Arabidopsis TBP-ASSOCIATED FACTOR 12 ortholog NOBIRO6 controls root elongation with unfolded protein response cofactor activity.

Plant root growth is indeterminate but continuously responds to environmental changes. We previously reported on the severe root growth defect of a double mutant in bZIP17 and bZIP28 (bz1728) modulating the unfolded protein response (UPR). To elucidate the mechanism by which bz1728 seedlings develop a short root, we obtained a series of bz1728 suppressor mutants, called nobiro, for rescued root growth. We focused here on nobiro6, which is defective in the general transcription factor component TBP-ASSOCIATED FACTOR 12b (TAF12b). The expression of hundreds of genes, including the bZIP60-UPR regulon, was induced in the bz1728 mutant, but these inductions were markedly attenuated in the bz1728nobiro6 mutant. In view of this, we assigned transcriptional cofactor activity via physical interaction with bZIP60 to NOBIRO6/TAF12b. The single nobiro6/taf12b mutant also showed an altered sensitivity to endoplasmic reticulum stress for both UPR and root growth responses, demonstrating that NOBIRO6/TAF12b contributes to environment-responsive root growth control through UPR.

Plant chromosome polytenization contributes to suppression of root growth in high polyploids.

Autopolyploidization, which refers to a polyploidization via genome duplication without hybridization, promotes growth in autotetraploids, but suppresses growth in high polyploids (autohexaploids or auto-octoploids). The mechanism underlying this growth suppression (i.e. 'high-ploidy syndrome') has not been comprehensively characterized. In this study, we conducted a kinematic analysis of the root apical meristem cells in Arabidopsis thaliana autopolyploids (diploid, tetraploid, hexaploid, and octoploid) to determine the effects of the progression of genome duplication on root growth. The results of the root growth analysis showed that tetraploidization increases the cell volume, but decreases cell proliferation. However, cell proliferation and volume growth are suppressed in high polyploids. Whole-mount fluorescence in situ hybridization analysis revealed extensive chromosome polytenization in the region where cell proliferation does not usually occur in the roots of high polyploids, which is likely to be at least partly correlated with the suppression of endoreduplication. The study findings indicate that chromosome polytenization is important for the suppressed growth of high polyploids.

Sustained defense response via volatile signaling and its epigenetic transcriptional regulation.

Plants perceive volatiles emitted from herbivore-damaged neighboring plants to urgently adapt or prime their defense responses to prepare for forthcoming herbivores. Mechanistically, these volatiles can induce epigenetic regulation based on histone modifications that alter the transcriptional status of defense genes, but little is known about the underlying mechanisms. To understand the roles of such epigenetic regulation of plant volatile signaling, we explored the response of Arabidopsis (Arabidopsis thaliana) plants to the volatile ß-ocimene. Defense traits of Arabidopsis plants toward larvae of Spodoptera litura were induced in response to ß-ocimene, through enriched histone acetylation and elevated transcriptional levels of defense gene regulators, including ethylene response factor genes (ERF8 and ERF104) in leaves. The enhanced defense ability of the plants was maintained for 5 d but not over 10 d after exposure to ß-ocimene, and this coincided with elevated expression of those ERFs in their leaves. An array of histone acetyltransferases, including HAC1, HAC5, and HAM1, were responsible for the induction and maintenance of the anti-herbivore property. HDA6, a histone deacetylase, played a role in the reverse histone remodeling. Collectively, our findings illuminate the role of epigenetic regulation in plant volatile signaling.

Novel whole-mount FISH analysis for intact root of Arabidopsis thaliana with spatial reference to 3D visualization.

