Shoot Apical Meristem Cells Research Articles

Single-cell transcriptomic analysis of pea shoot development and cell-type-specific responses to boron deficiency.

Understanding how nutrient stress impacts plant growth is fundamentally important to the development of approaches to improve crop production under nutrient limitation. Here we applied single-cell RNA sequencing to shoot apices of Pisum sativum grown under boron (B) deficiency. We identified up to 15 cell clusters based on the clustering of gene expression profiles and verified cell identity with cell-type-specific marker gene expression. Different cell types responded differently to B deficiency. Specifically, the expression of photosynthetic genes in mesophyll cells (MCs) was down-regulated by B deficiency, consistent with impaired photosynthetic rate. Furthermore, the down-regulation of stomatal development genes in guard cells, including homologs of MUTE and TOO MANY MOUTHS, correlated with a decrease in stomatal density under B deficiency. We also constructed the developmental trajectory of the shoot apical meristem (SAM) cells and a transcription factor interaction network. The developmental progression of SAM to MC was characterized by up-regulation of genes encoding histones and chromatin assembly and remodeling proteins including homologs of FASCIATA1 (FAS1) and SWITCH DEFECTIVE/SUCROSE NON-FERMENTABLE (SWI/SNF) complex. However, B deficiency suppressed their expression, which helps to explain impaired SAM development under B deficiency. These results represent a major advance over bulk-tissue RNA-seq analysis in which cell-type-specific responses are lost and hence important physiological responses to B deficiency are missed. The reported findings reveal strategies by which plants adapt to B deficiency thus offering breeders a set of specific targets for genetic improvement. The reported approach and resources have potential applications well beyond P. sativum species and could be applied to various legumes to improve their adaptability to multiple nutrient or abiotic stresses.

N6‐methyladenosine modification changes during the recovery processes for Paulownia witches' broom disease under the methyl methanesulfonate treatment

AbstractPhytoplasmas induce diseases in more than 1000 plant species and cause substantial ecological damage and economic losses, but the specific pathogenesis of phytoplasma has not yet been clarified. N6‐methyladenosine (m6A) is the most common internal modification of the eukaryotic Messenger RNA (mRNA). As one of the species susceptible to phytoplasma infection, the pathogenesis and mechanism of Paulownia has been extensively studied by scholars, but the m6A transcriptome map of Paulownia fortunei (P. fortunei) has not been reported. Therefore, this study aimed to explore the effect of phytoplasma infection on m6A modification of P. fortunei and obtained the whole transcriptome m6A map in P. fortunei by m6A‐seq. The m6A‐seq results of Paulownia witches' broom (PaWB) disease and healthy samples indicate that PaWB infection increased the degree of m6A modification of P. fortunei. The correlation analysis between the RNA‐seq and m6A‐seq data detected that a total of 315 differentially methylated genes were predicted to be significantly differentially expressed at the transcriptome level. Moreover, the functions of PaWB‐related genes were predicted by functional enrichment analysis, and two genes related to maintenance of the basic mechanism of stem cells in shoot apical meristem were discovered. One of the genes encodes the receptor protein kinase CLV2 (Paulownia_LG2G000076), and the other gene encodes the homeobox transcription factor STM (Paulownia_LG15G000976). In addition, genes F‐box (Paulownia_LG17G000760) and MSH5 (Paulownia_LG8G001160) had exon skipping and mutually exclusive exon types of alternative splicing in PaWB‐infected seedling treated with methyl methanesulfonate, and m6A modification was found in m6A‐seq results. Moreover, Reverse Transcription–Polymerase Chain Reaction (RT‐PCR) verified that the alternative splicing of these two genes was associated with m6A modification. This comprehensive map provides a solid foundation for revealing the potential function of the mRNA m6A modification in the process of PaWB. In future studies, we plan to verify genes directly related to PaWB and methylation‐related enzymes in Paulownia to elucidate the pathogenic mechanism of PaWB caused by phytoplasma invasion.

