The regulation of 1-aminocyclopropane-1-carboxylic acid synthase gene expression during the transition from system-1 to system-2 ethylene synthesis in tomato

1-Aminocyclopropane-1-carboxylic acid synthase (ACS) is one of the key regu latory enzymes involved in the synthesis of the hormone ethylene and is enc oded by a multigene family containing at least eight members in tomato (Lyc opersicon esculentum). Increased ethylene production accompanies ripening i n tomato, and this coincides with a change in the regulation of ethylene sy nthesis from auto-inhibitory to autostimulatory. The signaling pathways tha t operate to bring about this transition from so-called system-1 to system- 2 ethylene production are unknown, and we have begun to address these by in vestigating the regulation of ACS expression during ripening. Transcripts c orresponding to four ACS genes, LEACS1A, LEACS2, LEACS4, and LEACS6, were d etected in tomato fruit, and expression analysis using the ripening inhibit or (rin) mutant in combination with ethylene treatments and the Never-ripe (Nr) mutant has demonstrated that each is regulated in a unique way. A prop osed model suggests that system-1 ethylene is regulated by the expression o f LEACS1A and LEACS6, In fruit a transition period occurs in which the RIN gene plays a pivotal role leading to increased expression of LEACS1A and in duction of LEACS4. System-2 ethylene synthesis is subsequently initiated an d maintained by ethylene-dependent induction of LEACS2.