Whole-mount fluorescent in situ hybridization (WM-FISH) is an effective tool to observe chromosome behavior in tissues or organs. However, it is difficult to obtain a precise spatial profile of fluorescent signals in roots using conventional WM-FISH mainly because of the severe damage caused during the processing. To address this problem, we established a novel WM-FISH analysis for intact roots of Arabidopsis thaliana and successfully obtained a precise spatial profile of nuclear size and centromere signals. The two main improvements in the novel WM-FISH analysis are: (i) hybridization was performed directly on MAS-coated glass slides covered with silicon wells and (ii) conditions for enzyme treatment were optimized (37 °C, 45 s). After the WM-FISH using a centromere probe, we analyzed the results by 3D data processing to quantify the nuclear volume and number of centromere signals of the obtained cortical cell files and determined the position of each nucleus in intact roots. Then we plotted the nuclear volume and number of centromere signals versus distance from the quiescent center to evaluate the precise spatial profile of each parameter.

The SMC5/6 Complex Subunit NSE4A Is Involved in DNA Damage Repair and Seed Development.

The maintenance of genome integrity over cell divisions is critical for plant development and the correct transmission of genetic information to the progeny. A key factor involved in this process is the STRUCTURAL MAINTENANCE OF CHROMOSOME5 (SMC5) and SMC6 (SMC5/6) complex, related to the cohesin and condensin complexes that control sister chromatid alignment and chromosome condensation, respectively. Here, we characterize NON-SMC ELEMENT4 (NSE4) paralogs of the SMC5/6 complex in Arabidopsis (Arabidopsis thaliana). NSE4A is expressed in meristems and accumulates during DNA damage repair. Partial loss-of-function nse4a mutants are viable but hypersensitive to DNA damage induced by zebularine. In addition, nse4a mutants produce abnormal seeds, with noncellularized endosperm and embryos that maximally develop to the heart or torpedo stage. This phenotype resembles the defects in cohesin and condensin mutants and suggests a role for all three SMC complexes in differentiation during seed development. By contrast, NSE4B is expressed in only a few cell types, and loss-of-function mutants do not have any obvious abnormal phenotype. In summary, our study shows that the NSE4A subunit of the SMC5-SMC6 complex is essential for DNA damage repair in somatic tissues and plays a role in plant reproduction.

Two combinatorial patterns of telomere histone marks in plants with canonical and non-canonical telomere repeats.

Telomeres, nucleoprotein structures at the ends of linear eukaryotic chromosomes, are crucial for the maintenance of genome integrity. In most plants, telomeres consist of conserved tandem repeat units comprising the TTTAGGG motif. Recently, non-canonical telomeres were described in several plants and plant taxons, including the carnivorous plant Genlisea hispidula (TTCAGG/TTTCAGG), the genus Cestrum (Solanaceae; TTTTTTAGGG), and plants from the Asparagales order with either a vertebrate-type telomere repeat TTAGGG or Allium genus-specific CTCGGTTATGGG repeat. We analyzed epigenetic modifications of telomeric histones in plants with canonical and non-canonical telomeres, and further in telomeric chromatin captured from leaves of Nicotiana benthamiana transiently transformed by telomere CRISPR-dCas9-eGFP, and of Arabidopsis thaliana stably transformed with TALE_telo C-3×GFP. Two combinatorial patterns of telomeric histone modifications were identified: (i) an Arabidopsis-like pattern (A. thaliana, G. hispidula, Genlisea nigrocaulis, Allium cepa, Narcissus pseudonarcissus, Petunia hybrida, Solanum tuberosum, Solanum lycopersicum) with telomeric histones decorated predominantly by H3K9me2; (ii) a tobacco-like pattern (Nicotiana tabacum, N. benthamiana, C. elegans) with a strong H3K27me3 signal. Our data suggest that epigenetic modifications of plant telomere-associated histones are related neither to the sequence of the telomere motif nor to the lengths of the telomeres. Nor the phylogenetic position of the species plays the role; representatives of the Solanaceae family are included in both groups. As both patterns of histone marks are compatible with fully functional telomeres in respective plants, we conclude that the described specific differences in histone marks are not critical for telomere functions.

The formation of perinucleolar bodies is important for normal leaf development and requires the zinc-finger DNA-binding motif in Arabidopsis ASYMMETRIC LEAVES2.

