A tomato NAC transcription factor, SlNAM1, positively regulates ethylene biosynthesis and the onset of tomato fruit ripening

Abstract

SUMMARY Fruit ripening in tomato (Solanum lycopersicum) is the result of selective expression of ripening‐related genes, which are regulated by transcription factors (TFs). The NAC (NAM, ATAF1/2, and CUC2) TF family is one of the largest families of plant‐specific TFs and members are involved in a variety of plant physiological activities, including fruit ripening. Fruit ripening‐associated NAC TFs studied in tomato to date include NAC‐NOR (non‐ripening), SlNOR‐like1 (non‐ripening like1), SlNAC1, and SlNAC4. Considering the large number of NAC genes in the tomato genome, there is little information about the possible roles of other NAC members in fruit ripening, and research on their target genes is lacking. In this study, we characterize SlNAM1, a NAC TF, which positively regulates the initiation of tomato fruit ripening via its regulation of ethylene biosynthesis. The onset of fruit ripening in slnam1‐deficient mutants created by CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats and CRISPR‐associated protein 9) technology was delayed, whereas fruit ripening in OE‐SlNAM1 lines was accelerated compared with the wild type. The results of RNA‐sequencing (RNA‐seq) and promoter analysis suggested that SlNAM1 directly binds to the promoters of two key ethylene biosynthesis genes (1‐aminocyclopropane‐1‐carboxylate synthase: SlACS2 and SlACS4) and activates their expression. This hypothesis was confirmed by electrophoretic mobility shift assays and dual‐luciferase reporter assay. Our findings provide insights into the mechanisms of ethylene production and enrich understanding of the tomato fruit ripening regulatory network.

The PavNAC56 transcription factor positively regulates fruit ripening and softening in sweet cherry (Prunus avium)

“…For instance, NAC‐NOR binds to the promoter of SlPL and activates its expression, thereby regulating fruit softening (Gao et al, 2020). SlNAC4 binds to the promoters of SlCEL2 and SlEXP1 , which encode cell wall degrading enzymes, to promote tomato fruit softening (Gao et al, 2021). In vitro and in vivo analyses showed that NOR‐like1 binds to the promoters of SlPG2a, SlPL, SlCEL2 , and SlEXP1 , which encode cell wall‐modifying enzymes, to activate their expression and promote fruit softening (Gao et al, 2018).…”

“…In addition, NAC TFs participate in the regulation of fleshy fruit ripening-associated processes in banana (Shan et al, 2012), tomato (Kou et al, 2018), peach (Zhou et al, 2015), melon (Wang et al, 2022), apple (Migicovsky et al, 2021), and strawberry (Martín-Pizarro et al, 2021). For instance, NOR (non-ripening), a NAC TF, regulates tomato fruit ripening (Gao et al, 2020) alongside other NAC family TFs, including SlNAC1, SlNAC4, NOR-like1, and SlNAM1 (Gao et al, 2018(Gao et al, , 2021Kou et al, 2018;Ma et al, 2014). In apple, the NOR ortholog MdNAC18.1 is thought to be a major determinant of fruit softening and harvest date (Migicovsky et al, 2021).…”

“…Fruit ripening, a key trait for many fleshy fruits, is a complex, genetically programmed developmental process involving multiple physiological, biochemical, and structural changes. Such changes include softening, pigment accumulation, the degradation of starch into soluble sugars, and the production of aroma volatiles, with corresponding changes in fruit appearance, flavor, texture, color, aroma, and nutritional status (Gapper et al, 2013;Giovannoni et al, 2017;Li et al, 2021). Fleshy fruits can be classified as climacteric or non-climacteric; climacteric fruits show induction of respiration and an ethylene burst at the onset of ripening, while non-climacteric fruits do not (Giovannoni, 2004).…”

“…A number of transcription factors (TFs) that are responsive to ethylene and ABA affect the † Xiliang Qi and Yuanxin Dong have contributed equally to this study. expression of genes related to flavonoid biosynthesis, cell wall metabolism, and regulatory proteins and play important roles during fruit ripening (Li et al, 2021). Such TFs include AP2/ERFs (APETALA 2/Ethylene Response Factors) (Karlova et al, 2011), MADS-box (MCM1/AGAMOUS/DEFICIENT/SRF) (Li et al, 2019), MYB (V-myb avian myeloblastosis viral oncogene homolog) (Fan et al, 2018), SBP (SQUAMOSA promoter binding protein) (Lai et al, 2020), bHLHs (basic helix-loop-helix) (Xiao et al, 2018), and NACs (NAM, ATAF1/2, and CUC2) (Liu et al, 2022).…”

NAC Transcription Factor Family Regulation of Fruit Ripening and Quality: A Review

“…Recently, Gao et al [70] identified a new NAC TF, SlNAM1, in tomato and obtained homozygous gene-edited mutants of SlNAM1 using CRISPR/Cas9 technology. It was found that the timing of the ethylene peak and ripening onset in SlNAM1 mutant fruits were both delayed, whereas they were advanced in SlNAM1-overexpressing fruit.…”