GRAS transcription factors regulate cell division planes in moss overriding the default rule.

Plant cells are surrounded by a cell wall and do not migrate, which makes the regulation of cell division orientation crucial for development. Regulatory mechanisms controlling cell division orientation may have contributed to the evolution of body organization in land plants. The GRAS family of transcription factors was transferred horizontally from soil bacteria to an algal common ancestor of land plants. SHORTROOT (SHR) and SCARECROW (SCR) genes in this family regulate formative periclinal cell divisions in the roots of flowering plants, but their roles in nonflowering plants and their evolution have not been studied in relation to body organization. Here, we show that SHR cell autonomously inhibits formative periclinal cell divisions indispensable for leaf vein formation in the moss Physcomitrium patens, and SHR expression is positively and negatively regulated by SCR and the GRAS member LATERAL SUPPRESSOR, respectively. While precursor cells of a leaf vein lacking SHR usually follow the geometry rule of dividing along the division plane with the minimum surface area, SHR overrides this rule and forces cells to divide nonpericlinally. Together, these results imply that these bacterially derived GRAS transcription factors were involved in the establishment of the genetic regulatory networks modulating cell division orientation in the common ancestor of land plants and were later adapted to function in flowering plant and moss lineages for their specific body organizations.

H3K4me3 plays a key role in establishing permissive chromatin states during bud dormancy and bud break in apple.

Bud dormancy helps woody perennials survive winter and activate robust plant development in the spring. For apple (Malus × domestica), short-term chilling induces bud dormancy in autumn, then prolonged chilling leads to dormancy release and a shift to a quiescent state in winter, with subsequent warm periods promoting bud break in spring. Epigenetic regulation contributes to seasonal responses such as vernalization. However, how histone modifications integrate seasonal cues and internal signals during bud dormancy in woody perennials remains largely unknown. Here, we show that H3K4me3 plays a key role in establishing permissive chromatin states during bud dormancy and bud break in apple. The global changes in gene expression strongly correlated with changes in H3K4me3, but not H3K27me3. High expression of DORMANCY-ASSOCIATED MADS-box (DAM) genes, key regulators of dormancy, in autumn was associated with high H3K4me3 levels. In addition, known DAM/SHORT VEGETATIVE PHASE (SVP) target genes significantly overlapped with H3K4me3-modified genes as bud dormancy progressed. These data suggest that H3K4me3 contributes to the central dormancy circuit, consisting of DAM/SVP and abscisic acid (ABA), in autumn. In winter, the lower expression and H3K4me3 levels at DAMs and gibberellin metabolism genes control chilling-induced release of dormancy. Warming conditions in spring facilitate the expression of genes related to phytohormones, the cell cycle, and cell wall modification by increasing H3K4me3 toward bud break. Our study also revealed that activation of auxin and repression of ABA sensitivity in spring are conditioned at least partly through temperature-mediated epigenetic regulation in winter.

Adaptive Optics Microscopy with Wavefront Sensing Based on Neighbor Correlation.

Complex structures in living cells and tissues induce wavefront errors when light waves pass through them, and images observed with optical microscopes are undesirably blurred. This problem is especially serious for living plant cells because images are strikingly degraded even within a single cell. Adaptive optics (AO) is expected to be a solution to this problem by correcting such wavefront errors, thus enabling high-resolution imaging. In particular, scene-based AO involves wavefront sensing based on the image correlation between subapertures in a Shack-Hartmann wavefront sensor and thus does not require an intense point light source. However, the complex 3D structures of living cells often cause low correlation between subimages, leading to loss of accuracy in wavefront sensing. This paper proposes a novel method for scene-based sensing using only image correlations between adjacent subapertures. The method can minimize changes between subimages to be correlated and thus prevent inaccuracy in phase estimation. Using an artificial test target mimicking the optical properties of a layer of living plant cells, an imaging performance with a Strehl ratio of approximately 0.5 was confirmed. Upon observation of chloroplast autofluorescence inside living leaf cells of the moss Physcomitrium patens, recovered resolution images were successfully obtained even with complex biological structures. Under bright-field illumination, the proposed method outperformed the conventional method, demonstrating the future potential of this method for label- and damage-free AO microscopy. Several points for improvement in terms of the effect of AO correction are discussed.

