Fig. 3.

Simplified pathway of ethylene biosynthesis in plants showing auto-inhibition (inhibiting its own production) and auto-induction of ethylene (inducing its own production). These two systems are referred as system 1 and system 2 of ethylene production respectively. In system 1, ethylene inhibits its own production by inhibiting () ACS (ACC-synthase) expression/activity. It may be noted that the ACO (ACC-oxidase) activity is enhanced during system 1 but due to the absence of any enhancement in the activity of ACS there is no auto-induction. In system 2, ethylene induces more of its own production by stimulating () the expression/activity of both of the enzymes (ACS and ACO) simultaneously and this thereby enhances the overall ethylene production (). 2, 4- norbornadiene and 1-MCP (1-methylcyclopropene) [action inhibitors of ethylene] and response mutants of ethylene block () the action/response of the ethylene and thereby inhibit the system 2 of ethylene production (Source: Adapted and modified from Srivastava 2001)