The simplest plant growth and development hormone, ethylene, belongs (together with S.A. and JA) to key molecules governing the plant defense response. Production of ethylene is stimulated by wounding, flooding, metal ions, senescense and abscission processes, pathogene attack and ethylene itself. High level of ethylene induces the expression of many classes of pathogenesis-related proteins (PR). In order to properly function, ethylene must be precisely regulated, especially at the biosynthesis and signalling pathways. Two main enzymes involved in ethylene biosynthesis are ACC synthase and ACC oxidase. The ethylene signal transduction pathway is very complicated and needs to be further investigated. Ethylene binds to its receptors ETRs, then the signal is transduced to CTR1 and through phosforylating kinases cascades to EINs and EREBPs, directly binding to DNA GCC boxes. As a consequence, many PR proteins are activated.