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Plant Physiology 144:1-2 (2007) © 2007 American Society of Plant Biologists On the InsideViagra Blocks the Effects of Gravity in Germinating Fern SporesGerminating spores of the fern Ceratopteris richardii respond to gravity. The earliest detected response in this system is a gravity-directed Ca2+ current that rapidly reverses direction when the cells are turned upside-down. The orientation of this current predicts the direction of cell polarization, including the direction of nuclear migration, and, 48 h later, the direction of primary rhizoid growth, which is indicated by the site where Ca2+ entered the cell. Salmi et al. (pp. 94–104) present pharmacological results that support a role for nitric oxide (NO) and cGMP in mediating the responses of germinating Ceratopteris spores to gravity. The authors report that Viagra, a cGMP phosphodiesterase inhibitor that increases NO-dependent cGMP levels, disrupted the gravity-directed changes in cell polarity in a dose-dependent manner. The effects of Viagra effects were antagonized by application of NO scavengers. To identify other components of the signaling system, the authors analyzed gene expression changes induced by Viagra treatment using microarrays and quantitative real-time reverse transcription-PCR. Preliminary microarray analyses revealed that the expression of several genes, possibly involved in actin dynamics and chaperone function, was significantly altered by Viagra treatment. Thus, NO and cGMP appear to act as downstream effectors linking the gravity stimulus to polarized growth in C. richardii spores. Readers interested in NO may also wish to read the contribution by Zhao et al. (pp. 206–217) in which they characterize the roles of NO in mediating the responses of Arabidopsis (Arabidopsis thaliana) to salt stress. More specifically, they compare the effects of NaCl on wild-type plants versus a mutant (Atnoa1) that exhibits impaired NO synthase activity in vivo and reduced endogenous NO levels. Germination of Atnoa1 seeds was more sensitive to NaCl than that of wild-type seeds, and wild-type plants exhibited higher survival rates than Atnoa1 plants when grown under salt stress. A wide range of pharmacological experiments indicated that disruption of NO synthase-dependent NO production is associated with reduced salt tolerance in Arabidopsis. Genomics of Suberin Biosynthesis and Cork Differentiation Cork formation involves proliferation and commitment of the phellogen derivatives, cell expansion and extensive deposition of suberin and waxes, and an irreversible program of senescence ending in cell death. Cork cells are made impervious by the deposition of suberin onto cell walls. Although suberin deposition and cork formation are essential for survival of land plants, few molecular studies have been conducted on this tissue. To gain insight into the molecular biology underlying suberin production and cork formation in cork trees (Quercus suber), Soler et al. (pp. 419–431) used a two-step strategy. First, by means of suppression subtractive hybridization, they obtained a library of ESTs that are preferentially induced in cork. Then, these ESTs were printed on a microarray and subsequently used for a global comparison between a suberin-producing (cork/phellem) and a non-suberin-producing (wood/xylem) tissue. Based on these results, a list of candidate genes involved in cork formation and function was obtained. This list includes genes for the synthesis, transport, and polymerization of suberin monomers, such as components of the fatty acid elongase complexes, ABC transporters, and acyltransferases, as well as a number of regulatory genes, including MYB, NAM, and WRKY transcription factors. Actin and Vacuolar Dynamics in Papilla Cells Although considerable advances have been made in understanding the molecular biology underlying self-incompatibility in Brassica rapa, the cellular mechanisms used by papilla cells to block self-pollen germination and to facilitate cross-pollen germination remain unclear. Iwano et al. (pp. 72–81) have visualized and compared the configurations of actin filaments in stigmatic papilla cells before and after self- and cross-pollination by staining actin filaments with rhodamine-phalloidin and by the transiently expressing GFP-mTalin in papilla cells. The authors report that cross-pollination induces actin polymerization, whereas self-pollination induces actin depolymerization or, at least, reorganization. Additionally, three-dimensional electron microscopic tomography revealed a close association of the actin cytoskeleton with an apical vacuole network. Self-pollination disrupted the vacuole network, while cross-pollination led to vacuolar rearrangements toward the site of pollen attachment (Fig. 1 ). These findings suggest that self- and cross-pollination differentially affect the dynamics of actin cytoskeleton, leading to changes in vacuolar structure associated with pollen hydration and germination.
