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. 2015 Oct 28;169(4):2380–2390. doi: 10.1104/pp.15.01361

Figure 4.

Figure 4.

Two different systems of ethylene production operate during fruit development and ripening. At immature stages, ethylene biosynthesis is mediated by system 1, whereas system 2 takes over during ripening and is characterized by autocatalytic ethylene production. The main genes involved in system 1 are ACS6 and ACS1A, both genes being down-regulated by ethylene. From mature green stage onward, system 2 ethylene production is driven mainly by ACS2 and ACS4, the expression of which is stimulated by ethylene. ACS1A transcripts show a transient increase at the onset of ripening, suggesting that this gene may be important in regulating the transition from system 1 to system 2. ACO1 and ACO4 transcript levels are low in immature green stages, but undergo sharp increase at the climacteric peak when system 2 ethylene production is operating. IMG, Immature green; MG, mature green; B, breaker; OR, orange; R, red. Arrowheads represent positive regulatory interactions, and bar heads represent negative regulation.