Hiroyuki Nonogaki

Seed development, dormancy and germination.

Chapter 1. Genetic Control of Seed Development and Seed Mass. Masa--aki Ohto1, Sandra L. Stone2 and John J. Harada2. 1Department of Plant Sciences, College of Agricultural and Environmental Sciences and 2Section of Plant Biology, College of Biological Sciences, University of California, One Shields Avenue, Davis, CA 95616, USA . Chapter 2. Seed Coat Development and Dormancy. Isabelle Debeaujon, Loic Lepiniec, Lucille Pourcel and Jean--Marc Routaboul. Laboratoire de Biologie des Semences, Unite Mixte de Recherche 204 Institut National de la Recherche Agronomique/Institut National Agronomique Paris--Grignon, 78026 Versailles, France. Chapter 3. Definitions and Hypotheses of Seed Dormancy. Henk W.M. Hilhorst. Laboratory of Plant Physiology, Wageningen University, Arboretumlaan 4, 6703 BD, Wageningen, The Netherlands . Chapter 4. Modeling of Seed Dormancy. Phil S. Allen1, Roberto L. Benech--Arnold2, Diego Batlla2 and Kent J. Bradford3. 1Department of Plant & Animal Sciences, Brigham Young University, 275 WIDB, Provo, UT 84602--5253, USA 2IFEVA--Catedra de Cerealicultura, Facultad de Agronomia, Universidad de Buenos Aires/CONICET,Av. San Martin 4453, 1417 Buenos Aires, Argentina 3Seed Biotechnology Center, University of California, One Shields Avenue, Davis, CA 95616--8780, USA . Chapter 5. Genetic Aspects of Seed Dormancy. Leonie Bentsink1, Wim Soppe2 and Maarten Koornneef2,3. 1Department of Molecular Plant Physiology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands 2 Max Planck Institute for Plant Breeding Research, Carl--von--Linne--Weg 10, 50829 Cologne, Germany and 3Laboratory of Genetics, Wageningen University, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands. Chapter 6. Lipid Metabolism in Seed Dormancy. Steven Penfield, Helen Pinfield--Wells and Ian A. Graham. Centre for Novel Agricultural Products, Department of Biology, University of York, PO Box 373, York YO10 5YW, UK. . Chapter 7. Nitric Oxide in in Seed Dormancy and Germination. Paul C. Bethke1, Igor G.L. Libourel2 and Russell L. Jones1. 1Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720--3102, USA and 2Department of Plant Biology, Michigan State University, East Lansing, MI 48824, USA. Chapter 8. A Merging of Paths: Abscisic Acid and Hormonal Cross--talk in the Control of Seed Dormancy Maintenance and Alleviation. J. Allan Feurtado and Allison R. Kermode. Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6. Chapter 9. Regulation of ABA and GA Levels during Seed Development and Germination in Arabidopsis. Shinjiro Yamaguchi, Yuji Kamiya and Eiji Nambara. Plant Science Center, RIKEN, Growth Physiology Group, Laboratory for Cellular Growth & Development, 1--7--22 Suehirocho, Tsurumi--ku, Yokohama, 230--0045 Japan. Chapter 10. De--repression of Seed Germination by GA Signaling. Camille M. Steber. U.S. Department of Agriculture--Agricultural Research Service and Department of Crop and Soil Science and Graduate Program in Molecular Plant Sciences, Washington State University, Pullman, WA 99164--6420, USA. Chapter 11. Mechanisms and Genes Involved in Germination Sensu Stricto. Hiroyuki Nonogaki1, Feng Chen2 and Kent J. Bradford3. 1Department of Horticulture, Oregon State University, Corvallis, OR 97331, USA 2Department of Plant Sciences, University of Tennessee, Knoxville, TN 37996--4561, USA 3Seed Biotechnology Center, University of California, One Shields Avenue, Davis, CA 95616--8780, USA. Chapter 12. Sugar and Abscisic Acid Regulation of Germination and Transition to Seedling Growth. Bas J.W. Dekkers and Sjef C.M. Smeekens. Department of Molecular Plant Physiology, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands.

A Novel Endo-β-Mannanase Gene in Tomato LeMAN5 Is Associated with Anther and Pollen Development

Endo-β-mannanase (EC 3.2.1.78) is involved in cell wall disassembly and the weakening of plant tissues by degrading mannan polymers in the cell walls. Endo-β-mannanase genes are expressed in tomato (Lycopersicon esculentum) seeds (LeMAN1 and LeMAN2) and fruits (LeMAN3 and LeMAN4). A novel endo-β-mannanase gene (termed LeMAN5) was found in the tomato genome by genome-walking PCR and bacterial artificial chromosome library screening. The 5′-upstream region of this endo-β-mannanase gene contained four copies of the pollen-specific cis-acting elements POLLEN1LELAT52 (AGAAA). A GUS-reporter gene driven with the putative LeMAN5 promoter (-543 to +38) was activated in anthers and pollen of transgenic Arabidopsis, with the highest β-glucuronidase activity detected in pollen. β-Glucuronidase expression was detected in mature pollen retained in sporangia, discharged pollen, and elongating pollen tubes in transgenic Arabidopsis. Consistently, expression of LeMAN5 mRNA and endo-β-mannnanase activity was detected in tomato anthers and pollen. In anthers, the highest mRNA expression and endo-β-mannanase activity were detected during late stages of anther development, when pollen maturation occurred. Endo-β-mannanase activity was present in discharged pollen, which was easily eluted in a buffer, indicating that the enzyme proteins are probably secreted from, and deposited on, the surface of pollen. These data suggest that the LeMAN5 endo-β-mannanase is associated with anther and pollen development.

