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Plant Physiology 147:933-934 (2008) © 2008 American Society of Plant Biologists On the InsideThe Function of Extracellular Carbonic Anhydrase in a Dinoflagellate
Marine phytoplankton are responsible for about half the total photosynthetic carbon fixation in the world. The marine environment, however, imposes serious restraints on carbon fixation. First, the equilibrium between CO2 and bicarbonate is pH dependent, and in seawater, which normally is slightly alkaline, [CO2] is typically low. Second, the rate of CO2 diffusion in seawater is slow, so unless cells are able to take up bicarbonate efficiently, photosynthesis quickly becomes carbon limited. Dinoflagellates are a major constituent of the oceanic phytoplankton. Unlike all other eukaryotic phytoplankton, dinoflagellates employ a form of Rubisco that has a lower CO2 affinity and lower CO2/O2 specificity than the more common form. The properties of this form of Rubisco would seem to potentially exacerbate the problem of carbon unavailability in marine environments. Carbonic anhydrase (CA), which mediates the rapid interconversion of CO2 and HCO3–, is an important part of most mechanisms used to alleviate the restriction of CO2 near Rubisco in marine phytoplankton. Indeed, CA activity is so important for carbon metabolism that most algae probably employ several different CA isoforms acting in concert to ensure an adequate supply of CO2 to Rubisco. In this issue, Lapointe et al. (pp. 1427–1436) identify a Antioxidant Properties of Raffinose and Galactinol Certain environmental stresses lead to the expression of enzymes involved in the biosynthesis of galactinol and raffinose family oligosaccharides (RFOs). Galactinol synthase (GolS) and raffinose synthase (RS) are important enzymes in the synthesis of RFOs. GolS catalyzes the first committed step in the biosynthesis of RFOs and plays a key regulatory role in the carbon partitioning between Suc and RFOs. RS catalyzes the synthesis of raffinose from Suc and galactinol. It has previously been reported that the expression of Arabidopsis (Arabidopsis thaliana) GolS1 and GolS2 is regulated by a heat shock transcription factor (HsfA2). To explore the relationship between the roles of galactinol and/or raffinose and oxidative stress conditions, Nishizawa et al. (pp. 1251–1263) have studied the expression of GolS and RS isoenzymes and levels of galactinol and raffinose in HsfA2-overexpressing plants and in wild-type plants under control growth conditions. In leaves of Arabidopsis plants overexpressing HsfA2, the transcription of several GolS1 isoenzymes and an RS isoenzyme was highly induced, and, thus, levels of galactinol and raffinose increased compared with those in wild-type plants under control growth conditions. High intracellular levels of galactinol and raffinose in the transgenic plants were correlated with increased tolerance to methylviologen treatment and salinity or chilling stress. Galactinol and raffinose also were found to effectively protect salicylate from attack by hydroxyl radicals in vitro. The present findings suggest a novel function for galactinol and raffinose as hydroxyl radical scavengers that help protect plant cells from oxidative damage caused by methylviologen treatment, salinity, or chilling. Phytoremediation of a Vinyl Chloride Precursor The haloalkane 1,2-dichloroethane (1,2-DCA), used in the synthesis of vinyl chloride, is a carcinogen that pollutes soil and groundwater. Although plants lack the enzymatic activity required to metabolize and dehalogenate 1,2-DCA, the bacterium Xanthobacter autotrophicus does have the ability to dehalogenate a range of halogenated aliphatics, including 1,2-DCA. In a reaction catalyzed by haloalkane dehalogenase (DhlA), 1,2-DCA is initially dehalogenated by the substitution of one of the two terminal chlorine atoms by a hydroxyl group to form chloroethanol. Chloroethanol is then converted to chloroacetic acid by two sequential dehydrogenation steps catalyzed by alcohol dehydrogenase and aldehyde dehydrogenase. Chloroacetic acid is potentially toxic to the bacterium but is converted to the glyoxylate cycle intermediate glycolate by the action of L-haloacid dehalogenase (DhlB). In this issue, Mena-Benitez et al. (pp. 1192–1198) demonstrate that the introduction of both DhlA and DhlB, in combination with endogenous alcohol dehydrogenase and aldehyde dehydrogenase activities, into tobacco (Nicotiana tabacum) plants can create a complete pathway for the degradation of 1,2-DCA. Plants expressing DhlA only produced phytotoxic levels of chlorinated intermediates and died, whereas plants expressing DhlA together with DhlB thrived at levels of 1,2-DCA that were toxic to DhlA-expressing plants. These results represent a significant advance in the development of a low-cost, phytoremedial approach toward the clean-up of halogenated organic pollutants from contaminated soil and groundwater. Function of Cell Wall Invertase in Plant Defense When plant cells are attacked by pathogens, a network of energetically expensive, carbohydrate-requiring cellular reactions is initiated. Thus, plants have evolved mechanisms to modulate their carbohydrate fluxes in response to infections. Several lines of evidence suggest that plants establish high hexose levels in response to invading pathogens, which in turn support