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Plant Physiology 142:807-808 (2006) © 2006 American Society of Plant Biologists On the InsideVisualizing Extracellular ATPExtracellular ATP (eATP) has been implicated in a variety of plant cell processes, including the closure of the Venus fly trap, the inhibition of root gravitropism and polar auxin transport, the formation of reactive oxygen species (ROS), the responses of plants to wounding, and changes in gene expression. Kim et al. (pp. 984–992) have developed an elegant technique for visualizing eATP directly in root hairs of Medicago truncatula using a novel reporter construct. The construct was created by fusing a cellulose-binding domain peptide to the ATP-requiring enzyme luciferase, thereby allowing the visualization of eATP in the presence of the substrate luciferin. Luciferase activity could be detected in the interstitial spaces between plant epidermal cells and predominantly at the regions of actively growing cells. The levels of eATP were closely correlated with regions of active growth and cell expansion. Various calcium-regulating drugs revealed that ATP release is a Ca2+-dependent process and may occur through vesicular fusion, an important step in the polar growth of actively growing root hairs. The application of exogenous apyrase (ATPase) decreased ROS activity, suggesting that cytoplasmic Ca2+ gradients and ROS activity are closely associated with eATP release. Epigenetic Regulation of Seed Size Epigenesis refers to heritable changes in gene expression that do not involve changes in gene sequence. DNA methylation, the covalent addition of a methyl group to cytosine, plays a major role in epigenesis. In plants, the epigenetic modification of cytosine has been shown to be important for gene imprinting, gene silencing, seed viability, and development. DNA methyltransferases catalyze the transfer of a methyl group to DNA. In Arabidopsis (Arabidopsis thaliana), the primary DNA methyltransferase is DNA METHYLTRANSFERASE1 (MET1). Xiao et al. (pp. 1160–1168), by performing reciprocal crosses between antisense MET1 transgenic and wild-type plants, have found that DNA hypomethylation produces a strong, parent-of-origin effect on seed size. Crosses between wild-type and homozygous met1-6 (a loss-of-function recessive null allele) parents show that the hypomethylation of maternal and paternal genomes results in significantly larger and smaller F1 seeds, respectively (Fig. 1 ). An analysis of crosses between wild-type and heterozygous MET1/met1-6 parents revealed that hypomethylation in the female or the male gametophytic generation was sufficient to influence F1 seed size. A recessive mutation in another gene that reduces DNA methylation, DECREASE IN DNA METHYLATION1, also causes parent-of-origin effects on F1 seed size. These results show that maternal and paternal genomes play distinct roles in the regulation of seed size in Arabidopsis.
Ca2+ Entry Mediated by a Plant Glutamate Receptor
The Arabidopsis genome contains a family of 20 glutamate-receptor (GLR) genes homologous to the ionotropic (ion-conducting) glutamate receptors (iGluRs) that mediate synaptic transmission in the central nervous systems of mammals. Glutamate triggers strong, rapid changes in cytosolic Ca2+ levels in Arabidopsis cells. The increase in Ca2+ triggered by glutamate is accompanied by a large, transient membrane depolarization that is due at least in part to Ca2+ influx across the plasma membrane. Although there is much electrophysiological and pharmacological evidence supporting the existence of functional iGluRs in the plasma membranes of plant cells, there is a need for more research linking these pharmacological and electrophysiological effects to the products of GLR genes. Qi et al. (pp. 963–971) report that the membrane depolarization triggered by glutamate was greatly reduced by mutations in GLR3.3, one of the 20 GLR genes in Arabidopsis. The same mutations completely blocked the associated rise in cytosolic Ca2+. These results provide genetic evidence for the participation of a glutamate receptor in the rapid ionic responses to glutamate. Six amino acids commonly present in soils (glutamate, glycine, alanine, serine, asparagine, and cysteine) as well as the tripeptide glutathione ( Green Light Down-Regulates Plastid Transcripts The early events of seedling photomorphogenesis are mediated by a suite of photoreceptors that monitor ambient light conditions and appropriately tune gene expression profiles to those that best support seedling emergence. It has long been known that ultraviolet, blue, red, and far-red light rapidly and strongly suppress early stem growth. Green light (GL), however, causes a dramatic, rapid, yet transient increase in stem growth rate. Photophysiological and genetic tests indicated that GL-induced growth promotion is likely mediated by an undefined light sensor, as the response persists in all photomorphogenic mutants tested. In an effort to elucidate the mechanism underlying GL-mediated photomorphogenesis, Dhingra et al. (pp. 1256–1266) examined transcriptome changes in Arabidopsis in response to pulses of GL. Their results indicate that GL-induced stem growth is coincident with the well-known accumulation of phytochrome A-induced, nuclear-encoded