Functional interplay of histone lysine 2-hydroxyisobutyrylation and acetylation in Arabidopsis under dark-induced starvation.

Lysine 2-hydroxyisobutyrylation (Khib) is a novel type of histone acylation whose prevalence and function in plants remain unclear. Here, we identified 41 Khib sites on histones in Arabidopsis thaliana, which did not overlap with frequently modified N-tail lysines (e.g. H3K4, H3K9 and H4K8). Chromatin immunoprecipitation-sequencing (ChIP-seq) assays revealed histone Khib in 35% of protein-coding genes. Most Khib peaks were located in genic regions, and they were highly enriched at the transcription start sites. Histone Khib is highly correlated with acetylation (ac), particularly H3K23ac, which it largely resembles in its genomic and genic distribution. Notably, co-enrichment of histone Khib and H3K23ac correlates with high gene expression levels. Metabolic profiling, transcriptome analyses, and ChIP-qPCR revealed that histone Khib and H3K23ac are co-enriched on genes involved in starch and sucrose metabolism, pentose and glucuronate interconversions, and phenylpropanoid biosynthesis, and help fine-tune plant response to dark-induced starvation. These findings suggest that Khib and H3K23ac may act in concert to promote high levels of gene transcription and regulate cellular metabolism to facilitate plant adaption to stress. Finally, HDA6 and HDA9 are involved in removing histone Khib. Our findings reveal Khib as a conserved yet unique plant histone mark acting with lysine acetylation in transcription-associated epigenomic processes.

Topoisomerase 1α is required for synchronous spermatogenesis in Physcomitrium patens.

DNA topoisomerase 1 (TOP1) plays general roles in DNA replication and transcription by regulating DNA topology in land plants and metazoans. TOP1 is also involved in specific developmental events; however, whether TOP1 plays a conserved developmental role among multicellular organisms is unknown. Here, we investigated the developmental roles of TOP1 in the moss Physcomitrium (Physcomitrella) patens with gene targeting, microscopy, 3D image segmentation and crossing experiments. We discovered that the disruption of TOP1α, but not its paralogue TOP1ß, leads to a defect in fertilisation and subsequent sporophyte formation in P. patens. In the top1α mutant, the egg cell was functional for fertilisation, while sperm cells were fewer and infertile with disordered structures. We observed that the nuclei volume of wild-type sperm cells synchronously decreases during antheridium development, indicating chromatin condensation towards the compact sperm head. By contrast, the top1α mutant exhibited attenuated cell divisions and asynchronous and defective contraction of the nuclei of sperm cells throughout spermatogenesis. These results indicate that TOP1α is involved in cell division and chromatin condensation during spermatogenesis in P. patens. Our results suggest that the regulation of DNA topology by TOP1 plays a key role in spermatogenesis in both land plants and metazoans.

Single-shot common-path off-axis digital holography: applications in bioimaging and optical metrology [Invited].

The demand for single-shot and common-path holographic systems has become increasingly important in recent years, as such systems offer various advantages compared to their counterparts. Single-shot holographic systems, for example, reduce computational complexity as only a single hologram with the object information required to process, making them more suitable for the investigation of dynamic events; and common-path holographic systems are less vibration-sensitive, compact, inexpensive, and high in temporal phase stability. We have developed a single-shot common-path off-axis digital holographic setup based on a beam splitter and pinhole. In this paper, we present a concise review of the proposed digital holographic system for several applications, including the quantitative phase imaging to investigate the morphological and quantitative parameters, as a metrological tool for testing of micro-optics, industrial inspection and measurement, and sound field imaging and visualization.

