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PubMed2004

Erosion and landscape development affect plant nutrient status in the Hawaiian Islands.

We quantified variation in plant nutrient concentrations and provenance along catenas in landscapes of three different ages (0.15, 1.4, and 4.1 ma) in the Hawaiian Islands. Strontium (Sr) isotopes demonstrate that erosion provides a renewed source of rock-derived nutrients to slopes in landscapes of all ages, in some cases reversing a million years of ecosystem development in a distance of 100 m. However the effects of this input vary with landscape age. Plants on uneroded surfaces in a 0.15-ma landscape derive approximately 20% of their Sr from local bedrock (foliar 87Sr/86Sr approximately 0.7085), while on adjacent slopes this increases to approximately 80% (foliar 87Sr/86Sr approximately 0.7045). Despite this shift in provenance, foliar N and P do not vary systematically with slope position. Conversely, eroded slopes in a 4.1-ma landscape show smaller increases in rock-derived cations relative to stable uplands (foliar 87Sr/86Sr approximately 0.7075 vs 0.7090), but have >50% higher foliar N and P. These results demonstrate both that erosion can greatly increase nutrient availability in older landscapes, and that the ecological effects of erosion vary with landscape age. In addition, there can be as much biogeochemical variation on fine spatial scales in eroding landscapes as there is across millions of years of ecosystem development on stable surfaces.

Geological Phenomena↗
PubMed2005

Characterization of the Arabidopsis clb6 mutant illustrates the importance of posttranscriptional regulation of the methyl-D-erythritol 4-phosphate pathway.

The biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate, the two building blocks for isoprenoid biosynthesis, occurs by two independent pathways in plants. The mevalonic pathway operates in the cytoplasm, and the methyl-d-erythritol 4-phosphate (MEP) pathway operates in plastids. Plastidic isoprenoids play essential roles in plant growth and development. Plants must regulate the biosynthesis of isoprenoids to fulfill metabolic requirements in specific tissues and developmental conditions. The regulatory events that modulate the plant MEP pathway are not well understood. In this article, we demonstrate that the CHLOROPLAST BIOGENESIS6 (CLB6) gene, previously shown to be required for chloroplast development, encodes 1-hydroxy-2-methyl-butenyl 4-diphosphate reductase, the last-acting enzyme of the MEP pathway. Comparative analysis of the expression levels of all MEP pathway gene transcripts and proteins in the clb6-1 mutant background revealed that posttranscriptional control modulates the levels of different proteins in this central pathway. Posttranscriptional regulation was also found during seedling development and during fosmidomycin inhibition of the pathway. Our results show that the first enzyme of the pathway, 1-deoxy-d-xylulose 5-phosphate synthase, is feedback regulated in response to the interruption of the flow of metabolites through the MEP pathway.

Amino Acid Sequence↗
PubMed2003

Nitrate-specific and cytokinin-mediated nitrogen signaling pathways in plants.

Inorganic nitrogen is a limiting factor in plant growth and development. Plants constantly sense changes in nitrogen availability and respond appropriately by modulating gene expression. Plants employ multiple routes for the long-distance signaling and communication of nitrogen status. One of these depends on nitrate itself (nitrate-specific signaling), while another uses cytokinin as a messenger. Recent studies suggest that nitrate-specific signaling functions predominantly in the context of the synthesis of amino acids and nucleic acids. This pathway includes the control of the expression of a wide variety of genes. On the other hand, cytokinin-mediated signaling is related mainly to the control of nitrogen partitioning and development. Nitrogen-dependent cytokinin accumulation and the involvement of His-Asp phosphorelay systems are characteristic of this pathway. The coordination of both regulatory pathways seems to be crucially important for the integration of nitrogen signals at the whole plant level.

Cytokinins↗
PubMed2002

WUSCHEL signaling functions in interregional communication during Arabidopsis ovule development.

