Cryptochromes suppress leaf senescence in response to blue light in Arabidopsis.

The induction and progression of leaf senescence are effectively changed according to the light environment. The leaf senescence response is enhanced when plants are grown under a dense shade cast by neighboring vegetation. Although the fluence rate of the red and blue regions in the light spectrum is strongly attenuated under shade, photosensory mechanisms that underpin the blue light response are still unclear. In this study, we analyzed leaf senescence in response to blue light in Arabidopsis (Arabidopsis thaliana). We found that leaf senescence was promoted by the elimination of active phytochrome Pfr by pulsed far-red (FR) light, whereas irradiation with blue light suppressed leaf senescence in the wild type but not in the cryptochrome (CRY)-deficient mutant, cry1 cry2. Hence, two light-sensing modes contributed to the suppression of leaf senescence that was dependent on light spectrum features. First was the leaf senescence response to blue light, which was mediated exclusively by cryptochromes. Second was the phytochrome-mediated leaf senescence response to red/FR light. Physiological analysis of transgenic plants expressing green fluorescent protein (GFP)-tagged CRY2 revealed that photo-activation of cryptochromes was required to suppress leaf senescence in response to blue light. Transcriptomic analysis further uncovered the molecular and cellular processes involved in the regulation of leaf senescence downstream of cryptochromes. Furthermore, analysis of cryptochrome-downstream components indicated that ELONGATED HYPOCOTYL 5 (HY5) and PHYTOCHROME INTERACTING FACTOR (PIF) 4 and PIF5 were required for suppression and promotion of leaf senescence, respectively.

Regulation of Sugar and Storage Oil Metabolism by Phytochrome during De-etiolation.

Exposure of dark-grown (etiolated) seedlings to light induces the heterotrophic-to-photoautotrophic transition (de-etiolation) processes, including the formation of photosynthetic machinery in the chloroplast and cotyledon expansion. Phytochrome is a red (R)/far-red (FR) light photoreceptor that is involved in the various aspects of de-etiolation. However, how phytochrome regulates metabolic dynamics in response to light stimulus has remained largely unknown. In this study, to elucidate the involvement of phytochrome in the metabolic response during de-etiolation, we performed widely targeted metabolomics in Arabidopsis (Arabidopsis thaliana) wild-type and phytochrome A and B double mutant seedlings de-etiolated under R or FR light. The results revealed that phytochrome had strong impacts on the primary and secondary metabolism during the first 24 h of de-etiolation. Among those metabolites, sugar levels decreased during de-etiolation in a phytochrome-dependent manner. At the same time, phytochrome upregulated processes requiring sugars. Triacylglycerols are stored in the oil bodies as a source of sugars in Arabidopsis seedlings. Sugars are provided from triacylglycerols through fatty acid ß-oxidation and the glyoxylate cycle in glyoxysomes. We examined if and how phytochrome regulates sugar production from oil bodies. Irradiation of the etiolated seedlings with R and FR light dramatically accelerated oil body mobilization in a phytochrome-dependent manner. Glyoxylate cycle-deficient mutants not only failed to mobilize oil bodies but also failed to develop thylakoid membranes and expand cotyledon cells upon exposure to light. Hence, phytochrome plays a key role in the regulation of metabolism during de-etiolation.

RAD-seq-Based High-Density Linkage Map Construction and QTL Mapping of Biomass-Related Traits in Sorghum using the Japanese Landrace Takakibi NOG.

Sorghum [Sorghum bicolor (L.) Moench] grown locally by Japanese farmers is generically termed Takakibi, although its genetic diversity compared with geographically distant varieties or even within Takakibi lines remains unclear. To explore the genomic diversity and genetic traits controlling biomass and other physiological traits in Takakibi, we focused on a landrace, NOG, in this study. Admixture analysis of 460 sorghum accessions revealed that NOG belonged to the subgroup that represented Asian sorghums, and it was only distantly related to American/African accessions including BTx623. In an attempt to dissect major traits related to biomass, we generated a recombinant inbred line (RIL) from a cross between BTx623 and NOG, and we constructed a high-density linkage map based on 3,710 single-nucleotide polymorphisms obtained by restriction-site-associated DNA sequencing of 213 RIL individuals. Consequently, 13 fine quantitative trait loci (QTLs) were detected on chromosomes 2, 3, 6, 7, 8 and 9, which included five QTLs for days to heading, three for plant height (PH) and total shoot fresh weight and two for Brix. Furthermore, we identified two dominant loci for PH as being identical to the previously reported dw1 and dw3. Together, these results corroborate the diversified genome of Japanese Takakibi, while the RIL population and high-density linkage map generated in this study will be useful for dissecting other important traits in sorghum.

