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The synthesis of (22E,24R)-5 alpha-ergosta-2,22-dien-6-one (2), a key intermediate to the preparation of 24-epibrassinolide (1), was achieved through the 3,6-diketone 8 in a 40% overall yield in five steps starting from ergosterol (3).
A new brassinolide inhibitor, named KM-01, was isolated from the culture filtrates of two fungal species, Drechslera avenae and Pycnoporus coccineus, and the structure with absolute stereochemistry was elucidated as the fatty acid ester of an eremophilane sesquiterpene, bipolaroxin, based on spectroscopic analysis, chemical degradation, and synthesis of the fatty acid.
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Mechanisms of plant developmental programmed cell death (PCD) have been intensively studied in recent years. Most plant developmental PCD is triggered by plant hormones, and the 'death signal' may be transduced by hormonal signaling pathways. Although there are some fundamental differences in the regulation of developmental PCD in various eukaryotes of different kingdoms, hormonal control and death signal transduction via pleiotropic signaling pathways constitute a common framework. However, plants possess a unique process of PCD execution that depends on vacuolar lytic function. Comparisons of the developmental PCD mechanisms of plants and other organisms are providing important insights into the detailed characteristics of developmental PCD in plants.
A cDNA clone, CanTUB, encoding a putative beta-tubulin protein was isolated from a cDNA library constructed from 5-day old chickpea (Cicer arietinum) epicotyls. Analysis of its deduced amino acid sequence showed all the typical structural motifs of plant beta-tubulins. Putative sequences for autoregulation and tubulin mRNA stability, GTP and Ca2+/MAPs (microtubule-associated proteins) binding sites were present. Southern blot analysis of chickpea genomic DNA revealed that there are multiple beta-tubulin genes. The level of expression of beta-tubulin genes was correlated with the rate of growth in either seedlings and adult plants. The transcript levels of beta-tubulin genes were higher in actively elongating tissues such as etiolated epicotyls, roots and stem tissues of adult plants. Brassinolide-induced growth in chickpea epicotyls was accompanied by promotion of the expression of the gene coding for beta-tubulin.
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After a brief survey of the literature on the recently discovered brassinolide, some of our results with corticosteroids are given. A number of structural requirements of corticosteroids were deduced in a bioassay involving elongation growth and lateral root formation. To explain the activity of glucocorticoids, the involvement of a receptor protein is suggested as well as an enhanced RNA synthesis. However, the structural requirements found are not applicable to experiments on adventitious root formation, implying a different mode of action of corticosteroids in this test system.
Plant culture in oxygen concentrations below ambient is known to stimulate vegetative growth, but apart from reports on increased leaf number and weight, little is known about development at subambient oxygen concentrations. Arabidopsis thaliana (L.) Heynh. (cv. Columbia) plants were grown full term in pre-mixed atmospheres with oxygen partial pressures of 2.5, 5.1, 10.1, 16.2, and 21.3 kPa O2, 0.035 kPa CO2 and the balance nitrogen under continuous light. Fully expanded leaves were harvested and processed for light and transmission electron microscopy or for starch quantification. Growth in subambient oxygen concentrations caused changes in leaf anatomy (increased thickness, stomatal density and starch content) that have also been described for plants grown under carbon dioxide enrichment. However, at the lowest oxygen treatment (2.5 kPa), developmental changes occurred that could not be explained by changes in carbon budget caused by suppressed photorespiration, resulting in very thick leaves and a dwarf morphology. This study establishes the leaf parameters that change during growth under low O2, and identifies the lower concentration at which O2 limitation on transport and biosynthetic pathways detrimentally affects leaf development. Grant numbers: NAG5-3756, NAG2-1020, NAG2-1375.
A role for 5alpha-reduction in androgen physiology was first established with the recognition that dihydrotestosterone, the 5alpha-reduced metabolite of testosterone, is formed in many androgen target tissues, binds to the androgen receptor with greater affinity than testosterone, and plays an essential role in virilization of the urogenital sinus and urogenital tubercle during male development. Two 5alpha-reductases perform this reaction, and both isoenzymes utilize NADPH as cofactor and have broad specificity for steroids containing a delta4,3-keto configuration. 5alpha-Reduction, which is essentially irreversible, flattens the steroid molecule because of altered relation of the A and B rings, and stabilizes the hormone-receptor complex. Studies involving in vitro reporter gene assays and intact mice in which both isoenzymes are disrupted, indicate that the fundamental effect of dihydrotestosterone formation is to amplify hormonal signals that can be mediated by testosterone at higher concentrations. 5alpha-Reduction also plays a role in the action of other steroid hormones, including the plant growth hormone, brassinolide, the boar pheromones, androstanol and androstenol, progesterone (in some species), and, possibly, aldosterone and cortisol. The fact that the reaction is important in plants and animals implies a fundamental role in steroid hormone action.
