumber cotyledons

Most trichomes on the surface of cucumber (Cucumis sativus L.) cotyledons consist of three cells. We previously showed that continuous UV-B (290-320 nm) irradiation induces rapid cellular expansion and the accumulation of polyphenolic compounds, possibly stress lignin, in epidermal cells around these trichomes.(1)) To examine the mechanism of the UV-B-induced cellular expansion and to determine which step is stimulated by UV-B irradiation in the lignin synthesis pathway, we investigated relative DNA contents in epidermal cells, including trichomes, and enzyme activity and gene expression in the phenylpropanoid pathway. UV-B irradiation increased the ploidy level over 15 days, specifically in the epidermal cells surrounding trichomes, but not in the other epidermal cells or trichomes. In epidermal cells surrounding trichomes, UV-B irradiation induced peroxidase (POX) activity from days 7 to 15. In cotyledons, UV-B exposure induced CS-POX1 and CS-POX3 gene expression within 2 days, and it also induced two other enzymes in the phenylpropanoid pathway, sinapyl alcohol dehydrogenase and coniferyl alcohol dehydrogenase, from days 9 to 11. Thus, exposure to UV-B induces expansion, endoreduplication, POX activity, and the accumulation of polyphenolic compounds in epidermal cells surrounding the trichomes of cucumber cotyledons. Because polyphenolic compounds such as lignin absorb UV-B, our data indicate a physiological protective mechanism against UV-B irradiation in cucumber.

The Arabidopsis cell division cycle

Plant cells have evolved a complex circuitry to regulate cell division. In many aspects, the plant cell cycle follows a basic strategy similar to other eukaryotes. However, several key issues are unique to plant cells. In this chapter, both the conserved and unique cellular and molecular properties of the plant cell cycle are reviewed. In addition to division of individual cells, the specific characteristic of plant organogenesis and development make that cell proliferation control is of primary importance during development. Therefore, special attention should be given to consider plant cell division control in a developmental context. Proper organogenesis depends on the formation of different cell types. In plants, many of the processes leading to cell differentiation rely on the occurrence of a different cycle, termed the endoreplication cycle, whereby cells undergo repeated full genome duplication events in the absence of mitosis and increase their ploidy. Recent findings are focusing on the relevance of changes in chromatin organization for a correct cell cycle progression and, conversely, in the relevance of a correct functioning of chromatin remodelling complexes to prevent alterations in both the cell cycle and the endocycle.

Ethylene is an endogenous stimulator of cell division in the cambial meristem of Populus

The plant hormone ethylene is an important signal in plant growth responses to environmental cues. In vegetative growth, ethylene is generally considered as a regulator of cell expansion, but a role in the control of meristem growth has also been suggested based on pharmacological experiments and ethylene-overproducing mutants. In this study, we used transgenic ethylene-insensitive and ethylene-overproducing hybrid aspen (Populus tremula x tremuloides) in combination with experiments using an ethylene perception inhibitor [1-methylcyclopropene (1-MCP)] to demonstrate that endogenous ethylene produced in response to leaning stimulates cell division in the cambial meristem. This ethylene-controlled growth gives rise to the eccentricity of Populus stems that is formed in association with tension wood.

Ethylene-auxin interactions regulate lateral root initiation and emergence in Arabidopsis thaliana

Plant root systems display considerable plasticity in response to endogenous and environmental signals. Auxin stimulates pericycle cells within elongating primary roots to enter de novo organogenesis, leading to the establishment of new lateral root meristems. Crosstalk between auxin and ethylene in root elongation has been demonstrated, but interactions between these hormones in root branching are not well characterized. We find that enhanced ethylene synthesis, resulting from the application of low concentrations of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), promotes the initiation of lateral root primordia. Treatment with higher doses of ACC strongly inhibits the ability of pericycle cells to initiate new lateral root primordia, but promotes the emergence of existing lateral root primordia: behaviour that is also seen in the eto1 mutation. These effects are correlated with decreased pericycle cell length and increased lateral root primordia cell width. When auxin is applied simultaneously with ACC, ACC is unable to prevent the auxin stimulation of lateral root formation in the root tissues formed prior to ACC exposure. However, in root tissues formed after transfer to ACC, in which elongation is reduced, auxin does not rescue the ethylene inhibition of primordia initiation, but instead increases it by several fold. Mutations that block auxin responses, slr1 and arf7 arf19, render initiation of lateral root primordia insensitive to the promoting effect of low ethylene levels, and mutations that inhibit ethylene-stimulated auxin biosynthesis, wei2 and wei7, reduce the inhibitory effect of higher ethylene levels, consistent with ethylene regulating root branching through interactions with auxin.

A model describing cell polyploidization in tissues of growing fruit as related to cessation of cell proliferation

Endoreduplication is a phenomenon, widespread among plants, which consists of an incomplete cell cycle without mitosis and leads to the increase of the nuclear DNA content. In this work, a model was developed describing cell proliferation and DNA endoreduplication over the whole fruit development, from the pre-anthesis period until maturation. In each mitotic cycle of duration tau, the proportion of cells proceeding through division depends on a constant parameter rho and on the progressive decline of the proliferating capacity . The non-dividing cells may either stop the reduplication fully, or switch to repeated syntheses of DNA without cell division, resulting in cell endoreduplication. A single constant parameter sigma describes the proportion of cells that moves from one to the next class of DNA content after each lapse of time tauE, considered to be the minimum time required for an endocycle. The model calculates the total number of cells and their distribution among eight classes of ploidy level. The dynamic patterns of cell proliferation and ploidy were compared with those obtained experimentally on two contrasting tomato genotypes. The approach developed in this model should allow the future integration of new knowledge concerning the genetic and environmental control of the switch from complete to incomplete cell cycle.

