Sung Soo Whang

Age-Dependent Action of an ABA-Inducible Receptor Kinase, RPK1, as a Positive Regulator of Senescence in Arabidopsis Leaves

Leaf senescence, which constitutes the final stage of leaf development, involves programmed cell death and is intricately regulated by various internal and environmental signals that are incorporated with age-related information. ABA plays diverse and important physiological roles in plants, and is involved in various developmental events and stress responses. ABA has long been regarded as a positive regulator of leaf senescence. However, the cellular mediators of ABA-induced senescence have not been identified. We sought to understand the ABA-induced senescence signaling process in Arabidopsis by examining the function of an ABA- and age-induced gene, RPK1, which encodes a membrane-bound, leucine-rich repeat-containing receptor kinase (receptor protein kinase 1). Loss-of-function mutants in RPK1 were significantly delayed in age-dependent senescence. Furthermore, rpk1 mutants exhibited reduced sensitivity to ABA-induced senescence but little change to jasmonic acid- or ethylene-induced senescence. RPK1 thus mediates ABA-induced leaf senescence as well as age-induced leaf senescence. Conditional overexpression of RPK1 at the mature stage clearly accelerated senescence and cell death, whereas induction of RPK1 at an early developmental stage retarded growth without triggering senescence symptoms. Therefore, RPK1 plays different roles at different stages of development. Consistently, exogenously applied ABA affected leaf senescence in old leaves but not in young leaves. The results, together, showed that membrane-bound RPK1 functions in ABA-dependent leaf senescence. Furthermore, the effect of ABA and ABA-inducible RPK1 on leaf senescence is dependent on the age of the plant, which in part explains the mechanism of functional diversification of ABA action.

Variation of silica bodies in leaf epidermal long cells within and among seventeen species of Oryza (Poaceae).

Morphometric procedures were used with scanning electron microscopy backscattered images to study silica bodies in epidermal long cells of four different leaf veins of 17 of the 20 species of Oryza. The veins studied were midrib, large vein, small vein, and marginal vein. Image analysis was used to study morphological variations among the silica bodies. Statistical analyses were based on 11 variables. Even within a single leaf, silica bodies were not uniform. However, the degree of morphological variation normally showed a distribution of morpholgical types around one modal shape. The most significant differences observed were between silica bodies of the midrib and those of other veins. Bodies varied with respect to both size and shape. Computer-assisted image analysis is an effective tool for categorizing basic data and for statistical analysis of variation among silica bodies. Morphological variation among silica bodies of a single leaf may be related to water-conducting systems and their influence on silica availability and phytolith formation.

Agathis (Araucariaceae) macrofossils from Cainozoic sediments in south-eastern Australia

Organically preserved Cainozoic leaf fossils previously referred to Agathis are re-examined, and in all cases their affinity with that genus is confirmed. Previously undescribed organically preserved leaf fossils from several Cainozoic sites in south-eastern Australia are compared with Agathis and Wollemia and two new species of Agathis are described. Intraspecific variation in leaf cuticle morphology is examined in extant A. macrophylla in particular, and is found to be much higher than previously recorded. This makes assignment of fossil Agathis leaves to species difficult, especially when only leaf fragments are available. The new fossils extend the record of organically preserved Agathis macro-remains back to the Late Paleocene, but do not significantly extend the known spatial distribution.

Presence of a mannoprotein, MnpAp, in the hyphal cell wall of Aspergillus nidulans

The presence of a mannoprotein, MnpAp, in the hyphal cell wall of Aspergillus nidulans was examined by immunogold electron microscopy using a mnpA-null mutant as a negative control. The hyphal cell wall of wild type consisted of two layers—an electron-dense smooth outer layer and an electron-translucent inner layer—while the hyphal cell wall of the mnpA-null mutant had an electron-dense irregular outer layer together with the electron-translucent inner layer. In wild type, MnpAp was present throughout the electron-translucent layer of the hyphal cell wall but was absent from the conidial cell wall. In the mnpA-null mutant, MnpAp was absent from the cell walls of both cell types. These results indicate that MnpAp is present in the hyphal cell wall and that it influences cell wall surface structure.