Development and Application of a High-Resolution Imaging Mass Spectrometer for the Study of Plant Tissues.

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) or imaging mass spectrometry (imaging MS) has been a powerful tool to map the spatial distribution of molecules on the surface of biological materials. This technique has frequently been applied to animal tissue slices for the purpose of mapping proteins, peptides, lipids, sugars or small metabolites to find disease-specific biomarkers or to study drug metabolism. Recently, it has also been applied to intact plant tissues or thin slices thereof using commercial mass spectrometers. The present work is concerned with the refinement of MALDI/laser desorption/ionization (LDI)-Fourier transform ion cyclotron resonance (FTICR)-MS incorporating certain specific features namely, ultra-high mass resolution (>100,000), ultra-high molecular mass accuracy (<1 p.p.m.) and high spatial resolution (<10 µm) for imaging MS of plant tissues. Employing an in-house built mass spectrometer, the imaging MS analysis of intact Arabidopsis thaliana tissues, namely etiolated seedlings and roots of seedlings, glued to a small transparent ITO (indium tin oxide)-coated conductive glass was performed. A matrix substance was applied to the vacuum-dried intact tissues by sublimation prior to the imaging MS analysis. The images of various small metabolites representing their two-dimensional distribution on the dried intact tissues were obtained with or without different matrix substances. The effects of MALDI matrices on the ionization of small metabolites during imaging MS acquisition are discussed.

Tissue-autonomous promotion of palisade cell development by phototropin 2 in Arabidopsis.

Light is an important environmental information source that plants use to modify their growth and development. Palisade parenchyma cells in leaves develop cylindrical shapes in response to blue light; however, the photosensory mechanism for this response has not been elucidated. In this study, we analyzed the palisade cell response in phototropin-deficient mutants. First, we found that two different light-sensing mechanisms contributed to the response in different proportions depending on the light intensity. One response observed under lower intensities of blue light was mediated exclusively by a blue light photoreceptor, phototropin 2 (PHOT2). Another response was elicited under higher intensities of light in a phototropin-independent manner. To determine the tissue in which PHOT2 perceives the light stimulus to regulate the response, green fluorescent protein (GFP)-tagged PHOT2 (P2G) was expressed under the control of tissue-specific promoters in the phot1 phot2 mutant background. The results revealed that the expression of P2G in the mesophyll, but not in the epidermis, promoted palisade cell development. Furthermore, a constitutively active C-terminal kinase fragment of PHOT2 fused to GFP (P2CG) promoted the development of cylindrical palisade cells in the proper direction without the directional cue provided by light. Hence, in response to blue light, PHOT2 promotes the development of cylindrical palisade cells along a predetermined axis in a tissue-autonomous manner.

CT image-based 3D inflorescence estimation of Chrysanthemum seticuspe.

To study plant organs, it is necessary to investigate the three-dimensional (3D) structures of plants. In recent years, non-destructive measurements through computed tomography (CT) have been used to understand the 3D structures of plants. In this study, we use the Chrysanthemum seticuspe capitulum inflorescence as an example and focus on contact points between the receptacles and florets within the 3D capitulum inflorescence bud structure to investigate the 3D arrangement of the florets on the receptacle. To determine the 3D order of the contact points, we constructed slice images from the CT volume data and detected the receptacles and florets in the image. However, because each CT sample comprises hundreds of slice images to be processed and each C. seticuspe capitulum inflorescence comprises several florets, manually detecting the receptacles and florets is labor-intensive. Therefore, we propose an automatic contact point detection method based on CT slice images using image recognition techniques. The proposed method improves the accuracy of contact point detection using prior knowledge that contact points exist only around the receptacle. In addition, the integration of the detection results enables the estimation of the 3D position of the contact points. According to the experimental results, we confirmed that the proposed method can detect contacts on slice images with high accuracy and estimate their 3D positions through clustering. Additionally, the sample-independent experiments showed that the proposed method achieved the same detection accuracy as sample-dependent experiments.

Antagonistic regulation of leaf flattening by phytochrome B and phototropin in Arabidopsis thaliana.