In Arabidopsis, the ASYMMETRIC LEAVES2 (AS2) protein plays a key role in the formation of flat symmetric leaves via direct repression of the abaxial gene ETT/ARF3. AS2 encodes a plant-specific nuclear protein that contains the AS2/LOB domain, which includes a zinc-finger (ZF) motif that is conserved in the AS2/LOB family. We have shown that AS2 binds to the coding DNA of ETT/ARF3, which requires the ZF motif. AS2 is co-localized with AS1 in perinucleolar bodies (AS2 bodies). To identify the amino acid signals in AS2 required for formation of AS2 bodies and function(s) in leaf formation, we constructed recombinant DNAs that encoded mutant AS2 proteins fused to yellow fluorescent protein. We examined the subcellular localization of these proteins in cells of cotyledons and leaf primordia of transgenic plants and cultured cells. The amino acid signals essential for formation of AS2 bodies were located within and adjacent to the ZF motif. Mutant AS2 that failed to form AS2 bodies also failed to rescue the as2-1 mutation. Our results suggest the importance of the formation of AS2 bodies and the nature of interactions of AS2 with its target DNA and nucleolar factors including NUCLEOLIN1. The partial overlap of AS2 bodies with perinucleolar chromocenters with condensed ribosomal RNA genes implies a correlation between AS2 bodies and the chromatin state. Patterns of AS2 bodies in cells during interphase and mitosis in leaf primordia were distinct from those in cultured cells, suggesting that the formation and distribution of AS2 bodies are developmentally modulated in plants.

SQAP, an acyl sulfoquinovosyl derivative, suppresses expression of histone deacetylase and induces cell death of cancer cells under hypoxic conditions.

Sulfoglycolipid, SQAP, is a radiosensitizing agent that makes tumor cells more sensitive to radiation therapy. A previous study revealed that SQAP induced the degradation of hypoxia-inducible factor-1α (HIF-1α) and inhibited angiogenesis in a hepatoma model mouse. Herein, we examined the biological activities of SQAP against hepatocarcinoma cells under low oxygen conditions. Cell growth inhibition of SQAP under hypoxic conditions was significantly higher than that under normoxic conditions. In addition, SQAP was found to impair the expression of histone deacetylase (HDAC) under low oxygen conditions. Our present data suggested that SQAP induced the degradation of HIF-1α and then decreased the expression of HDAC1. Unlike known HDAC inhibitors, SQAP increased the acetylation level of histone in cells without inhibition of enzymatic activity of HDACs. Our data demonstrated hypoxia-specific unique properties of SQAP.

Whole-Tissue Three-Dimensional Imaging of Rice at Single-Cell Resolution.

The three-dimensional (3D) arrangement of cells in tissues provides an anatomical basis for analyzing physiological and biochemical aspects of plant and animal cellular development and function. In this study, we established a protocol for tissue clearing and 3D imaging in rice. Our protocol is based on three improvements: clearing with iTOMEI (clearing solution suitable for plants), developing microscopic conditions in which the Z step is optimized for 3D reconstruction, and optimizing cell-wall staining. Our protocol successfully 3D imaged rice shoot apical meristems, florets, and root apical meristems at cellular resolution throughout whole tissues. Using fluorescent reporters of auxin signaling in rice root tips, we also revealed the 3D distribution of auxin signaling events that are activated in the columella, quiescent center, and multiple rows of cells in the stele of the root apical meristem. Examination of cells with higher levels of auxin signaling revealed that only the central row of cells was connected to the quiescent center. Our method provides opportunities to observe the 3D arrangement of cells in rice tissues.

Deep Imaging Analysis in VISUAL Reveals the Role of YABBY Genes in Vascular Stem Cell Fate Determination.

