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Plant Physiology 149:1205-1206 (2009) © 2009 American Society of Plant Biologists On the InsideSystem Potentials: A New Form of Long-Distance Electrical Signal in Plants Both electrical and chemical signals are known to be involved in long-distance signaling in plants. Action potentials (APs) are all-or-none electrical depolarizations that travel without decrement. As a single binary event, a sole AP probably does not carry much information regarding the kind of threat or stress that caused it; it may, however, serve as a general stress signal. Variation potentials (VPs) are yet another form of long-distance electrical depolarization arising from chemicals released from areas of severe wounding. In this issue, Zimmermann et al. (pp. 1593–1600) report on a new type of systemic electrical signal that they have named "system potentials" (SPs). These electrical variations can be triggered by substances added to mechanically injured leaves of barley (Hordeum vulgare) and fava bean (Vicia faba). They are shown to propagate from leaf to leaf at rates of 5 to 10 cm/min. The initial polarity of SPs is hyperpolarizing with respect to the cell interior. Moreover, they do not obey the all-or-none rule, but depend on both concentration and the type of substance added. Thus, they are not APs. Their ability to travel long distances from organ to organ means that they are not localized receptor potentials. Since the initial direction of the signal is hyperpolarizing (with respect to the cell interior), SPs also do not appear to be VPs in the classical sense. Apoplastic ion flux analyses revealed that, in contrast to APs or VPs, all of the investigated ion movements (Ca2+, K+, H+, Cl–) occur after the voltage change begins. Pharmacological experiments with fusicoccin and vanadate suggest that SPs involve stimulation of plasma membrane H+-ATPases. These wound-induced SPs appear to represent a third type of electrical long-distance signaling in higher plants.
The oxygen-evolving complex (OEC) of PSII is one of the most vulnerable complexes of the photosynthetic electron transport chain. When the OEC is inactivated by heat stress, for example, the supply of electrons from water to the oxidized primary donor, P680+, is interrupted and the residual electron transport activity of PSII is rapidly lost. The accumulation of P680+ results in the loss of the capacity for primary charge separation and the degradation of the D1 protein of PSII. A short heat pulse has two advantages insofar as studying OEC inactivation in isolation: It allows the PSII reaction centers to retain their activity, and other secondary effects, such as desiccation, adaptation, or partial recovery, are avoided. Previously, it has been found that the PSII of barley leaves after OEC inhibition is supplied by electrons from a large pool of alternative donors. In vitro studies have shown that ascorbate (Asc) can replace water, the terminal donor of PSII, when the OEC is inactivated. These findings raise the question of whether Asc can serve in vivo as an alternative electron donor in whole leaves bearing inactivated OECs. Tóth et al. (pp. 1568–1578) have monitored the activity of the two photosystems with fast chlorophyll a fluorescence and 820-nm absorbance transient measurements in wild-type Arabidopsis (Arabidopsis thaliana) and its Asc-deficient mutant, vtc2-1, with and without externally supplied Asc. They have also compared thylakoid membranes isolated from heat-treated leaves with Tris-washed samples and investigated the pathway of electron donation from Asc to PSII. Their data provide evidence for the existence of an Asc-dependent electron transport via the redox-active Tyr residue TyrZ, a mechanism that appears to be present in plants and green algae and might protect PSII reaction centers from photooxidation, especially under conditions of moderate heat stress.