Coordinating the behaviors of different cell populations is essential for multicellular development. One important example for this can be found in ovule development in higher plants. Ovules give rise to the gametophyte in the distal nucellus and form protective sporophytic organs from the underlying chalaza. We show that the WUSCHEL (WUS) homeobox gene provides a mechanism to coordinate these events. WUS is expressed in the nucellus and our loss- and gain-of-function analyses show that WUS is not only necessary but also sufficient for integument formation from the chalaza. WUS protein is retained in the nucellus, indicating that WUS activity in the nucellus generates a downstream signal that non-cell-autonomously regulates integument initiation in the chalaza. This signal appears to act locally, thus determining the position of organ formation from chalazal cells adjacent to the nucellus. Analysis of WUS and AINTEGUMENTA functions indicates that integument initiation requires inputs from different ovule regions. Together with previous findings for shoot and floral meristems, where WUS signaling establishes a stem cell niche, our results indicate that WUS defines a signaling mechanism that is used repeatedly during plant development in coordinating the behavior of adjacent cell groups.

Arabidopsis↗
PubMed1988

Evidence for involvement of phytochrome in tumor development on plants.

The regulation of nonpathogenic tumorous growths on tomato plants by red and far-red radiation was studied using leaf discs floated on water and irradiated from beneath. It was found that red light (600-700 nanometers) was required for the induction of tumors on tomato (Lycopersicon hirsutum Humb. & Bonpl. Plant Introduction LA 1625), while both blue (400-500 nanometers) and green (500-600 nanometers) light had little effect on tumor development. Detailed studies with red light demonstrated that tumor development increased with increasing photon flux and duration, though duration was the more significant factor. It was observed that tumor development could be prevented by the addition of far-red irradiance to red irradiance or by providing far-red irradiance immediately following red irradiance. The effectiveness of red and far-red irradiance in the regulation of tumor development indicates phytochrome involvement in this response. These findings should provide additional insight into the multiplicity of physiological factors regulating the development of nonpathogenic tumorous growths in plants.

Dose-Response Relationship, Radiation↗
PubMed2006

New nodulation mutants responsible for infection thread development in Lotus japonicus.

Legume plants develop specialized root organs, the nodules, through a symbiotic interaction with rhizobia. The developmental process of nodulation is triggered by the bacterial microsymbiont but regulated systemically by the host legume plants. Using ethylmethane sulfonate mutagenesis as a tool to identify plant genes involved in symbiotic nodule development, we have isolated and analyzed five nodulation mutants, Ljsym74-3, Ljsym79-2, Ljsym79-3, Ljsym80, and Ljsym82, from the model legume Lotus japonicus. These mutants are defective in developing functional nodules and exhibit nitrogen starvation symptoms after inoculation with Mesorhizobium loti. Detailed observation revealed that infection thread development was aborted in these mutants and the nodules formed were devoid of infected cells. Mapping and complementation tests showed that Ljsym74-3, and Ljsym79-2 and Ljsym79-3, were allelic with reported mutants of L. japonicus, alb1 and crinkle, respectively. The Ljsym82 mutant is unique among the mutants because the infection thread was aborted early in its development. Ljsym74-3 and Ljsym80 were characterized as mutants with thick infection threads in short root hairs. Map-based cloning and molecular characterization of these genes will help us understand the genetic mechanism of infection thread development in L. japonicus.

Flowers↗
PubMed1986

Stability of the determined state.

During development parts of plants become determined for specific fates which can persist in populations of dividing cells. Students of plant development have tended to favour the view that the determined state is stabilized at the supracellular level. We provide evidence for the alternative view that determination can be inherited at the cellular level. This conclusion is based on the observation that cultured cells derived from the leaf lamina and cortex of the stem of tobacco plants inherit different states of cytokinin requirement. Plant regeneration experiments show that the cytokinin-requiring (C-) state characteristic of leaf cells and the cytokinin autotrophic (C+) state characteristic of cortex cells are stable but not permanent. Progeny of both cell types can give rise to complete plants with tissues exhibiting the cytokinin requirement of comparable tissues of seed grown plants. Pith cells can shift from the C- to the C+ state by a process known as habituation. Evidence is presented that this epigenetic change is stabilized by a positive-feedback mechanism in which cytokinins appear to promote their own biosynthesis. A single dominant, Mendelian gene, H1-2, controls expression of the C- and C+ phenotypes. There are hints that this gene changes state in cultured cells at rates expected for epigenetic modifications. This suggests that the C- and C+ state can also be stabilized by genetic mechanisms.

