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Plant Physiology 137:829-830 (2005) © 2005 American Society of Plant Biologists On the InsideProteomics of S-Nitrosylated ProteinsAlthough nitric oxide (NO) is now recognized as a key signaling molecule in plants, studies of NO-dependent processes in animal systems have demonstrated that protein S-nitrosylation of Cys residues is an important regulatory mechanism for many animal proteins. S-Nitrosylation is now regarded as posttranslational modification similar to phosphorylation. In this issue, Lindermayr et al. (pp. 921–930) employ a proteomics approach to examine the question of whether S-nitrosylation is common in Arabidopsis as well. They have identified 63 proteins from cell cultures and 52 proteins from leaves that represent candidates for S-nitrosylation, including stress-related, redox-related, signaling/regulating, cytoskeleton, and metabolic proteins. Strikingly, about 60% of these proteins have been identified previously as targets of S-nitrosylation in animals. At the enzymatic level, a case study demonstrated NO-dependent reversible inhibition of plant glyceraldehyde-3-phosphate dehydrogenase, suggesting that this enzyme could be affected by S-nitrosylation. This research represents an important starting point for further investigations into the signaling pathways and other cellular processes regulated by protein S-nitrosylation in plants. Role of Microtubules during Xylogenesis Tracheary elements possess elaborately patterned secondary cell walls that give rise to their characteristic appearances as well as their abilities to withstand high negative pressure. Cortical microtubules lying directly beneath the secondary walls are known to participate in the regulation of secondary wall deposition. The clustering of these microtubules is found to precede cell wall thickening and to be orientated parallel with the cellulose microfibrils and patterned secondary walls. If these cortical microtubules are disrupted, irregular secondary walls are observed. Previous studies of microtubules and secondary walls have been restricted to observations of chemically fixed cells. Oda et al. (pp. 1027–1036) have established a system to induce and observe the differentiation of tracheary elements in living Arabidopsis suspension cells stably expressing a GFP-tubulin fusion protein (Fig. 1). Approximately 30% of the cells differentiated into tracheary elements 96 h after culture in auxin-free media containing 1 µM brassinolide. Using this differentiation system, the authors have been able to elucidate the sequence of microtubule arrangement during secondary wall thickening. They report that a single microtubule bundle first appears beneath the secondary wall and then develops into two separate bundles located along both sides of the developing secondary wall. In response to colchicine, the secondary wall patterns turn rough and the smooth rims of the secondary walls are destroyed. In contrast, taxol-treated tracheary elements tended to form simpler and more oblique secondary wall patterns. These results suggest that the pair of microtubule bundles adjacent to the secondary wall plays a crucial role in the regulation of secondary wall development.
ETR1 and Hydrogen Peroxide Signaling Hydrogen peroxide (H2O2) is an important player in several processes in plants such as stomatal closure, root growth, gravitropism, and responses to pathogen challenge. Although it has been suggested that oxidative modification of reactive Cys residues within proteins may be the means by which H2O2 signaling may activate responses such as gene expression and reversible protein phosphorylation, the precise linkage between H2O2 perception and intracellular signaling remains to be elucidated. Exogenous H2O2 has previously been shown to up-regulate the expression of genes encoding elements of both two-component signal transduction pathways and ethylene signaling, suggesting the possibility of cross-talk between the H2O2 and ethylene signaling pathways. His kinases (HKs), such as the ethylene receptor ETR1, are part of the two-component systems that transduce environmental signals into cellular responses. Hybrid HKs consist of an N-terminal signal input domain, an HK domain, and a C-terminal response regulator domain. In this issue, Desikan et al. (pp. 831–834) report genetic and physiological data that demonstrate a novel function for the Arabidopsis ETR1, that of mediating H2O2 signaling in stomatal guard cells. Stomata in the loss-of-function etr1-7 mutant do not close in response to H2O2, and mutation of a Cys residue in the N-terminal region of ETR1 disrupts H2O2 signaling in both plants and yeast (Saccharomyces cerevisiae). These findings demonstrate an unexpected role for ETR1, that of mediating stomatal closure in response to H2O2. Until now, ETR1 has been associated solely with ethylene perception and signaling. The discovery that ETR1 can, in fact, mediate cellular responses to two different signaling molecules, namely, ethylene and H2O2, indicates multiple functions for this single protein. Transport of Jasmonate Precursors into Peroxisomes