Single-cell transcriptome analysis of Physcomitrella leaf cells during reprogramming using microcapillary manipulation.

Next-generation sequencing technologies have made it possible to carry out transcriptome analysis at the single-cell level. Single-cell RNA-sequencing (scRNA-seq) data provide insights into cellular dynamics, including intercellular heterogeneity as well as inter- and intra-cellular fluctuations in gene expression that cannot be studied using populations of cells. The utilization of scRNA-seq is, however, restricted to cell types that can be isolated from their original tissues, and it can be difficult to obtain precise positional information for these cells in situ. Here, we established single cell-digital gene expression (1cell-DGE), a method of scRNA-seq that uses micromanipulation to extract the contents of individual living cells in intact tissue while recording their positional information. With 1cell-DGE, we could detect differentially expressed genes (DEGs) during the reprogramming of leaf cells of the moss Physcomitrella patens, identifying 6382 DEGs between cells at 0 and 24 h after excision. Furthermore, we identified a subpopulation of reprogramming cells based on their pseudotimes, which were calculated using transcriptome profiles at 24 h. 1cell-DGE with microcapillary manipulation can be used to analyze the gene expression of individual cells without detaching them from their tightly associated tissues, enabling us to retain positional information and investigate cell-cell interactions.

Single-shot common-path off-axis dual-wavelength digital holographic microscopy.

A single-shot common-path off-axis self-interference dual-wavelength digital holographic microscopic (DHM) system based on a cube beam splitter is demonstrated to expand the phase range in a stepped microstructure and for simultaneous measurement of the refractive index and physical thickness of a specimen. In the system, two laser beams with wavelengths of 532 nm and 632.8 nm are used. These laser beams are combined to transilluminate the object under study, then the object beam is divided into two beams by using a beam splitter oriented in such a way that both the beams propagate in almost the same direction, with an appropriate lateral separation between them. One of the object beams is spatially filtered at its Fourier plane, using a pinhole to generate a reference spherical beam free from the object information. The reference beam interferes with the object beam to form a digital hologram at the faceplate of the image sensor. The phase information is extracted from a single recorded digital hologram using the phase aberration compensation method that is based on principal component analysis (PCA). Owing to the common-path configuration, the system shows high temporal phase stability and it is less vibration-sensitive compared to counterparts such as a Mach-Zehnder type DHM. The performance of the dual-wavelength DHM system is verified in two different application fields by conducting the experiments using microsphere beads and living plant cells.

The Polycomb group protein CLF emerges as a specific tri-methylase of H3K27 regulating gene expression and development in Physcomitrella patens.

Packaging of eukaryotic DNA largely depends on histone modifications that affect the accessibility of DNA to transcriptional regulators, thus controlling gene expression. The Polycomb group (PcG) chromatin remodeling complex deposits a methyl group on lysine 27 of histone 3 leading to repressed gene expression. Plants encode homologs of the Enhancer of zeste (E(z)), a component of the PcG complex from Drosophila, one of which is a SET domain protein designated CURLY LEAF (CLF). Although this SET domain protein exhibits a strong correlation with the presence of the H3K27me3 mark in plants, the methyl-transferase activity and specificity of its SET domain have not been directly tested in-vivo. Using the evolutionary early-diverged land plant model species Physcomitrella patens we show that abolishment of a single copy gene PpCLF, as well as an additional member of the PcG complex, FERTILIZATION-INDEPENDENT ENDOSPERM (PpFIE), results in a specific loss of tri-methylation of H3K27. Using site-directed mutagenesis of key residues, we revealed that H3K27 tri-methylation is mediated by the SET domain of the CLF protein. Moreover, the abolishment of H3K27me3 led to enhanced expression of transcription factor genes. This in turn led to the development of fertilization-independent sporophyte-like structures, as observed in PpCLF and PpFIE null mutants. Overall, our results demonstrate the role of PpCLF as a SET protein in tri-methylation of H3K27 in-vivo and the importance of this modification in regulating the expression of transcription factor genes involved in developmental programs of P. patens.

