Figure 1.

The activation of the plant innate immune system requires tree steps; immune recognition, signal integration and defense response. Plants use numerous cell surface and intracellular immune receptors to recognize microorganism/host-derived molecular patterns (MAMPs and DAMPs), or apoplastic/avirulance effectors (AE). Cell surface pattern recognition receptors (PRRs) bind to MAMPs or DAMPs or AE directly through their extracellular domain while NOD- like receptors (NLRs) recognize effectors delivered inside host cells by directly binding effectors or sensing modulation of effector host targets. PRR- mediated recognition of MAMPs or DAMPs elicits pattern- triggered immunity (PTI), and NLR- mediated pathways trigger effector-triggered immunity (ETI). Activation of immune receptors subsequently initiates the second phase of immune system. In this phase, various immune signaling events such as calcium fluxes, activation of mitogen- activated protein kinase (MAPK) cascades, alteration of host transcription and phytohormone signaling trigger the defense response in each cellular compartment in plants. Hormone- dependent response generally activates a large set of plant defense-related genes against biotrophs. For instance, hormone accumulation in plants triggers hypersensitive response (HR) which cause rapid local death of the infected and surrounding cells to restrict the spread of pathogens to other parts of the plant. Accumulation of hormones and pathogenesis-related proteins in the plants can also induce long-lasting protection against a broad spectrum of pathogens, called systemic acquired resistance (SAR). In this resistance, putative SAR signal molecules move from the infected systemic organs to non-infected distant parts of the plant where it make more resistance to pathogens prior to infection. Induced Systemic Resistance (ISR) is another defense response increasing physical or chemical barriers of the host plant against pathogens rather than direct killing or inhibiting the invading pathogen. RNA interference (RNAi) is the last plant resistance mechanism activated during the viral infection.