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Plant Physiology 135:2038-2039 (2004) © 2004 American Society of Plant Biologists Graft Transmission of a CONSTANS-Derived Floral Stimulant CONSTANS (CO) has been implicated as a mediator between photoperiod perception and the transition to flowering in the long-day plant Arabidopsis. Plants carrying mutant alleles of CO are late flowering in long days. Consistent with a role in promoting flowering in long days, CO mRNA accumulates under long-day conditions, relative to short days, and oscillates in response to circadian rhythms. To test the role of CO in long-distance signaling, Ayre and Turgeon (pp. 2271–2278) expressed CO in noninduced plants using a promoter specific to the companion cells of the smallest veins of mature leaves. This expression in tissues at the beginning of the phloem translocation stream was sufficient to accelerate flowering at the apical meristem under noninductive (short-day) conditions. Grafts that conjoined the vegetative stems of plants with different flower-timing phenotypes demonstrated that the expression of CO in minor veins is able to substitute for inductive photoperiods in generating a mobile flowering signal. These results suggest that a CO-derived signal(s), or possibly CO itself, may be florigen.
Reporter genes are useful tools for studying gene expression in transformed organisms. Fluorescent proteins allow for the noninvasive detection of gene expression in living cells without the addition of substrates. The potential of fluorescent proteins, such as green fluorescent protein (GFP) and red fluorescent protein (DsRED) to monitor dynamic gene regulation, however, is limited by their high stability. The recently made DsRED-E5 variant overcomes this problem. DsRED-E5 changes its emission spectrum over time from green to red, in a concentration independent manner. Therefore, the green to red fluorescence ratio indicates the age of the protein and can be used as fluorescent timer to monitor dynamics of gene expression. In this issue, Mirabella et al. (pp. 1879–1887) have analyzed the potential of DsRED-E5 as a reporter in plant cells. They show that in Vigna unguiculata mesophyll protoplasts DsRED-E5 shifts its fluorescence in a similar way as in animal cells and that the timing of this shift is suitable for studying developmental processes in plants. To test whether DsRED-E5 can be used to monitor gene regulation in plant organs, the authors placed DsRED-E5 under the control of promoters that are either up- or down-regulated (MtACT4 and LeEXT1 promoters) or constitutively expressed (MtACT2 promoter) during root hair development in Medicago truncatula. Analysis of the fluorescence ratios clearly provided more accurate insight into the timing of promoter activity.
Plant defensins are small (45–54 amino acids) Cys-rich proteins implicated in host defense against fungal pathogens. Plant defensins are highly varied in their primary amino acid sequences with only eight structure-stabilizing Cys residues in common. The variation in the different primary sequences may account for the different biological activities reported for plant defensins including antifungal activity, antibacterial activity, proteinase activity, and amylase inhibitory activity. The structural determinants in plant defensins that govern their antifungal activity and the mechanisms by which they inhibit fungal growth remain unclear. Alfalfa seed defensin, MsDef1, strongly inhibits the growth of Fusarium graminearum in vitro, whereas MtDef2, which shares 65% amino acid sequence identity with MsDef1, lacks antifungal activity against F. graminearum. In this issue, Spelbrink et al. (pp. 2055–2067) characterize the in vitro antifungal activity of chimeras containing portions of the MsDef1 and MtDef2 proteins and show that the major determinants of antifungal activity reside in the carboxy-terminal region (amino acids 31–45) of MsDef1. They also report that MsDef1 blocks a mammalian L-type Ca2+ channel in a similar manner to a structurally unrelated antifungal toxin KP4 from Ustilago maydis. This blockage was found to be very strong (up to 90% blockage) and highly specific for the L-type channel. The structurally similar MtDef2 and the radish seed defensin, Rs-AFP2, however, failed to block the L-type Ca2+ channel. From these results, it appears that the two unrelated antifungal proteins, KP4 and MsDef1, have evolutionarily converged upon the same molecular target, whereas the two structurally related antifungal plant defensins, MtDef2 and Rs-AFP2, have diverged to attack different targets in fungi.
