Figure 6

A proposed model for the ripening of non-climacteric capsicum. The ethylene pathway generally involves different isoforms of ACO and ACS to produce ethylene before it is perceived by ETR for other downstream responses. In tomato, System 1 ethylene production during development will be followed by System 2 ethylene production to induce climacteric ripening [8]. However, in non-climacteric capsicum ripening, System 2 may be absent based on the limited expression (dotted arrows) of certain System 2-associated isoforms such as CaACS2 and CaACO1 during ripening and upon ethylene treatment (compared to their homologue expression in tomato). The expression of CaACS1 and CaACO4 during capsicum ripening was associated with System 1 due to their significant reduction upon ethylene treatment at either or both G and B stages. This is in contrast to the CaACO4 homologue in tomato, LeACO4, which is a System 2-associated isoform [22]. Therefore System 1 may be predominantly operating in capsicum to produce the basal ethylene level (while inhibiting System 2 ethylene production). The basal ethylene level may be needed to maintain the rate of ETR turnover, particularly subfamily II CaETR4 and CaETR5, as ethylene perception removal (through 1-MCP treatment) severely affected the CaETR expression as well as other possible downstream CaACO and CaACS isoforms. These subfamily II CaETRs may become saturated, and perhaps together with ethylene independent pathways upon ripening onset, non-climacteric capsicum ripening could be initiated. The ethylene independent pathways may also involve some ripening regulators such as RIN transcription factors and their presence may induce the expression of CaACO4 upon ripening onset.