Graph metric learning quantifies morphological differences between two genotypes of shoot apical meristem cells inArabidopsis

AbstractWe present a method for learning ‘spectrally descriptive’ edge weights for graphs. We generalize a previously known distance measure on graphs (graph diffusion distance [GDD]), thereby allowing it to be tuned to minimize an arbitrary loss function. Because all steps involved in calculating this modified GDD are differentiable, we demonstrate that it is possible for a small neural network model to learn edge weights which minimize loss. We apply this method to discriminate between graphs constructed from shoot apical meristem images of two genotypes of Arabidopsis thaliana specimens: wild-type and trm678 triple mutants with cell division phenotype. Training edge weights and kernel parameters with contrastive loss produce a learned distance metric with large margins between these graph categories. We demonstrate this by showing improved performance of a simple k-nearest-neighbour classifier on the learned distance matrix. We also demonstrate a further application of this method to biological image analysis. Once trained, we use our model to compute the distance between the biological graphs and a set of graphs output by a cell division simulator. Comparing simulated cell division graphs to biological ones allows us to identify simulation parameter regimes which characterize mutant versus wild-type Arabidopsis cells. We find that trm678 mutant cells are characterized by increased randomness of division planes and decreased ability to avoid previous vertices between cell walls.

Comparative Transcriptome Analysis of Grafted Tomato with Drought Tolerance.

Grafting is a method used in agriculture to improve crop production and tolerance to biotic and abiotic stress. This technique is widely used in tomato, Solanum lycopersicum L.; however, the effects of grafting on changes in gene expression associated with stress tolerance in shoot apical meristem cells are still under-discovered. To clarify the effect of grafting, we performed a transcriptomic analysis between non-grafted and grafted tomatoes using the tomato variety Momotaro-scion and rootstock varieties, TD1, GS, and GF. Drought tolerance was significantly improved not only by a combination of compatible resistant rootstock TD1 but also by self-grafted compared to non-grafted lines. Next, we found the differences in gene expression between grafted and non-grafted plants before and during drought stress treatment. These altered genes are involved in the regulation of plant hormones, stress response, and cell proliferation. Furthermore, when comparing compatible (Momo/TD1 and Momo/Momo) and incompatible (Momo/GF) grafted lines, the incompatible line reduced gene expression associated with phytohormones but increased in wounding and starvation stress-response genes. These results conclude that grafting generates drought stress tolerance through several gene expression changes in the apical meristem.

Roles of plant growth regulators on flowering of rose (Rosa hybrida L.’Red Rose’)

Rose is the most popular ornamental flower all over the world, which is used as garden plants and cut flowers. In the case of Rosa hybrida L. ’Red Rose’, flowering provides the major developmental transition from the vegetative to the reproductive stage, and reproduction is one of the most important phases in an organism’s life cycle. In this study, the morphological and physiological changes during the flower development of rose, which is planted in the garden, and roles of plant growth regulators on the flowering of in vitro vegetative shoots of rose were analyzed. The development of a flower includes three stages: the shoot apical meristem, floral meristem, floral bud. Levels of cytokinin, auxins, and gibberellins increased in the transition of meristem from the shoot apical meristem to the floral meristem stage. Plant growth regulators have important effects on the shoot apical meristem cell division and flowering. The combination of 0.5 mg.L−1 GA3, 0.1 mg.L−1 NAA, 2.5 or 3.0 mg.L−1 BA to Murashige and Skoog (MS) medium induces the floral transition of the in vitro vegetative shoots with the highest percentage (41%) as well as growth and development in comparison to the other treatments after 10 weeks. Then, the in vitro floral meristem continuously developed into a flower bud after 12 weeks.

Sucrose interferes with endogenous cytokinin homeostasis and expression of organogenesis-related genes during de novo shoot organogenesis in kohlrabi

Cross-talk between phytohormones and sugars is intensely involved in plant metabolism, growth and regeneration. We documented alterations in cytokinin (CK) homeostasis in four developmental stages during de novo shoot organogenesis (DNSO) of kohlrabi (Brassica oleracea var. gongylodes cv. Vienna Purple) seedlings induced by exogenous CKs, trans-zeatin (transZ) and thidiazuron (TDZ), added together with elevated sucrose concentration (6% and 9%). Significant impact of CK and sucrose treatment and their interaction was recorded in all investigated stages, including plantlet development before calli formation (T1 and T2), calli formation (T3) and shoot regeneration (T4). Results showed remarkable increase in total CK levels for transZ treatment, particularly with 9% sucrose. This trend was observed for all physiological and structural groups of CKs. Application of TDZ contributed to little or no increase in CK levels regardless of sucrose concentration. Analysis of expression profiles of organogenesis-related genes involved in auxin transport, CK response, shoot apical meristem formation and cell division revealed that higher sugar concentration significantly downregulated the analysed genes, particularly in T3. This continued on TDZ, but transZ induced an opposite effect with 9% sucrose in T4, increasing gene activity. Our results demonstrated that phytohormone metabolism might be triggered by sucrose signalling in kohlrabi DNSO.