WOX13-like genes are required for reprogramming of leaf and protoplast cells into stem cells in the moss Physcomitrella patens.

Many differentiated plant cells can dedifferentiate into stem cells, reflecting the remarkable developmental plasticity of plants. In the moss Physcomitrella patens, cells at the wound margin of detached leaves become reprogrammed into stem cells. Here, we report that two paralogous P. patens WUSCHEL-related homeobox 13-like (PpWOX13L) genes, homologs of stem cell regulators in flowering plants, are transiently upregulated and required for the initiation of cell growth during stem cell formation. Concordantly, Δppwox13l deletion mutants fail to upregulate genes encoding homologs of cell wall loosening factors during this process. During the moss life cycle, most of the Δppwox13l mutant zygotes fail to expand and initiate an apical stem cell to form the embryo. Our data show that PpWOX13L genes are required for the initiation of cell growth specifically during stem cell formation, in analogy to WOX stem cell functions in seed plants, but using a different cellular mechanism.

A flowering integrator, SOC1, affects stomatal opening in Arabidopsis thaliana.

Stomatal movements are regulated by multiple environmental signals. Recent investigations indicate that photoperiodic flowering components, such as CRY, GI, CO, FT and TSF, are expressed in guard cells and positively affect stomatal opening in Arabidopsis thaliana. Here we show that SOC1, which encodes a MADS box transcription factor and integrates multiple flowering signals, also exerts a positive effect on stomatal opening. FLC encodes a potent repressor of FT and SOC1, and FRI acts as an activator of FLC. Thus, we examined stomatal phenotypes in FRI-Col, which contains an active FRI allele of accession Sf-2 by introgression. We found higher expression of FLC and lower expression of FT, SOC1 and TSF in guard cells from FRI-Col than in those from Col. Light-induced stomatal opening was significantly suppressed in FRI-Col. Interestingly, vernalization of FRI-Col partially restored light-induced stomatal opening, concomitant with a decrease of FLC and increase of FT, SOC1 and TSF. Furthermore, we observed the constitutive open-stomata phenotype in transgenic plants overexpressing SOC1-GFP (green fluorescent protein) in guard cells (SOC1-GFP overexpressor), and found that light-induced stomatal opening was significantly suppressed in a soc1 knockout mutant. RNA sequencing using epidermis from the SOC1-GFP overexpressor revealed that the expression levels of several genes involved in stomatal opening, such as BLUS1 and the plasma membrane H(+)-ATPases, were higher than those in background plants. From these results, we conclude that SOC1 is involved in the regulation of stomatal opening via transcriptional regulation in guard cells.

Epigenetic maintenance of the vernalized state in Arabidopsis thaliana requires LIKE HETEROCHROMATIN PROTEIN 1.

Vernalization is the process by which sensing a prolonged exposure to winter cold leads to competence to flower in the spring. In winter annual Arabidopsis thaliana accessions, flowering is suppressed in the fall by expression of the potent floral repressor FLOWERING LOCUS C (FLC). Vernalization promotes flowering via epigenetic repression of FLC. Repression is accompanied by a series of histone modifications of FLC chromatin that include dimethylation of histone H3 at Lys9 (H3K9) and Lys27 (H3K27). Here, we report that A. thaliana LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) is necessary to maintain the epigenetically repressed state of FLC upon return to warm conditions typical of spring. LHP1 is enriched at FLC chromatin after prolonged exposure to cold, and LHP1 activity is needed to maintain the increased levels of H3K9 dimethylation at FLC chromatin that are characteristic of the vernalized state.

Growth habit determination by the balance of histone methylation activities in Arabidopsis.

