Plant immune system is entirely based on the immunities of the individual cells in which systemic signals originate from the infection sites. Powdery mildew disease is one of the agents causing these infection sites, resulting in significant yield losses, if disease develops. Understanding the molecular basis of plant-pathogen interactions is the new trend for fighting against plant pathogens, since classical methods used in selection of resistant plants are becoming less and less efficient nowadays. Thus, finding out the genes which are responsible in plant&rsquo / s resistance is becoming very important. In this thesis, effect of &lsquo / General Control Nondepressible-2&rsquo / (GCN2) homolog protein in barley defense mechanism was aimed to be studied. The GCN2 of yeast was v previously identified in our laboratory as an interacting protein when the yeast cDNA library was screened with a putative yellow rust R gene (Yr10) fragment. There are reports available in the literature for the function of GCN2 protein, which makes it a good candidate for a role in disease resistance. Thus, the barley homologue of GCN2 might have a role in the R protein mediated early disease response of which may be proceeding via Programmed Cell Death (PCD). In order to observe such function of HvGCN2 in barley, silencing of its expression via Virus Induced Gene Silencing (VIGS) was investigated. Therefore, the GCN2 homologue was found to function as dampening the severity of the disease. The silencing with triple technical replicates was observed in 5 of the 6 samples, at an average of 43.2% by qRT-PCR analysis. The pathogen growth levels at different time points were analyzed under light microscope on the silenced and the control samples by measuring the primary and secondary hyphae lengths. The total of 24 seedlings and 292 individual spores were analyzed, and then the level of disease formation was quantitated with 603 primary hyphae and 106 secondary hyphae measurements. Up to 25% hyphae growth rate differences between the control and silenced groups were observed with a probability value less than 0.05 on t-test.