Analysis of the role of the HvMaPK4 gene in the barley biotic stress response
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Mitogen-activated protein kinase (MAPK) cascades play a crucial role in several developmental and physiological processes in plant life, including responses to different biotic and abiotic stresses. Expression of the barley HvMAPK4 gene is known to be up-regulated after the infection by the blast pathogen Magnaporthe oryzae, thus in order to investigate the role of this gene in barley pathogen defence mechanism, different transgenic barley lines have been generated that have either a constitutive expression of HvMAPK4 or down regulation of HvMAPK4 by means of an antisense strategy. Both HvMAPK4 overexpression and antisense lines were challenged with the blast pathogen, as well as barley wildtype and transgenic line with the empty binary vector (pWBVec.8) as controls. The results showed enhanced disease resistance in antisense lines compared to overexpression lines and controls. The level of resistance in antisense lines was accompanied by an elevated level of endogenous salicylic acid and hydrogen peroxide after infection. Enhanced expression of pathogenesis-related protein (PR1) post-inoculation was seen in these lines along with a significant reduction in catalase activity. The opposite was found in HvMAPK4 overexpression plants which produced lower amounts of salicylic acid and hydrogen peroxide and showed elevated production of ethylene and an increase in catalase activity in response to the pathogen. The level of jasmonic acid was found to be elevated in these lines, even in the absence of biotic stress. These results suggest that the barley MAPK (HvMAPK4) acts as a negative regulator in barley resistance to the hemibiotrophic pathogen M. oryzae, and may act by regulating salicylic acid levels in the plant. HvMAPK4 is also a positive regulator of jasmonic acid and ethylene production, important compounds in the plant response to wounding and necrotrophic pathogen.