Stem cells undergo cell division and differentiation to ensure organized tissue development. Because plant cells are immobile, plant stem cells ought to decide their cell fate prior to differentiation, to locate specialized cells in the correct position. In this study, based on a chemical screen, we isolated a novel secondary cell wall indicator BF-170, which binds to lignin and can be used to image in vitro and in situ xylem development. Use of BF-170 to observe the vascular differentiation pattern in the in vitro vascular cell induction system, VISUAL, revealed that adaxial mesophyll cells of cotyledons predominantly generate ectopic xylem cells. Moreover, phloem cells are abundantly produced on the abaxial layer, suggesting the involvement of leaf adaxial-abaxial polarity in determining vascular cell fate. Analysis of abaxial polarity mutants highlighted the role of YAB3, an abaxial cell fate regulator, in suppressing xylem and promoting phloem differentiation on the abaxial domains in VISUAL. Furthermore, YABBY family genes affected in vivo vascular development during the secondary growth. Our results denoted the possibility that such mediators of spatial information contribute to correctly determine the cell fate of vascular stem cells, to conserve the vascular pattern of land plants.

Insights into cortical microtubule nucleation and dynamics in Arabidopsis leaf cells.

Plant microtubules (MTs) are nucleated from the γ-tubulin-containing ring complex (γTuRC). In cortical MT arrays of interphase plant cells, γTuRC is preferentially recruited to the lattice of preexisting MTs, where it initiates MT nucleation in either a branch- or bundle-forming manner, or dissociates without mediating nucleation. In this study, we analyzed how γTuRCs influence MT nucleation and dynamics in cotyledon pavement cells of Arabidopsis thaliana We found that γTuRC nucleated MTs at angles of ∼40° toward the plus-ends of existing MTs, or in predominantly antiparallel bundles. A small fraction of γTuRCs was motile and tracked MT ends. When γTuRCs decorated the depolymerizing MT end, they reduced the depolymerization rate. Non-nucleating γTuRCs associated with the MT lattice promoted MT regrowth after a depolymerization phase. These results suggest that γTuRCs not only nucleate MT growth but also regulate MT dynamics by stabilizing MT ends. On rare occasions, a non-MT-associated γTuRC was pushed in the direction of the MT minus-end, while nucleating a new MT, suggesting that the polymerizing plus-end is anchored to the plasma membrane.

Visualization of extracellular vesicles in the regenerating caudal fin blastema of zebrafish using in vivo electroporation.

Zebrafish have high regenerative ability in several organs including the fin. Although various mechanisms underlying fin regeneration have been revealed, some mechanisms remain to be elucidated. Recently, extracellular vesicles (EVs) have been the focus of research with regard to their role in cell-to-cell communication. It has been suggested that cells in regenerating tissues communicate using EVs. In this study, we examined the involvement of EVs in the caudal fin regeneration of zebrafish using an in vivo electroporation method. The process of regeneration appeared normal after in vivo electroporation, and the transferred plasmid showed mosaic expression in the blastema. We took advantage of this mosaic expression to observe the distribution of exosomal markers in the blastema. We transferred exosomal markers by in vivo electroporation and identified EVs in the regenerating caudal fin. The results suggest that blastemal cells communicate with other cells via EVs during caudal fin regeneration.

LSD1-LIKE1-Mediated H3K4me2 Demethylation Is Required for Homologous Recombination Repair.

Homologous recombination is a key process for maintaining genome integrity and diversity. In eukaryotes, the nucleosome structure of chromatin inhibits the progression of homologous recombination. The DNA repair and recombination protein RAD54 alters the chromatin structure via nucleosome sliding to enable homology searches. For homologous recombination to progress, appropriate recruitment and dissociation of RAD54 is required at the site of homologous recombination; however, little is known about the mechanism regulating RAD54 dynamics in chromatin. Here, we reveal that the histone demethylase LYSINE-SPECIFIC DEMETHYLASE1-LIKE 1 (LDL1) regulates the dissociation of RAD54 at damaged sites during homologous recombination repair in the somatic cells of Arabidopsis (Arabidopsis thaliana). Depletion of LDL1 leads to an overaccumulation of RAD54 at damaged sites with DNA double-strand breaks. Moreover, RAD54 accumulates at damaged sites by recognizing histone H3 Lys 4 di-methylation (H3K4me2); the frequency of the interaction between RAD54 and H3K4me2 increased in the ldl1 mutant with DNA double-strand breaks. We propose that LDL1 removes RAD54 at damaged sites by demethylating H3K4me2 during homologous recombination repair and thereby maintains genome stability in Arabidopsis.

