| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
Plant Physiology 147:1761-1762 (2008) © 2008 American Society of Plant Biologists On the InsideArg as a Nitric Oxide SourceLocalized increases in nitric oxide (NO) mediate many processes in plant development, including the formation of lateral and adventitious roots. Although it is known that auxin induces NO accumulation, it is not known how auxin does this or, indeed, if the production is enzymatic. For example, nonenzymatic production could result from localized auxin-induced increases in acidity or the reduction of nitrite to NO by ascorbate. The reduction of nitrite by nitrate reductase is also an established enzymatic source of NO in plants, and NO can also be generated from nitrite acting as a terminal acceptor of mitochondrial electron transport. The story of NO production in plants is becoming even more complicated by accumulating evidence that Arg may also serve as a NO source. For example, a previous study has detected an Arg-dependent NO-producing activity in extracts of pea (Pisum sativum) seedlings and demonstrated that this type of NO production was inhibited by a known inhibitor of animal NO synthase. In this issue, Flores et al. (pp. 1936–1946) report that T-DNA insertion mutation of either of the two arginase structural genes of Arabidopsis (Arabidopsis thaliana; ARGAH1 or ARGAH2) results in increased NO accumulation and a greater propensity to form lateral and adventitious roots, traits known to be under the control of auxin signaling. The authors propose that Arg, or an Arg derivative, is a potential NO source, and that reduced arginase activity in the mutants results in greater conversion of Arg to NO, thereby potentiating auxin action in roots. The model is supported by the ability of Arg supplementation to both induce adventitious roots and increase NO accumulation in argah1-1 and argah2-1 mutants. Metabolite Profiling in Opium Poppy More than 80 benzylisoquinoline alkaloids occur in opium poppy (Papaver somniferum; Fig. 1 ), and many, including morphine and codeine, the antimicrobial agent sanguinarine, the muscle relaxant papaverine, and the antitumorigenic agent noscapine, have potent pharmacological properties. Despite the pharmacological importance of opium poppy, the regulation of its alkaloid metabolism remains largely unknown. As part of their functional genomics program, Hagel et al. (pp. 1805–1821) have applied 1H NMR metabolite profiling to investigate opium poppy metabolism. Not all opium poppies are the same, however. Many natural and induced mutant varieties of opium poppy with altered alkaloid profiles exist. Preliminary screening of several opium poppy varieties identified six candidates with unique alkaloid phenotypes. Analyses of the 1H NMR spectra revealed that morphinan alkaloids were largely responsible for the variance in latex extracts. Relatively few differences were found in the levels of other metabolites, indicating that the variation was specific for alkaloid metabolism. Real-time PCR analysis of 42 genes involved in primary and secondary metabolism showed differential gene expression mainly associated with alkaloid biosynthesis. These results suggest that variations in alkaloid profiles are generally associated with relatively few differences in the steady-state levels of primary metabolites.
Cry No More: Onions Just Got Healthier (and Tearless) Allium species synthesize a unique set of secondary sulfur metabolites derived from Cys. When the tissues of any Allium species are disrupted, these amino acid derivatives are cleaved by the enzyme alliinase into their corresponding sulfenic acids and volatile sulfur compounds that give the respective flavors and bioactivities of these species. In the case of onion (Allium cepa), one of the volatiles released is propanthial S-oxide (lachrymatory factor [LF]). LF is the chemical responsible for inducing tearing. Morever, it is hypothesized that LF production causes the absence of otherwise predicted sulfur volatiles, analogues of which in garlic (Allium sativum) are known for their health attributes. Current "tearless" onion cultivars (e.g. Vidalia) are achieved through deficient uptake and partitioning of sulfur and/or growth in sulfur-deficient soils, but in so doing they accumulate fewer secondary sulfur compounds in the bulb, reducing their sensory and health qualities compared with more pungent high-sulfur cultivars. Eady et al. (pp. 2096–2106) set out with the goal of making a healthier and tearless onion by reducing lachrymatory factor synthase (LFS) and preventing the conversion of 1-propenyl sulfenic acid to the undesirable LF. By means of RNA interference, LFS activity in onions was reduced by up to 1,544-fold, so that when wounded the onions produced significantly reduced levels of LF. The authors then confirmed that RNA interference silencing of LFS shifted sulfur metabolism so that more 1-propenyl sulfenic acid was converted into di-1-propenyl thiosulfinate, which in turn gave rise to a cascade of predicted secondary compounds that had not been detected previously in onion or only in trace amounts. The authors hope to initiate formal taste evaluation trials of these tearless onions following regulatory approval. Are Golgi Vesicles Involved in Monolignol Export? Secondary xylem (wood) formation in gymnosperms requires that the developing tracheids reinforce their secondary cell walls with lignin, an amorphous product of the random radical coupling of monolignols, most prominently coniferin. Both the precise chemical identity and the mechanism by which monolignols are exported from their site of synthesis in the cytoplasm to their site of polymerization in the apoplast are poorly understood. Kaneda et al. (pp. 1750–1760) undertook to