Seed dormancy and germination—emerging mechanisms and new hypotheses

Seed dormancy has played a significant role in adaptation and evolution of seed plants. While its biological significance is clear, molecular mechanisms underlying seed dormancy induction, maintenance and alleviation still remain elusive. Intensive efforts have been made to investigate gibberellin and abscisic acid metabolism in seeds, which greatly contributed to the current understanding of seed dormancy mechanisms. Other mechanisms, which might be independent of hormones, or specific to the seed dormancy pathway, are also emerging from genetic analysis of “seed dormancy mutants.” These studies suggest that chromatin remodeling through histone ubiquitination, methylation and acetylation, which could lead to transcription elongation or gene silencing, may play a significant role in seed dormancy regulation. Small interfering RNA and/or long non-coding RNA might be a trigger of epigenetic changes at the seed dormancy or germination loci, such as DELAY OF GERMINATION1. While new mechanisms are emerging from genetic studies of seed dormancy, novel hypotheses are also generated from seed germination studies with high throughput gene expression analysis. Recent studies on tissue-specific gene expression in tomato and Arabidopsis seeds, which suggested possible “mechanosensing” in the regulatory mechanisms, advanced our understanding of embryo-endosperm interaction and have potential to re-draw the traditional hypotheses or integrate them into a comprehensive scheme. The progress in basic seed science will enable knowledge translation, another frontier of research to be expanded for food and fuel production.

Expression of a GALACTINOL SYNTHASE Gene in Tomato Seeds Is Up-Regulated before Maturation Desiccation and Again after Imbibition whenever Radicle Protrusion Is Prevented

Raffinose family oligosaccharides (RFOs) have been implicated in mitigating the effects of environmental stresses on plants. In seeds, proposed roles for RFOs include protecting cellular integrity during desiccation and/or imbibition, extending longevity in the dehydrated state, and providing substrates for energy generation during germination. A gene encoding galactinol synthase (GOLS), the first committed enzyme in the biosynthesis of RFOs, was cloned from tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds, and its expression was characterized in tomato seeds and seedlings. GOLS (LeGOLS-1) mRNA accumulated in developing tomato seeds concomitant with maximum dry weight deposition and the acquisition of desiccation tolerance.LeGOLS-1 mRNA was present in mature, desiccated seeds but declined within 8 h of imbibition in wild-type seeds. However, LeGOLS-1 mRNA accumulated again in imbibed seeds prevented from completing germination by dormancy or water deficit. Gibberellin-deficient (gib-1) seeds maintainedLeGOLS-1 mRNA amounts after imbibition unless supplied with gibberellin, whereas abscisic acid (ABA) did not prevent the loss of LeGOLS-1 mRNA from wild-type seeds. The presence of LeGOLS-1mRNA in ABA-deficient (sitiens) tomato seeds indicated that wild-type amounts of ABA are not necessary for its accumulation during seed development. In all cases,LeGOLS-1 mRNA was most prevalent in the radicle tip. LeGOLS-1 mRNA accumulation was induced by dehydration but not by cold in germinating seeds, whereas both stresses induced LeGOLS-1mRNA accumulation in seedling leaves. The physiological implications ofLeGOLS-1 expression patterns in seeds and leaves are discussed in light of the hypothesized role of RFOs in plant stress tolerance.

microRNA, seeds, and Darwin?: diverse function of miRNA in seed biology and plant responses to stress

microRNAs (miRNAs) are small, single-stranded RNAs that down-regulate target genes at the post-transcriptional level. miRNAs regulate target genes by guiding mRNA cleavage or by repressing translation. miRNAs play crucial roles in a broad range of developmental processes in plants. Multiple miRNAs are present in germinating seeds and seedlings of Arabidopsis, some of which are involved in the regulation of germination and seedling growth by plant hormones such as abscisic acid (ABA) and auxin. The involvement of miRNAs in ABA responses is not limited to the early stages of plant development but seems to be important for general stress responses throughout the plant life cycle. This Darwin review summarizes recent progress in miRNA research focusing on seed and stress biology, two topics which were of interest to Charles Darwin.

