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光合产氧:新方法带来的遗忘通量
2018-5-10
来源:英驰科技
点击数: 522          作者:英驰科技
  • 光合产氧:新方法带来的遗忘通量(原文请自行下载或者来电索取,以下为部分内容截取)

    Photosynthetic Oxygen Production New Method Brings to Light Forgotten flux


    Oxygen (O2) is evolved during photosynthetic electron
    transport when water is split by the oxygenevolving
    complex to provide protons and electrons to
    the chloroplastic electron chain, thereby generating
    ATP and NADPH—the energy source and reducing
    power for plant metabolism. The majority of this
    chemical energy is used to drive photosynthetic carbon
    metabolism, which consists of ribulose-1,5-bisphosphate
    carboxylation (photosynthetic carbon reduction
    cycle) and oxygenation (photosynthetic carbon oxidation
    cycle); with a combined electron requirement = JA.
    Four electrons are required for every O2 evolved so that
    gross O2 production (GOP) is related to linear electron
    transport (J) according to J/4. When linear electron
    transport is used only to drive CO2
    fixation, the consumption
    of O2 and the release of CO2 by photosynthetic
    carbon oxidation and mitochondrial respiration is
    such that net O2 production (NOP) is equal to net CO2
    assimilation (Anet; provided the respiratory quotient is
    1, but see Tcherkez et al., 2017).
    Additionally, electrons can be used for alternative
    noncyclic electron transport (ANCET), including, for
    example, the photoreduction of O2 itself forming reactive
    oxygen species (Mehler-peroxidase reactions or
    “water-water cycle”; Asada, 1999), chloroplastic anabolism
    (e.g. lipids; Stumpf et al., 1963), the reduction of
    oxaloacetate to malate (which is exported to the mitochondria;
    Scheibe, 2004), and nitrogen assimilation
    (Bloom et al., 1989). ANCET has been hypothesized
    both as a way to regulate ATP/NADPH ratio to meet
    the changing energy demands of cellular metabolism
    and as a mechanism to prevent photodamage through
    utilizing excess reductant when the photon flux density
    exceeds the energy requirement of CO2
    fixation (e.g.
    under high irradiance, cold temperatures, water stress
    closing stomata; e.g. Badger, 1985; Ort and Baker, 2002;
    Robinson, 1988). Importantly, there is no formal evidence
    for how electron flows interact, particularly under
    fluctuating light conditions (Morales et al., 2018).
    As ANCET allows for greater rates of linear electron
    transport to be sustained, total electron transport (Jt)
    will be greater than JA. Conversely, the effect on O2
    uptake will be dependent on the metabolic pathway
    involved. For example, in the Mehler-peroxidase reactions,
    there is no net change in O2 so that NOP will remain
    equal to Anet. But in the reduction of nitrate,
    the ratio between N-linked O2 production and O2 consumption
    is highly dependent on the amino acid synthesized
    (Noctor and Foyer, 1998). In this case, NOPwill
    not always equal Anet because O2 and CO2 may not be
    balanced in metabolism(Skillman, 2008). Consequently,
    concomitant measurements of CO2 and O2
    fluxes are
    important to the understanding of how plants regulate
    the use of light energy, with different fates having very
    different metabolic outcomes.
    The earliest measurements of O2 evolution were unable
    to distinguish GOP from uptake of O2 (Hill, 1937).
    The mass spectrometry method established by Mehler
    and Brown (1952) solved this problem by employing
    O2 isotope tracers to independently monitor fluxes
    of 16O2 and 18O2. In this method, pure 18O2 was supplied
    to the gas headspace of a closed chamber, and the
    decline in 18O2 was attributed to O2 uptake. O2 evolved
    carries the same isotopic composition as the water from
    which it is generated; in this case, the dominant isotope
    in the water was 16O (Fig. 1). The 18O-labeling approach
    was further applied to leaf disks (e.g. Tourneux and
    Peltier, 1995), whole excised leaves (e.g. Volk and Jackson,
    1972), and entire plants (Gerbaud and André, 1980),
    illuminating the fate of O2 in vivo.
    The limitation of closed gas exchange systems is that
    measurements can only be undertaken for short periods
    of time (seconds to minutes) before the CO2 concentration
    is depleted. Consequently, CO2:O2 is not constant,
    which changes the relative rates of carboxylation
    and oxygenation so that estimates of GOP and O2 uptake
    will be inaccurate. This limitation was overcome in
    the mass spectrometry approach by replacing CO2
    consumed through periodic influx of CO2 into the
    chamber, allowing for steady-state quantification and
    extending the ability to measure O2
    fluxes under a
    range of conditions and physiological states (Canvin
    et al., 1980). At the same time, advances were being
    made in the use of chlorophyll fluorescence, which
    provides information on PSII quantum yield (Baker,
    2008). Genty et al. (1989) provided the empirical link
    between fluorescence and electron transport rate,
    replacing the need to directly measure O2 evolution.
    Chlorophyll fluorescence is now one of the most popular
    techniques in plant physiology because of its ease
    of use and relatively low cost. This has been aided by
    the capacity to multiplex fluorescence measurements
    with H2O and CO2 gas exchange in portable, commercially
    available instruments, opening up the possibility
    of measuring plant function outside of the laboratory.
    Consequently, in vivo measurements of O2
    fluxes have
    substantially declined over the last 20 years.
    In this issue of Plant Physiology, Gauthier et al. (2018)
    remind us why it is so important to return our attention
    to O2, providing us with a new, elegant open-path system
    to measure O2
    fluxes. Their method is a “reverse”
    isotopic approach, involving 18O-labeling of leaf water
    rather than the air so that the isotopic composition of O2
    that is evolved during water splitting has a signature

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