localize monolignols in intact developing tracheids of lodgepole pine (Pinus contorta) during lignification using transmission electron microscopy. Since antibodies specific to lignin detect only polymeric forms, they were unable to detect monolignols prior to polymerization. Therefore, they employed a novel autoradiographic approach to localize monolignols. They fed 3H-Phe to dissected cambium/developing wood from Pinus seedlings, then rapidly froze the cells and performed autoradiography to detect the locations of the monolignols responsible for lignification. The authors found that the secondary cell walls of developing tracheids were heavily labeled when incubated with 3H-Phe, whereas within the protoplast the cytoplasm was most strongly labeled followed by Golgi. Following the inhibition of protein synthesis, cell wall lignification was not changed but the amount of Golgi label dropped dramatically. In contrast, in samples where phenylpropanoid metabolism was inhibited, wall lignification was strongly decreased but there was no change in Golgi label. Thus, the Golgi signal appears to be attributable to protein rather than to phenylpropanoid metabolites. These findings support a model whereby unknown membrane transporters, rather than Golgi vesicles, export monolignols. Interactions between E3 Ubiquitin Ligases, Receptor Kinases, and Abscisic Acid Disruptions in ubiquitin-mediated protein degradation can lead to prolonged activity of a target protein and affect plant growth and development. E3 ubiquitin ligases are important components that define the substrate specificity of this pathway. Based on known E3 ligase motifs, there are at least 1,300 predicted E3 ligase genes in the Arabidopsis genome. The U-box, a modified ring finger that ubiquitinates substrates in the presence of the appropriate E1 and E2, is a conserved E3 ligase motif. The plant U-box (PUB) family can be further divided based on the presence of other distinguishing domains, such as ARM repeats. The PUB-ARM family comprises the largest E3 ubiquitin ligase family. Connections are emerging between PUB-ARM proteins and receptor kinases. For example, the Brassica ARC1 protein, which is a U-box/ARM-repeat-containing E3 ligase, binds to S-receptor kinase and is required for the Brassica self-incompatibility response. Samuel et al. (pp. 2084–2095) hypothesize that the Arabidopsis S-domain receptor kinase family may potentially utilize the numerous AtPUB-ARM family members as downstream signaling components. To investigate this idea, they performed yeast two-hybrid analyses of various S-domain receptor kinase family members with representative AtPUB-ARMs. The kinase domains from S-domain receptor kinases were found to interact with ARM repeat domains from AtPUB-ARM proteins. These kinase domains along with MLPK (M-locus protein kinase), a positive regulator of self-incompatibility response, were also able to phosphorylate the ARM repeat domains in in vitro phosphorylation assays. Changes in subcellular localization patterns were also detected in the presence of interacting kinases. Interestingly, AtPUB9 displayed a redistribution to the plasma membrane of tobacco (Nicotiana tabacum) BY-2 cells when either treated with abscisic acid (ABA) or coexpressed with the active kinase domain of ARK1. T-DNA insertion mutants for ARK1 and AtPUB9 lines were also altered in their ABA sensitivity during germination, suggesting a potential involvement of these proteins in ABA responses. Synthetic Lipid Vesicles Accumulate in the Cell Plate Region The cell plate that forms during plant cell cytokinesis is formed by the fusion of membrane vesicles of approximately 60 nm in diameter that contain a variety of hemicelluloses and pectins as well as membrane-localized callose and cellulose synthesizing enzyme complexes. The cell plate is formed in the middle of a complex structure called the phragmoplast, a cytoplasmically dense area containing microtubules, actin filaments, endoplasmic reticulum (ER), and cell plate forming vesicles. It has been suggested that phragmoplast microtubules are directly responsible for vesicle transport through the phragmoplast. However, the injection of the actin-binding protein profilin indicates that actin filaments in the phragmoplast are crucial for the transport of cell plate forming vesicles. Apart from being vehicles for transport, the tight mesh of microtubules and actin filaments in the phragmoplast might also act as a sieve, allowing only properly sized vesicles to move into the region. In an effort to shed light on the physical properties of cell plate forming vesicles, Esseling-Ozdoba et al. (pp. 1699–1709) microinjected fluorescent synthetic lipid vesicles that were made of 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] into Tradescantia virginiana stamen hair cells. These synthetic vesicles partially mimic the behavior of endogenous vesicles during cell plate formation in that they are transported to and through the phragmoplast and accumulate in the region of the existing growing cell plate. However, unlike endogenous vesicles, synthetic vesicles redistribute into the cytoplasm of the daughter cells upon completion of cytokinesis. These results suggest that the phragmoplast is not selective for lipid membrane composition or for integral membrane proteins. Since at this stage of cytokinesis, microtubules are virtually absent from that region, while actin filaments are present, it appears likely that actin filaments play a role in the transport of vesicles toward the cell plate.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900267 Related articles in Plant Physiol.:
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ASPB Publications | PLANT PHYSIOLOGY® | THE PLANT CELL | |
|---|---|---|---|