Correction to: Heat and chilling stress induce nucleolus morphological changes.

The article Heat and chilling stress induce nucleolus morphological changes, written by Kohma Hayashi and Sachihiro Matsunaga, was originally published Online First without Open Access. After publication in volume 132, issue 3, page 395-403 the author decided to opt for Open Choice and to make the article an Open Access publication.

Interspecies hormonal control of host root morphology by parasitic plants.

Parasitic plants share a common anatomical feature, the haustorium. Haustoria enable both infection and nutrient transfer, which often leads to growth penalties for host plants and yield reduction in crop species. Haustoria also reciprocally transfer substances, such as RNA and proteins, from parasite to host, but the biological relevance for such movement remains unknown. Here, we studied such interspecies transport by using the hemiparasitic plant Phtheirospermum japonicum during infection of Arabidopsis thaliana Tracer experiments revealed a rapid and efficient transfer of carboxyfluorescein diacetate (CFDA) from host to parasite upon formation of vascular connections. In addition, Phtheirospermum induced hypertrophy in host roots at the site of infection, a form of enhanced secondary growth that is commonly observed during various parasitic plant-host interactions. The plant hormone cytokinin is important for secondary growth, and we observed increases in cytokinin and its response during infection in both host and parasite. Phtheirospermum-induced host hypertrophy required cytokinin signaling genes (AHK3,4) but not cytokinin biosynthesis genes (IPT1,3,5,7) in the host. Furthermore, expression of a cytokinin-degrading enzyme in Phtheirospermum prevented host hypertrophy. Wild-type hosts with hypertrophy were smaller than ahk3,4 mutant hosts resistant to hypertrophy, suggesting hypertrophy improves the efficiency of parasitism. Taken together, these results demonstrate that the interspecies movement of a parasite-derived hormone modified both host root morphology and fitness. Several microbial and animal plant pathogens use cytokinins during infections, highlighting the central role of this growth hormone during the establishment of plant diseases and revealing a common strategy for parasite infections of plants.

Characterization of somatic embryogenesis initiated from the Arabidopsis shoot apex.

Somatic embryogenesis is one of the best examples of the remarkable developmental plasticity of plants, in which committed somatic cells can dedifferentiate and acquire the ability to form an embryo and regenerate an entire plant. In Arabidopsis thaliana, the shoot apices of young seedlings have been reported as an alternative tissue source for somatic embryos (SEs) besides the widely studied zygotic embryos taken from siliques. Although SE induction from shoots demonstrates the plasticity of plants more clearly than the embryo-to-embryo induction system, the underlying developmental and molecular mechanisms involved are unknown. Here we characterized SE formation from shoot apex explants by establishing a system for time-lapse observation of explants during SE induction. We also established a method to distinguish SE-forming and non-SE-forming explants prior to anatomical SE formation, enabling us to identify distinct transcriptome profiles of these two explants at SE initiation. We show that embryonic fate commitment takes place at day 3 of SE induction and the SE arises directly, not through callus formation, from the base of leaf primordia just beside the shoot apical meristem (SAM), where auxin accumulates and shoot-root polarity is formed. The expression domain of a couple of key developmental genes for the SAM transiently expands at this stage. Our data demonstrate that SE-forming and non-SE-forming explants share mostly the same transcripts except for a limited number of embryonic genes and root genes that might trigger the SE-initiation program. Thus, SE-forming explants possess a mixed identity (SAM, root and embryo) at the time of SE specification.

The coordination of ploidy and cell size differs between cell layers in leaves.