Light is one of the most important environmental factors regulating the growth and development of leaves. As the primary photosynthetic organs, leaves have a laminar structure in many dicotyledonous plants. The regulation of leaf flatness is a key mechanism for the efficient absorption of light under low light conditions. In the present study, we demonstrated that phytochrome B (phyB) promoted the development of curled leaves. Wild-type leaves gently curled downwards under white light, whereas the phyB-deficient mutant (phyB) constitutively exhibited flatter leaves. In the wild type, leaf flattening was promoted by end-of-day far-red light (EODFR) treatment, which rapidly eliminates the active Pfr phytochrome. Interestingly, the curled-leaf phenotype in a phototropin-deficient mutant was almost completely suppressed by the phyB mutation as well as by EODFR. Thus, phototropin promotes leaf flattening by suppressing the leaf-curling activity of phyB. We examined the downstream components of phyB and phototropin to assess their antagonistic regulation of leaf flatness further. Consequently, we found that a phototropin signaling transducer, NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3), was required to promote leaf flattening in phyB. The present study provides new insights into a mechanism in which leaf flatness is regulated in response to different light environmental cues.

Involvement of auxin and brassinosteroid in the regulation of petiole elongation under the shade.

Plants grown under a canopy recognize changes in light quality and modify their growth patterns; this modification is known as shade avoidance syndrome. In leaves, leaf blade expansion is suppressed, whereas petiole elongation is promoted under the shade. However, the mechanisms that control these responses are largely unclear. Here, we demonstrate that both auxin and brassinosteroid (BR) are required for the normal leaf responses to shade in Arabidopsis (Arabidopsis thaliana). The microarray analysis of leaf blades and petioles treated with end-of-day far-red light (EODFR) revealed that almost half of the genes induced by the treatment in both parts were previously identified as auxin-responsive genes. Likewise, BR-responsive genes were overrepresented in the EODFR-induced genes. Hence, the auxin and BR responses were elevated by EODFR treatment in both leaf blades and petioles, although opposing growth responses were observed in these two parts. The analysis of the auxin-deficient doc1/big mutant and the BR-deficient rot3/cyp90c1 mutant further indicates that auxin and BR were equally required for the normal petiole elongation response to the shade stimulus. In addition, the spotlight irradiation experiment revealed that phytochrome in leaf blades but not that in petioles regulated petiole elongation, which was probably mediated through regulation of the auxin/BR responses in petioles. On the basis of these findings, we conclude that auxin and BR cooperatively promote petiole elongation in response to the shade stimulus under the control of phytochrome in the leaf blade.

A chromosome-level genome sequence of Chrysanthemum seticuspe, a model species for hexaploid cultivated chrysanthemum.

Chrysanthemums are one of the most industrially important cut flowers worldwide. However, their segmental allopolyploidy and self-incompatibility have prevented the application of genetic analysis and modern breeding strategies. We thus developed a model strain, Gojo-0 (Chrysanthemum seticuspe), which is a diploid and self-compatible pure line. Here, we present the 3.05 Gb chromosome-level reference genome sequence, which covered 97% of the C. seticuspe genome. The genome contained more than 80% interspersed repeats, of which retrotransposons accounted for 72%. We identified recent segmental duplication and retrotransposon expansion in C. seticuspe, contributing to arelatively large genome size. Furthermore, we identified a retrotransposon family, SbdRT, which was enriched in gene-dense genome regions and had experienced a very recent transposition burst. We also demonstrated that the chromosome-level genome sequence facilitates positional cloning in C. seticuspe. The genome sequence obtained here can greatly contribute as a reference for chrysanthemum in front-line breeding including genome editing.

Genetic Interaction Among Phytochrome, Ethylene and Abscisic Acid Signaling During Dark-Induced Senescence in Arabidopsis thaliana.

Leaf senescence is induced by various internal and external stimuli. Dark-induced senescence has been extensively investigated, but the detailed mechanism underlying it is not well understood. The red light/far-red light receptor phytochrome B and its downstream transcription factors, PYHTOCHROME INTERACTING FACTORs (PIFs) 4 and 5, are known to play an important role in dark-induced senescence. Furthermore, the senescence-inducing phytohormones, ethylene and abscisic acid (ABA) are reported to be involved in dark-induced senescence. In this study, we analyzed the relationship between ethylene, ABA and PIFs in dark-induced leaf senescence. A triple mutant of the core ABA signaling components; SNF1-related protein kinases 2D (SRK2D), SRK2E, and SRK2I, displayed an ABA insensitive phenotype in ABA-induced senescence, whilst the ethylene insensitive mutant ein2 demonstrated low sensitivity to ABA, suggesting that ethylene signaling is involved in ABA-induced senescence. However, the pif4 pif5 mutant did not display low sensitivity to ABA, suggesting that PIF4 and PIF5 act upstream of ABA signaling. Although PIF4 and PIF5 reportedly regulate ethylene production, the triple mutant ein2 pif4 pif5 showed a stronger delayed senescence phenotype than ein2 or pif4 pif5, suggesting that EIN2 and PIF4/PIF5 partially regulate leaf senescence independently of each other. While direct target genes for PIF4 and PIF5, such as LONG HYPOCOTYL IN FAR-RED1 (HFR1) and PHYTOCHROME INTERACTING FACTOR 3-LIKE 1 (PIL1), showed transient upregulation under dark conditions (as is seen in the shade avoidance response), expression of STAY GREEN1 (SGR1), ORESARA1 (ORE1) and other direct target genes of PIF5, continued to increase during dark incubation. It is possible that transcription factors other than PIF4 and PIF5 are involved in the upregulation of SGR1 and ORE1 at a later stage of dark-induced senescence. Possible candidates are senescence-induced senescence regulators (SIRs), which include the NAC transcription factors ORE1 and AtNAP. In fact, ORE1 is known to bind to the SGR1 promoter and promotes its expression. It is therefore inferred that the phytochrome-PIF pathway regulates initial activation of senescence and subsequently, induced SIRs reinforce leaf senescence during dark-induced senescence.

