Abstract
Pioneered by Clark's microelectrode more than half a century ago, there has been substantial interest in developing new, miniaturized optical methods to detect molecular oxygen inside cells. While extensively used for animal tissue measurements, applications of intracellular optical oxygen biosensors are still scarce in plant science. A critical aspect is the strong autofluorescence of the green plant tissue that interferes with optical signals of commonly used oxygen probes. A recently developed dual-frequency phase modulation technique can overcome this limitation, offering new perspectives for plant research. This review gives an overview on the latest optical sensing techniques and methods based on phosphorescence quenching in diverse tissues and discusses the potential pitfalls for applications in plants. The most promising oxygen sensitive probes are reviewed plus different oxygen sensing structures ranging from micro-optodes to soluble nanoparticles. Moreover, the applicability of using heterologously expressed oxygen binding proteins and fluorescent proteins to determine changes in the cellular oxygen concentration are discussed as potential non-invasive cellular oxygen reporters.
Highlights
Molecular oxygen (O2; here often referred to as “oxygen”) is an essential factor for most life forms on Earth and its vital role in respiratory energy metabolism has been well described
While the oxygen distribution is widely studied in animals and mammalian tissues, it is much less well characterized in plant cells
A custom-made micro-optode, with a tip diameter of less than 20 μm was successfully applied to measure the oxygen concentration in living root nodules of Lotus japonicus. This fiber employed the Pt-complex of tetrapentafluorophenylporphyrins (PtPFPP)-dye as oxygen sensitive dye, whose phosphorescent signal normally interferes with the plant autofluorescence but can be overcome by applying the two-frequency phase modulation technique
Summary
Molecular oxygen (O2; here often referred to as “oxygen”) is an essential factor for most life forms on Earth and its vital role in respiratory energy metabolism has been well described. Hypoxic conditions can occur when the resupply of oxygen to a cell does not keep pace with its oxygen consumption. The internal oxygen concentration of cells in the center of plant tissue can be about five times less than the concentration of cells at the surface [1,2] In addition to these steep internal oxygen concentration gradients, external oxygen deficiency or depletion is regularly occurring in the rooting area after rain or flooding, when oxygen diffusion is hampered due to water-filled spaces in the soil. The development of oxygen sensors has greatly contributed to our knowledge on the oxygen distribution in plant tissue and the regulation of oxygen consumption by plant cells. This review discusses the state of the art of optical oxygen sensor methods with a special emphasis on the problems of their application in plants
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