Ethylene and fruit ripening

Abstract

The latest advances in our understanding of the relationship between ethylene and fmit ripening are reviewed. Considerable progress has been made in the characterisation of genes encoding the key ethylene biosynthetic enzymes, ACC synthase (ACS) and ACC oxidase (ACO) and in the isolation of genes involved in the ethylene signal transduction pathway, particularly those encoding ethylene receptors (ETR). These have allowed the generation of transgenic fmit with reduced ethylene production and the identification of the Nr tomato ripening mutant as an ethylene receptor mutant. Through these tools, a clearer picture of the role of ethylene in fmit ripening is now emerging. In climacteric fmit, the transition to autocatalytic ethylene production appears to result from a series of events where developmentally regulated ACO and ACS gene expression initiates a rise in ethylene production, setting in motion the activation of autocatalytic ethylene production. Differential expression of ACS and ACO gene family members is probably involved in such a transition. Finally, we discuss evidence suggesting that the NR ethylene perception and transduction pathway is specific to a defined set of genes expressed in ripening climacteric fmit and that a distinct ETR pathway regulates other ethylene-regulated genes in both immature and ripening climacteric fruit as well as in non-climacteric fmit. The emerging picture is one where both ethylene-dependent and -independent pathways coexist in both climacteric and non-climacteric fmits. Further work is needed in order to dissect the molecular events involved in individual ripening processes and to understand the regulation of the expression of both ethylene-dependent and -independent genes.

Characterisation of ethylene pathway components in non-climacteric capsicum

“…CaACO4 , which may be considered to be a System 1-associated isoform, was up-regulated during ripening onset (Figure 1A) which suggests that its up-regulation might be closely associated with these ripening regulators in an ethylene-independent manner. Two pathways, ethylene dependent- and independent- pathways, have been suggested to operate in climacteric fruit but only the latter pathway may be conserved in non-climacteric fruit to induce ripening [6,33]. The main regulators of the ethylene-independent pathways especially in non-climacteric fruit are still unknown but the Le MADS-RIN transcription factor (RIN) has been proposed to be one of the regulators [1,34].…”

“…ACC is synthesised by ACC synthase (ACS) from S-adenosyl methionine (SAM) which originates from the amino acid methionine [1]. Differences between the capability of climacteric and non-climacteric fruit to produce ethylene probably lie with the presence of two systems of ethylene production exclusively in climacteric fruit [4,6]. …”

“…During fruit development, ethylene is produced at a basal level in climacteric and non-climacteric fruit alike, a process known as System 1 [4,6]. When climacteric fruit reach maturity, another process called System 2 is initiated to produce a burst in ethylene production to promote ripening while non-climacteric fruit are thought to remain in System 1 [4,6,7].…”

“…When climacteric fruit reach maturity, another process called System 2 is initiated to produce a burst in ethylene production to promote ripening while non-climacteric fruit are thought to remain in System 1 [4,6,7]. The regulation of these two systems has been associated with the differential expression of ACO and ACS isoforms, especially when first characterised during tomato ripening [8,9].…”

“…Furthermore, System 1 is also known to be an auto-inhibitory system whereas System 2 is an auto-stimulatory system [1,4]. In climacteric tomato, System 1-associated isoforms (such as LeACS1A ) are negatively regulated by high ethylene whereas System 2-associated isoforms (such as LeACO1 and LeACS2 ) are positively regulated [6,8]. Given that these ACO and ACS isoforms were regulated by the presence of ethylene, its perception also appears integral to climacteric ripening.…”