Volume 1 Number 2 2007

CONTENTS AND ABSTRACTS

Kenji Henmi, Mototsugu Yanagida, Ken’ichi Ogawa (Japan) Roles of Reactive Oxygen Species and Glutathione in Plant Development (pp 185-193)

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Invited Review: Endogenous levels of reactive oxygen species (ROS), which are toxic to cells, increase in response to various environmental and developmental stimuli, inducing changes in cellular redox state that is maintained by the redox buffer glutathione. Such redox changes are used in signals for the regulation of plant growth and stress responses. In this review, in order to give further insight into the mechanism underlying ROS- and glutathione-associated physiological events, we present recent findings on and discuss the roles of ROS and glutathione in plant growth and development.

Ramamurthy Mahalingam, Niranjani Jambunathan Anuradha Penaganti (USA) Pyridine Nucleotide Homeostasis in Plant Development and Stress (pp 194-201)

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Invited Review: The traditional role of pyridine nucleotides (PNs) - NAD(H) and NADP(H), as the key redox couples in the cytoplasm of all living cells is well known. The reversible conversion between oxidized and reduced forms of PNs does not affect the net levels of these molecules. Resynthesis of these metabolites arises due to their participation in signaling reactions. Recent discoveries in animal systems have shown that derivatives of PNs such as ADP-ribose, cyclic ADP-ribose, o-acetyl ADP-ribose, nicotinic acid adenine dinucleotide phosphate are important signaling molecules. Some of these metabolites are reported in plant responses to environmental perturbations. In animal systems it has been shown that derivatives of PNs are involved in at least three important post-translational modifications of proteins - poly(ADP)-ribosylation, mono(ADP) ribosylation and o-acetylation. Role of these modifications in plants are gaining increasing attention. Levels of these PNs in plant cells show considerable changes during various developmental stages indicating these molecules can serve as metabolic read out of cell fate. Recent studies in animal systems show that the redox state of PNs can serve as global metabolic regulators of gene expression. Arabidopsis mutants with altered PN levels show massive changes in gene expression further supporting the animal studies. Our understanding of the role of PNs in plants is only beginning. A systems biology approach will facilitate a thorough understanding of the versatility of these labile redox molecules in plant health and sickness.

Cristina Alexandre, Lars Hennig (Switzerland) FLC-Independent Vernalization Responses (pp 202-211)

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Invited Review: Varieties of many plant species show a requirement for prolonged exposure to low temperatures in order to accelerate flowering, a process termed vernalization. In Arabidopsis, the early-flowering phenotype of vernalized plants results from the combined action of three MADS-domain proteins - FLC, AGL19 and AGL24, each assigned to an independent vernalization pathway. Both AGL19 and AGL24 function to promote flowering, and are activated during vernalization, while FLC acts to delay flowering and therefore is repressed by a vernalizing-treatment. One conspicuous attribute of vernalization is a delayed effect that is coupled to a cellular memory-mechanism. For both FLC and AGL19 pathways this cellular memory has been found to be based on epigenetic modifications. One model is that two distinct histone-modifying Polycomb repressive complexes - the VRN2- and EMF2-complexes - introduce repressive histone H3 lysine 27 trimethylation marks at specific locations in the FLC and AGL19 chromatin, respectively, leading to mitotically stable transcriptional repression. Vernalization acts differentially on each complex, and the coordinated action of both is necessary for a complete vernalization response. As homologs of the Arabidopsis vernalization genes are being identified in other species, it may soon be revealed whether the same mechanisms are shared by distinct plant groups. However, it is believed that vernalization responses evolved independently in different plant groups, and in grasses, epistatic interactions between two loci, VRN1 and VRN2 that are unrelated to the Arabidopsis VRN1 and VRN2 genes, mainly determine the vernalization requirement. Whether epigenetic mechanisms are also involved in the vernalization response outside Arabidopsis remains to be determined. Importantly, VRN1 in grasses encodes a MADS-domain protein. Thus, MADS-domain proteins play central roles in various vernalization pathways.

