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Plant Physiology 137:1-2 (2005) © 2005 American Society of Plant Biologists On the InsidePhotodamage and Repair of PSIIThe exposure of photosynthetic organisms to excessively strong light results in damage to the PSII complex. This photodamage has been studied extensively in vitro, for example, in isolated thylakoid membranes. In intact cells of photosynthetic organisms, however, photoinhibition is more complex because photodamaged PSII is rapidly repaired. The main feature of the repair process is the replacement of the D1 protein in the photodamaged PSII by newly synthesized D1 and reassembly of active PSII. Thus, in living photosynthetic organisms, the extent of photoinhibition represents a balance between rates of photodamage and the repair of PSII. In this issue, Allakhverdiev et al. (pp. 263–273) examine the roles of electron transport and ATP synthesis in these two processes by monitoring them separately and systematically in the cyanobacterium Synechocystis sp. PCC 6803. They report that the rate of photodamage, which is proportional to light intensity, is unaffected by inhibition of the electron transport in PSII, by acceleration of electron transport in PSI, or by inhibition of ATP synthesis. In contrast, the rate of repair and the synthesis of D1 protein are reduced upon inhibition of the synthesis of ATP either via PSI or PSII. Evidence is provided that ATP synthesis might influence the repair of PSII by regulating translation of psbA genes, which encode for a precursor of the D1 protein. Insect-Induced Conifer Defense The white pine weevil (Pissodes strobi) is a major insect pest of many conifers, particularly pines and spruces. The larvae inflict the most damage by killing the leaders of the infested trees (Fig. 1). Although much remains to be learned about the molecular mechanisms involved in the terpenoid defenses that are activated in response to white pine weevil infestation, it does appear that methyl jasmonate (MeJA) plays a major role. For example, treatment with MeJA induces formation of traumatic resin ducts in developing stem xylem and increases accumulation of oleoresin terpenoids in stem tissues. In general, however, MeJA treatment is only a partial mimic of insect damage, and there are often important qualitative and quantitative differences between the two. To compare insect- and MeJA-induced terpenoid responses, Miller et al. (pp. 369–382) analyzed traumatic oleoresin mixtures, emissions of terpenoid volatiles, and expression of terpenoid synthase (TPS) genes in Sitka spruce (Picea sitchensis) following attack by white pine weevils or application of MeJA. Overall, weevils and MeJA induced similar, but not identical, terpenoid defense responses in Sitka spruce. Both insects and MeJA caused traumatic resin accumulation in stems, with more accumulation induced by the weevils. Increased levels of weevil- and MeJA-induced TPS transcripts accompanied major changes in terpenoid accumulation in stems. Weevil-induced terpenoid emission profiles were also more complex than emissions induced by MeJA. Weevil feeding caused a rapid release of a blend of monoterpene olefins. These compounds were not found in MeJA-induced emissions.
Imaging Abscisic Acid Pools in Planta Our understanding of the role of abscisic acid (ABA) in plant physiological processes would be greatly facilitated by greater information concerning the dynamics and distribution of physiologically active ABA pools in planta. In an effort to provide such information, Christmann et al. (pp. 209–219) have developed a noninvasive system with single cell resolution that monitors the generation and distribution of pools of physiologically active ABA at the whole plant level. The system utilizes a luciferase reporter gene under the control of ABA-specific promoters. In the absence of water stress, low levels of ABA-dependent reporter activation were observed in the columella cells and quiescent center of the root, as well as in the vascular tissues and stomata of cotyledons, suggesting a nonstress related role for ABA in these cell types. Exposure of seedlings to exogenous ABA resulted in a uniform pattern of luciferase expression. In marked contrast, reporter expression in response to drought stress was predominantly confined to the vasculature and stomata. Surprisingly, water stress applied to the root system resulted in the generation of ABA pools in the shoot but not in the root. These results conflict with many current models of drought stress that postulate that the root is the sensor of dehydration and that the ABA generated within the root is subsequently translocated to the shoot where it regulates transpiration. The authors speculate that water stress sensed by the root may induce a long distance-acting signal, which triggers ABA biosynthesis in the shoot. Such a concept is supported by previous grafting experiments that revealed that ABA-biosynthesis of tomato (Lycopersicon esculentum) roots exposed to low-water potentials is not sufficient to induce proper stomatal closure in ABA-deficient shoots. Chilling and Hydrogen Peroxide Effects on Aquaporins Root hydraulic conductance decreases dramatically in response to cold temperatures. Although such decreases in hydraulic conductance are temporary in chilling-resistant species, there is little or no recovery in chilling-sensitive