One of the main goals of the plant research community is to determine the function of every Arabidopsis gene by the end of the year 2010. Gutiérrez et al. (pp. 1888–1892) propose that Arabidopsis proteins that are unique to plants should be a major priority for future studies because many of these proteins have unknown functions, and are not likely to be studied in other model organisms. In an effort to aid the functional characterization of the Arabidopsis proteins that are specific to plants the authors have created the Plant-Specific Database (PLASdb). PLASdb is a relational database of the nuclear-encoded Arabidopsis proteins classified according to their pattern of sequence similarity in the protein sets of the following organisms: Homo sapiens, Rattus norvegicus, Drosophila melanogaster, Caenorhabditis elegans, Mus musculus, Schizosaccharomyces pombe, Saccharomyces cerevisiae, a combined set of 88 species of bacteria, and a combined set of 16 species of Archaea. The curators of the database determine the phylogenetic profile of each Arabidopsis protein sequence as a vector of nine values that indicates the similarity of the Arabidopsis protein to the proteins in each of the other sets. After excluding proteins encoded in retroelements or transposable elements, 3,848 Arabidopsis proteins were selected and classified as plant-specific proteins. PLASdb contains extensive information compiled from multiple public data sources (e.g. annotation information, expression in organs based on microarray data), and generated with predictive algorithms (e.g. protein families and subcellular localization). The authors hope that this new resource stimulates further research by identifying and providing quick and easy access to available information about the Arabidopsis plant-specific proteins.
Induced anti-herbivory defenses may be direct or indirect. Direct defenses, such as increases in secondary metabolites or defense-associated proteins, lower the food quality for herbivores. Plants, however, may also acquire protection indirectly by the production of herbivore-induced volatiles that attract the natural enemies of the herbivores, such as predators and parasitoids. Ament et al. (pp. 2025–2037) have taken advantage of the tomato (Lycopersicon esculentum) mutant def-1, which is deficient in induced jasmonic acid (JA) accumulation upon wounding or herbivory, to study the role of JA in the direct and indirect defense responses of tomato plants to the phytophagous mite Tetranychus urticae. In contrast to earlier studies, the authors report that spider mites laid as many eggs and caused as much damage on def-1 as on wild-type plants, even though the hatching rate of eggs on def-1 was significantly higher. These results suggest that JA-dependent direct defenses somehow enhance egg mortality or increase the time needed for embryonic development. The def-1 mutant was also found to have lower levels of JA-related transcripts after 1 d of spider mite infestation. Most importantly, the indirect defense response was absent in def-1, since the five typical spider mite-induced tomato-volatiles were not induced and the predatory mite Phytoseiulus persimilis did not discriminate between infested and uninfested def-1 tomatoes as it did with wild-type tomatoes. Thus, through a combination of transcriptomics, volatile-metabolomics, and behavioral analyses, this study provides unambiguous evidence for an essential role for JA as a plant defense-component for the induction of host-plant odors that are associated with spider mite herbivory and predator recruitment.
Although much recent evidence supports a direct role for aquaporins in plant water relations, relatively little is known about the traffic pathways that deliver aquaporins to their destination membranes. Endosomal trafficking of plant aquaporins could conceivably play a role in the regulation and turnover of aquaporins at their target membrane under conditions of osmotic stress. Indeed, in animals, regulatory cycling between membranes has been demonstrated for several aquaporins. In the ice plant (Mesembryanthemum crystallinum), changes in membrane distribution of the tonoplast aquaporin McTIP1;2 have previously been shown to be correlated with changes in osmotic potential. In this issue, Vera-Estrella et al. (pp. 2318–2329) demonstrate that ice plant McTIP1;2 is differentially regulated by osmotic and salt stress and that the membrane distribution of McTIP1;2 is altered by osmotic stress. Differences in the response of McTIP1;2 to salt and osmotic stress suggest that the ionic component of the salt stress initiates a separate and independent signaling pathway resulting in decreased expression of the McTIP1;2 protein. Osmotic stress, without the ionic component inherent in salt stress, results in the upregulation and apparent redistribution of McTIP1;2. Pharmacological studies reveal that the mannitol-induced redistribution of McTIP1;2 is arrested by pretreatment with brefeldin A, wortmannin, and cytochalasin D, inhibitors of vesicle trafficking-related processes. Sequence analysis of ice plant McTIP1;2 identified a possible protein kinase A phosphorylation site motif at the N terminus of the protein. Treatment with forskolin, an activator of adenylyl cyclase, known to elicit cyclic AMP-dependent physiological responses by increasing intracellular cyclic AMP, and 8-Br cyclic AMP, a cell permeable cyclic AMP analog that activates protein kinase A, mimicked the effect of mannitol-induced osmotic stress on McTIP1;2 membrane redistribution. Inhibitors of adenylyl cyclase or protein kinase A appeared to prevent the redistribution of McTIP1;2 protein. These studies indicate the involvement of a cAMP-dependent signaling pathway in McTIP1;2 redistribution during osmotic stress. The authors propose that McTIP1;2 redistribution to endosomal compartments may be part of a homeostatic process that helps to restore and maintain cellular osmolarity under osmotic stress conditions.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900118. Related articles in Plant Physiol.:
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