O(2) photoreduction by photosynthetic electron transfer, the Mehler reaction, was observed in all groups of oxygenic photosynthetic organisms, but the electron transport chain mediating this reaction remains unidentified. We provide the first evidence for the involvement of A-type flavoproteins that reduce O(2) directly to water in vitro. Synechocystis sp. strain PCC 6803 mutants defective in flv1 and flv3, encoding A-type flavoproteins, failed to exhibit O(2) photoreduction but performed normal photosynthesis and respiration. We show that the light-enhanced O(2) uptake was not due to respiration or photorespiration. After dark acclimation, photooxidation of P(700) was severely depressed in mutants Deltaflv1 and Deltaflv3 but recovered after light activation of CO(2) fixation, which gives P(700) an additional electron acceptor. Inhibition of CO(2) fixation prevented recovery but scarcely affected P(700) oxidation in the wild-type, where the Mehler reaction provides an alternative route for electrons. We conclude that the source of electrons for O(2) photoreduction is PSI and that the highly conserved A-type flavoproteins Flv1 and Flv3 are essential for this process in vivo. We propose that in cyanobacteria, contrary to eukaryotes, the Mehler reaction produces no reactive oxygen species and may be evolutionarily related to the response of anaerobic bacteria to O(2).

17 O of the O 2 evolved were essentially identical to those of the substrate water. The fractionation slopes for the oxygenase reaction of Rubisco and respiration were identical (0.518 6 0.001) and that of glycolate oxidation was 0.503 6 0.002. There was a considerable difference in the slopes of O 2 photoreduction (the Mehler reaction) in the cyanobacterium Synechocystis sp. strain PCC 6803 (0.497 6 0.004) and that of pea (Pisum sativum) thylakoids (0.526 6 0.001). These values provided clear and independent evidence that the mechanism of O 2 photoreduction differs between higher plants and cyanobacteria. We used our method to assess the magnitude of O 2 photoreduction in cyanobacterial cells maintained under conditions where photorespiration was negligible. It was found that electron flow to O 2 can be as high as 40% that leaving photosystem II, whereas respiratory activity in the light is only 6%. The implications of our findings to the evaluation of specific O 2 -producing or -consuming reactions, in vivo, are discussed. O/ 16 O ratios in air bubbles in polar ice cores or dissolved O 2 in water bodies have been used to assess the photosynthetic rates on broad spatial and temporal scales (Luz et al., 1999;Luz and Barkan, 2000;Blunier et al., 2002). To accurately assess these rates, it is essential to know, with high precision, the isotope fractionation effects due to biological producing and consuming mechanisms. To date, precise measurements of these fractionations are available only for oxygen uptake in dark respiration by the cytochrome oxidase and the alternative oxidase pathways (Angert et al., 2003;Luz and Barkan, 2005 (Helman et al., 2003). In higher plants, electron transfer from PSI to oxygen, either from the Fx center of PSI or ferredoxin (Foyer and Noctor, 2000), results in the formation of superoxide. The latter is disproportionated by superoxide dismutase, and the H 2 O 2 produced is reduced to water by ascorbate peroxidase (Asada, 1999(Asada, , 2000. In cyanobacteria, NADPH donates electrons to A-type flavoproteins that reduce the oxygen directly to water without the formation of reactive oxygen intermediates (Vicente et al., 2002;Helman et al., 2003). Hence, the isotopic fractionation during photoreduction might differ between plants and cyanobacteria.In this study, we measured the triple isotopic O 2 fractionation by oxygen-producing and -consuming reactions in order to assess the overall photosynthetic oxygen production and the extent of these reactions in vivo. We show that the Mehler reaction of cyanobacteria has a different triple isotopic signature of oxygen compared to other oxygen-consuming processes. Thus, besides serving as an important tool for measurements of photosynthetic productivity, the relationship between the three oxygen isotopes can be used to assess the magnitude of the Mehler reaction under natural conditions. Determination of Triple Isotope Fractionations Fractionations during O 2 Uptake in the Absence of PhotosynthesisIn cases where O 2 -consuming reaction occurs withou...