The Arabidopsis Shoot Apex Cell Atlas

Plant stem cell niches are embedded within an organized group of dividing cells known as the meristem. The shoot apical meristem allows for reiterative development and the formation of new aerial structures throughout the life cycle of a plant. As in animals, plant stem cells possess the capacity to self-renew and at the same time to differentiate into lateral organs through transit-amplifying cells. Here, we use single-cell RNA sequencing to define the cellular taxonomy of the Arabidopsis vegetative shoot apex at the transcriptome level. We find that the shoot apex is composed of highly heterogeneous cells, which can be partitioned into 7 broad populations with 23 transcriptionally distinct cell clusters. We delineate cell cycle continuums and developmental trajectories of epidermal cells, vascular tissue and leaf mesophyll cells, and infer transcription factors and gene expression signatures associated with cell fate decisions. Moreover, panoramic integration analysis of shoot and root apical meristem cell populations reveals common vascular cell types and uncovers shoot-specific parenchymatous bundle sheath cell cluster. Our results thus provide a molecular definition of the cell populations making up the shoot apex, and offer an unprecedented opportunity to investigate conserved principles underlying cell division and differentiation in multicellular organisms at single-cell resolution.

Suppression of RNA-dependent RNA polymerase 6 in tomatoes allows potato spindle tuber viroid to invade basal part but not apical part including pluripotent stem cells of shoot apical meristem.

RNA-dependent RNA polymerase 6 (RDR6) is one of the key factors in plant defense responses and suppresses virus or viroid invasion into shoot apical meristem (SAM) in Nicotiana benthamiana. To evaluate the role of Solanum lycopersicum (Sl) RDR6 upon viroid infection, SlRDR6-suppressed (SlRDR6i) ‘Moneymaker’ tomatoes were generated by RNA interference and inoculated with intermediate or lethal strain of potato spindle tuber viroid (PSTVd). Suppression of SlRDR6 did not change disease symptoms of both PSTVd strains in ‘Moneymaker’ tomatoes. Analysis of PSTVd distribution in shoot apices by in situ hybridization revealed that both PSTVd strains similarly invade the basal part but not apical part including pluripotent stem cells of SAM in SlRDR6i plants at a low rate unlike a previous report in N. benthamiana. In addition, unexpectedly, amount of PSTVd accumulation was apparently lower in SlRDR6i plants than in control tomatoes transformed with empty cassette in early infection especially in the lethal strain. Meanwhile, SlRDR6 suppression did not affect the seed transmission rates of PSTVd. These results indicate that RDR6 generally suppresses PSTVd invasion into SAM in plants, while suppression of RDR6 does not necessarily elevate amount of PSTVd accumulation. Additionally, our results suggest that host factors such as RDR1 other than RDR6 may also be involved in the protection of SAM including pluripotent stem cells from PSTVd invasion and effective RNA silencing causing the decrease of PSTVd accumulation during early infection in tomato plants.

Physiological mechanism and developmental events in differentiating floral buds of Sophora tonkinensis gagnep

