PROLINE AND GABA AS ANTIOXIDANTS

PROLINE GAMMMA AMINO BUTYRIC ACID (GABA)

STRUCTURE

PROLINE Alpha amino acid. Involves in protein synthesis. Cellular homeostasis. Can act as signaling molecule to modulate mitochondrial functions. Influnces cell proliferation. Triggers specific gene expression.

REACTIVE OXYGEN SPECIES (ROS)

Aerobic metabolism constantly generates ROS which are confined to the different plant cellular compartments, like the chloroplast, mitochondria and peroxisomes. The major members of the ROS family include free radicals like O•−

2, OH• and non-radicals like H2O2 and 1O2. Act as secondary messengers. Also induce oxidative damages under environmental stress conditions. The cellular damages are manifested in the form of degradation of biomolecules. Estimates show that only 1–2% of the O2 consumption by plant tissues, leads to the formation of ROS.

 FACTORS RESPONSIBLE FOR GENERATION OF ROS

REF: Kaushik Das et al. Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants, Frontiers in Environmental Science, 2014, 2: 53

DIFFERENT MEMBERS OF THE ROS FAMILY AND ATTRIBUTES

REF: Kaushik Das et al. Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants, Frontiers in Environmental Science, 2014, 2: 53

VARIOUS  TARGETS OF ROS

REF: Kaushik Das et al. Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants, Frontiers in Environmental Science, 2014, 2: 53

To ensure survival, plants have developed efficient antioxidant machinery having two arms:

1. Enzymatic antioxidants

2. Non enzymatic antioxidants

ENZYMATIC AND NON - ENZYMATIC ANTIOXIDANT

REF: Kaushik Das et al. Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants, Frontiers in Environmental Science, 2014, 2: 53

Non-enzymatic antioxidants

PLANT STRESS Plant stress has been defined by Lichtenthaler (1996) as any unfavorable environmental conditions that alter plant growth, development and metabolism, and even may lead to plant death.

Salt and soil drought conditions are major abiotic stress factors that limits crop productivity.

GLUTAMATE

GLUTAMATE-SEMIALDEHYDE(GSA)

PYRROLINE-5-CARBOXYLATE(P5

C)

PROLINE

PYRROLINE-5-CARBOXYLATE

SYNTHETASE(P5CS)

SPONTANEOUSE

PYRROLINE-5-CARBOXYLATE REDUCTASE(P5C REDUCTASE) PYRROLINE-5-

CARBOXYLATE(P5C)

GLUTAMATE

PROLINE DEHYDROGENASE(PDH)

PROLINE OXIDASE

PYRROLINE-5-CARBOXYLATE DEHYDROGENASE(P5CDH)

SYNTHESIS AND DEGRADATION OF PROLINE

DEGRA

DATION IN

MITOCH

ODRIA

PROLINE METABOLISM IN HIGHER PLANTS

REF: Laszlo Szabados et al. Proline: a multifunctional amino acid, Trends in Plant Science, 2009, 15: 89-97

LOCALISATION OF ENZYMES P5CS1 accumulates in chloroplast under salt or osmotic stress. P5CS2 predominantly localised in the cytosol. P5CR protein and activity detected in both cytosol and chloroplast. PDH and P5CDH are mitochondrial enzymes that use FAD and NAD+ as electron acceptors and generate FADH2 and NADH respectively, delivering elctrons for mitochondrial respiration. When PDH activity is limited, the P5C-proline cycle can transfer more electrons to the mitochondrial ETC and generate reactive oxygen species (ROS)

MULTIPLE FUNCTIONS OF PROLINE IN PLANTS

REF: Laszlo Szabados et al. Proline: a multifunctional amino acid, Trends in Plant Science, 2009, 15: 89-97

PROLINE AS ANTIOXIDANT Proline has the ROS scavenging activity by which it protect the cells from oxidative damage.

Proline treatment can diminish ROS levels in fungi and yeast, thus preventing programmed cell death, and can reduce lipid peroxidation in alga cells exposed to heavy metals.

Proline pretreatment also alleviated mercuric toxicity in rice through scavenging ROS, such as H2O2 (hydrogen peroxide).

PROLINE AS ANTIOXIDANT AND PERHAPS A PROTECTANT

The damaging effects of singlet oxygen and hydroxyl radicals on photosystem 2 (PS2) can be reduced by proline in isolated thylakoid membranes.