The evolution of multicellularity in animals required the production of extracellular matrices that serve to spatially organize cells according to function. In corals, three matrices are involved in spatial organization: (i) an organic ECM, which facilitates cell-cell and cell-substrate adhesion; (ii) a skeletal organic matrix (SOM), which facilitates controlled deposition of a calcium carbonate skeleton; and (iii) the calcium carbonate skeleton itself, which provides the structural support for the 3D organization of coral colonies. In this report, we examine the production of these three matrices by using an in vitro culturing system for coral cells. In this system, which significantly facilitates studies of coral cell physiology, we demonstrate in vitro excretion of ECM by primary (nondividing) tissue cultures of both soft (Xenia elongata) and hard (Montipora digitata) corals. There are structural differences between the ECM produced by X. elongata cell cultures and that of M. digitata, and ascorbic acid, a critical cofactor for proline hydroxylation, significantly increased the production of collagen in the ECM of the latter species. We further demonstrate in vitro production of SOM and extracellular mineralized particles in cell cultures of M. digitata. Inductively coupled plasma mass spectrometry analysis of Sr/Ca ratios revealed the particles to be aragonite. De novo calcification was confirmed by following the incorporation of 45 Ca into acid labile macromolecules. Our results demonstrate the ability of isolated, differentiated coral cells to undergo fundamental processes required for multicellular organization.aragonite ͉ cell culture ͉ cnidaria ͉ calcification ͉ C orals (class, Anthozoa) are the most basal cnidarians and the first animal phylum with an organized neural system and complex active behavior (1). The embryonic gastrula develops to form an outer ectoderm and an inner endoderm separated by the mesoglea, a noncellular fibrous jelly-like material (2). The two germ layers are spatially structured by an ECM in which embedded, interstitial (stem) cells give rise to nematocysts, mucous glands, and sensory or nerve cells (2, 3). Many corals also precipitate calcium carbonate in the form of aragonite on a skeletal organic matrix (SOM) template (4, 5). The precipitation pattern is highly controlled between colonies, giving rise to morphological structures that are used as primary phenotypic markers of species in extant reefs and fossil samples.The basic cellular processes responsible for the production of ECM, SOM, and calcium carbonate skeleton remain largely unknown. Molecular, genetic, and physiological analyses of cellular processes in corals have been elusive mainly because it is difficult to grow corals under controlled conditions in the laboratory and because of the genetic and physiological complexities inherent in associations of the animals with symbionts and parasites. All zooxanthellate corals harbor intracellular symbiotic dinoflagellates (zooxanthellae) within their endoderm cells; the al...

Bacteria are able to sense their population's density through a cell-cell communication system, termed 'quorum sensing' (QS). This system regulates gene expression in response to cell density through the constant production and detection of signalling molecules. These molecules commonly act as auto-inducers through the up-regulation of their own synthesis. Many pathogenic bacteria, including those of plants, rely on this communication system for infection of their hosts. The finding that the countering of QS-disrupting mechanisms exists in many prokaryotic and eukaryotic organisms offers a promising novel method to fight disease. During the last decade, several approaches have been proposed to disrupt QS pathways of phytopathogens, and hence to reduce their virulence. Such studies have had varied success in vivo, but most lend promising support to the idea that QS manipulation could be a potentially effective method to reduce bacterial-mediated plant disease. This review discusses the various QS-disrupting mechanisms found in both bacteria and plants, as well as the different approaches applied artificially to interfere with QS pathways and thus protect plant health.

The ability to move on solid surfaces provides ecological advantages for bacteria, yet many bacterial species lack this trait. We found that Xanthomonas spp. overcome this limitation by making use of proficient motile bacteria in their vicinity. Using X. perforans and Paenibacillus vortex as models, we show that X. perforans induces surface motility, attracts proficient motile bacteria and 'rides' them for dispersal. In addition, X. perforans was able to restore surface motility of strains that lost this mode of motility under multiple growth cycles in the lab. The described interaction occurred both on agar plates and tomato leaves and was observed between several xanthomonads and motile bacterial species. Thus, suggesting that this motility induction and hitchhiking strategy might be widespread and ecologically important. This study provides an example as to how bacteria can rely on the abilities of their neighboring species for their own benefit, signifying the importance of a communal organization for fitness.