Fig. 2.

Simplified pathway of ethylene biosynthesis in plants showing auto-inhibition (inhibiting its own production) and auto-induction of ethylene (inducing its own production). These systems are referred as system 1 and system 2 of ethylene production respectively. In system 1, ethylene inhibits its own production by inhibiting (⊝) ACS expression/activity. It may be noted that the ACO 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