The Barley mlo-gene:: an important powdery mildew resistance source

This review briefly summarises recently generated knowledge about mlo powdery mildew resistance in barley. Barley mlo resistance has remained highly effective since commercial spring barley varieties with the resistance were first released in 1979. Currently, this resistance is the most used resistance in spring barley grown throughout Europe. Barley mlo resistance confers nearly total resistance against fungal penetration attempts. However, the efficiency of the resistance depends on several factors including epidermal cell type, host genetic background, environmental conditions and fungal genotype. Recently, the barley Mlo-gene has been cloned, but the exact function of the gene is not known. The Mlo-gene most likely regulates several mechanisms involved in penetration resistance against powdery; mildew, and mlo mutations cause disfunction of the wild type Mlo-protein leading to increased resistance. The resistance mechanisms involved probably include earlier deposition and increased size of the host papilla response, callose deposition, production of phenolic compounds and cell wall strengthening by cross binding.