Valerie M. Sponsel

Gibberellin Biosynthesis and Inactivation

The gibberellins (GAs1) are defined by chemical structure. Naturallyoccurring tetracyclic diterpenoid acids with structures based on the entgibberellane carbon skeleton (Fig. 1) are assigned gibberellin “A numbers” in chronological order of their identification (45) (http://www.planthormones.info/gibberellin_nomenclature.htm). At the present time there are 136 fully characterized GAs, designated gibberellin A1 (GA1) through GA136, that have been identified from 128 different species of vascular plants, and also from seven bacteria and seven fungi (44) (http://www.planthormones.info/ga1info.htm).

The major antennal chemosensory protein of red imported fire ant workers

Some chemosensory proteins (CSPs) are expressed in insect sensory appendages and are thought to be involved in chemical signalling by ants. We identified 14 unique CSP sequences in expressed sequence tag (EST) libraries of the red imported fire ant, Solenopsis invicta. One member of this group (Si‐CSP1) is highly expressed in worker antennae, suggesting an olfactory function. A shotgun proteomic analysis of antennal proteins confirmed the high level of Si‐CSP1 expression, and also showed expression of another CSP and two odorant‐binding proteins (OBPs). We cloned and expressed the coding sequence for Si‐CSP1. We used cyclodextrins as solubilizers to investigate ligand binding. Fire ant cuticular lipids strongly inhibited Si‐CSP1 binding to the fluorescent dye N‐phenyl‐naphthylamine, suggesting cuticular substances are ligands for Si‐CSP1. Analysis of the cuticular lipids showed that the endogenous ligands of Si‐CSP1 are not cuticular hydrocarbons.

A Century of Gibberellin Research.

Gibberellin research has its origins in Japan in the 19th century, when a disease of rice was shown to be due to a fungal infection. The symptoms of the disease including overgrowth of the seedling and sterility were later shown to be due to secretions of the fungus Gibberella fujikuroi (now reclassified as Fusarium fujikuroi), from which the name gibberellin was derived for the active component. The profound effect of gibberellins on plant growth and development, particularly growth recovery in dwarf mutants and induction of bolting and flowering in some rosette species, prompted speculation that these fungal metabolites were endogenous plant growth regulators and this was confirmed by chemical characterisation in the late 1950s. Gibberellins are now known to be present in vascular plants, and some fungal and bacterial species. The biosynthesis of gibberellins in plants and the fungus has been largely resolved in terms of the pathways, enzymes, genes and their regulation. The proposal that gibberellins act in plants by removing growth limitation was confirmed by the demonstration that they induce the degradation of the growth-inhibiting DELLA proteins. The mechanism by which this is achieved was clarified by the identification of the gibberellin receptor from rice in 2005. Current research on gibberellin action is focussed particularly on the function of DELLA proteins as regulators of gene expression. This review traces the history of gibberellin research with emphasis on the early discoveries that enabled the more recent advances in this field.

Expression of gibberellin 20-oxidase1 (AtGA20ox1) in Arabidopsis seedlings with altered auxin status is regulated at multiple levels

Bioactive gibberellins (GAs) affect many biological processes including germination, stem growth, transition to flowering, and fruit development. The location, timing, and level of bioactive GA are finely tuned to ensure that optimal growth and development occur. The balance between GA biosynthesis and deactivation is controlled by external factors such as light and by internal factors that include auxin. The role of auxin transport inhibitors (ATIs) and auxins on GA homeostasis in intact light-grown Arabidopsis thaliana (L.) Heynh. seedlings was investigated. Two ATIs, 1-N-naphthylthalamic acid (NPA) and 1-naphthoxyacetic acid (NOA) caused elevated expression of the GA biosynthetic enzyme AtGA20-oxidase1 (AtGA20ox1) in shoot but not in root tissues, and only at certain developmental stages. It was investigated whether enhanced AtGA20ox1 gene expression was a consequence of altered flow through the GA biosynthetic pathway, or was due to impaired GA signalling that can lead to enhanced AtGA20ox1 expression and accumulation of a DELLA protein, Repressor of ga1-3 (RGA). Both ATIs promoted accumulation of GFP-fused RGA in shoots and roots, and this increase was counteracted by the application of GA4. These results suggest that in ATI-treated seedlings the impediment to DELLA protein degradation may be a deficiency of bioactive GA at sites of GA response. It is proposed that the four different levels of AtGA20ox1 regulation observed here are imposed in a strict hierarchy: spatial (organ-, tissue-, cell-specific) > developmental > metabolic > auxin regulation. Thus results show that, in intact auxin- and auxin transport inhibitor-treated light-grown Arabidopsis seedlings, three other levels of regulation supersede the effects of auxin on AtGA20ox1.