The Janus face of ethylene: growth inhibition and stimulation

The gaseous plant hormone ethylene modulates many internal processes and growth responses to environmental stimuli. Ethylene has long been recognized as a growth inhibitor, but evidence is accumulating that ethylene can also promote growth. Therefore, the concept of ethylene as a general growth inhibitor needs reconsideration: a close examination of recent literature can help to understand the two contrasting faces of growth control by ethylene. Here, we propose a hypothesis that integrates growth inhibition and growth stimulation into one biphasic ethylene response model. Focusing on photosynthesis and cell expansion, we highlight several mechanisms through which ethylene affects plant growth, thereby interacting with various other signal transduction routes.

Mitosis-specific promoter of the alfalfa cyclin-dependent kinase gene (Medsa;CDKB2;1) is activated by wounding and ethylene in a non-cell division-dependent manner

Cyclin-dependent serine/threonine kinases (CDKs) have pivotal roles in regulating the eukaryotic cell cycle. Plants possess a unique class of CDKs (B-type CDKs) with preferential protein accumulation at G2/M-phases; however, their exact functions are still enigmatic. Here we describe the functional characterization of a 360-bp promoter region of the alfalfa (Medicago sativa) CDKB2;1 gene in transgenic plants and cell lines. It is shown that the activity of the analyzed promoter was characteristic for proliferating meristematic regions in planta and specific for cells in the G2/M-phases in synchronized cell cultures. Immunohistochemical analysis of transgenic root sections further confirmed the correlation of the expression of the CDKB2;1 promoter-linked reporter genes with the accumulation of the correspondent kinase. It was found that, in addition to auxin (2,4-dichlorophenoxyacetic acid) treatment, wounding could also induce both the reporter and endogenous genes in transgenic leaf explants. Furthermore, ethylene, known as a wound-response mediator, had a similar effect. The gene activation in response to wounding or ethephon was faster and occurred without the induction of cell cycle progression in contrast to the control auxin treatment. In silico analysis of this promoter indeed revealed the presence of a set of cis-elements, indicating not only cell cycle- but wound- and ethylene-dependent regulation of this CDK gene. Based on the presented data, we discuss the functional significance of the complex regulation of mitosis-specific CDK genes in plants.

Endopolyploidy in seed plants

Two main attempts have been suggested for the biological significance of endopolyploidy: (i) provision of high DNA amounts to support high synthetic demands in certain cells and (ii) compensation for a lack of nuclear DNA in species with small genomes. However, in seed plants, the positive correlation between DNA content and cell volume of endopolyploid cells suggests other possibilities. Cell size paralleled by the endopolyploidy level has an impact on growth and development. Endopolyploidy levels in turn are characteristic for a given species and even families, reflecting the adaptation to certain habitats during phylogeny. Furthermore, endopolyploidy levels vary to some degree between individuals of one species in response to different environmental conditions. In addition, endopolyploidy differs between different tissues suggests that a certain cell size is advantageous for a given cell function. This article reviews these findings and discusses more conclusive possible functions of endopolyploidy.

Efficient plant regeneration from cotyledon explants of bottle gourd (Lagenaria siceraria Standl.)

Using cotyledon explants excised from seedlings germinated in vitro, an efficient plant regeneration system via organogenesis was established for bottle gourd (Lagenaria siceraria Standl.). Maximum shoot regeneration was obtained when the proximal parts of cotyledons from 4-day-old seedlings were cultured on MS medium with 3 mg/l BA and 0.5 mg/l AgNO(3) under a 16-h photoperiod. After 3-4 weeks of culture, 21.9-80.7% of explants from the five cultivars regenerated shoots. Adventitious shoots were successfully rooted on a half-strength MS medium with 0.1 mg/l IAA for 2-3 weeks. Flow cytometric analysis revealed that most of the regenerated plants derived from culture on medium with AgNO(3) were diploid.

Arabidopsis EIN3-binding F-box 1 and 2 form ubiquitin-protein ligases that repress ethylene action and promote growth by directing EIN3 degradation

Ubiquitination of various intracellular proteins by ubiquitin-protein ligases (or E3s) plays an essential role in eukaryotic cell regulation primarily through its ability to selectively target proteins for degradation by the 26S proteasome. Skp1, Cullin, F-box (SCF) complexes are one influential E3 class that use F-box proteins to deliver targets to a core ligase activity provided by the Skp1, Cullin, and Rbx1 subunits. Almost 700 F-box proteins can be found in Arabidopsis, indicating that SCF E3s likely play a pervasive role in plant physiology and development. Here, we describe the reverse genetic analysis of two F-box proteins, EBF1 and -2, that work coordinately in SCF complexes to repress ethylene action. Mutations in either gene cause hypersensitivity to exogenous ethylene and its precursor 1-aminocyclopropane-1-carboxylic acid. EBF1 and -2 interact directly with ethylene insensitive 3 (EIN3), a transcriptional regulator important for ethylene signaling. Levels of EIN3 are increased in mutants affecting either EBF1 or -2, suggesting that the corresponding SCF complexes work together in EIN3 breakdown. Surprisingly, double ebf1 ebf2 mutants display a substantial arrest of seedling growth and have elevated EIN3 levels, even in the absence of exogenous ethylene. Collectively, our results show that the SCF(EBF1/EBF2)-dependent ubiquitination and subsequent removal of EIN3 is critical not only for proper ethylene signaling but also for growth in plants.

Transient exposure to ethylene stimulates cell division and alters the fate and polarity of hypocotyl epidermal cells

After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.