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Plant Physiology 135:1147-1148 (2004) © 2004 American Society of Plant Biologists A Halophyte Relative of Arabidopsis Salt cress (Thellungiella halophila), previously classified as Arabidopsis halophila, is a halophytic crucifer native to China. It is small winter annual with a short life cycle and which produces copious seeds, can self-pollinate, and can be genetically transformed by the simple floral dip procedure. Its small genome, about twice the size of Arabidopsis, shares high sequence identity (average 92%) with Arabidopsis. Unlike the glycophyte Arabidopsis, salt cress can reproduce after exposure to extreme saline conditions (500 mM NaCl). All of these attributes make it an ideal model system for studying the molecular processes underlying salt tolerance. Although Arabidopsis is a typical glycophyte and not particularly salt tolerant, a number of recent studies suggest that it may contain most, if not all, of the salt tolerance genes that one might expect to find in halophytes. These findings have led to the hypothesis that many halophytes may use the same mechanisms of salt tolerance found in glycophytes and that subtle differences in regulation may account for the large variations in tolerance or sensitivity between glycophytes and halophytes. To elucidate the differences in the regulation of salt tolerance mechanisms between salt cress and Arabidopsis, Taji et al. (pp. 1697–1709) analyzed the gene expression profiles in salt cress using full-length Arabidopsis cDNA microarrays. Only a few genes were induced by salinity stress in salt cress compared to Arabidopsis. The authors conclude that stress tolerance of salt cress may be due to the constitutive overexpression of many of the same types of stress-inducible genes in Arabidopsis that function in stress tolerance. Also, in this issue, Inan et al. (pp. 1718–1737) report on their isolation of several chemically derived mutants of salt cress that have reduced salinity tolerance. Studies of these mutants indicate that salt tolerance in salt cress is significantly affected by individual genetic loci. Analyses of salt cress expressed sequence tags provide evidence for the presence of paralogs, missing in the Arabidopsis genome, and for genes with abiotic stress-relevant functions. Consistent with the aforementioned findings of Taji et al., hybridizations of salt cress target RNA to an Arabidopsis whole-genome oligonucleotide array indicate that stress-associated transcripts are expressed at a noticeably higher level in unstressed salt cress plants as compared to unstressed Arabidopsis plants and are induced rapidly under stress.
Pollen tube growth requires a Ca2+ gradient, with elevated levels of cytosolic Ca2+ at the growing tip. Ca2+ influx into the growing tip is necessary for elongation and its magnitude oscillates out of phase with an oscillation in growth. It has been widely assumed that stretch-activated Ca2+ channels underlie this influx, but such channels have never been reported in either pollen grains or pollen tubes. In this issue, Dutta and Robinson (pp. 1398–1406) characterize stretch-activated Ca2+ channels from Lilium longiflorum pollen grain and tube tip protoplasts. The channels were localized to a small region of the grain protoplasts associated with the site of tube germination. In addition, they report the occurrence of a stretch-activated K+ channel as well as a spontaneous K+ channel distributed over the entire grain surface. Neither of the K+ channels were present at the germination site or at the tip. The stretch-activated channels were inhibited both by Gd3+ ions and by a 3,000-fold dilution of the venom from the tarantula Grammostola spatulata (Fig. 1) that has been shown to block stretch-activated channels in pituitary cells, but the spontaneous channel was unaffected by the venom. The venom also stopped pollen tube germination and elongation and blocked Ca2+ entry into the growing tip. Grammostola venom may prove useful in isolating mutants with altered stretch-activated channels.
Do Plants Detect Green Light? Scattered evidence in the literature suggests that monochromatic green light (GL) is capable of influencing a number of plant physiological processes. For example, GL has been shown to act as a signal in inhibiting seedling mass, plant cell culture growth, and light-induced gravitropic root elongation. Moreover, it has recently been shown that GL can reverse blue light-induced stomatal opening. Although phytochromes and cryptochromes absorb GL weakly, the action spectra for GL-induced responses exhibit a peak between 540 and 550 nm and thus are inconsistent with the absorption spectra for phytochromes, cryptochromes, and phototropins and the action spectra for the responses they govern. In this issue, Folta (pp. 1407–1416) reports, based on high-resolution analyses of early growth kinetics, that GL irradiation causes a rapid increase in early stem elongation rate in Arabidopsis, a response that is opposite to that induced by all other light conditions studied. The transient growth promotion is evident within 15 min of irradiation, and its magnitude is regulated in a dose-dependent manner. Genetic analyses indicate that GL-stimulated stem elongation cannot be completely attributed to any known photoreceptors. Mutations in the cryptochromes, phototropins, and the major phytochromes do not eliminate the GL response. This work suggests that there may be an as yet unidentified GL receptor in plants and underscores the need for extreme care in the use of green safelights in photobiological studies.