Growth and developmental processes are occasionally accompanied by multiple rounds of DNA replication, known as endoreduplication. Coordination between endoreduplication and cell size regulation often plays a crucial role in proper organogenesis and cell differentiation. Here, we report that the level of correlation between ploidy and cell volume is different in the outer and inner cell layers of leaves of Arabidopsis thaliana using a novel imaging technique. Although there is a well-known, strong correlation between ploidy and cell volume in pavement cells of the epidermis, this correlation was extremely weak in palisade mesophyll cells. Induction of epidermis cell identity based on the expression of the homeobox gene ATML1 in mesophyll cells enhanced the level of correlation between ploidy and cell volume to near that of wild-type epidermal cells. We therefore propose that the correlation between ploidy and cell volume is regulated by cell identity.

Pyrenocine A induces monopolar spindle formation and suppresses proliferation of cancer cells.

Pyrenocine A, a phytotoxin, was found to exhibit cytotoxicity against cancer cells with an IC50 value of 2.6-12.9 µM. Live cell imaging analysis revealed that pyrenocine A arrested HeLa cells at the M phase with characteristic ring-shaped chromosomes. Furthermore, as a result of immunofluorescence staining analysis, we found that pyrenocine A resulted in the formation of monopolar spindles in HeLa cells. Monopolar spindles are known to be induced by inhibitors of the kinesin motor protein Eg5 such as monastrol and STLC. Monastrol and STLC induce monopolar spindle formation and M phase arrest via inhibition of the ATPase activity of Eg5. Interestingly, our data revealed that pyrenocine A had no effect on the ATPase activity of Eg5 in vitro, which suggested the compound induces a monopolar spindle by an unknown mechanism. Structure-activity relationship analysis indicates that the enone structure of pyrenocine A is likely to be important for its cytotoxicity. An alkyne-tagged analog of pyrenocine A was synthesized and suppressed proliferation of HeLa cells with an IC50 value of 2.3 µM. We concluded that pyrenocine A induced monopolar spindle formation by a novel mechanism other than direct inhibition of Eg5 motor activity, and the activity of pyrenocine A may suggest a new anticancer mechanism.

Heat and chilling stress induce nucleolus morphological changes.

The nucleolus, where components of the ribosome are constructed, is known to play an important role in various stress responses in animals. However, little is known about the role of the plant nucleolus under environmental stresses such as heat and chilling stress. In this study, we analyzed nucleolus morphology by determining the distribution of newly synthesized rRNAs with an analog of uridine, 5-ethynyl uridine (EU). When EU was incorporated into the root of the Arabidopsis thaliana, EU signals were strongly localized in the nucleolus. The results of the short-term incorporation of EU implied that there is no compartmentation among the processes of transcription, processing, and construction of rRNAs. Nevertheless, under heat and chilling stress, EU was not incorporated into the center of the nucleolus. Morphological analyses using whole rRNA staining and differential interference contrast observations revealed speckled and round structures in the center of the nucleolus under heat and chilling stress, respectively.

Intracellular localization of histone deacetylase HDA6 in plants.

Histone modification is an important epigenetic mechanism in eukaryotes. Histone acetyltransferase and deacetylase regulate histone acetylation levels antagonistically, leading to dynamic control of chromatin structure. One of the histone deacetylases, HDA6, is involved in gene silencing in the heterochromatin regions, chromocenter formation, and metabolic adaptation under drought stress. Although HDA6 plays an important role in chromatin control and response to drought stress, its intracellular localization has not been observed in detail. In this paper, we generated transformants expressing HDA6-GFP in the model plant, Arabidopsis thaliana, and the crops, rice, and cassava. We observed the localization of the fusion protein and showed that HDA6-GFP was expressed in the whole root and localized at the nucleus in Arabidopsis, rice, and cassava. Remarkably, HDA6-GFP clearly formed speckles that were actively colocalized with chromocenters in Arabidopsis root meristem. In contrast, such speckles were unlikely to be formed in rice or cassava. Because AtHDA6 directly binds to the acetate synthesis genes, which function in drought tolerance, we performed live imaging analyses to examine the cellular dynamics of pH in roots and the subnuclear dynamics of AtHDA6 responding to acetic acid treatment. The number of HDA6 speckles increased during drought stress, suggesting a role in contributing to drought stress tolerance.