Background: Sophora tonkinensis Gagnep is an important medicinal plant in China. Previous research has focused on the rapid propagation and quality analysis of in vitro tissue culture plantlets. Few studies have focused on floral bud differentiation. Physiological and molecular mechanisms regulating floral bud differentiation have not been elucidated. Objective: We used paraffin sections and RNA-seq and then analyzed changes in physiological, biochemical, and endogenous hormones to examine floral bud differentiation. Materials and Methods: Buds were harvested in mid-March or early April. After sorting and cleaning, flower buds physiological and biochemical properties were measured and hormones were determined. Morphological observation of S. tonkinensis was carried out using paraffin sections. Results: The transition from the meristematic to the reproductive phase included changes in the shoot apical meristem morphogenesis and cell differentiation. The process includes floral buds initial differentiation and flower primordium differentiation stages; we defined eight stages in total. There was no significant change in soluble content, but soluble protein content gradually decreased. Activities of antioxidant enzymes changed significantly. Total chlorophyll (Chl a + b) content increased significantly. Endogenous hormone levels changed differently during floral bud differentiation. We identified 104,519 expressed genes and 24 were involved in flowering. MADS-box and AP family genes are involved in flower formation and 40 differentially expressed genes associated with floral bud differentiation were identified. Conclusion: The higher soluble sugar, protein, and chlorophyll content and the higher peroxidase activity were beneficial to floral bud differentiation of S. tonkinensis. The dynamic changes in hormone content contribute to differentiating floral buds.

Decreased morphogenetic potential in peach palm stem-like cells in long-term in vitro conditions

Peach palm (Bactris gasipaes Kunth) has been micropropagated from pre-procambial cells that provide stem-like cell niches, (i.e., pre-procambial cells), multipotent, pluripotent and totipotent for direct vascularization, adventitious buds and somatic embryogenesis, respectively. The direct induction of adventitious buds and somatic embryogenesis reduces the frequency of mutations when compared to indirect morphogenesis. Long-term in vitro cultivation of perennial species such as peach palm cause the clones to age and deteriorate; however, the consequences for morphogenesis potential are not fully clear. The morphogenic potential of peach palm clones established and in vitro cultivated for 8 years (regeneration of adventitious buds without callus formation) was investigated in leaves, roots and stem bases using histological and histochemical analyses. Data from long-term cultures (8-years-old) was compared to data from short-term cultures (1-year-old). Morphogenic pathways monitoring for direct induction of somatic embryos and adventitious buds revealed a strong morphogenic reduction potential in the pre-procambial cells, parenchyma cells in the proximal region of stem bases, and external cells of leaf sheaths. Initial cells of shoot apical meristems and pre-procambial cells commit cell reprogramming to the undifferentiated state and subsequent acquisition of cellular competence. These results are applicable in the micropropagation of peach palm, with consideration to obtaining clones and their long-term in vitro culture.

LCM-seq reveals the crucial role of LsSOC1 in heat-promoted bolting of lettuce (Lactuca sativa L.).

Lettuce (Lactuca sativa L.) is one of the most economically important vegetables. The floral transition in lettuce is accelerated under high temperatures, which can significantly decrease yields. However, the molecular mechanism underlying the floral transition in lettuce is poorly known. Using laser capture microdissection coupled with RNA sequencing, we isolated shoot apical meristem cells from the bolting-sensitive lettuce line S39 at four critical stages of development. Subsequently, we screened specifically for the flowering-related gene LsSOC1 during the floral transition through comparative transcriptomic analysis. Molecular biology, developmental biology, and biochemical tools were combined to investigate the biological function of LsSOC1 in lettuce. LsSOC1 knockdown by RNA interference resulted in a significant delay in the timing of bolting and insensitivity to high temperature, which indicated that LsSOC1 functions as an activator during heat-promoted bolting in lettuce. We determined that two heat shock transcription factors, HsfA1e and HsfA4c, bound to the promoter of LsSOC1 to confirm that LsSOC1 played an important role in heat-promoted bolting. This study indicates that LsSOC1 plays a crucial role in the heat-promoted bolting process in lettuce. Further investigation of LsSOC1 may be useful for clarification of the bolting mechanism in lettuce.

Cytoskeleton in the Parasitic Plant Cuscuta During Germination and Prehaustorium Formation.