The root exudate-rich region around the root, the rhizosphere, supports large microbial populations capable of exerting beneficial, neutral, or detrimental effects on plant growth. Trichoderma spp. are free-living fungi that are common denizens of the rhizosphere. They have the capacity to help plants by producing antibiotics, parasitizing other fungi and competing with deleterious plant microorganisms. Until recently, these traits were considered to be the basis for how Trichoderma fungi exert their beneficial effects on crop productivity. It now appears, however, that certain strains also have substantial direct influences on plant architecture. In maize (Zea mays) plants, for example, Trichoderma inoculation affects root system architecture by enhancing root biomass production and root hair development. Many lines of evidence strongly support a role for auxin in the regulation of root system architecture. For example, the auxin applications increase lateral root and root hair development, whereas auxin transport inhibitors reduce root branching. Auxin-resistant mutants also produce fewer lateral roots than wild-type plants. Since auxin has been identified as a major player in controlling root architecture, Contreras-Cornejo et al. (pp. 1579–1592) have evaluated the response of Arabidopsis to inoculation with two Trichoderma species with the purpose of determining whether the architectural changes induced by the fungus are mediated by auxin. They report that both types of fungi promoted Arabidopsis seedling growth under axenic conditions and that this promotion of plant growth was correlated with the proliferation of lateral roots. A role for auxin signaling in mediating the observed developmental alterations induced by Trichoderma virens inoculation in plants was inferred from tests using auxin-responsive marker constructs and by the analysis of various auxin-related mutants of Arabidopsis. The authors also show that T. virens is able to produce indolic compounds, including indole-3-acetic acid, which may play a role in mediating plant growth promotion by this fungus.
Rhodococcus fascians is a biotrophic, Gram-positive actinomycete that infects plants. Unlike necrotrophic pathogens, biotrophs rely on living tissues for survival and multiplication. Upon infection with R. fascians, the architecture of plants is dramatically changed by the production of neoplastic shooty outgrowths. In the first of two articles, Depuydt et al. (pp. 1366–1386) compared the response of Arabidopsis upon presentation of one of two near-isogenic R. fascians strains, a wild-type line containing a linear virulence plasmid and a plasmid-free nonpathogenic derivative. Microarray analyses of infected Arabidopsis plants in combination with the profiling of primary metabolites revealed that R. fascians alters the cytokinin balance of the host as evidenced by the pronounced alterations in the expression levels of genes related to cytokinin perception, signal transduction, and homeostasis. The data also suggest that the virulent forms of R. fascians are able to actively suppress an oxidative burst involved in plant defense. The data indicate that the levels of free sugars and amino acids are greatly increased upon infection, presumably for use by the bacteria. This buildup of metabolites appears to inhibit the expression of photosynthesis-related genes in infected leaves. Interestingly, the high sugar levels that result from the induced sink metabolism also induce a number of defense-related genes. In a second article, Depuydt et al. (pp. 1387–1398) explore the question of how the leaf architecture of the host plant is changed upon infection by R. fascians. In infected Arabidopsis, the newly formed leaves exhibit small narrow lamina and serrated margins. The symptomatic leaves are less differentiated and display more, but smaller cells that accumulate in the G1 phase of the cell cycle. Thus, the growth of infected leaves appears to occur primarily through mitotic cell division and not via cell expansion. The extremely reduced phenotypical response of a cyclind3;1-3 triple knockout mutant suggests that the D-type cyclin/retinoblastoma/E2F transcription factor pathway, a major mediator of cell growth and cell cycle progression, plays a key role in symptom development in infected leaves.
Leaf discs grow as rapidly as intact leaf tissue and respond strongly to alterations of external cues. Moreover, leaf disc assays have a number of distinct advantages over whole-plant assays. For example, leaf discs require little space, allowing higher numbers of replicates. The application of chemical treatments is simple, as they do not have to be taken up by the root and transported to the leaves. Instead, leaf discs can float on solutions of active ingredients that are presented in, for example, microtiter plates. Finally, image-based measurements of growth benefit from the simple shape of leaf discs. Dicot leaves grow with pronounced diel (24-h) cycles that are controlled by a complex network of factors. It is an open question to what extent leaf growth dynamics are controlled by long-range or by local signals. To address this question, Biskup et al. (pp. 1452–1461) established a stereoscopic imaging system, GROWSCREEN 3D, which quantifies the surface growth of 458 isolated leaf discs floating on nutrient solution in wells of microtiter plates. A camera system is automatically displaced across the array of leaf discs; visualization and camera displacement takes about 12 seconds for each leaf disc, resulting in a time interval of 1.5 h for consecutive size analyses. Leaf discs showed a diel leaf growth cycle similar to that of intact leaves. Hence, it appears that the timing of leaf growth is regulated by local rather than by systemic control processes.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900284 Related articles in Plant Physiol.:
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