Wim Grunewald (Belgium), Boris Parizot (France), Dirk Inzé, Godelieve Gheysen, Tom Beeckman (Belgium) Developmental Biology of Roots: One Common Pathway for All Angiosperms? (pp 212-225)

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Invited Review: The primary root meristem is formed during embryogenesis and supports the first growth of the seedling into the soil. However, the continuous growth of the above-ground plant parts imposes the establishment of an elaborate root system in order to mine for additional water and nutrients. This can only be achieved by de novo installation of extra root meristems soon after germination. In general, two fundamentally different types of root systems can be found in the plant kingdom. The taproot system is characterized by an elongating primary root with developing lateral roots while in plants with a fibrous root system, the primary root is early on replaced by a plethora of shoot-borne roots. Recently our knowledge on the formation and patterning of the primary root meristem has improved seriously. From early embryogenesis onwards, a response maximum of the cell-fate instructive plant hormone auxin is formed that activates specific patterning genes and leads to the establishment of a root stem cell niche. During lateral and shoot-borne root formation, embryonic gene activities are employed again and as a consequence, these processes can be seen as a replay of embryonic pattern formation. Moreover, the onset of lateral and shoot-borne root formation from apparently fully differentiated cells is in sharp contrast with the embryogenic origin of the primary root and has been a point of controversy for many root biologists. In this review we will give an overview on the parallel mechanisms that might exist between embryonic and post-embryonic root development and will evaluate the potential existence of conserved molecular mechanisms between taproot versus fibrous root development.

Kristiina Himanen, Getnet Dino Adem, Mieke Van Lijsebettens (Belgium) Genetic and Epigenetic Control of Leaf Size and Shape (pp 226-238)

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Invited Review: Leaf shapes and sizes vary naturally from simple with smooth, serrated or lobed margins to compound with a few or a lot of leaflets. Simple leaves develop through gradients of cell division and cell expansion from tip to base, resulting in a fully differentiated mature leaf without meristematic activity, referred to as a determinate structure. Cell numbers and cell expansion influence leaf size and shape as observed by manipulation of the core cell cycle or cell wall extensibility. However, mechanisms exist that compensate changes in leaf growth by affecting cell expansion or number, which indicates that leaf size is also under supracellular control. Foliar morphology is used for classification in botany, demonstrating its genetic and evolutionary basis. A developmental biology approach is taken to identify the molecular control of leaf size and shape by using a limited number of model species. Mutational and transgene analysis in Arabidopsis thaliana has uncovered more than 100 loci important for simple leaf development. Regulatory genes, such as transcription factors, have been shown to regulate leaf growth and development, of which some might act upstream of the hormonal responses and core cell cycle. Chromatin modification complexes are involved in the control of leaf growth and might form the interface with developmental and environmental cues to influence leaf formation - a phenomenon known as leaf plasticity. In contrast to simple leaves, compound leaves develop discernable meristems that will form leaflets along the rachis. Molecular-genetic work in snapdragon (Antirrhinum majus), tomato (Solanum lycopersicum), and pea (Pisum sativum) identified independent molecular pathways for compound leaf development. These regulatory pathways have previously been shown to be important for meristem identity and suggest that compound leaves can be considered as transitional forms from determinate simple leaves to indeterminate shoots. The use of developmental genes for applications in agriculture, horticulture and ornamentals will be discussed.

Humberto F. Causin, Atilio J. Barneix (Argentina) The Role of Oxidative Metabolism in the Regulation of Leaf Senescence by the Light Environment (pp 239-244)

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Invited Mini-Review: Senescence is a genetically controlled process whose regulation depends on many factors. Leaf senescence begins with the catabolism of chlorophyll and macromolecules such as proteins and membrane lipids, a process that is accompanied by the alteration of organelles and, ultimately, the breakdown of nuclear components. In annual plants, reproduction is the main event that triggers leaf senescence. However, several environmental factors may initiate and/or modulate it both in the vegetative as well as the reproductive stages. An important exogenous factor is light, and both its senescence-inhibiting or -promoting qualities are described. Both the darkening of individual leaves as well as exposure to high light intensities can induce senescence, and it has been proposed that changes in the redox status of cells associated with the overproduction of reactive oxygen species (ROS) might be the basis of the senescence symptoms. In dense stands, basal leaves frequently senesce before anthesis, and there is evidence that this phenomenon is regulated by changes in light quantity and quality. While models about putative signalling pathways leading to either acclimation or to cell death involving the accumulation of ROS in response to excess light have been recently developed, less information is available with regards to the biochemistry of leaf senescence induced by changes in light spectral quality. In the present review we summarize a series of studies that have contributed to the view that changes in either light quantity or quality may modulate leaf senescence through signals derived from oxidative metabolism.

