Proteomic analysis during capsicum ripening reveals differential expression of ACC oxidase isoform 4 and other candidates

Capsicum (Capsicum annuum L.) is categorised as a non-climacteric fruit that exhibits limited ethylene production during ripening and the molecular mechanisms associated with this process are poorly understood. A proteomic approach was used to identify the differentially expressed proteins during various ripening stages (Green (G), Breaker Red 1 (BR1) and Light Red (LR)) and the genes associated with their synthesis. From 2D gel electrophoresis (2DGE), seven protein spots were identified as selectively present either in G or BR1 and are involved in carbon metabolism, colour and fruit development, protein synthesis and chaperones or biosynthesis of amino acids and polyamines. One candidate of interest, 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO) is known to be involved in ethylene biosynthesis and was only present in BR1 and is related to the tomato ACO isoform 4 (LeACO4) and hence named CaACO4. CaACO4 RNA expression as well as total ACO protein expression in multiple stages of ripening (G, Breaker (B), BR1, Breaker Red 2 (BR2), LR and Deep Red (DR)) corresponded to the 2DGE protein spot abundance in breaker stages. Our findings highlight the involvement of the ethylene pathway in non-climacteric fruit ripening.