In Arabidopsis, the rapid-flowering summer-annual versus the vernalization-requiring winter-annual growth habit is determined by natural variation in FRIGIDA (FRI) and FLOWERING LOCUS C (FLC). However, the biochemical basis of how FRI confers a winter-annual habit remains elusive. Here, we show that FRI elevates FLC expression by enhancement of histone methyltransferase (HMT) activity. EARLY FLOWERING IN SHORT DAYS (EFS), which is essential for FRI function, is demonstrated to be a novel dual substrate (histone H3 lysine 4 (H3K4) and H3K36)-specific HMT. FRI is recruited into FLC chromatin through EFS and in turn enhances EFS activity and engages additional HMTs. At FLC, the HMT activity of EFS is balanced by the H3K4/H3K36- and H3K4-specific histone demethylase (HDM) activities of autonomous-pathway components, RELATIVE OF EARLY FLOWERING 6 and FLOWERING LOCUS D, respectively. Loss of HDM activity in summer annuals results in dominant HMT activity, leading to conversion to a winter-annual habit in the absence of FRI. Thus, our study provides a model of how growth habit is determined through the balance of the H3K4/H3K36-specific HMT and HDM activities.

Targeted single-cell gene induction by optimizing the dually regulated CRE/loxP system by a newly defined heat-shock promoter and the steroid hormone in Arabidopsis thaliana.

Multicellular organisms rely on intercellular communication systems to organize their cellular functions. In studies focusing on intercellular communication, the key experimental techniques include the generation of chimeric tissue using transgenic DNA recombination systems represented by the CRE/loxP system. If an experimental system enables the induction of chimeras at highly targeted cell(s), it will facilitate the reproducibility and precision of experiments. However, multiple technical limitations have made this challenging. The stochastic nature of DNA recombination events, especially, hampers reproducible generation of intended chimeric patterns. Infrared laser-evoked gene operator (IR-LEGO), a microscopic system that irradiates targeted cells using an IR laser, can induce heat shock-mediated expression of transgenes, for example, CRE recombinase gene, in the cells. In this study, we developed a method that induces CRE/loxP recombination in the target cell(s) of plant roots and leaves in a highly specific manner. We combined IR-LEGO, an improved heat-shock-specific promoter, and dexamethasone-dependent regulation of CRE. The optimal IR-laser power and irradiation duration were estimated via exhaustive irradiation trials and subsequent statistical modeling. Under optimized conditions, CRE/loxP recombination was efficiently induced without cellular damage. We also found that the induction efficiency varied among tissue types and cellular sizes. The developed method offers an experimental system to generate a precisely designed chimeric tissue, and thus, will be useful for analyzing intercellular communication at high resolution in roots and leaves.

Polycomb proteins regulate the quantitative induction of VERNALIZATION INSENSITIVE 3 in response to low temperatures.

Vernalization, the promotion of flowering in response to low temperatures, is one of the best characterized examples of epigenetic regulation in plants. The promotion of flowering is proportional to the duration of the cold period, but the mechanism by which plants measure time at low temperatures has been a long-standing mystery. We show that the quantitative induction of the first gene in the Arabidopsis vernalization pathway, VERNALIZATION INSENSITIVE 3 (VIN3), is regulated by the components of Polycomb Response Complex 2, which trimethylates histone H3 lysine 27 (H3K27me3). In differentiated animal cells, H3K27me3 is mostly associated with long-term gene repression, whereas, in pluripotent embyonic stem cells, many cell lineage-specific genes are inactive but exist in bivalent chromatin that carries both active (H3K4me3) and repressive (H3K27me3) marks on the same molecule. During differentiation, bivalent domains are generally resolved to an active or silent state. We found that H3K27me3 maintains VIN3 in a repressed state prior to cold exposure; this mark is not removed during VIN3 induction. Instead, active VIN3 is associated with bivalently marked chromatin. The continued presence of H3K27me3 ensures that induction of VIN3 is proportional to the duration of the cold, and that plants require prolonged cold to promote the transition to flowering. The observation that Polycomb proteins control VIN3 activity defines a new role for Polycomb proteins in regulating the rate of gene induction.