Apple (Malus domestica) fruits undergo a loss of firm texture during prolonged storage. Because pectic substances are enriched in the walls of fruit cells and because they constitute major components of the middle lamellae, these acidic polysaccharides have long been suspected to play key roles in fruit ripening and overripening. In this issue, Peña and Carpita (pp. 1305–1313) examine the chronology of the biochemical events associated with the development of overripening in four apple cultivars that vary markedly in their storage behavior. The cell walls of the edible part of apple fruits appear to have a composition and architecture characteristic of the primary cell wall of a typical dicot cell (i.e., Type I), consisting of xyloglucan-cellulose microfibril networks embedded in matrix of several complex pectic polysaccharides. The authors show that growth and maturation of the edible cortical cells of apple fruits are accompanied by a selective loss of pectin-associated (1
The abscisic acid (ABA) biosynthesis pathway in higher plants involves the oxidative cleavage of 9-cis-epoxycarotenoids. One of the key regulatory steps in this pathway is catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). Han et al. (pp. 1574–1582) have developed a new competitive inhibitor of ABA biosynthesis that specifically targets NCED. This inhibitor, named abamine (abscisic acid biosynthesis inhibitor) offers many advantages over prior compounds (e.g. fluridone and norflurazon) that previously have been used to inhibit ABA biosynthesis but which have the side effect of inhibiting carotene biosynthesis also. The authors report that under conditions of osmotic stress (0.4 M mannitol), abamine inhibits stomatal closure in spinach (Spinacea oleracea) leaves, and that this inhibition can be overcome by the exogenous application of ABA. In Arabidopsis, the expression of the endogenous RD29B gene containing ABA-responsive elements in the promoter region is increased by drought stress and exogenous ABA treatment. As expected, RD29B expression was down-regulated by the application of abamine. These results and others reported by the authors suggest that abamine is a novel ABA biosynthesis inhibitor that targets the enzyme that catalyzes the oxidative cleavage of 9-cis-epoxycarotenoids. Abamine should prove to be a powerful tool in understanding ABA-regulated process in plants and in isolating ABA signaling mutants.
High-throughput genotype screening of plants is rapidly becoming a standard research tool in the post-genomic era. A major bottleneck in this process is that tissue samples from living plants must be collected manually, one plant at a time. Moreover, it is necessary to obtain tissue samples from individual plants in a manner that does not kill the plants. In addition, each tissue sample must be kept separate so that an accurate genotype for each individual can be determined. In this issue, Krysan (pp. 1162–1169) describes a clever method for harvesting tissue samples from living seedlings that eliminates this bottleneck. The method has been named Ice-Cap to reflect the fact that ice is used to capture the tissue samples. A key component of this process is a 96-well plate with funnel-shaped holes in the bottoms of the wells. The holes in the bottom of this plate are initially sealed with a removable adhesive film, and agar growth substrate is deposited in the lower portion of the wells. A single seed is then deposited on the surface of the solidified agar in each well by using a novel seed loading device. Once the roots start to penetrate the agar substrate, the sealing film is removed from the bottom of the plate, and the plate containing the seedlings is stacked on top of a second 96-well plate that contains water. After 2 weeks, the roots are growing in the water contained in the lower of the two plates. Root tissue samples from each plant are then collected by transferring the stacked plates to a 96-well thermal block resting in a dry ice/ethanol bath. The seedlings in this upper plate remain fully viable following this treatment. An example of this new technique in use is provided.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.900114. Related articles in Plant Physiol.:
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4)-
-D-galactan from the cell walls, whereas a selective loss of highly branched (1
-L-arabinans occurs after ripening and in advance of the loss of firm texture. In contrast to the decrease in arabinan content, the loss of rhamnogalacturonan I branching is tightly correlated with loss of firm texture in all cultivars regardless of storage time. In vitro cell separation assays show that structural proteins, perhaps via their phenolic residues, and homogalacturonans also contribute to cell adhesion.