Tomato is a model for studying the climate for fruit development and ripening. Down-regulation of a tomato bell-like homeodomain 4 (SlBL4) resulted in a slightly darker green fruit phenotype and increased accumulation of starch, fructose and glucose. Chlorophyll content analysis and TEM observation confirmed these phenotypes, indicating that SlBL4 was involved in the chlorophyll accumulation and chloroplast formation in tomato. SlBL4-i fruits had noticeably decreased firmness and have larger intercellular spaces and thinner cell walls in the ripened fruits. RNA-Seq had identified differential expression genes involved in chlorophyll metabolism, chloroplast development, cell wall metabolism and carotenoid metabolism. ChIP-seq identified (G/A) GCCCA (A/T/C) and (C/A/T) (C/A/T) AAAAA (G/A/T) (G/A) motifs. SlBL4 directly inhibited protoporphyrinogen oxidase (SlPPO), magnesium chelatase H subunit (SlCHLD), pectinesterase (SlPE) protochlorophyllide reductase (SlPOR), chlorophyll a/b binding protein (SlCAB-3B) and homeobox protein knotted 2 (TKN2) and expressions. In addition, SlBL4 positively regulated squamosa promoter binding protein-like colorless non-ripening (LeSPL-CNR) expression. Our study indicated that SlBL4 was involved in the chlorophyll accumulation, chloroplast development, cell wall metabolism and carotenoids accumulation during tomato fruit ripening. Our data reveal novel evidence for the transcriptional regulation mechanism of BELL mediated fruit growth and ripening.

Chlorophylls and carotenoids are essential and beneficial substances for both plant and human health. Identifying the regulatory network of these pigments is necessary for improving fruit quality. In a previous study, we identified an R2R3-MYB transcription factor, SlMYB72, that plays an important role in chlorophyll and carotenoid metabolism in tomato fruit. Here, we demonstrated that the SlMYB72-interacting protein SlZHD17, which belongs to the zinc-finger homeodomain transcription factor family, also functions in chlorophyll and carotenoid metabolism. Silencing SlZHD17 in tomato improved multiple beneficial agronomic traits, including dwarfism, accelerated flowering, and earlier fruit harvest. More importantly, downregulating SlZHD17 in fruits resulted in larger chloroplasts and a higher chlorophyll content. Dual-luciferase, yeast one-hybrid and electrophoretic mobility shift assays clarified that SlZHD17 regulates the chlorophyll biosynthesis gene SlPOR-B and chloroplast developmental regulator SlTKN2 in a direct manner. Chlorophyll degradation and plastid transformation were also retarded after suppression of SlZHD17 in fruits, which was caused by the inhibition of SlSGR1, a crucial factor in chlorophyll degradation. On the other hand, the expression of the carotenoid biosynthesis genes SlPSY1 and SlZISO was also suppressed and directly regulated by SlZHD17, which induced uneven pigmentation and decreased the lycopene content in fruits with SlZHD17 suppression at the ripe stage. Furthermore, the protein–protein interactions between SlZHD17 and other pigment regulators, including SlARF4, SlBEL11, and SlTAGL1, were also presented. This study provides new insight into the complex pigment regulatory network and provides new options for breeding strategies aiming to improve fruit quality.

No abstract