FLC: a hidden polycomb response element shows up in silence.

A sizeable fraction of eukaryotic genomes is regulated by Polycomb group (PcG) and trithorax group (trxG) proteins, which play key roles in epigenetic repression and activation, respectively. In Drosophila melanogaster, homeotic genes are well-documented PcG targets; they are known to contain cis-acting elements termed Polycomb response elements (PREs), which bind PcG proteins and satisfy three defined criteria, and also often contain binding sites for the trithorax (trx) protein. However, the presence of PREs, or an alternative mode for PcG/trxG interaction with the genome, has not been well documented outside Drosophila. In Arabidopsis thaliana, PcG/trxG regulation has been studied extensively for the flowering repressor gene FLOWERING LOCUS C (FLC). Here we evaluate how PRE-like activities that reside within the FLC locus may satisfy the defined Drosophila criteria, by analyzing four FLC transcription states. When the FLC locus is not transcribed, the intrinsic PcG recruitment ability of the coding region can be attributed to two redundant cis-acting elements (Modules IIA and IIB). When FLC is highly expressed, trxG recruitment is to a region overlapping the transcription start site (Module I). Exposure to prolonged cold converts the active FLC state into a repressed state that is maintained after the cold period finishes. These two additional transcriptional states also rely on the same three modules for PcG/trxG regulation. We conclude that each of Modules I, IIA and IIB partially fulfills the PRE function criteria, and that together they represent the functional FLC PRE, which differs structurally from canonical PREs in Drosophila.

Arabidopsis trithorax-related3/SET domain GROUP2 is required for the winter-annual habit of Arabidopsis thaliana.

The winter-annual habit of Arabidopsis thaliana requires active alleles of flowering locus C (FLC), which encodes a potent flowering repressor, and FRIGIDA (FRI), an activator of FLC. FLC activation by FRI is accompanied by an increase in specific histone modifications, such as tri-methylation of histone H3 at lysine 4 (H3K4me3), and requires three H3K4 methyltransferases, the Drosophila Trithorax-class Arabidopsis trithorax1 (ATX1) and ATX2, and yeast Set1-class ATX-related7/set domain group25 (ATXR7/SDG25). However, lesions in all of these genes failed to suppress the enhanced FLC expression caused by FRI completely, suggesting that another H3K4 methyltransferase may participate in the FLC activation. Here, we show that ATXR3/SDG2, which is a member of a novel class of H3K4 methyltransferases, also contributes to FLC activation. An ATXR3 lesion suppressed the enhanced FLC expression and delayed flowering caused by an active allele of FRI in non-vernalized plants. The decrease in FLC expression in atxr3 mutants was accompanied by reduced H3K4me3 levels at FLC chromatin. We also found that the rapid flowering of atxr3 was epistatic to that of atxr7, suggesting that ATXR3 functions in FLC activation in sequence with ATXR7. Our results indicate that the novel-class H3K4 methyltransferase, ATXR3, is a transcriptional activator that plays a role in the FLC activation and establishing the winter-annual habit. In addition, ATXR3 also contributes to the activation of other FLC clade members, such as flowering locus M/MADS affecting flowering1 (FLM/MAF1) and MAF5, at least partially explaining the ATXR3 function in delayed flowering caused by non-inductive photoperiods.

ARABIDOPSIS TRITHORAX-RELATED7 is required for methylation of lysine 4 of histone H3 and for transcriptional activation of FLOWERING LOCUS C.