Cytokinins↗
PubMed2000

Developmental expression of the Arabidopsis thaliana CycA2;1 gene.

The associations of cyclins with highly conserved cyclin-dependent kinases are key events in the regulation of cell cycle progression. The spatio-temporal expression of an Arabidopsis thaliana (L.) Heynh. mitotic cyclin, Arath;CycA2;1, was studied by histochemical beta-glucuronidase (GUS) analysis and in-situ hybridizations. The CycA2,1] promoter was active in the egg apparatus before fertilization. During embryogenesis, CycA2;1:gus expression was found in the embryo and the developing endosperm. Throughout plant development, CycA2;1 transcripts were found in both dividing and non-dividing cells, indicating that the expression of this cyclin is not a limiting factor for cell division. In the pericycle and stelar parenchyma, CycA2;1 transcripts were located at the xylem poles, a position that can be correlated with competence for lateral root formation. In addition, CycA2;1:gus expression was upregulated in roots by auxins and in the shoot apex by cytokinins. Transcription of CycA2;1 was shown by reverse transcription-polymerase chain reaction to be strongly induced by sucrose in A. thaliana cell suspensions.

Arabidopsis↗
PubMed2004

Molecular physiological analysis of the two plastidic ATP/ADP transporters from Arabidopsis.

Arabidopsis (Arabidopsis thaliana) possesses two isoforms of plastidic ATP/ADP transporters (AtNTT1 and AtNTT2) exhibiting similar biochemical properties. To analyze the function of both isoforms on the molecular level, we examined the expression pattern of both genes by northern-blot analysis and promoter-beta-glucuronidase fusions. AtNTT1 represents a sugar-induced gene mainly expressed in stem and roots, whereas AtNTT2 is expressed in several Arabidopsis tissues with highest accumulation in developing roots and young cotyledons. Developing lipid-storing seeds hardly contained AtNTT1 or -2 transcripts. The absence of a functional AtNTT1 gene affected plant development only slightly, whereas AtNTT2T-DNA, AtNTT1-2T-DNA, and RNA interference (RNAi) plants showed retarded plant development, mainly characterized by a reduced ability to generate primary roots and a delayed chlorophyll accumulation in seedlings. Electron microscopic examination of chloroplast substructure also revealed an impaired formation of thylakoids in RNAi seedlings. Moreover, RNAi- and AtNTT1-2T-DNA plants showed reduced accumulation of the nuclear-encoded protein CP24 during deetiolation. Under short-day conditions reduced plastidic ATP import capacity correlates with a substantially reduced plant growth rate. This effect is absent under long-day conditions, strikingly indicating that nocturnal ATP import into chloroplasts is important. Plastidic ATP/ADP transport activity exerts significant control on lipid synthesis in developing Arabidopsis seeds. In total we made the surprising observation that plastidic ATP/ADP transport activity is not required to pass through the complete plant life cycle. However, plastidic ATP/ADP-transporter activity is required for both an undisturbed development of young tissues and a controlled cellular metabolism in mature leaves.

Arabidopsis↗
PubMed2001

Actin and actin-binding proteins in higher plants.