plants, and such plants may become severely wilted and die. Aroca et al. (pp. 341–353) investigated the effect of chilling on the respective root hydraulic conductances of two maize (Zea mays) genotypes differing in chilling tolerance, and compared these effects to the effects of chilling on the osmotic water permeability of isolated root cortex protoplasts, aquaporin gene expression, aquaporin abundance and aquaporin phosphorylation, H2O2 accumulation in the roots, and electrolyte leakage from the roots. Because chilling can cause H2O2 accumulation, the authors also studied the effects of a short H2O2 treatment of the roots on the same parameters. The response of aquaporin proteins to chilling and H2O2 treatments was the same in both genotypes. Both treatments significantly increased the abundance of aquaporins and the phosphorylation state of PIP2 aquaporin proteins. Although higher levels of aquaporins and greater phosphorylation may help explain the recovery from chilling in the chilling-tolerant genotype, they fail to account for why the chilling-sensitive genotype failed to recover. The chilling-tolerant genotype also had the capacity to avoid H2O2 accumulation during chilling, whereas the sensitive genotype did not. Thus, the recovery of hydraulic conductance during chilling in the chilling-tolerant genotype may be made possible by a greater abundance and/or activity of aquaporins and by avoiding or repairing oxidative damage to membranes. Membrane Microdomains in Arabidopsis Plasma Membrane Certain lipids, in particular sphingolipids and cholesterol, self-associate in tight clusters in membranes and become segregated from surrounding phospholipids. The lipid raft hypothesis postulates that the more ordered sterol- and sphingolipid-rich phases form discrete microdomains or lipid rafts within the membrane. In animal and yeast cells, lipid rafts are believed to function as sorting platforms for proteins destined for the plasma membrane. In addition to protein targeting, lipid rafts have been implicated in numerous cell-surface processes, including signal transduction, pathogen entry, secretion, and endocytosis. Lipid rafts and the proteins associated with them can be separated from nonraft membranes by suitable detergent extraction. The resulting fraction of detergent-resistant membranes (DRMs) is thought to consist of aggregates of the microdomains. In this issue, Borner et al. (pp. 104–116) developed a protocol to prepare DRMs from Arabidopsis and investigated their composition using immunoblots, proteomics, and lipid analysis. The results strongly support the hypothesis that Arabidopsis DRMs are predominantly derived from plasma membrane sphingolipid- and sterol-rich lipid rafts. Sterols and sphingolipids were 4- to 5-fold enriched in DRMs. Moreover, the DRMs were highly enriched in many well-known proteins that are specific to the plant cell plasma membrane. The authors also identified a plant homolog of flotillin, a major mammalian DRM protein, suggesting a conserved role for this protein in lipid domain phenomena in eukaryotic cells. Mechanical Properties of Callose in Pollen Tubes Germinating pollen must resist both tension stress and compression stress. Tension stress in the cell wall is created by turgor pressure and by the continuous insertion of new cell wall material at the growing tube tip. At the same time, in planta, compression stress is exerted by the surrounding tissue. Parre and Geitmann (pp. 274–286) have assessed the possible role of callose, a cell wall polymer that occurs in exceptionally large amounts in pollen, in resisting tension and compression stress in germinating pollen of Solanum chacoense and Lilium orientalis. The pollen of these two species showed very different patterns of callose deposition and very different responses to lyticase, an enzyme that specifically hydrolyses callose. Germination of Solanum pollen grains was stimulated by lyticase, and higher enzyme concentrations caused bursting of the grains at the aperture. This finding suggests that acidic pectins, the most abundant component next to callose, are unable to withstand the cellular turgor once callose is digested. Callose, therefore, does seem to be the main tension stress-bearing structure in the aperture cell wall of Solanum pollen grains. Pollen grains of Lilium, on the other hand, lacked prominent callose accumulations at the site of the emerging pollen tube, and their germination was not stimulated by any of the lyticase concentrations tested. Moreover, Lilium pollen grains remained morphologically intact when treated with high lyticase concentrations, even though germination was inhibited. To investigate whether or not callose is able to provide mechanical resistance against compression stress, the authors subjected pollen tubes to local deformation by microindentation. The data revealed that lowering the amount of callose resulted in reduced cellular stiffness and increased viscoelasticity, thus, indicating that callose is able to resist compression stress. Together, these data reveal the capacity of cell wall callose to resist both tension and compression stress and suggest that callose may play a mechanical role in at least some types of growing plant cells.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 Related articles in Plant Physiol.:
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