Although cytoskeleton is a driving force for cell division and growth in higher plants, there is little evidence about its components in parasitic angiosperms. Microtubules and actin filaments in cells of shoot apical meristem and root-like structure of stem holoparasites European (C. europaea L.) and Eastern (C. monogyna Vahl.) dodders, as well as in prehaustorium, the specific organ adapted to parasitism, were visualized for the first time by immunolabeling and fluorescence microscopy. The significance of cytoskeletal elements during germination and prehaustorium formation was addressed by treatments with taxol, oryzalin, latrunculin B, cytochalasin B/D, jasplakinolide, and 2,3-butanedione monoxime. In shoot apical meristem many dividing cells were observed, in contrast to root-like structure, devoid of cell divisions. Cortical microtubules were oriented transversely and/or obliquely, while actin filaments were randomly distributed in cells of both organs. Furthermore, longitudinal cortical microtubules were present in digitate cells of prehaustorium, and transverse arrays were found in its file cells. Long and short random actin filaments were also observed in prehaustorium cells. Thus, it was shown that the cytoskeleton in dodder shoot cells is organized in a similar way to non-parasitic dicots, while cytoskeletal organization has some peculiarities in quickly senescing root-like structure and prehaustorium.

A Non-Classical Member of the Protein Disulfide Isomerase Family, PDI7 of Arabidopsis thaliana, Localizes to the cis-Golgi and Endoplasmic Reticulum Membranes.

Members of the protein disulfide isomerase (PDI)-C subfamily are chimeric proteins containing the thioredoxin (Trx) domain of PDIs, and the conserved N- and C-terminal Pfam domains of Erv41p/Erv46p-type cargo receptors. They are unique to plants and chromalveolates. The Arabidopsis genome encodes three PDI-C isoforms: PDI7, PDI12 and PDI13. Here we demonstrate that PDI7 is a 65 kDa integral membrane glycoprotein expressed throughout many Arabidopsis tissues. Using a PDI7-specific antibody, we show through immunoelectron microscopy that PDI7 localizes to the endoplasmic reticulum (ER) and Golgi membranes in wild-type root tip cells, and was also detected in vesicles. Tomographic modeling of the Golgi revealed that PDI7 was confined to the cis-Golgi, and accumulated primarily at the cis-most cisterna. Shoot apical meristem cells from transgenic plants overexpressing PDI7 exhibited a dramatic increase in anti-PDI7 labeling at the cis-Golgi. When N- or C-terminal fusions between PDI7 and the green fluorescent protein variant, GFP(S65T), were expressed in mesophyll protoplasts, the fusions co-localized with the ER marker, ER-mCherry. However, when GFP(S65T) was positioned internally within PDI7 (PDI7-GFPint), the fusion strongly co-localized with the cis-Golgi marker, mCherry-SYP31, and faintly labeled the ER. In contrast to the Golgi-resident fusion protein (Man49-mCherry), PDI7-GFPint did not redistribute to the ER after brefeldin A treatment. Protease protection experiments indicated that the Trx domain of PDI7 is located within the ER/Golgi lumen. We propose a model where PDI-C isoforms function as cargo receptors for proteins containing exposed cysteine residues, cycling them from the Golgi back to the ER.

Loss of developmental pluripotency occurs in two stages during leaf aging in Arabidopsis thaliana

Aging in plants and animals can result in the loss of cellular pluripotency and the inability to regenerate multiple organs from differentiated, somatic tissues. Detached cotyledons from Arabidopsis thaliana (L.) Heynh. seedlings can proliferate to form callus as well as root and shoot organs in the presence of the phytohormones auxin and cytokinin or their synthetic analogs. In this study, detached cotyledons from the ecotype Landsberg erecta (Ler-0) demonstrated two separate developmental intervals of pluripotency loss. During a 48- to 60-h interval, occurring approximately 4 to 6 d after germination (DAG), detached cotyledons from Ler-0 could proliferate new cells (callus) but lost shoot regeneration competency, which the ecotype Nossen-0 (No-0) retained. In older cotyledons, the pluripotency loss was more severe, as detached cotyledons failed to proliferate callus. In the first interval, Ler-0 cotyledons lost the ability to respond to treatment with a high concentration of synthetic auxin, a level previously hypothesized to promote dedifferentiation. However, both young and old Ler-0 cotyledons were found to induce WUSCHEL::NLS-GUS, a marker for undifferentiated shoot apical meristem cells. The late-stage interval positively correlated to the timing of slowed growth in cotyledons still attached to intact seedlings. Also, cytokinin overexpression could rescue (or delay) this late-stage callus proliferation failure, but not the early-stage decline in shoot regeneration ability. Combined, the results demonstrate that loss of pluripotency in vitro occurs in distinct temporal stages and involves auxin and/or cytokinin signaling. Furthermore, there is genetic variation for this loss of pluripotency within Arabidopsis.