In the winter-annual accessions of Arabidopsis thaliana, presence of an active allele of FRIGIDA (FRI) elevates expression of FLOWERING LOCUS C (FLC), a repressor of flowering, and thus confers a vernalization requirement. FLC activation by FRI involves methylation of Lys 4 of histone H3 (H3K4) at FLC chromatin. Many multicellular organisms that have been examined contain two classes of H3K4 methylases, a yeast (Saccharomyces cerevisiae) Set1 class and a class related to Drosophila melanogaster Trithorax. In this work, we demonstrate that ARABIDOPSIS TRITHORAX-RELATED7 (ATXR7), a putative Set1 class H3K4 methylase, is required for proper FLC expression. The atxr7 mutation partially suppresses the delayed flowering of a FRI-containing line. The rapid flowering of atxr7 is associated with reduced FLC expression and is accompanied by decreased H3K4 methylation and increased H3K27 methylation at FLC. Thus, ATXR7 is required for the proper levels of these histone modifications that set the level of FLC expression to create a vernalization requirement in winter-annual accessions. Previously, it has been reported that lesions in ATX1, which encodes a Trithorax class H3K4 methylase, partially suppress the delayed flowering of winter-annual Arabidopsis. We show that the flowering phenotype of atx1 atxr7 double mutants is additive relative to those of single mutants. Therefore, both classes of H3K4 methylases appear to be required for proper regulation of FLC expression.

Functional and expressional analyses of apple FLC-like in relation to dormancy progress and flower bud development.

We previously identified the FLOWERING LOCUS C (FLC)-like gene, a MADS-box transcription factor gene that belongs to Arabidopsis thaliana L. FLC clade, in apple (Malus $\times$  domestica Borkh.), and its expression in dormant flower buds is positively correlated with cumulative cold exposure. To elucidate the role of the MdFLC-like in the dormancy process and flower development, we first characterized the phenotypes of MdFLC-like overexpressing lines with the Arabidopsis Columbia-0 background. The overexpression of MdFLC-like significantly delayed the bolting date and reduced the plant size, but it did not significantly affect the number of rosette leaves or flower organ formation. Thus, MdFLC-like may affect vegetative growth and development rather than flowering when expressed in Arabidopsis, which is not like Arabidopsis FLC that affects development of flowering. We compared seasonal expression patterns of MdFLC-like in low-chill 'Anna' and high-chill 'Fuji' and 'Tsugaru' apples collected from trees grown in a cold winter region in temperate zone and found an earlier upregulation in 'Anna' compared with 'Fuji' and 'Tsugaru'. Expression patterns were also compared in relation to developmental changes in the flower primordia during the chilling accumulation period. Overall, MdFLC-like was progressively upregulated during flower primordia differentiation and development in autumn to early winter and reached a maximum expression level at around the same time as the genotype-dependent chilling requirements were fulfilled in high-chill cultivars. Thus, we hypothesize MdFLC-like may be upregulated in response to cold exposure and flower primordia development during the progress of endodormancy. Our study also suggests MdFLC-like may have a growth-inhibiting function during the end of endodormancy and ecodormancy when the temperature is low and unfavorable for rapid bud outgrowth.

Digital Holographic Multimodal Cross-Sectional Fluorescence and Quantitative Phase Imaging System.

We present a multimodal imaging system based on simple off-axis digital holography, for simultaneous recording and retrieval of cross-sectional fluorescence and quantitative phase imaging of the biological specimen. Synergism in the imaging capabilities can be achieved by incorporating two off-axis digital holographic microscopes integrated to record different information at the same time. The cross-sectional fluorescence imaging is realized by a common-path configuration of the single-shot off-axis incoherent digital holographic system. The quantitative phase imaging, on the other hand, is achieved by another off-axis coherent digital holographic microscopy operating in transmission mode. The fundamental characteristics of the proposed multimodal system are confirmed by performing various experiments on fluorescent beads and fluorescent protein-labeled living cells of the moss Physcomitrella patens lying at different axial depth positions. Furthermore, the cross-sectional live fluorescence and phase imaging of the fluorescent beads are demonstrated by the proposed multimodal system. The experimental results presented here corroborate the feasibility of the proposed system and indicate its potential in the applications to analyze the functional and structural behavior of biological cells and tissues.