Ethylene biosynthesis and transduction pathway. Methionine is the precursor of ethylene (pink triangle). It is converted into S-adenosyl-methionine (SAM) by SAM synthetase (SAMS); SAM is converted into 1-aminocyclopropane-1-carboxylate (ACC) by ACC synthase (ACS). ACC is later converted into ethylene by ACC oxidase (ACO). Both auxin and cytokinin are known to promote ethylene production; the former up-regulates ACS expression whereas the latter prevents ACS degradation. Ethylene is known to modulate its own levels by stimulating (+) or inhibiting (-) ACO activity. ACC deaminase is an enzyme found in certain types of bacteria; it converts ACC into ammonia and α-ketobutyrate and both may be used as nutrients by the bacteria. When no ethylene is sensed in a cell (circle 1), ethylene receptors (such as ETR1) located on the ER membrane promote CTR1 activity; in doing so, they allow CTR1 to phosphorylate EIN2 which inactivates it. Upon ethylene perception (circle 2), ETR1 is dephosphorylated and CTR1 is switched off. ETR1 can now bind efficiently to EIN2 which is activated. Its C-end terminus (small yellow star) is cleaved and moves to the nucleus where it stabilizes the transcription factors EIN3 and EIL1, which are then able as dimers to trigger the expression of ethylene-responsive genes, inducing many developmental and biochemical events. When ethylene is not perceived, EIN3 and EIL1 are targeted to proteasomes via the action of EBF1/2. MAP kinases 3 and 6 (MAPK3/6) are known to activate ACS enzymes and to phosphorylate EIN3 in Arabidopsis, protecting it from degradation. Jasmonic acid is known to work synergistically with ethylene by promoting the degradation of JAZ2, a protein known to repress EIN3/EIL1. Finally, EIN3/EIL1 inhibits salicylic acid synthesis as they bind to the promoter of SID2, one of its biosynthetic genes, thus inactivating it.