Promotive effect of brassinolide (BL) on green lamina inclination was concentration-dependent when excised rice (Oryza sativa L.) lamina was floated on BL solution under continuous light conditions. Protein kinase inhibitor staurosporine and Ca2+ channel blocker LaCl3 could completely, while Ca2+ chelator EGTA could partially inhibit the lamina inclination caused by BL. Two protein kinases with apparent molecular masses of 45 and 54 kDa were detected using an in-gel kinase assay with histone III-S as a substrate. In particular, the changes in 45 kDa protein kinase activity correlated with lamina inclination caused by BL. The 45 kDa kinase activity was inhibited by Ca2+ chelator EGTA, protein kinase inhibitor, staurosporine and calmodulin antagonist W-7. Therefore, this 45 kDa protein kinase was identified as a Ca2+ -dependent protein kinase (CDPK). Patterns of 2-dimensional PAGE after in vitro phosphorylation of crude extracts showed that the phosphorylation of 56 and 41 kDa proteins, which was Ca2+ -dependent, was strongly increased by BL treatment. These results suggested that CDPK and Ca2+ -dependent protein phosphorylation are involved in BL-induced rice lamina inclination.
A tobacco peroxidase gene tpoxN1 was reported to be expressed within 1 h after wounding in leaves [Hiraga et al. (2000a) Plant Cell Physiol. 41: 165]. We describe here further results on the wound-induced tpoxN1 expression. The quick tpoxN1 induction occurred preferentially in stems and petioles, but was negligible in leaf blades even 8 h after wounding. Induced GUS activity was also detected rapidly after wounding in the stem of transgenic tobacco plants carrying the tpoxN1 promoter::GUS fusion gene, localized mainly in the vascular systems where it was maintained this level for 14 d or more. Strong GUS activity was also found in the petiole and veinlet as well as the epidermal tissue in the stem. Treatment of known inducers for wound-responsive genes such as jasmonate, 1-aminocyclopropane-1-carboxylate, spermine, phytohormones and other stress treatments did not enhance wound-induced tpoxN1 gene expression in stems at all, but rather repressed it in some cases. Studies using metabolic inhibitors suggested that phosphorylation and dephosphorylation of proteins together with de novo protein synthesis are likely to be involved in the wound-induced tpoxN1 expression as well as some other wound-responsive genes. Thus, tpoxN1 is a unique wound-inducible and possible wound-healing gene which is rapidly expressed being maintained for a long time in veins via an unknown wound-signaling pathway(s).
Aquaporins in the plasma and vacuolar membranes play a key role in the intercellular and intracellular water transport in plants. First, we quantitated the absolute amounts for mRNAs of eight aquaporin isoforms in hypocotyls of radish seedlings. Then, we investigated the effects of salt and water stresses (150 mM NaCl, 300 mM mannitol and 20% polyethylene glycol) and phytohormones (gibberellic acid, abscisic acid and brassinolide) on the mRNA and protein levels of aquaporins in the plasma membrane (RsPIP1-1, 1-2, 1-3, 2-1, 2-2 and 2-3) and vacuolar membrane (RsTIP1-1 and 2-1). The mRNA and protein levels of RsTIP1-1, RsTIP2-1, RsPIP1-1, RsPIP1-2 and RsPIP1-3 were comparatively constant. In contrast, mannitol treatment altered the mRNA levels of RsPIP2-1, RsPIP2-2 and RsPIP2-3 in roots. Immunoblot analysis showed that the RsPIP2-1 protein level was increased by NaCl treatment and decreased by treatment with mannitol and polyethylene glycol. Gibberellic acid and abscisic acid suppressed the levels of mRNAs of RsPIP2-1, RsPIP2-2 and RsPIP2-3 and the protein level of RsPIP2-1 in roots. On the other hand, the protein levels of RsPIP1-group members and RsTIPs were scarcely changed by these phytohormones. In the case of hypocotyls and cotyledons, the mRNA and protein levels of eight isoforms were not markedly affected by any treatment. These results indicate that aquaporins in the root, especially the RsPIP2 group, may be a stress responsive type of aquaporin at least in the protein level.