Jasmonic acid (JA) and related jasmonates play important roles in plant defense, metabolism, and development. JA is formed from the unsaturated fatty acid linolenic acid (18:3), an abundant octadecanoid in higher plant membranes. Most of the enzymes involved in the 18:3 pathway have now been identified by a combination of classical biochemical and genetic approaches. In short, JA biosynthesis is initiated in the chloroplast and terminates with several rounds of Transcriptome of a Hypoxic Response Plant roots growing in poorly drained soil often experience hypoxia following transient flooding or irrigation. The deleterious effects associated with hypoxia include a decrease in cellular energy charge, a drop in cytoplasmic pH, and accumulation of toxic end products from anaerobic respiration. These changes, as well as the production of reactive oxygen species during recovery, are responsible for the slowed growth and reduced yield of many agriculturally important crops following flooding. To gain insight into how plants respond to low oxygen stress, Liu et al. (pp. 1115–1129) have examined gene expression at 7 intervals over 24 h, in wild-type and transgenic PSAG12:ipt Arabidopsis plants. The latter have previously been found to perform better during poststress recovery following flooding relative to wild-type plants. In this study, the authors found almost no difference in the transcriptional profiles of transgenic PSAG12:ipt and wild type in response to hypoxic conditions over the first 24 h of stress, emphasizing that the advantage PSAG12:ipt enjoys is probably limited to the poststress recovery phase. In both plant types, the levels of gene products involved in glycolysis and fermentation pathways, ethylene synthesis and perception, calcium signaling, nitrogen utilization, trehalose metabolism, and alkaloid synthesis were significantly altered in response to oxygen limitation. Further analysis suggested a significant down-regulation of genes whose functions are associated with cell walls, nucleosome structures, water channels and ion transporters, and a significant up-regulation of genes involved in transcriptional regulation, protein kinase activity, and auxin responses under conditions of oxygen shortage. This study represents one of the most comprehensive analyses conducted to date investigating hypoxia-responsive transcriptional networks in plants. Circadian Gating of a Cold-Induced Transcription Factor Circadian rhythms in chilling and freezing tolerance have long been known for several plant species, but the molecular biology underlying this phenomenon is poorly understood. At present, the best studied genetic system involved in cold acclimation is the Arabidopsis CBF cold response. Exposing Arabidopsis plants to low temperature results in rapid induction of a small family of transcriptional activators known as CBF1, 2, and 3 (CBF1-3) that induces expression of a battery of genes that increase plant freezing and chilling tolerance. Recently, it has been shown that basal levels of CBF3 transcripts and those of certain CBF-regulated genes exhibit circadian cycling. Fowler et al. (pp. 961–968) have further explored the regulation of CBF1-3 by the circadian clock and report some curious results. Their research indicates that the extent to which CBF1-3 transcripts accumulate in response to low temperature is dependent on the time of day that the plants were exposed to low temperature and that this daily variation is under circadian control. The highest and lowest levels of cold-induced CBF1-3 transcript accumulation occurred at 4 and 16 h after subjective dawn, respectively. Although the results presented establish that the circadian clock regulates the low temperature-induced production of CBF1-3, the biological significance of this regulation remains uncertain. It seems counterintuitive that the clock would dampen CBF1-3 expression in the evening, as temperatures are generally lowest during the night. Moreover, the cold responsiveness of two CBF-target genes, COR78 and COR6.6, was, at most, marginally affected by the clock, raising the question of whether the gating of cold-induced accumulation of CBF1-3 transcripts has a significant impact on the cold-regulated expression of genes downstream of CBF1-3. Nevertheless, a better understanding of the molecular bases for this regulation should provide insight into the nature of the cold signaling pathway responsible for CBF1-3 gene expression and shed new light on output pathways from the circadian clock.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 Related articles in Plant Physiol.:
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-oxidation in the peroxisome. In this issue, Theodoulou et al. (pp.