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Plant Physiology 145:573-574 (2007) © 2007 American Society of Plant Biologists On the InsidePhotoprotection in Lichens Lichens are often found growing on exposed rocks or trees, where they face high levels of irradiation while in the desiccated state. To grow and survive, lichens must resist photodamage while desiccated and resume photosynthesis soon after hydration. Since PSII is often a target of desiccation-induced damage, lichens must render PSII largely inactive and/or minimize the amount of solar radiation reaching it to maximize their endurance while desiccated and their recovery upon rehydration. PSII activity is often assessed in vivo by the measurement of variable chlorophyll (Chl) a fluorescence, which originates from active PSII reaction centers. A decrease in variable Chl a emission observed in desiccated lichens is likely associated with their phototolerance and may arise from multiple mechanisms. One mechanism involves structural changes that induce changes in light-scattering and "shading" properties. During desiccation the algae aggregate and change shape to limit exposure to light, while at the same time the lichen thallus curls to minimize the available surface area and reduce light absorbance. These mechanisms decrease the exposure of the photosynthetic apparatus of the photobiont to light. An alternative mechanism contributing to phototolerance is the dissipation of absorbed energy by the lichen's photosynthetic apparatus. Veerman et al. (pp. 997–1005) have investigated the origins of fluorescence quenching in the poikilohydric lichen Parmelia sulcata (Fig. 1 ) by means of steady-state, low temperature, and time-resolved Chl fluorescence spectroscopy. They found the most dramatic target of quenching to be PSII, which produces negligible levels of fluorescence in desiccated lichens. They also report that fluorescence decay in desiccated lichens was dominated by a short-lifetime, long-wavelength component energetically coupled to PSII. The long-wavelength-quenching species was responsible for most (about 80%) of the fluorescence quenching observed in desiccated lichens; the rest of the quenching was attributed to structural changes in the lichen thallus.
Functional Sieve Element Protoplasts The isolation of functional sieve element (SE) protoplasts has proven difficult. Conventional enzyme mixtures simply turn phloem tissues into a mash. Another problem is the unequivocal identification of SE protoplasts. They easily fragment into smaller protoplasts during isolation and, therefore, may be difficult to distinguish. Nevertheless, it remains tempting to isolate and identify SE protoplasts for several reasons. The SE plasma membrane contains numerous ion channels and carbohydrate carriers that are essential for sieve tube functioning. Ion channels are important not only for the control of ionic balance in SEs, but also for the regulation of photoassimilate transport rates and the long-distance propagation of electrical signals via the phloem. SE protoplasts would provide a powerful new tool for understanding SE function. SE protoplasts from successive phloem sections would also enable identification, characterization, and quantification of carbohydrate carriers in the SE plasma membrane at various sites along the phloem translocation pathway. Hafke et al. (pp. 703–711) employed Vicia faba phloem for isolation of SE protoplasts since the SEs in this species contain giant protein bodies that are useful for SE identification. The authors tested the integrity of the SE protoplasts using fluorochromes and osmotic treatments. Patch-clamp experiments revealed that a weak inward-rectifying potassium channel dominated the current-voltage relations of the SE protoplasts at negative membrane voltages.
Millimolar concentrations of ATP may be released into the extracellular space as a result of wounding. In plant as well as in animal systems, extracellular ATP (eATP) regulates a broad range of physiological processes. In the case of plants, eATP has been implicated in the inhibition of root gravitropism, polar auxin transport, pollen germination, and the regulation of growth. Inositol phosphates, Ca2+, cAMP, and nitric oxide (NO) are among the second messengers involved in eATP signaling in animal cells. Less is known about eATP signaling in plant cells, although previous studies have shown that Ca2+ and superoxide are required for ATP-induced responses in plants. In this issue, Foresi et al. (pp. 589–592) report that exogenous ATP induces NO production in tomato (Solanum lycopersicum) suspension-cultured cells, as it does in animal cells, via P2-like purinergic receptors. Pharmacological evidence suggests that both NO synthase and nitrate reductase activities are involved in ATP-induced NO production in tomato cells, and that ATP-induced NO production is mediated, as it is in animal cells, by P2 purinergic like-receptors. Although the pharmacological data is consistent with the presence of P2-like receptors in plants, it should be noted that such receptors have not yet been identified in plants.
The outermost cell layer of the stele is an anatomically distinct layer of thin-walled cells called the pericycle. A unique attribute of pericycle cells compared to other root cells that have left the meristematic zone is the competence of a subset of these cells to re-enter the cell cycle and become founder cells of lateral root meristems. Dembinsky et al. (pp. 575–588) have isolated pericycle cells from maize (Zea mays) roots via laser capture microdissection. Microarray experiments identified 32 genes preferentially expressed in pericycle versus all other root cells that have left the apical meristem. Seven genes were preferentially expressed in pericycle versus central cylinder cells of the same root region. The most abundant functional categories among pericycle-specific genes were transcription and protein synthesis. A proteomic analysis revealed that two proteins were preferentially expressed in the pericycle. Data are presented that are consistent with the idea that pericycle specification and lateral root initiation may be controlled by a different set of genes.
The external application of cytokinins to mosses has long been known to induce bud formation. However, a detailed profile of endogenous cytokinin titers in mosses is wanting, and it is unclear how the intracellular and extracellular distributions of cytokinins regulate moss development. von Schwartzenberg et al. (pp. 786–800) present a comprehensive analysis of the intracellular and extracellular distributions of cytokinins in the moss Physcomitrella patens, together with data on their activity, and the influence of cytokinin deficiency on its developmental processes. The most abundant intracellular cytokinins were cis-zeatin-riboside-O-glucoside, N6-(
The plasma membrane-associated proteins called remorins were named after the remora fish that attaches to sharks. Remorins were originally discovered in a screen for plasma membrane proteins that are differentially phosphorylated in the presence of oligogalacturonides. Several proteomic studies of PM preparations indicate that plant remorins also occur in association with PMs. Interestingly, remorins have also been found in detergent-resistant membrane fractions, called lipid rafts, which are membrane microdomains that that have been implicated in a wide diversity of cellular processes. Searches in databases of fully sequenced plant species reveal the existence of eight, 16, and 19 remorin genes in Populus trichocarpa, Arabidopsis, and Oryza sativa, respectively. Patterns of differentially regulated remorin genes and proteins have been reported in an increasing number of transcriptomic and proteomic studies. Since many of these are annotated as "unknown function," they need to be properly classified and given suitable nomenclature. Unfortunately, there is no published genetic evidence concerning any plant species that can help scientists decipher the putative functions of remorins. The few reported attempts to generate plant mutants in which one or more of the ubiquitously expressed remorin genes were knocked out either failed or did not lead to an obvious phenotype. Raffaele et al. (pp. 593–600) present a genome-wide survey of the remorin family in the fully sequenced plant genomes. They have combined a phylogenetic approach with various in silico sequence-scanning tools and analysis of available genetic and genomic data to identify a specific remorin signature and to define several groups within the remorin family. They have used this classification to propose a general nomenclature for remorins that should prove useful in differentiating between the different members in future studies.
Department of Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900241 Related articles in Plant Physiol.:
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2-isopentenyl)adenosine-5'-monophosphate, and trans-zeatin-riboside-O-glucoside. The most abundant extracellular cytokinin was iPRMP. The effects of overexpressing a heterologous cytokinin oxidase/dehydrogenase gene (AtCKX2 from Arabidopsis [Arabidopsis thaliana]) on the intracellular and extracellular distribution of cytokinins were also assessed. In cultures of CKX-transformed plants, there were pronounced reductions in the extracellular concentrations of N6-(