A pure line derived from a self-compatible Chrysanthemum seticuspe mutant as a model strain in the genus Chrysanthemum.

Asteraceae is the largest family of angiosperms, comprising approximately 24,000 species. Molecular genetic studies of Asteraceae are essential for understanding plant diversity. Chrysanthemum morifolium is the most industrially important ornamental species in Asteraceae. Most cultivars of C. morifolium are autohexaploid and self-incompatible. These properties are major obstacles to the genetic analysis and modern breeding of C. morifolium. Furthermore, high genome heterogeneity complicates molecular biological analyses. In this study, we developed a model strain in the genus Chrysanthemum. C. seticuspe is a diploid species with a similar flowering property and morphology to C. morifolium and can be subjected to Agrobacterium-mediated transformation. We isolated a natural self-compatible mutant of C. seticuspe and established a pure line through repeated selfing and selection. The resultant strain, named Gojo-0, was favorable for genetic analyses, including isolation of natural and induced mutants, and facilitated molecular biological analysis, including whole genome sequencing, owing to the simplicity and homogeneity of its genome. Interspecific hybridization with Chrysanthemum species was possible, enabling molecular genetic analysis of natural interspecific variations. The accumulation of research results and resources using Gojo-0 as a platform is expected to promote molecular genetic studies on the genus Chrysanthemum and the genetic improvement of chrysanthemum cultivars.

The unique function of the Arabidopsis circadian clock gene PRR5 in the regulation of shade avoidance response.

Shade avoidance response (S.A.R) is regulated by light and circadian clock. Circadian clock controls S.A.R by the transcriptional regulation of positive regulators of S.A.R, PIF4 and PIF5, to prevent plants from responding to 'light' of dark period. Thus, in many cases, deficits in circadian clock appear in abnormalities of hypocotyl and/or petiole elongation. Previously, interesting phenomena were reported that the triple mutants of PSEUDO RESPONSE REGULATORS9, 7 and 5, which are clock components, show longer petioles and smaller leaves under light/dark cycle than those under continuous lighting. These S.A.R-like phenotypes cannot be explained by their hyposensitivity to red light. We demonstrated detailed analyses of this mutant to reveal the leaf-specific S.A.R regulated by circadian clock. Expression analyses of S.A.R-related genes suggested that PRR5 functions as a repressor of S.A.R. Morphological analyses of leaves under different light condition revealed that PRR5 is involved in the inhibition of leaf expansion in S.A.R.

Photomorphogenesis of leaves: shade-avoidance and differentiation of sun and shade leaves.

Leaf shape is an important factor in optimal plant growth, because leaves are the main photosynthetic organs. Plants exhibit plasticity in leaf shape and structure, allowing them to optimize photosynthetic efficiency. In Arabidopsis thaliana(L.) Heynh., several types of leaves develop differentially, according to light intensity and quality. When shaded, the expansion of leaf lamina is inhibited, while the petiole elongation is enhanced. This phenomenon is part of the so-called shade-avoidance syndrome. Under low light, A. thaliana develops shade leaves with only one layer of palisade tissue, whereas under high light, it develops sun leaves that have nearly two complete layers of palisade tissue. Although the molecular mechanisms of these photomorphogenic phenomena in leaves are not well understood, recent studies of A. thaliana have provided some insight. For example, some cytochrome P450s may be involved in the specific control of the petiole length during photomorphogenesis. On the other hand, switching between sun and shade leaves is regulated by long-distance signaling from mature leaves in Chenopodium album. Here we provide an overview of the mechanisms of photomorphogenesis in leaves based on recent findings.

