Hydrogen peroxide signalingin plants

By

Muhammad Kaleem

Department of Botany

University of Agriculture Faisalabad

Introduction

• Recent biochemical and genetic studies confirm that hydrogen peroxide is a

signaling molecule in plants.

• H2O2 mediates responses to abiotic and biotic stresses.

• It is safe to say that ROS is “two-faced”, being “harmful” when produced in

excess and “beneficial” at lower concentrations.

• ROS at these “beneficial” levels plays a part in sensing the environment and

regulating development, growth, and environmental accumulation.

• Excess concentrations of ROS results in oxidative damage to or the apoptotic

death of cells.

• Development of an antioxidant defense system in plants protects them against

oxidative stress damage.

• Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses, all of which exert adverse effects on plant growth and development.

• Consequences of much stress is an increase in the cellular concentration of reactive oxygen species(ROS）, which is subsequently converted to hydrogen peroxide（H2O2).

• H2O2 plays versatile roles in plant normal physiological processes and resistance to stresses.

• Recently, H2O2 has been regarded as a signaling molecule and regulator of the expression of some genes in cells.

• H2O2 function, generation and scavenging, genes regulation .

• H2O2 participates in many resistance mechanisms, including reinforcement of the plant cell wall.

• Calcium and ROS Signalling contribute to the maintenance of hydrogen peroxide homeostasis.

• Signalling roles for hydrogen peroxide during abscisic-acid-mediated to.

Stomatal closure.

Auxin-regulated root gravitropism.

Tolerance of oxygen deprivation.

• Downstream Signalling events that are modulated by hydrogen peroxide.

Include calcium mobilization.

Protein phosphorylation.

Gene expression.

• The synthesis and action of hydrogen peroxide appear to be linked to those of nitric oxide.

• H2O2 has also been shown to act as a key regulator in a broad range of physiological processes such as

senescence

photorespiration and photosynthesis

Stomatal movement

cell cycle

growth and development

• Excess H2O2 accumulation can lead to oxidative stress in plants, which then triggers cell death PCD.

H2O2-scavenging mechanisms Enzymes• The development of efficient H2O2-scavenging

mechanisms Enzymes, including.

superoxide dismutase (SOD)

Catalase (CAT)

Peroxidase (POD)

Ascorbate peroxide (APX)

Glutathione reductase (GR)

• Some non enzymatic antioxidants also involved such as,

• Tocopherols

• Ascorbic acid (ABA), and glutathione (GSH) work in concert to detoxify H2O2.

H2O2-scavenging mechanisms Enzymes• The development of efficient H2O2-scavenging

mechanisms Enzymes, including.

superoxide dismutase (SOD)

Catalase (CAT)

Peroxidase (POD)

Ascorbate peroxide (APX)

Glutathione reductase (GR)

• Some non enzymatic antioxidants also involved such as,

• Tocopherols,

• Ascorbic acid (ABA), and glutathione (GSH) work in concert to detoxify H2O2.

Flow chart of the major free radical scavenging mechanisms and reactions in plants

Schematic diagram including the names of the specific enzymes commonly involved inremoval of free radicals such as hydrogen peroxide from plants, and the specificreactions they catalyze (Modified from Quan et al. 2008).s

H2O2 IN STRESS CONDITIONS AND AS SIGNALLING MOLECULE

• H2O2 concentration at an appropriate level can promote plant development and reinforce resistance to environment stressors.

• H2O2 modulates the expression of various genes.

• The H2O2 induced transcripts encoded proteins with functions such as metabolism, energy, protein destination and transport, cellular organization and biogenesis, cell rescue of defense, and transcription.

• Stimulates defense-genes in adjacent cells.

• It is well-documented that H2O2 plays a central role in responses to both abiotic and biotic stresses in plants.

• This molecule seems to be a master hormone” that controls a variety of stress responses and physiological adjustments.

Localization of H2O2 scavenging enzymes

• In plant cells, enzymes and redox metabolites act in synergy to carry out H2O2 scavenging Major ROS-scavenging enzymes of plants include superoxide dismutase (SOD), Ascorbate peroxidase (APX), catalase (CAT), and glutathione peroxidase (GPX). These enzymes provide cells with highly efficient machinery for detoxifying O2- and H2O2.

• The rate of H2O2 production depends on the strength and duration of the imposed stress.

• Moreover, H2O2 levels differ in various cell compartments and these levels are related to the type of stress, e.g. excess light stress is responsible for the overproduction of H2O2 mainly in the chloroplast.

• Plants pre-treated with H2O2 were more resistant to excess light and chilling stresses.

• Relative stability and higher concentrations of H2O2 in plant cells could point to the fact that H2O2 plays a key role as a signal transduction factor.

• However, signal molecules are usually present in cells in very low concentrations; the relatively high level of H2O2 in plant tissues supports the assumption that H2O2 is not only a Signalling molecule, but also plays a key role in primary plant metabolism.

• Moreover, H2O2 regulates the expression of various genes, including those encoding antioxidant enzymes and modulators of H2O2 production

Origin of H2O2

Reactive oxygen species (ROS), a collective term for radicals and other non-radical but reactive species derived from the oxygen molecule (O2).

CELLULAR AND EXTRACELLULAR SOURCES OF H2O2Hydrogen peroxide is produced not only through the disproportionation of superoxide, but also due to the reduction of O 2 ˉ˙by a reductant (X) such as ascorbate (Asc), thiols, ferredoxins and others.

