SIGNALING VIA PLANT PEROXIDASES

•Plant peroxidases are versatile enzymes. They are known to use hydrogen peroxide (H 2O 2 ) as an electron acceptor during the oxidation

•However, in certain conditions, they can also generate reactive oxygen species (ROS), namely, superoxide (O 2−. ), H 2O 2 , or hydroxyl radical ( . OH).

•Most of them are localized in cell walls, where they play a central role in ROS metabolism.

•Their participation in the oxidative burst (a rapid, transient, production of huge amounts of reactive oxygen species ) taking place in the apoplast has been shown in several situations,

•including defense against pathogens,

• abiotic stresses,

•wound healing, and

• cell death.

• It is now widely accepted that ROS have important signaling functions. Peroxidases can be considered as being at the crossroads between two different signal transduction systems in plant cells. First, they generate and interconvert the different ROS, thus modulating their signaling function in many physiological processes. Second, they are Ca 2+ -dependent enzymes, whose activity could be regulated by this second messenger.

H2O2-based signaling pathways by establishing connections between typical plant responses to abiotic stressors and responses to wounding events. In Arabidopsis, a mitogen-activated protein kinase 8 (MPK8) was shown to connect protein phosphorylation, calcium, and ROS in a wound-signaling pathway

mitogen-activated protein kinase 8 (MPK8) and how it connects to protein phosphorylation, calcium, and ROS in a wound-signaling pathway that begins with herbivore attack and ends with ROS production.

•Firstly, mechanical wounding of the plant triggered an increase in cytosolic calcium in local cells, which promoted the binding of calmodulins and the subsequent formation of active Ca2+/CaM complexes.

•These complexes then activated MPK8, which was also phosphorylated and activated by a MAPKK MKK3 in the typical MAPK cascade.

• Figure provides a visual summary of this calcium dependent pathway that begins with a typical herbivore attack.

• then considered the effect of activated MPK8 on expression levels of three major molecular markers for wounding and H2O2-response: RbohD, zinc finger protein-encoding Zat12, and the serine/threonine kinase-encoding OXI1. 

•In wild-type plants, they observed induction of expression of all three markers; however, in mutant mpk8 species they observed a relative increase in this induction.

•This precise MAPK pathway negatively regulates buildup of reactive species such as hydrogen peroxide.

• It is important to note the discrepancy between this finding and the previously described involvement of MAPK in positively facilitating responses to environmental stressors such as cold temperatures and osmotic stress.

Key points

•Hydrogen peroxide, H2O2, is an endogenous molecule in plants that is part of a group of cellular components referred to as reactive oxygen species (ROS).

•H2O2 activates MAPK cascades and mediates UV-B-induced gene expression, as indicated by down-regulation of UV-induced gene PDF1 in Arabidopsis plants exposed to antioxidants and UV-B light.

•In wound pathways, H2O2 serves as a local signal for hypersensitive cell death and cell wall stiffening, stimulates defense-genes in adjacent cells, and is linked to up-regulation of phenylpropanoid genes expressing phenolic defense compounds .

•The polyamine oxidase gene (NbPAO) is essential for H2O2 generation and subsequent hypersensitive response, and may be part of a larger Rboh gene network that includes other respiratory burst oxidase homologs .

•Many exogenous and endogenous compounds have been implicated in H2O2- mediated stress and defense pathways, including several enzyme and micronutrients. Future research should report additional influential structures and clarify known pathways.