Phospholipase C-Dependent Proline Accumulation A common response to hyperosmotic stress in plants is a decrease in the intracellular water potential resulting from the accumulation of compatible osmolytes such as ions, amino acids, and soluble sugars. Among these osmolytes, the amino acid Pro is the most widely accumulated in plants. To identify potential components of the signaling pathways required for the regulation of Pro accumulation, Parre et al. (pp. 503–512) screened a variety of pharmaceutical compounds for their effects on Pro accumulation in Arabidopsis. The aminosteroid U73122, a commonly employed specific inhibitor of phospholipase C (PLC), reduced the accumulation of pyrroline-5-carboxylate synthetase, a key enzyme in Pro synthesis, as well as Pro levels in salt-treated seedlings. This effect was not seen in seedlings stressed with mannitol. The authors also provide evidence that PLC signaling in response to salt stress involves inositol trisphosphate-gated Ca2+ release from internal stores but not phosphatidic acid signaling from diacylglycerol. The inhibitory effect of U73122 was reversed by the application of extracellular calcium or by modifying the intracellular calcium homeostasis with cyclopiazonic acid, a blocker of plant type IIA calcium pumps. These results suggest that PLC-based signaling is involved in Pro accumulation in response to ionic (NaCl) hyperosmotic conditions, but this signal transduction pathway is not involved in nonionic (mannitol) hyperosmotic stress. UV Radiation Drives CO2 Fixation in Marine Phytoplankton Photosynthesis by phytoplankton in aquatic environments accounts for more than 40% of global primary production. Within the euphotic zone, however, phytoplankton are exposed not only to photosynthetically active radiation (PAR; 400–700 nm) but also to UV radiation (UVR; 280–400 nm) that can penetrate to considerable depths. In contrast to PAR, which energizes photosynthesis, UVR is usually regarded as a stressor. Solar UVR is known to reduce photosynthetic rates, and damage cellular components such as D1 proteins and DNA molecules. On the other hand, positive effects of UVR, especially of UV-A (315–400 nm), have also been reported. For example, UV-A enhances carbon fixation of phytoplankton under reduced or rapidly fluctuating solar irradiance and allows for the photorepair of UV-B-induced DNA damage. Furthermore, UV-A increases biomass production of some cyanobacteria as compared to that under PAR alone. Positive effects of UVR have also been reported in corals and their algal symbionts and in diatoms. However, despite the reports of positive effects of solar UVR on these aquatic photosynthetic organisms, there is little information concerning to what extent and by what mechanisms UVR is utilized by phytoplankton. Gao et al. (pp. 54–59) provide evidence that UVR, although occasionally causing photoinhibition at high PAR levels, can also act as an additional source of energy for photosynthesis in tropical marine phytoplankton. These results indicate that moderate levels of UVR can enhance primary production of phytoplankton. Therefore, oceanic carbon fixation estimates may be underestimated by a large percentage if UVR is not taken into account. How Does Photorespiration Prevent Photoinhibition? Excessive light absorption can damage PSII and cause photoinhibition. Photorespiration is an important mechanism for protecting PSII from photoinhibition. A number of photorespiratory pathway mutants have been isolated by their inability to grow at air versus high CO2 conditions, and it has been shown that the photorespiratory pathway is indispensable for growth and survival of C3 plants under current atmospheric conditions. To avoid photoinhibition of PSII, photoprotective mechanisms are used by plants to both suppress the photodamage to PSII and to facilitate the repair of photodamaged PSII. The extent of photoinhibition, therefore, represents a dynamic balance between photodamage to PSII and its repair. It is believed that the consumption of photochemical energy, such as ATP and NADPH, through photorespiration helps avoid the photooxidative damage to PSII by highly toxic singlet oxygen. To further elucidate the role of photorespiration in ameliorating photoinhibition, Takahashi et al. (pp. 487–494) have examined the effect of the impairment of the photorespiratory pathway on the photoinhibition process. This was achieved using four Arabidopsis mutants of the photorespiratory pathway that impair Fd-GOGAT, Ser hydroxymethyltransferase, Glu/malate transporter (DiT2), and glycerate kinase. The authors report that reduced photorespiration enhances photoinhibition by inhibiting the de novo synthesis of D1 protein at the translation step. Thus, contrary to previous beliefs, impairment of photorespiration increases photoinhibition not by accelerating photodamage to PSII but by suppressing the repair of photodamaged PSII.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900224 Related articles in Plant Physiol.:
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