Dacrycarpus (Podocarpaceae) macrofossils from Miocene sediments at Elands, eastern Australia

Vegetative remains and male cones containing pollen grains from Miocene sediments at Elands in northern New South Wales are assigned to the new Podocarpaceae species Dacrycarpus elandensis. This species represents the most complete and best preserved Dacrycarpus macrofossil remains known. These fossils are morphologically most similar to extant D. imbricatus and fossil D. latrobensis and D. linifolius, but the phylogenetic significance of many of the characters used to make this comparison is unknown. Dacrycarpus is now extinct in Australia, but D. elandensis confirms a presence for it in eastern Australia at a time when rainforest was drying and retracting and Eucalyptus was expanding its range in the region.

Identification of Differentially Expressed Genes in Flower Buds of Calanthe discolor and C. sieboldii

The genus Calanthe includes species of terrestrial orchids that produce attractive flowers with diverse floral traits. Breeding programs have been established to improve the horticultural value of various Calanthe species, but studies to identify the genetic components contributing to the key phenotypic characteristics have not been undertaken. To understand the molecular mechanisms underlying floral development associated with floral morphology, color, and fragrance production, the flower buds of two typical Korean Calanthe species, C. discolor and C. sieboldii, were subjected to gene expression analysis by differential display RT-PCR (DDRT-PCR). A total of 66 non-redundant differentially expressed genes (DEGs) were isolated and sequenced. Of these, 26 and 40 DEGs were found to be highly expressed in C. discolor and C. sieboldii, respectively. Moreover, the expression patterns of a subset of genes presumably implicated in signal transduction, metabolic pathways, and hormonal signaling differed between the two species. The data presented here may improve our understanding of the mechanisms underlying floral development and contribute to advances in orchid biotechnology.

Late Palaeocene Cupressaceae Macrofossils at Lake Bungarby, New South Wales

Four species of conifer macrofossils are described from the Late Palaeocene Lake Bungarby sediments in southern New South Wales. Two are assigned to the Cupressaceae and are considered to represent new species of Libocedrus, L. acutifolius and L. obtusifolius, although it is recognised that both share some characters with Austrocedrus. Two other fossils are considered to have less certain affinities with the Cupressaceae, although both have cuticular micromorphology that contains synapomorphies for that family. Both are assigned to new genera. Bungarbia linifolius has entire, petiolate, univeined leaves that resemble Metasequoia leaves, although the cuticular morphology is very distinct from that genus. Unfortunately, the phyllotaxis of Bungarbia is unknown. Monarophyllum has leaves in apparently opposite pairs, with the leaves bilaterally flattened. This combination of phyllotaxis and leaf flattening is currently unknown in the Cupressaceae.

Confocal Microscopy Study of Arabidopsis Embryogenesis Using GFP:mTn

Embryogenesis in transgenic Arabidopsis plants with GFP:mTn, a chimeric fusion of soluble shifted green fluorescent protein and a mouse actin binding domain, was studied. Confocal laser scanning microscopy was used to determine patterns of formation and cellular responses during asymmetric cell division. Before such cells divide, the nucleus moves to the position where new cell walls are to be formed. The apical–basal axis of the embryo develops mainly at the zygote to octant stage, and these events are associated with asymmetric divisions of the zygote and hypophyseal cells. Formation of the radial axis is established from the dermatogen to the globular-stage embryo via tangential cell division within the upper tiers. Bilateral symmetry of the embryo primarily happens at the triangular stage through zig-zag cell divisions of initials of the cotyledonary primordia. All stages of embryogenesis are described in detail here.

Comparison of Cell Wall Ultrastructures of Aspergillus nidulans in Presence and Absence of a MnpAp Mannoprotein

Abstract The ultrastructure of Aspergillus nidulans cell wall in relation to a mannoprotein was studied by scanning and transmission electron microscopy. An mnpAp null‐mutant, DMPV1, was used as a negative control of a wild type VER7. To analyze whether the mannoprotein in the cell wall during the development of an mnpAp null‐mutant is present or not, immunogold microscopy was also adopted. The surface sculpturing of various cell types ‐ hyphae, conidium, Hülle cell, and ascospore ‐ were not very different between the wild type and the mnpAp‐null mutant (DMPV1) as examined by scanning electron microscopy. These results were comparable to those examined by transmission electron microscopy, in that the hyphal cell wall was not indentical between two strains, probably caused by the MnpA protein (MnpAp). MnpAp was absent in both the hyphal cell wall of the DMPV1 strain and the conidial cell wall of a wide type, but clearly recognized in the hyphal cell wall of a wild type.