defense responses of the host. Cell wall invertase (cwINV) is a sink-specific enzyme that catalyzes the irreversible cleavage of Suc into hexoses. It has been shown previously that a rapid induction of cwINV is one of the early defense-related reactions in resistant tobacco source leaves after infection with the oomycete Phytophthora nicotianae, a soil-borne plant pathogen. Yet, it still remains unclear whether cwINV activity and the related reprogramming of the carbon metabolism are indispensable elements of defense in source leaves. To address this question, Essmann et al. (pp. 1288–1299) generated transgenic tobacco plants in which cwINV activity in source leaves is constitutively suppressed by an RNA interference construct. They then investigated changes in the carbohydrate status, callose deposition, and hypersensitive reaction/cell death in source leaves during an interaction with the oomycete P. nicotianae. They report that the resistant wild type exhibited hypersensitive lesions about 24 h postinoculation. In contrast, the transgenic tobacco plants in which cwINV activity was repressed by 90% exhibited weaker hypersensitive reaction symptoms and were less tolerant of the pathogen, even though they developed normally under standard growth conditions. These results strongly support the hypothesis that insufficient carbohydrate supply can delay, hinder, or even completely suppress hypersensitive cell death, and that cwINV plays a key role in the acquisition of carbohydrates during plant defense. Zinc and Programmed Cell Death In many types of animal cells, zinc is an important regulator of programmed cell death (PCD). Specifically, zinc depletion commits cells to death. In contrast, the involvement of zinc in the control of plant PCD remains unknown. The earliest function of PCD in plant ontogenesis occurs when the embryo proper and its suspensor develop during early embryogenesis. While the embryo proper develops eventually into a plant, the embryo suspensor is committed to death and elimination. Autophagic PCD is essential for the complete elimination of fully differentiated suspensors during late embryogenesis. Helmersson et al. (pp. 1158–1167) have used somatic embryos of Norway spruce (Picea abies) to investigate the role of zinc in developmental PCD. Staining of the early embryos with zinc-specific molecular probes revealed high accumulation of zinc in the proliferating cells of the embryo proper and abrupt decreases of zinc content in the dying suspensor cells. Exposure of early embryos to a membrane-permeable zinc chelator led to embryonic lethality. The death of suspensor cells involves the loss of plasma membrane integrity, metacaspase-like proteolytic activity, and nuclear DNA fragmentation. Supplementation with extra zinc suppressed terminal differentiation and death of the suspensors, thereby causing a delay of suspensor elimination and embryo maturation. These data demonstrate that perturbation of zinc homeostasis disrupts the balance between cell proliferation and PCD required for plant embryogenesis. Thus, zinc is an important cue governing cell fate decisions in plants. Retrotransposon Silencing in Arabidopsis Transposable elements, first discovered in plants, account for a large proportion of some plant genomes. Long terminal repeat (LTR) retrotransposons, such as the tobacco Tnt1 element, are retrovirus-like elements that represent the most widespread class of mobile elements in plants. LTR retrotransposons are generally silent in plant genomes. However, they often constitute a large proportion of repeated sequences in plants. This suggests that their silencing is set up after a certain copy number is reached and/or that it can be released in some circumstances. The genome of Arabidopsis contains relatively few retrotransposons, most of them being methylated and inactive. Previous studies have shown that transposable elements are inactivated by silencing mechanisms involving DNA or histone modifications. Pérez-Hormaeche et al. (pp. 1264–1278) have introduced the tobacco LTR retrotransposon Tnt1 into Arabidopsis, thus mimicking the horizontal transfer of a retrotransposon into a new host species and allowing them to study the regulatory mechanisms controlling its amplification. In the present study, the authors show that Tnt1 is regulated by transcriptional gene silencing dependent on the number of Tnt1 copies per plant. The silencing of Tnt1 is released in polymerase IV and DNA methylation mutants but not in methyltransferase1 mutants. Thus, the silencing of Tnt1 appears to be associated with non-CG methylation of its promoter and with small RNAs targeting the LTR regions. These findings suggest the involvement of an RNA-dependent DNA methylation process directed to the Tnt1 promoter. Silencing is reset at each generation, allowing the reactivation of the element when the number of copies declines by genetic segregation. Tnt1 silencing is also locally released by wounding.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900265 Related articles in Plant Physiol.:
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-CA from the marine dinoflagellate Lingulodinium polyedrum that appears to play a role in carbon acquisition. They report that the CA activity of whole Lingulodinium cells was susceptible to the poorly membrane-permeable CA inhibitor acetazolamide, suggesting that the