Induction of 9-cis-epoxycarotenoid dioxygenase in Arabidopsis thaliana seeds enhances seed dormancy

Full understanding of mechanisms that control seed dormancy and germination remains elusive. Whereas it has been proposed that translational control plays a predominant role in germination, other studies suggest the importance of specific gene expression patterns in imbibed seeds. Transgenic plants were developed to permit conditional expression of a gene encoding 9-cis-epoxycarotenoid dioxygenase 6 (NCED6), a rate-limiting enzyme in abscisic acid (ABA) biosynthesis, using the ecdysone receptor-based plant gene switch system and the ligand methoxyfenozide. Induction of NCED6 during imbibition increased ABA levels more than 20-fold and was sufficient to prevent seed germination. Germination suppression was prevented by fluridone, an inhibitor of ABA biosynthesis. In another study, induction of the NCED6 gene in transgenic seeds of nondormant mutants tt3 and tt4 reestablished seed dormancy. Furthermore, inducing expression of NCED6 during seed development suppressed vivipary, precocious germination of developing seeds. These results indicate that expression of a hormone metabolism gene in seeds can be a sole determinant of dormancy. This study opens the possibility of developing a robust technology to suppress or promote seed germination through engineering pathways of hormone metabolism.

MicroRNA Gene Regulation Cascades During Early Stages of Plant Development

MicroRNAs (miRNAs) regulate various developmental programs of plants. This review focuses on miRNA involvement in early events of plant development, such as seed germination, seedling development and the juvenile to adult phase transition. miR159 and miR160 are involved in the regulation of seed germination through their effects on the sensitivity of seeds to ABA. miR156 and miR172 play critical roles in the emergence of vegetative leaves at post-germinative stages, which is important for the transition to autotrophic growth. The phase transition from the juvenile to adult stage in both monocots and dicots is also regulated by miR156 and miR172. In these early developmental processes, there are miRNA gene regulation cascades where the miR156 pathway acts upstream of the miR172 pathway. Moreover, targets of miR156 and miR172 exert positive feedback on the expression of MIR genes that suppress themselves. The early events of plant development appear to be controlled by complex mechanisms involving sequential expression of different miRNA pathways and feedback loops among miRNAs and their target genes.

An Endo-[beta]-Mannanase Develops Exclusively in the Micropylar Endosperm of Tomato Seeds Prior to Radicle Emergence.

A galactomannan-hydrolyzing enzyme that develops pregerminatively in the micropylar region of the endosperm of the tomato (Lycopersicon esculentum [L.] Mill.) seed was characterized. The enzyme was endo-[beta]-mannanase (EC 3.2.1.78), since it hydrolyzed galactomannan into oligosaccharides with no release of galactose and mannose. The mobility of this pregerminative enzyme in sodium dodecyl sulfate and native polyacrylamide gel electrophoresis was not identical to that of any of the three endo-[beta]-mannanases that develop in the same tissue (endosperm) after germination (H. Nonogaki, M. Nomaguchi, Y. Morohashi [1995] Physiol Plant 94: 328–334). There were also some differences in the products of galactomannan hydrolysis between the pregerminative and the postgerminative enzymes, indicating that the action pattern is different between the two types of enzymes. The pregerminative enzyme began to develop in the micropylar region of the endosperm at about 18 h postimbibition and increased up to the time immediately before radicle protrusion (24 h postimbibition). This enzyme was not present in the lateral part of the endosperm at any stage before or after germination. It is proposed that the enzyme develops prior to germination specifically at the micropylar region of the endosperm.

Seed Biology in the 21st Century: Perspectives and New Directions

Seeds are a fundamental component of the plant life cycle, as they store the genetic information necessary for the next generation of plants to disperse, establish, develop and eventually reproduce to maintain the species. However, these mysterious genetic capsules contain many secrets that have yet to be revealed. For instance, the mechanism(s) by which certain seeds undergo extreme desiccation without losing viability remain unclear. Further, the molecular network of genes conferring on seeds the ability to remain dormant for the required periods of time has yet to be fully elucidated. This special focus issue is dedicated to the latest research on seed biology aimed at tackling some of these unanswered questions.

The Endo-β-Mannanase gene families in Arabidopsis, rice, and poplar

Mannans are widespread hemicellulosic polysaccharides in plant cell walls. Hydrolysis of the internal β-1,4-d-mannopyranosyl linkage in the backbone of mannans is catalyzed by endo-β-mannanase. Plant endo-β-mannanase has been well studied for its function in seed germination. Its involvement in other plant biological processes, however, remains poorly characterized or elusive. The completed genome sequences of Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and poplar (Populus trichocarpa) provide an opportunity to conduct comparative genomic analysis of endo-β-mannanase genes in these three species. In silico sequence analysis led to the identification of eight, nine and 11 endo-β-mannanase genes in the genomes of Arabidopsis, rice, and poplar, respectively. Sequence comparisons revealed the conserved amino acids and motifs that are critical for the active site of endo-β-mannanases. Intron/exon structure analysis in conjunction with phylogenetic analysis implied that both intron gain and intron loss has played roles in the evolution of endo-β-mannanase genes. The phylogenetic analysis that included the endo-β-mannanases from plants and other organisms implied that plant endo-β-mannanases have an ancient evolutionary origin. Comprehensive expression analysis of all Arabidopsis and rice endo-β-mannanase genes showed divergent expression patterns of individual genes, suggesting that the enzymes encoded by these genes, while carrying out the same biochemical reaction, are involved in diverse biological processes.