Plant development: From cells to individuals

As the development of research tools in molecular biology, plant developmental biology has been changed its focus from the descriptive analysis of plant morphology to the cellular and gene regulation level. Plants have distinct postembryonic development patterns compare to the animal, which give the plant a flexible developmental plasticity in response to different growth environments. In the long-term of evolution, plants are adapted to the environment changes through the continuous adjustment of their development strategy, which makes the plant world highly diverse. The growth and development of multicellular organisms depend on the maintenance and constant differentiation of stem cells. In plants, most of the organs originate from stem cells where resided in the shoot apical meristem, root apical meristem and cambium. The functional conservation of stem cells in those diverse plants is the basis for ontogenesis. However, from the perspective of evolutionary development, the diversity of key regulatory genes expression in different stem cell populations fulfills the continuous adjustments of development strategy to adapt to environmental changes. The plasticity in stem cell regulations determines the flexibility of plant development, which is conserved in plant kingdom from moss to angiosperms. Of course, the mechanism of stem cell maintenance and differentiation are even more complicated in angiosperms. In model plant of Arabidopsis thanian , the molecular mechanism of stem cell regulations has been extensively studied over the past decades. Multiple signaling molecules and transcription factors are found to tightly control the stem cell fate in Arabidopsis thaliana . The homeodomain transcription factor WUSCHEL ( WUS ) where expressed in the organizing center (OC) is a key regulator for plant stem cell fate determination. While, stem cells expressed secreted peptide CLV3, negatively regulates the expression of WUS . They form a negative feedback loop that tightly control plant stem cell fate. In addition, the classic phytohormones cytokinin and auxin also play essential roles in the maintenance of stem cells in shoot apical meristem, root apical meristem and cambium, and exhibit complex functional interactions. The molecular organization of the RAM is quite similar to that of the shoot. WOX5 ( WUSCHEL- RELATED HOMEOBOX 5 ), a homologue of WUS , is expressed in the QC, and induces root stem cell fate in the surrounding cells. The GRAS family transcription factor SHORT-ROOT ( SHR ) is expressed in the innermost tissue of the root, and SHR moves to the surrounding cell layer activating SCARECROW ( SCR ), together with PLT1 and PLT2 , defines the stem cells fate. In this review, we focused on the functions of plant stem cells in plant postembryonic development, and covered recent findings on plant hormonal regulation in stem cells. We also discussed the integration of endogenous genetic information and external environmental factors in plant development, and how they affected the development of organs, morphology and yield of crops.

Robust plant cell tracking using local spatio-temporal context

Automated segmentation and tracking of cells in actively developing plant tissues can provide high-throughput and quantitative spatio-temporal measurements of a range of cell behaviors. In this paper, we propose an automated segmentation and tracking method for the shoot apical meristem cells based on the cells׳ spatio-temporal contextual information. The cells are properly segmented and then tracked by using the proposed Triangle Neighborhood Structure matching method, which exploits the cells’ spatial context and turns out to be more robust than the other local graph matching methods. The tracking output acts as an indicator of the quality of segmentation and, in turn, the over-segmentation and under-segmentation errors are corrected by a local adjustment method, which is proved to be much more accurate and efficient than the global correction method. Furthermore, the cells׳ lineage tracklets are associated by using the cells׳ spatio–temporal contextual information to obtain long-term lineages. Our results on two datasets validate the effectiveness of the proposed method and we are able to track 99% of the plant cells across a long-term time period.

Expression and genomic integration of transgenes after Agrobacterium-mediated transformation of mature barley embryos.