Anthony R. Gendall, John W. Forster (Australia) Genetics of Reproductive Development in Forage Legumes (pp 245-252)

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Invited Mini-Review: Temperate forage legumes include members of the Trifolium, Medicago and Lotus genera. Aspects of reproductive development such as flowering time and seed yield are important breeding objectives for commercial seed producers. The two most advanced model species for legume genetics and genomics Medicago truncatula (barrel medic), and Lotus japonicus are close relatives of cultivated species, and provide important sources of information for breeding improvement through translational genetics and genomics. Genetic control of floral initiation is poorly understood in forage species, but genes regulating flowering time and floral development have recently been identified in a number of model species, providing the basis for functional evaluation in forages. Some of these genes appear to be genuinely orthologous, exerting effects on the predicted processes when functionally tested in heterologous systems. Reproductive development genes are hence likely to be conserved between legumes and other higher plant species. Genetic variation for reproductive developmental traits has been assessed in Medicago, Lotus and Trifolium species, and molecular genetic marker-based linkage groups have been used to identify quantitative trait loci. The information presented in this review suggests that comparative genomics between forage and model legumes is now sufficiently developed to allow prediction of the candidate genes contributing to variation for important agronomic traits.

Rebecca Grumet (USA), Nurit L. Katzir (Israel), Holly A. Little (USA), Vitaly Portnoy, Yoseph Burger (Israel) New Insights into Reproductive Development in Melon (Cucumis melo L.) (pp 253-264)

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Invited Review: Melon, Cucumis melo L., is a high value crop prized throughout the world for sweet, flavorful, dessert-like fruits. Production of such fruits ultimately depends on a series of steps in reproductive development, flowering, fruit set, fruit growth, maturation, and ripening. New approaches, utilizing molecular, genetic, plant transformation, and genomics tools, have led to increased understanding of the underlying processes responsible for the various stages of reproductive development, and promise to further our understanding in the future. Recent studies have led to mapping, cloning, and identification of key genes associated with sex expression, and floral and fruit development; have identified transcriptional and enzymatic changes associated with various stages of development; and have furthered our understanding of the role of ethylene production and perception in floral sex determination and development, and fruit set, maturation and ripening. In this review we first examine melon flower development with an emphasis on sex expression and development of carpel-bearing flowers. We next discuss fruit development and ripening processes, including factors that contribute to production of high quality fruits such as sweetness, flavor, texture, and aroma.

Nuria Alburquerque, Jose Egea, Lorenzo Burgos (Spain) Apricot Floral Development: Keys for a Good Yield (pp 265-273)

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Invited Mini-Review: Different aspects related to floral development have a close link to fruit set failures in apricot and other fruit trees. In this work we review studies on flower bud development, pollen viability and germinability, ovule development and longevity, fertilization, as well as on molecular bases of self- and cross-(in)compatibility. A high flower buds drop and a scarce flower bud production of some apricot cultivars affect negatively fruit set. Climatic conditions during flowering influence pollen germination and pollen tubes growth. A delay in the ovule development at anthesis has been found as genotype-dependent in apricot and this trait has been related to low percentages of fertilisation. Also differences in fruit set observed between cultivars may be related to the ability of the ovules to develop and mature after anthesis. Abnormal embryo sacs or ovules have been observed in apricot at different stages of flower development, which is a direct cause of low fruit set. Different plant growth regulators can be useful to regulate different processes. The knowledge of the incompatibility phenotype for many apricot cultivars has allowed advising about the planting of single-cultivar orchards. The study of the inheritance of this and other traits in apricot and other fruit trees has allowed planning of hybridisations to minimise or eliminate the production of undesirable seedlings, increasing the efficiency of the breeding programmes. Furthermore, molecular studies and characterization of S-alleles in apricot have allowed the development of molecular markers for self-(in)compatibility, a successful tool for breeding. All together, these studies on the floral development in apricot have provided valuable information to help select the appropriate cultivars for producers and to avoid losses caused by an inadequate cultivar selection.

José Pissarra, Claudia Pereira, Diana Soares da Costa, Raquel Figueiredo, Patrícia Duarte, Jorge Teixeira, Susana Pereira (Portugal) From Flower to Seed Germination in Cynara cardunculus: A Role for Aspartic Proteinases (pp 274-281)