The actin cytoskeleton is a complex and dynamic structure that participates in diverse cellular events which contribute to plant morphogenesis and development. Plant actins and associated actin-binding proteins are encoded by large, differentially expressed gene families. The complexity of these gene families is thought to have been conserved to maintain a pool of protein isovariants with unique properties, thus providing a mechanistic basis for the observed diversity of plant actin functions. Plants contain actin-binding proteins which regulate the supramolecular organization and function of the actin cytoskeleton, including monomer-binding proteins (profilin), severing and dynamizing proteins (ADF/cofilin), and side-binding proteins (fimbrin, 135-ABP/villin, 115-ABP). Although significant progress in documenting the biochemical activities of many of these classes of proteins has been made, the precise roles of actin-binding proteins in vivo awaits clarification by detailed mutational analyses.

Actins↗
PubMed2000

Nitric oxide in plants. To NO or not to NO.

The current knowledge on the occurrence and activity of NO in plants is reviewed. The multiplicity of nitrogen monoxide species and implications for differentiated reactivity are indicated. Possible sources of NO are evaluated, and the evidence for the presence of nitric oxide synthase in plants is summarised. The regulatory role of NO. in plant development and in plant interactions with microorganisms, involving an interplay with other molecules, like ethylene or reactive oxygen species is demonstrated. Finally, some other suggestions on potential functions of NO. in plants are indicated.

Animals↗
PubMed2000

Promoter analysis of pyk20, a gene from Arabidopsis thaliana.

The gene pyk20 which has been isolated from Arabidopsis thaliana encodes a protein with a glutamine-rich domain in the C-terminal region. The transcription of this gene was shown to be induced in feeding sites of root-parasitic nematodes (Heterodera schachtii), in roots infected by a fungus-like organism (Plasmodiophora brassicae), by plant hormone treatment, and by wounding. In order to identify functional promoter regions seven different 5' and 3' pyk20 promoter (ppyk20) deletion fragments were fused to the uidA gene (gus) and transformed into A. thaliana plants. Histochemical analysis of plants containing the different ppyk20::uidA reporter constructs was performed during plant development in different plant tissues. Comparison of the promoter deletion constructs showed that the region between -277 and -1 bp is necessary to enhance the level of the GUS expression in nematode feeding sites and by plant hormone treatment. The region between -1912 and -278 is essential to provide specificity of GUS expression. Conserved regulatory elements were identified in the ppyk20 by sequence analysis. The activation pattern of ppyk20 makes it well suited to engineer resistance against nematodes and other pathogens.

Journal Article↗
PubMed2004

A family of auxin-conjugate hydrolases that contributes to free indole-3-acetic acid levels during Arabidopsis germination.

Auxins are hormones important for numerous processes throughout plant growth and development. Plants use several mechanisms to regulate levels of the auxin indole-3-acetic acid (IAA), including the formation and hydrolysis of amide-linked conjugates that act as storage or inactivation forms of the hormone. Certain members of an Arabidopsis amidohydrolase family hydrolyze these conjugates to free IAA in vitro. We examined amidohydrolase gene expression using northern and promoter-beta-glucuronidase analyses and found overlapping but distinct patterns of expression. To examine the in vivo importance of auxin-conjugate hydrolysis, we generated a triple hydrolase mutant, ilr1 iar3 ill2, which is deficient in three of these hydrolases. We compared root and hypocotyl growth of the single, double, and triple hydrolase mutants on IAA-Ala, IAA-Leu, and IAA-Phe. The hydrolase mutant phenotypic profiles on different conjugates reveal the in vivo activities and relative importance of ILR1, IAR3, and ILL2 in IAA-conjugate hydrolysis. In addition to defective responses to exogenous conjugates, ilr1 iar3 ill2 roots are slightly less responsive to exogenous IAA. The triple mutant also has a shorter hypocotyl and fewer lateral roots than wild type on unsupplemented medium. As suggested by the mutant phenotypes, ilr1 iar3 ill2 imbibed seeds and seedlings have lower IAA levels than wild type and accumulate IAA-Ala and IAA-Leu, conjugates that are substrates of the absent hydrolases. These results indicate that amidohydrolases contribute free IAA to the auxin pool during germination in Arabidopsis.

Amidohydrolases↗