During favorable conditions, the level of reactive spices in the cell is limited to what is required for normal cellular activities. They act as important components of signaling pathways. Plants control some important processes such as defense, hormonal signaling and development by using them as signaling molecules. And An equilibrium is steblished between antioxidant system and ros formation. But when plant feels an external stress like, drought,cold, salt etc. the level of reactive specease increases above the basal level a situation that we call oxidative stress. These reactive molecules during oxidative stress, they react with biomolecules like as carbohydrates, unsaturated lipids, proteins, nucleic acids. Proteins are the most abundant cellular targets of the oxidative species, more than DNA and lipids, making up 68% of the oxidized molecules in the cell. Ros reacts with proteins which results in protein modification called redox PTMs.