The Deoxyxylulose Phosphate Pathway for the Biosynthesis of Plastidic Isoprenoids: Early Days in Our Understanding of the Early Stages of Gibberellin Biosynthesis

The identification of a novel pathway for isopentenyl diphosphate synthesis by Rohmer, Arigoni and colleagues in the early 1990s has led to a reappraisal of terpenoid biosynthesis in many organisms. It is now apparent that in plants there are two biosynthetic routes to isopentenyl diphosphate-the classical mevalonate pathway in the cytosol and the deoxyxylulose phosphate pathway in plastids. Sesquiterpenoids and sterols are predominantly synthesized in the cytosol by the mevalonate pathway whereas monoterpenoids, diterpenoids, the phytol side-chain of chlorophyll, carotenoids, and the nonaprenyl side-chain of plastoquinone-9 are synthesized within plastids by the deoxyxylulose phosphate pathway. Our assumptions that the early stages of gibberellin biosynthesis are plastid-localized has led to several attempts to demonstrate that the deoxyxylulose phosphate pathway is the biosynthetic route to gibberellins. Although definitive evidence is still not available there is a growing body of evidence, mostly from transgenic plants and from the use of the inhibitor, fosmidomycin, that gibberellins are synthesized from deoxyxylulose phosphate-derived isopentenyl diphosphate. However, there is evidence that a small amount of cross-talk between the two pathways may occur, implying that the pathways are not totally autonomous. Implications for the regulation of the early stages of gibberellin biosynthesis are discussed.

Systematic identification of functional modules and cis-regulatory elements in Arabidopsis thaliana

BackgroundSeveral large-scale gene co-expression networks have been constructed successfully for predicting gene functional modules and cis-regulatory elements in Arabidopsis (Arabidopsis thaliana). However, these networks are usually constructed and analyzed in an ad hoc manner. In this study, we propose a completely parameter-free and systematic method for constructing gene co-expression networks and predicting functional modules as well as cis-regulatory elements.ResultsOur novel method consists of an automated network construction algorithm, a parameter-free procedure to predict functional modules, and a strategy for finding known cis-regulatory elements that is suitable for consensus scanning without prior knowledge of the allowed extent of degeneracy of the motif. We apply the method to study a large collection of gene expression microarray data in Arabidopsis. We estimate that our co-expression network has ~94% of accuracy, and has topological properties similar to other biological networks, such as being scale-free and having a high clustering coefficient. Remarkably, among the ~300 predicted modules whose sizes are at least 20, 88% have at least one significantly enriched functions, including a few extremely significant ones (ribosome, p < 1E-300, photosynthetic membrane, p < 1.3E-137, proteasome complex, p < 5.9E-126). In addition, we are able to predict cis-regulatory elements for 66.7% of the modules, and the association between the enriched cis-regulatory elements and the enriched functional terms can often be confirmed by the literature. Overall, our results are much more significant than those reported by several previous studies on similar data sets. Finally, we utilize the co-expression network to dissect the promoters of 19 Arabidopsis genes involved in the metabolism and signaling of the important plant hormone gibberellin, and achieved promising results that reveal interesting insight into the biosynthesis and signaling of gibberellin.ConclusionsThe results show that our method is highly effective in finding functional modules from real microarray data. Our application on Arabidopsis leads to the discovery of the largest number of annotated Arabidopsis functional modules in the literature. Given the high statistical significance of functional enrichment and the agreement between cis-regulatory and functional annotations, we believe our Arabidopsis gene modules can be used to predict the functions of unknown genes in Arabidopsis, and to understand the regulatory mechanisms of many genes.

Soil-Strength Enhancements from Polymer-Infused Roots

AbstractThis research investigates the use of polymer-infused roots for soil-improvement applications. By infusing polymer through the easily accessible above-surface plant stems, polymer-infused roots can be created without subsurface excavations. Evaluation of this technique involves identifying the improvements from polymer-infused roots by measuring in situ shear strength of soil using a vane shear apparatus and by measuring the tensile strength using split-tension tests in the laboratory. Roots of Ruellia squarrosa and Artemisia annua plants were infused with a mixture of epoxy resin and polyoxyalkylamine blend hardener. Compared with noninfused roots, polymer-infused roots provided an additional 22 kPa (28%) of shear strength for elastic silt and an additional 13.1 kPa (25%) of shear strength for low-plasticity clay with a corresponding 13.6-kPa (55%) increase in tensile strength for the low-plasticity clay. Acid hydrolysis testing was performed to ascertain the potential durability of the polymer-i...

Strength enhancement of plant roots through polymer infusions

By infusing a thermoset epoxy polymer into the root system of a plant, we aim to create an in situ plant-polymer composite material of high tensile strength resistant to biodegradation and suitable for soil stabilization applications. Microscopy imaging, volume and mass measurements, thermogravimetric analysis, and tensile testing were conducted to characterize the effects of infusing polymer into the plant system. The polymer appears to travel predominantly through xylem vessels of the plant system during infusions. Maximum tensile strength and modulus of elasticity are increased by 107% and 92%, respectively, with a polymer content of 59% by mass after infusions were conducted.

Erratum: The deoxyxylulose phosphate pathway for the biosynthesis of plastidic isoprenoids: Early days in our understanding of the early stages of gibberellin biosynthesis (Journal of Plant Growth Regulation (2002) 20:4 (332-345))

In the article ‘‘The Deoxyxylulose Phosphate Pathway for the Biosynthesis of Plastidic Isoprenoids: Early Days in Our Understanding of the Early Stages of Gibberellin Biosynthesis’’ by Valerie M. Sponsel (published in Journal of Plant Growth Regulation, Volume 20, Number 4, 2002, pp 332–345), the following errors were introduced as a result of a file transfer error during the conversion of the online to the print version of the article. Springer-Verlag deeply regrets the errors and the corrections are listed as follows: