Synthetic chemical fungicides have long served as agents for reducing the incidence of plant disease; however they are costly, cause environmental pollution and pathogens develop quick resistance. Alternative biological control offers an attractive alternative or supplement to fungicides for the management of plant disease, without the negative effects of chemical control. Biological control includes the use of organisms or its active components. The results are not consistent when whole organism, since it requires the maintenance linked surveillance which may be a difficult task to achieve in the environment. The use of active component such as proteins polysaccharides and metabolites attracts great importance which needs no conditions to work efficiently.

Plant hypersensitive response (HR) is a complex, early defense response that causes necrosis and cell death to restrict the growth of a biotrophic pathogen. Resistance (R) genes detect the pathogen and change the membrane potential and ion permeability of the plasma membrane. The outward K+ and inward Ca+2 and H+ ion flux are dependent and trigger the HR, resulting in cell death and formation of local lesions. The HR precedes the secondary resistance response, the systemic acquired response (SAR) leads to the biochemical changes include generation of oxidative burst, cell death, overproduction of lignin-related materials, and the induction of certain pathogen related (PR) proteins.

In plants, systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. Systemic acquired resistance (SAR) is the activation of defences in uninfected parts of the plant. As a result the entire plant is more resistant to a secondary infection. SAR is important for plants to resist disease, as well as to recover from disease once formed. SAR can be induced by infection of plant parts with low effective necrotic pathogens or molecules from pathogens like HR inducing proteins. SAR is long lasting and often confers broad-based resistance to different pathogens. Salicylic acid may be a signaling compound involved in transmission of the defence response throughout the plant to produce SAR. SAR includes the production defence enzymes of antifungal chitinases, β-1,3-glucanases, PR-1, PR-5.

Harpins are one class of heat stable proteins produced by gram negative plant pathogenic bacteria and elicit hypersensitive response (HR) in non-host plants. The HR inducing ability is not because of toxicity of harpin, but due to biochemical changes induced by protein in the plant. Harpin is a leucine and glycine-rich protein lacking cysteine with no reported enzymatic activity. It retains activity even after treating at 100°C for 10'. Harpin was first isolated from Erwinia amylovora, a fire blight pathogen of apples and pears and later it is reported form other gram negative plant pathogenic bacteria like Pseudomonas, Xanthomonas and others. Another interesting feature of harpin is any truncated part of the protein is able to induce the HR in yeast as like full length protein. Harpin, by contrast, elicits a protective response in the plant that makes it resistant to a wide range of fungal, bacterial, and viral diseases. Harpin does not interact directly with disease pests so these organisms are not expected to develop resistance to it, unlike chemicals. Harpin does not alter the DNA of treated plants, but instead activates a natural defense mechanism in the host plant, referred to as systemic acquired resistance (SAR).

Harpin elicits disease resistance by activating multiple defense signal transduction pathways leading to activation of defense genes, analogous to a broad spectrum immune response in animals.
The use of harpin is already proved to be protective against several diseases caused by bacteria, fungi and virus etc.

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