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Invited Mini-Review: Cardosins are plant aspartic proteinases (APs) isolated from the flowers of Cynara cardunculus L. (cardoon) and are responsible for their milk-clotting activity used to manufacture ewe’s cheese. Cardosin A is mainly accumulated in protein storage vacuoles of the stigmatic papillae being suggested a possible role in pollen-pistil interaction. Cardosin B has been localized to the extracellular matrix of stylar transmitting tissue and a role in the remodelling or degradation of pistil extracellular matrix, during pollen tube growth, has been suggested. Also, cardosin B localization is closely correlated with programmed cell death (PCD) events in the nucellus of C. cardunculus, suggesting involvement in ovule and embryo sac development. Therefore, both cardosin A and B may fulfil important roles during sexual reproduction of the plant. In seeds APs may participate in protein hydrolysis, but, as they are present since early seed maturation they may take part both in zymogen activation and localised PCD in seed tissues, having a crucial role in the regulation of protein degradation and embryo nourishing. In the embryo cardosin A precursor form is accumulated in protein bodies and cell walls, a different localization from the described in cardoon flowers, suggesting a tissue-dependent pattern of accumulation of the protein. Furthermore, APs Plant Specific Insert seems to have a preponderant role in membrane reorganisation events and during water uptake and solute leakage, which supports the recently proposed bifunctional role of the AP precursor molecule. In this review, we intend to characterise cardosins developmental regulation in organs of C. cardunculus from the flower to post embryonic development and explore the putative roles assigned to these APs.

Min Qin Hu, Yang Han, Chao Tian Xie, Hui Qiao Tian (China) Male Sterility of Chinese Cabbage, Wheat and Rice: Cell Biological Research on the Process of Anther Abortion (pp 282-286)

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Invited Review: Male sterility of higher plants is a valuable trait used to improve agricultural crops through hybridization. Pollen abortion is a complex and complicated process and the mechanisms involved are actively being studied. Recently, new data on the structure and function of the tapetal cell, changes in Ca²⁺ distribution, ATPase activity distribution and programmed cell death in anther cells were obtained using cell biology. These data have helped identify the process of pollen abortion and illuminated the mechanisms of male sterility of higher plants, and provide an important link between research on male sterility at the individual and molecular levels. This paper summarizes the recent data regarding the aborting process of male-sterile anthers of Chinese cabbage, wheat and rice obtained using cell biology.

Li Li Ge, Chao Tian Xie, Dong Mei Wei, Yi Lan Qiu, Hui Qiao Tian (China) Calcium Distribution during Anther Development of Tobacco (pp 287-293)

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Original Research Paper: Calcium distribution during anther development of tobacco was investigated using potassium antimonite technique. Before the appearance of microspore mother cells, no calcium-induced precipitates ware found in sporogenous cells and somatic cells of the anther wall. Prior to the onset of microsporogenesis, abundant minute calcium precipitates appeared in the callose wall. At this stage, there were also many precipitates that accumulated in the vacuoles of the tapetum. However, fewer calcium precipitates appeared in young microspores. With microspore development many calcium precipitates accumulated in the germ pore, and then appeared in the small vacuoles of the cytoplasm. At a later microspore stage, a large vacuole formed and some precipitates were observed in the vacuole. The number of precipitates in tapetal cells decreased as microsporogenesis proceeded. After microgametogenesis began, calcium precipitates in the cytoplasm of pollen grains decreased. As pollen grains accumulated starch, a fewer were evident in the cytoplasm. These results displayed the spatial and temporal features of calcium distribution during anther development of Nicotiana tabacum L., suggesting that it may be related with microspore development.

H. Raja Naika, V. Krishna (India) Plant Regeneration from Leaf Explants of Clematis gouriana Roxb. - An Endemic Medicinal Plant of Western Ghats (pp 294-297)

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Original Research Paper: Studies were undertaken to evaluate the regenerative capacity of leaf explants of Clematis gouriana Roxb. The leaf explants were excised from healthy plants and cultured on Murashige and Skoog medium (MS medium) variously supplemented with growth regulators. Regeneration occurred via organogenesis on MS medium supplemented with 3.5 to 5.5 mg l^-1 6-furfuryl aminopurine (FAP), 0.2 to 0.6 mg l^-1 indole-3-butyric acid (IBA) and 0.2-0.6 mg l^-1 thidiazuron (TDZ). The frequency of shoot bud production was the highest (mean of 10.90 ± 0.27 shoots per explants) at 4.5 mg l^-1, FAP 0.4 mg l^-1 IBA and 0.4 mg l^-1 TDZ. Regenerated shoots formed complete plantlets on medium containing 0.1 to 0.5 mg l^-1 IBA as well as on MS basal medium without growth regulators. Matured plants were established, acclimatized, and thrived in greenhouse conditions and grew normally without any morphological variation. The regeneration protocol developed in this study provides a basis for germplasm conservation and for further investigation of medicinally active constituents of C. gouriana, a medicinal plant of Western Ghats.