As an alternative to direct ROS scavenging feature, proline can protect and stabilise proteins and perhaps ROS scavenging enzymes and activate alternative detoxification pathways.

Proline function as a molecular chaperone to protect protein integrity and enhance the activities of different enzymes. E.g. ribonucleases, proteases, nitrate reductase, lactate dehydrogenase.

GAMMA AMINO BUTYRIC ACID(GABA)

NON PROTEINOGENIC AMINO ACID INHIBITORY NEUROTRANSMITTER IN CNS ALSO EXPRESSED IN NON- NEURAL CELLS DEFENCE AGAINST STRESS IN PLANTS REGULATOR OF FLIGHT OR FIGHT STRESS RESPONSE MOSTLY FOUND AS ZWITTER ION i.e CARBOXYL GROUP DEPROTONATED AND AMINO GROUP PROTONATED BIOACTIVE CONSTITUENT OF FRUITS, VEGETABLES AND CEREALS CELL SIGNALING IN PLANTS

BIOSYNTHESIS OF GABA

GABA shunt reactions are responsible for the synthesis, conservation and metabolism of GABA. GABA-T, GABA α-oxoglutarate transaminase; GAD, glutamic acid decarboxylase; SSADH, succinic semialdehyde dehydrogenase

GABA SHUNT PATHWAY• In cytosol, glutamate decarboxylase(GAD) catalyses glutamate to GABA.•GABA is transported from cytosol to mitochondria and forms succinate semialdehyde(SSA), catalysed by 2-oxyglutarate-dependent GABA transaminase(GABA-T).•SSA is converted to succinate by SSA dehydrogenase(SSADH).•Succinate enters to TCA cycle

REF: Barry J. Shelp et al. Compartmentation of GABA metabolism raises intriguing questions, Trends in Plant Science, 2012, 17: 57-59

REDOX HOMEOSTASIS BY GABA

During stress conditions GABA levels increase because stress is generally associated with elevated ratios of NADH to NAD+ and NADPH to NADP+ , and frequently with cytosolic acidosis (e.g. hypoxia), the glyoxylate reductase (GLYRs), like glutamate decarboxylate (GAD), would be expected to function efficiently during stress.

PLANT UNDER STRESS

SIGNAL TRANSDUCTION PATHWAY

INCREASED CYTOSOLIC CALCIUM

CALCIUM /CALMODULIN DEPENDENT GLUTAMATE DECARBOXYLASE ACTIVITY

ACTIVATION

GABA SYNTHESIS

INCREASE IN HYDROGEN IONS CONC.

STIMULATE GLUTAMATE DEHYDROGENASE ACTIVITY

GABA ACCUMULATION

SUMMARY The crrent aerobic atmosphere leads to the formation of ROS, in plant tissues.

ROS induce oxidative damages under environmental stress conditions. Proline act as antioxidant by scavenging ROS and also function as molecular chaperone to protect protein integrity and enhance the activities of different enzymes.

GABA maintains the redox homeostasis by maintaining the ratios of NADH to NAD+ and NADPH to NADP+ during stress.

REFERENCES Kilani Ben Rejeb et al. How reactive oxygen species and proline face stress together, Plant Physiology and Biochemistry,2014,80: 278-284 Laszlo Szabados et al. Proline: a multifunctional amino acid, Trends in Plant Science, 2009, 15: 89-97 Shamsul Hayat et al. Role of proline under changing environments, Plant Signaling and Behavior, 2012, 7: 1456-1466 Simon Michaeli et al. Closing the loop on the GABA shunt in plants: are GABA metabolism and signaling entwined?, Frontiers in Plant Science, 2015, 6: 419 Barry J. Shelp et al. Compartmentation of GABA metabolism raises intriguing questions, Trends in Plant Science, 2012, 17: 57-59 Kaushik Das et al. Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants, Frontiers in Environmental Science, 2014, 2: 53

ACKNOWLEDGMENTI WOULD LIKE TO EXPRESS MY SINCERE GRATITUDE TO Prof . A.S.RAGHAVENDRA , Dr. IRFAN A.GHAZI AND Prof. C.H.VENKETRAMANA FOR THEIR GUIDANCE AND SUPPORT TO WORK ON THE PRESENTATION

Prof. A.S. RAGHAVENDRA Dr. IRFAN A. GHAZI

Prof. C.H.VENKETRAMANA H.O.DDEPARTMENT OF PLANT SCIENCES