Differential roles of SNAC4 and SNAC9 in ABA-mediated softening during tomato fruit ripening

Previous study suggested that SNAC4 (NM_001279348.2) and SNAC9 (NM_001365397.1) have opposite effects on carotenoid biosynthesis and softening in tomato (Solanum lycopersicum L.) fruit ripening. Here, we use an overexpressing system to explore the regulatory mechanism of SNAC4/9 in tomato fruit ripening and softening. Our results showed that the overexpression of SNAC4 and SNAC9 (OE-SNAC4/9) accelerated fruit ripening, promoted ethylene biosynthesis and carotenoid accumulation, but showed different functions on ABA content and fruit firmness. Genes related to ABA and fruit softening were up-regulated in OE-SNAC4, while down-regulated in OE-SNAC9. The structures of cell wall and peel were also different in these two OE lines. These data verified that SNAC4 and SNAC9 exhibit different regulation modes on fruit softening. Thus, we propose a working model that SNAC4/9 regulates fruit ripening and softening through three layers including cell wall modification-related genes, ethylene and ABA signaling. Our findings provide new insight into NACs involved in regulating fruit ripening.