The different growth responses of the Arabidopsis thaliana leaf blade and the petiole during shade avoidance are regulated by photoreceptors and sugar.

During the shade-avoidance response, leaf blade expansion is inhibited and petiole elongation is enhanced. In this study, we examined the roles of photoreceptors and sugar on the differential growth of the leaf blade and petiole in shade conditions. Under the conditions examined, cell expansion, not cell division, played a major role in the differential leaf growth. The enhanced cell expansion in the leaf blade is associated with an increase in the ploidy level, whereas cell elongation was stimulated in the petiole in dark conditions without an increase in the ploidy level. Analysis of phytochrome, cryptochrome and phototropin mutants revealed that phytochromes and cryptochromes specifically regulate the contrasting growth patterns of the leaf blade and petiole in shade. Examination of the effects of photo-assimilated sucrose on the growth of the leaf blade and petiole revealed growth-promotional effects of sucrose that are highly dependent on the light conditions. The leaf blades of abscisic acid-deficient and sugar-insensitive mutants did not expand in blue light, but expanded normally in red light. These results suggest that both the regulation of light signals and the modulation of responses to sugar are important in the control of the differential photomorphogenesis of the leaf blade and petiole.

CYP90C1 and CYP90D1 are involved in different steps in the brassinosteroid biosynthesis pathway in Arabidopsis thaliana.

Brassinosteroids (BRs) are plant hormones that are essential for a wide range of developmental processes in plants. Many of the genes responsible for the early reactions in the biosynthesis of BRs have recently been identified. However, several genes for enzymes that catalyze late steps in the biosynthesis pathways of BRs remain to be identified, and only a few genes responsible for the reactions that produce bioactive BRs have been identified. We found that the ROTUNDIFOLIA3 (ROT3) gene, encoding the enzyme CYP90C1, which was specifically involved in the regulation of leaf length in Arabidopsis thaliana, was required for the late steps in the BR biosynthesis pathway. ROT3 appears to be required for the conversion of typhasterol to castasterone, an activation step in the BR pathway. We also analyzed the gene most closely related to ROT3, CYP90D1, and found that double mutants for ROT3 and CYP90D1 had a severe dwarf phenotype, whereas cyp90d1 single knockout mutants did not. BR profiling in these mutants revealed that CYP90D1 was also involved in BR biosynthesis pathways. ROT3 and CYP90D1 were expressed differentially in leaves of A. thaliana, and the mutants for these two genes differed in their defects in elongation of hypocotyls under light conditions. The expression of CYP90D1 was strongly induced in leaf petioles in the dark. The results of the present study provide evidence that the two cytochrome P450s, CYP90C1 and CYP90D1, play distinct roles in organ-specific environmental regulation of the biosynthesis of BRs.

Genetic control of petiole length in Arabidopsis thaliana.

Shade-avoidance syndrome is characterized by the formation of elongated petioles and unexpanded leaf blades under low-intensity light, but the genetic basis for these responses is unknown. In this study, two-dimensional mutational analysis revealed that the gene for phytochrome B, PHYB, had opposing effects in the leaf petioles and leaf blades of Arabidopsis, while the ROT3, ACL2, and GAI genes influenced the length of leaf petioles more significantly than the length of leaf blades. Anatomical analysis revealed that the PHYB and ACL2 genes control the length of leaf petioles exclusively via control of the length of individual cells, while the GAI, GA1 and ROT3 genes appeared to control both the elongation and proliferation of petiole cells, in particular, under strong light. By contrast, both the size and the number of cells were affected by the mutations examined in leaf blades. The differential control of leaf petiole length and leaf blade expansion is discussed.

Improved method for visualizing coated pits, microfilaments, and microtubules in cryofixed and freeze-substituted plant cells.

We have optimized the conditions for visualizing microfilaments, microtubules, and coated pits in the cortical cytoplasm of high-pressure frozen and freeze-substituted plant cells, in both tobacco root tips and onion cotyledons, individual microfilaments and the supramolecular structure of coated pits can be seen clearly in freeze-substituted samples treated with OsO4 at 40 degrees C followed by 5% uranyl acetate. Treatment with uranyl acetate alone resulted in poorly stained cytoplasmic organelles, whereas microfilaments were difficult to discern in specimen treated with OsO4 alone. The combination of a 40 degrees C OsO4 staining step followed by staining with uranyl acetate at 4 degrees C should prove useful for more detailed plant cytoskeletal/membrane studies in the future.