Abstract
BackgroundClimacteric fruit exhibit high ethylene and respiration levels during ripening but these levels are limited in non-climacteric fruit. Even though capsicum is in the same family as the well-characterised climacteric tomato (Solanaceae), it is non-climacteric and does not ripen normally in response to ethylene or if harvested when mature green. However, ripening progresses normally in capsicum fruit when they are harvested during or after what is called the ‘Breaker stage’. Whether ethylene, and components of the ethylene pathway such as 1-aminocyclopropane 1-carboxylate (ACC) oxidase (ACO), ACC synthase (ACS) and the ethylene receptor (ETR), contribute to non-climacteric ripening in capsicum has not been studied in detail. To elucidate the behaviour of ethylene pathway components in capsicum during ripening, further analysis is therefore needed. The effects of ethylene or inhibitors of ethylene perception, such as 1-methylcyclopropene, on capsicum fruit ripening and the ethylene pathway components may also shed some light on the role of ethylene in non-climacteric ripening.ResultsThe expression of several isoforms of ACO, ACS and ETR were limited during capsicum ripening except one ACO isoform (CaACO4). ACS activity and ACC content were also low in capsicum despite the increase in ACO activity during the onset of ripening. Ethylene did not stimulate capsicum ripening but 1-methylcyclopropene treatment delayed the ripening of Breaker-harvested fruit. Some of the ACO, ACS and ETR isoforms were also differentially expressed upon treatment with ethylene or 1-methylcyclopropene.ConclusionsACS activity may be the rate limiting step in the ethylene pathway of capsicum which restricts ACC content. The differential expression of several ethylene pathway components during ripening and upon ethylene or 1-methylclopropene treatment suggests that the ethylene pathway may be regulated differently in non-climacteric capsicum compared to the climacteric tomato. Ethylene independent pathways may also exist in non-climacteric ripening as evidenced by the up-regulation of CaACO4 during ripening onset despite being negatively regulated by ethylene exposure. However, some level of ethylene perception may still be needed to induce ripening especially during the Breaker stage. A model of capsicum ripening is also presented to illustrate the probable role of ethylene in this non-climacteric fruit.
Highlights
Climacteric fruit exhibit high ethylene and respiration levels during ripening but these levels are limited in non-climacteric fruit
In this study we have investigated the expression of ACC oxidase (ACO), ACC synthase (ACS) and ethylene receptor (ETR) isoforms during capsicum
CaACO, CaACS and CaETR isoforms were differentially expressed during capsicum ripening Throughout capsicum ripening, the transcript expression of most ACO isoforms was limited except CaACO4 (Figure 1A)
Summary
Climacteric fruit exhibit high ethylene and respiration levels during ripening but these levels are limited in non-climacteric fruit. 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]. 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 appears integral to climacteric ripening. The regulation of these isoforms and the two ethylene production systems during nonclimacteric ripening is still inadequately documented and further research is needed
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