Mature embryos in tissue cultures are advantageous because of their abundance and rapid germination, which reduces genomic instability problems. In this study, 2-day-old isolated mature barley embryos were infected with 2 Agrobacterium hypervirulent strains (AGL1 and EHA105), followed by a 3-day period of co-cultivation in the presence of L-cystein amino acid. Chimeric expression of the b-glucuronidase gene (gusA) directed by a viral promoter of strawberry vein banding virus was observed in coleoptile epidermal cells and seminal roots in 5-day-old germinated seedlings. In addition to varying infectivity patterns in different strains, there was a higher ratio of transient b-glucuronidase expression in developing coleoptiles than in embryonic roots, indicating the high competency of shoot apical meristem cells in the mature embryo. A total of 548 explants were transformed and 156 plants developed to maturity on G418 media after 18-25 days. We detected transgenes in 74% of the screened plant leaves by polymerase chain reaction, and 49% of these expressed neomycin phosphotransferase II gene following AGL1 transformation. Ten randomly selected T0 transformants were analyzed using thermal asymmetric interlaced polymerase chain reaction and 24 fragments ranged between 200-600 base pairs were sequenced. Three of the sequences flanked with transferred-DNA showed high similarity to coding regions of the barley genome, including alpha tubulin5, homeobox 1, and mitochondrial 16S genes. We observed 70-200-base pair filler sequences only in the coding regions of barley in this study.

Translational control of Arabidopsis meristem stability and organogenesis by the eukaryotic translation factor eIF3h.

Essentially all aboveground plant tissues develop from the stem cells in the primary shoot apical meristem. Proliferation of the stem cell population in the Arabidopsis shoot apical meristem is tightly controlled by a feedback loop formed primarily by the homeodomain transcription factor WUSCHEL (WUS) and the CLAVATA ligand-receptor system. In this study, it is shown that mutation of a translation initiation factor, eIF3h, causes a tendency to develop a strikingly enlarged shoot apical meristem with elevated and ectopic expression of WUS and CLAVATA3 (CLV3). Many of the mRNAs that function in apical meristem maintenance possess upstream open reading frames (uORFs), translational attenuators that render translation partially dependent on eIF3h. Specifically, the mRNA for the receptor kinase, CLV1, is undertranslated in the eif3h mutant as shown by transient and transgenic expression assays. Concordant phenotypic observations include defects in organ polarity and in translation of another uORF-containing mRNA, ASYMMETRIC LEAVES 1 (AS1), in eif3h. In summary, the expression of developmental regulatory mRNAs is attenuated by uORFs, and this attenuation is balanced in part by the translation initiation factor, eIF3h. Thus, translational control plays a key role in Arabidopsis stem cell regulation and organogenesis.

English

Seeds of Taxus chinensis var. mairei are known for their deep dormancy that can be broken by a procedure involving variable temperature stratification. Changes of ultrastructure in seed embryo of T. mairei during stratification were investigated with a scan electricity microscope and a transmission electricity microscope. After 4 months of warm stratification, the following cytological changes were observed: an enlargement of the embryo and cotyledon. Two months of warm stratification following the cold treatment induced embryonic cells and apical meristems continued to divide. An abundance of electro-dense matter and a small amount of starch grain and mitochondria appeared, and the lipids in the embryonic cell gradually degraded. After four months of cold stratification treatment, one or two distinct nucleoli in the shoot apical meristem cells were observed, embryo increased in volume, the cotyledon became stretched, the lipids in the embryonic cell disappeared, the number of mitochondria increased, and the Golgi bodies appeared. We concluded that warm stratification with alternating temperatures increased the growth of embryos by cell expansion and enlargement. Cold stratification may induce a response to gibberellic acid and initiated cell division resulting in the release of physiological dormancy and subsequent germination of T. mairei seeds. Key words: Taxus chinensis var. mairei, seed, scan electricity microscope (SEM), transmission electricity microscope (TEM).

Molecular mechanisms controlling plant organ abscission

Plants shed organs by abscission, removing leaves, flowers or fruits when the organs are senescent, damaged, diseased or mature. Abscission also affects agriculture; for example, abscission of fruits or cereal grains can significantly reduce crop yield. Abscission of organs typically occurs in a predetermined tissue region, the abscission zone (AZ). Organ abscission can be disturbed in two ways, inhibition of AZ differentiation in the organ or suppression of abscission processes in AZ cells. Recent studies, mainly in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and tomato (Solanum lycopersicum), have identified many genes involved in regulation of AZ differentiation and activation of abscission of flowers or floral organs, seeds, and fruits. In this review, we discuss the functions of these genes, the developmental regulation of AZ tissues, and the signaling pathways that induce abscission. We also discuss the emerging concept that the regulation of abscission involves many of the same regulators that function in determination of shoot apical meristem cell fate.