Chloroplasts

The photosynthetic electron transport (PET) chain in the chloroplast is responsible for H2O2 production. ROS in chloroplasts, and the rate of O2

photoreduction depends on environmental conditions.

Peroxisomes

The main function of peroxisomes in the plant cell is photorespiration, which is light-dependent uptake of O2 and the associated release of CO2

connected with the generation of H2O2.

Mitochondria

In plant mitochondria superoxide anion radical production occurs mainly at two sites of the electron transport chain: NADPH dehydrogenases (complex I) and the cytochrome complex (com-plex III) .The amount of H2O2 produced in plant mitochondria is less than that of chloroplasts or peroxisomes when exposed to light , but in the dark or in non-green tissues, mitochondria can be a major source of ROS.

Other sources of H2O2 in the plant cell

• H2O2 is also produced in the cytoplasm, plasma membrane and in the extracellular matrix (ECM).

Transition between the oxygen molecule (O2), superoxide anion (O2-), hydrogen peroxide (H2O2) and hydroxyl radicals (.OH).

During oxidative burst, O2 is reduced to O2-, and then the O2- undergoes spontaneous dismutation at a higher rate and at acidic pH, which is also found in the cell wall . O2- is also catalyzed by superoxide dismutase (SOD) enzymes, which occur in the cytosol, chloroplasts, and mitochondria .O2 can also be reduced to H2O2 by protein kinase C (PKC), PKC exists in all organelle of plants (Juan et al. 2004). H2O2 reacts with Fe2+ leading to the H2O2-dependent formation of OH.

Mode of action of Hydrogen per oxide in plants

• Hydrogen peroxide (H2O2) has been regarded as the second messenger for gene activation in mammalian systems as well as in plant. In plants, increased H2O2 level induces the expressing not only of defense genes, but also other resistance genes.

• A subset of genes within the heat shock response might be triggered by increased levels of H2O2.

• Twenty genes were induced in response to H2O2 and under at least two stress conditions.

• DREB2A is known to be a key regulator of drought response.

• ZAT12 participates in regulation of cold-responsive genes and contributes to an increase in freezing tolerance.

• Among these genes, the genes encoding potential transcription factors should be emphasized due to their capacity for activating the expression of downstream target genes.

• H2O2 itself is a key signal molecule mediating a series of responses and activating many other important signal molecules (Ca2+, SA, ABA, JA, ethylene, NO) of plants.

• These signal molecules function together and play a complex role in signal transduction of resistance responses, and growth and development in plant.

• Hydrogen peroxide is also one of the earliest signaling molecules involved in wound-signaling pathways, serving as a local signal for hypersensitive cell death and cell wall stiffening events, and as a mobile signal that stimulates defense-genes in adjacent cells.

• Smaller levels of peroxide production also occur in the cytoplasm, plasma membrane, and the extracellular matrix of plant cells

Representation of a mitogen-activated protein kinase (MAPK) signaling cascade

• H2O2 participates in the physiological metabolism of plant and activate defense responses to various stresses.

• H2O2 is beginning to be accepted as a second messenger for signals generated by means of ROS because of its relatively long life and high permeability across membranes.

Biological processes leading to and regulated by H2O2.

Biological processes leading to and regulated by H2O2. Various developmental or

environmental signals (plant hormones, abiotic or biotic stress) can lead to H2O2

accumulation, which in turn triggers a variety of biological responses as developmental

processes, stress acclimation, or PCD. The H2O2 signal is mediated through alterations in

Ca2+ fluxes, redox changes, activation of MAPK cascades, and interactions with other

signaling molecules like salicylic acid and nitric oxide.Gechev T S , and Hille J J Cell Biol 2005;168:17-20

H2O2 signaling in plants. Grey arrow represents potential

synthesis and degradation, black arrow show potentialcellular effect of H2O2

• Under other stress conditions, which include UV-radiation, salt stress, drought stresses, light stress, metal stress, high or low temperature and so on. H2O2 production in plants induces resistance to various stresses and protects itself from being hurt.

• H2O2 can also regulate the plant cell cycle.

• H2O2 and activated MAPK protein. MAPK as a key signal protein regulates the cell cycle.

• It has been proven that there is more H2O2 accumulation in old leaf than young leaf.

• H2O2 also takes part in ABA-induced stomatal opening and closing.

• Distribution of H2O2

• Plant mitochondria as an“energy factory” is believed to be a major site of H2O2 production related to continuous physiological processes under aerobic conditions.

• Chloroplasts are also a major source for H2O2 production. Chloroplasts consist of pigment and protein, two photo reaction systems: photo-system Ⅰ(PSⅠ) and photo-system Ⅱ(PS Ⅱ)

Versatile roles of H2O2

• Hydrogen peroxide (H2O2) plays a dual role in plants: at low concentrations, it acts as a signal molecule involved in acclimatory signaling triggering tolerance against various abiotic and biotic stresses.

• And, at high concentrations, it orchestrates programmed cell death.

• H2O2 takes part in resistance mechanism, reinforcement of plant cell wall (lignification, cross-linking of cell wall structural proteins) phytoalexin production and resistance enhancement.

• H2O2 takes part in resistance mechanism, reinforcement of plant cell wall (lignification, cross-linking of cell wall structural proteins) phytoalexin production and resistance enhancement

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