Regulatory link of CaSGR–CaNOR controls fruit shelf-life in pepper

Abstract

STAY-GREEN (SGR), encoding the Mg-dechelatase, is a key genetic regulator of chlorophyll degradation in leaves and fruit. However, the extended functions of SGR in fruit ripening remain largely unexplored. Single-nucleotide polymorphisms were identified in the CaSGR of brown peppers (Capsicum spp.), and co-segregated with the recessive brown fruit color in an F₂ population. The brown pepper accessions showed chlorophyll retention in the ripening fruit and increased shelf-life. CaSGR-silenced and brown peppers delayed ethylene-dependent leaf senescence. CaSGR-silenced fruit resulted in chlorophyll retention, reduced carotenoid content, increased firmness, and extended shelf-life. The gene expression of chlorophyll catabolic (NOL, HCAR, PPH, PAO, CYP89A9), carotenoid biosynthetic (PSY1, PDS, ZISO, ZDS, CHYB, ZEP, CCS), and cell wall degradation-related (PG2a, CEL1, CEL2, EXP1) genes was downregulated in the CaSGR-silenced fruit. In addition, the flavor volatiles were altered in the CaSGR-silenced fruit, characterized by an absence of hexanal, nonanal, and nonanoic acid, and the accumulation of 4-methylpentyl 3-methylbutanoate. Given that CaSGR regulates fruit ripening, CaNOR, a homologue of the tomato NAC-NOR, was selected as a candidate regulator of CaSGR. Transcriptional activation assay demonstrated that CaNOR acts as a positive regulator of CaSGR expression. Furthermore, silencing of CaNOR reduced chlorophyll degradation, carotenoid accumulation, and fruit softening. The expression of chlorophyll catabolic, carotenoid biosynthetic, and cell wall degradation-related genes was down-regulated in CaNOR-silenced fruit. Furthermore, the reciprocal reduction of each gene expression in the silenced fruit suggests that a regulatory link between CaSGR and CaNOR coordinates fruit ripening. These findings will be helpful for improving fruit shelf-life in pepper.