A screen for Arabidopsis mutants that were insensitive to methyl jasmonate (MeJA) in an assay for seedling root growth yielded only alleles of previously isolated mutants jar1 and coi1, with one exception. Mapping of the locus and morphological characterization of the new mutant suggested it might be allelic to axr1, which had not previously been reported to show resistance to MeJA. The F(1) from a cross of the new mutant with axr1-3 did not show complementation, confirming that these are the same genes. The new allele is called axr1-24. In addition to MeJA and indole-3-acetic acid (IAA), axr1-24 had decreased sensitivity to 1-aminocyclopropane-1-carboxylic acid, 6-benzylamino-purine, epi-brassinolide, and abscisic acid. Both axr1-24 and the previously characterized axr1-3 allele were shown to be susceptible to the opportunistic pathogen Pythium irregulare, a trait found in other jasmonate response mutants, including jar1-1. The double mutant jar1-1/axr1-3 was more resistant to inhibition of root growth by MeJA and was more susceptible to P. irregulare infection than either single mutant, suggesting these genes might act in independent response pathways. In contrast, resistance to IAA in the double mutant was not different from axr1-3. Northern-blot analysis showed that IAA induced the jasmonate-responsive lipoxygenase 2, AOS, and AtVSP gene transcripts and induction was strongly impaired in axr1-3. However, transcript induction by MeJA was only minimally affected in axr1-3. This study demonstrates that in addition to auxin signaling, the AXR1 locus is involved in MeJA response, providing a mechanistic link between jasmonate and auxin-signaling pathways.
Abscisic acid (ABA) is involved in a number of critical processes in normal growth and development as well as in adaptive responses to environmental stresses. For correct and accurate actions, a physiologically active ABA level is controlled through fine-tuning of de novo biosynthesis and catabolism. The hydroxylation at the 8'-position of ABA is known as the key step of ABA catabolism, and this reaction is catalyzed by ABA 8'-hydroxylase, a cytochrome P450. Here, we demonstrate CYP707As as the P450 responsible for the 8'-hydroxylation of (+)-ABA. First, all four CYP707A cDNAs were cloned from Arabidopsis and used for the production of the recombinant proteins in insect cells using a baculovirus system. The insect cells expressing CYP707A3 efficiently metabolized (+)-ABA to yield phaseic acid, the isomerized form of 8'-hydroxy-ABA. The microsomes from the insect cells exhibited very strong activity of 8'-hydroxylation of (+)-ABA (K(m) = 1.3 microm and k(cat) = 15 min(-1)). The solubilized CYP707A3 protein bound (+)-ABA with the binding constant K(s) = 3.5 microm, but did not bind (-)-ABA. Detailed analyses of the reaction products confirmed that CYP707A3 does not have the isomerization activity of 8'-hydroxy-ABA to phaseic acid. Further experiments revealed that Arabidopsis CYP707A1 and CYP707A4 also encode ABA 8'-hydroxylase. The transcripts of the CYP707A genes increased in response to salt, osmotic, and dehydration stresses as well as ABA. These results establish that the CYP707A family plays a key role in regulating the ABA level through the 8'-hydroxylation of (+)-ABA.
Auxin affects many aspects of plant growth and development. We previously used chemical genetics to dissect auxin-signaling mechanisms and identified a small molecule, sirtinol, that constitutively activated auxin signaling (Y. Zhao et al. [2003], Science 301: 1107-1110). Here we describe the isolation, characterization, and cloning of an Arabidopsis mutant Atcand1-1 that emerged from a genetic screen for mutants insensitive to sirtinol. Loss-of-function mutants of AtCAND1 were resistant to sirtinol and auxin, but not to gibberellins or brassinolide. Atcand1 displayed developmental phenotypes similar to those of axr1, namely, short petioles, downwardly curling leaves, short inflorescence, and reduced fertility. AtCAND1 is homologous to human CAND1, a protein that is composed almost entirely of HEAT-repeat units and has been implicated in regulating the assembly and disassembly of the SCF protein degradation machinery. Taken together with previous biochemical studies, this work helps to elucidate the roles of AtCAND1 in protein degradation and auxin signaling.
The signal transduction pathways that control cytokinesis in plants are largely uncharacterized. Here, we provide genetic evidence that mitogen-activated protein kinase kinase kinases (MAPKKKs) play a role in the control of plant cell division. Using a reverse-genetic approach, we isolated plants carrying knockout alleles of the Arabidopsis MAPKKK genes ANP1, ANP2, and ANP3. The resulting single-mutant plants displayed no obvious abnormal phenotypes; two of the three double-mutant combinations displayed defects in cell division and growth; and the triple-mutant combination was not transmitted through either male or female gametes. The molecular and structural phenotypes displayed by the double mutants support a model in which the ANP family of MAPKKKs positively regulates cell division and growth and may negatively regulate stress responses.