transcripts, while a suite of plastid-based transcripts normally induced by light becomes repressed by light. The response is GL specific and cannot be attributed to any known classes of plant photoreceptors. Reactive Nitrogen Species in Chloroplasts Nitric oxide (NO) is a gaseous free radical that plays important roles in plant growth regulation, cell differentiation, stomatal closure, phytoalexin accumulation, and in plant responses against a variety of abiotic stresses, such as wounding, salinity, drought, and hypoxia. NO has also been shown to affect photorespiration in different plants. Jasid et al. (pp. 1246–1255) have undertaken research to determine the mechanism by which NO is generated in isolated soybean (Glycine max) chloroplasts. NO production was assessed by means of an electron paramagnetic resonance (EPR) spin-trapping technique. Both nitrite and L-arginine (Arg) were effective in inducing NO production in isolated chloroplasts. NO production by soybean chloroplasts in the presence of NaNO2 was inhibited more than 60% by the inhibition of photosynthetic electron flow by DCMU. NO production by chloroplasts incubated in Arg was inhibited when chloroplasts were incubated in the presence of nitric oxide synthase inhibitors, but this activity was unaffected by changes in Ca2+ and/or calmodulin in the incubation medium. The lack of dependence of NO generation on Ca2+ indicates that chloroplast NO generation is distinctive from other Arg-dependent nitric oxide synthase activities described in plants. These data suggest that at least two pathways for NO production are operating in chloroplasts, one dependent on a NO synthase-like enzyme activity and another that uses nitrite as a substrate. A Photorespiratory Enzyme Modulates Amino Acid Contents During photorespiration, glycolate-2-P is produced by the oxygenase activity of Rubisco. Photorespiration is often considered to be a wasteful process because at current atmospheric concentrations of O2 and CO2, photorespiration in C3 plants dissipates more than 25% of the carbon fixed by means of photosynthesis. Some researchers, however, have argued that photorespiration might be useful to plants insofar as mitigating stress-induced photoinhibition or altering amino acid levels. In regard to this last idea, photorespiratory transamination to glyoxylate, which is mediated by peroxisomal glutamate:glyoxylate aminotransferase (GGAT) and serine glyoxylate aminotransferase (SGAT), is believed to play an important role in the biosynthesis and metabolism of major amino acids. Serine and glycine are involved in protein biosynthesis and serve as precursors in a variety of important biosynthetic pathways, including phospholipid synthesis (serine) and purine formation (glycine). Serine and glycine are synthesized through two main pathways. One of these pathways involves the transamination of glyoxylate into glycine, which is in turn converted into serine. A possible source of glyoxylate is provided by the oxidation of glycolate during the photorespiratory cycle. To better understand the role of photorespiration in the regulation of amino acid levels, Igarashi et al. (pp. 901–910) produced 42 GGAT1 overexpression lines that express different levels of GGAT1 mRNA. The levels of free serine, glycine, and citrulline increased markedly in GGAT1 overexpression lines compared with levels in the wild-type plants. Moreover, the levels of these amino acids were strongly correlated with the levels of GGAT1 mRNA and with the GGAT activity in the leaves. These results suggest that the photorespiratory aminotransferase reactions catalyzed by GGAT and SGAT are important regulators of amino acid contents. Increased Seed Longevity Seed longevity is of paramount importance to the seed industry and in germplasm conservation efforts. To date, only genes that reduce seed longevity have been described. Among the genes whose overexpression might potentially increase seed longevity are those coding for small heat shock proteins (sHSPs) since they contribute to various processes that have been associated with seed longevity, such as heat and desiccation tolerance, membrane stabilization, and oxidative stress resistance. Previously, it has been shown that the transcription factor HaHSFA9 is specifically involved in the developmental regulation of sHSP genes in sunflower (Helianthus annuus) embryos. Prieto-Dapena et al. (pp. 1102–1112) have tested the effects of the seed-specific overexpression of HaHSFA9 in transgenic tobacco (Nicotiana tabacum) under the control of a late embryogenesis abundant (LEA) promoter. They report that HaHSFA9 overexpression increases seed longevity while having no adverse effects on plant growth, morphology, or seed production. These findings may lead to improved seed longevity in economically important crops.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900208. Related articles in Plant Physiol.:
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-glutamyl-cysteinyl-glycine) were also found to be strong agonists of the GLR3.3-mediated responses. These results suggest that the sensing of six amino acids in the rhizosphere and perhaps extracellular peptides is coupled to Ca2+ signaling through a GLR-dependent mechanism homologous to a fundamental component of neuronal signaling.