ROS signaling in plants: Are we missing a fundamental pathway?

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Reactive oxygen species (ROS) are important physiological regulators of intracellular signaling pathways in animals, plants, fungi and bacteria. The ROS hydrogen peroxide (H2O2) in particular is a potent initiator and transducer of an oxidising signal to the nucleus that is a major regulator of environmentally responsive gene expression. Major sources of intracellular H2O2 arise from chloroplasts, mitochondria, the endoplasmic reticulum (ER) lumen, the peroxisome, cytosol and the plasma membrane.
Mainly from studies on yeast and animal cells, the prevailing view is that H2O2 signaling follows the same rules and biochemistry across the Eukarya including higher plants. However, our recent research and thinking has led us to the radical conclusion that there is a yet to be described plant cell-specific H2O2-responsive signalling system IN ADDITION to that common to other eukaryotic cells. We hypothesise that this novel signaling arose because of a combination of higher plants' oxygenic photosynthesis and their sessile nature. These constraints required a greater range of responses to ROS in the face of a variable and stressful environment. If correct, this fundamental new insight would transform our understanding of how H2O2 signaling can potentially operate in eukaryotic systems and not just plant cells. At a minimum, this insight reveals a new basic rule of how plant cells integrate multiple environmental and internal cues and allow resolution of apparently contradictory findings regarding ROS signaling in plant cells.
This plant-specific signaling is characterised by the accumulation of H2O2 in the nucleus secreted from a sub-population of chloroplasts in very close proximity to it. In contrast, in animal cells, H2O2 does not accumulate in the nucleus. Therefore, this signaling system depends on a juxtaposition of chloroplasts in relation to the nucleus. This novel spatial component is its distinctive feature.
HYPOTHESIS: Multiple sources of H2O2 converge in the perinuclear space because once in the nucleus, its oxidising signal rapidly diverges into a large number of distinct transducing redox carriers that each have specific regulatory functions. This convergence point in the perinuclear space/ER lumen provides the cell (and us) with opportunities to modulate a highly complex response. We anticipate that the timing and amplitude of a convergent H2O2 signal and its rate of delivery to the nucleus provides a basis for signaling specificity. Furthermore, cells from specific tissues may show differences in spatial organisation of H2O2-specific signaling.
This hypothesis is novel. In the long term, our concept of a convergence of H2O2 in the perinuclear space will transform our thinking and approach to linking genome-wide responses to multiple environmental challenges. Organelle-to-nucleus signaling, involving molecules such as H2O2, has been described recently as holding great potential as a target for controlling plants' responses to fluctuating multiple stresses but only if its mechanics can be understood.