BEL1‐LIKE HOMEODOMAIN 11 regulates chloroplast development and chlorophyll synthesis in tomato fruit

Abstract

Chloroplast development and chlorophyll(Chl)metabolism in unripe tomato contribute to the growth and quality of the fruit, however these mechanisms are poorly understood. In this study, we initially investigated seven homeobox-containing transcription factors (TFs) with specific ripening-associated expression patterns using virus-induced gene silencing (VIGS) technology and found that inhibiting the expression of one of these TFs, BEL1-LIKE HOMEODOMAIN11 (SlBEL11), significantly increased Chl levels in unripe tomato fruit. This enhanced Chl accumulation was further validated by generating stable RNA interference (RNAi) transgenic lines. RNA sequencing (RNA-seq) of RNAi-SlBEL11 fruit at the mature green (MG) stage showed that 48 genes involved in Chl biosynthesis, photosynthesis and chloroplast development were significantly upregulated compared with the wild type (WT) fruit. Genomic global scanning for Homeobox TF binding sites combined with RNA-seq differential gene expression analysis showed that 22 of these 48 genes were potential target genes of SlBEL11 protein. These genes included Chl biosynthesis-related genes encoding for protochlorophyllide reductase (POR), magnesium chelatase H subunit (CHLH) and chlorophyllide a oxygenase (CAO), and chloroplast development-related genes encoding for chlorophyll a/b binding protein (CAB), homeobox protein knotted 2 (TKN2) and ARABIDOPSIS PSEUDO RESPONSE REGULATOR 2-LIKE (APRR2-like). Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation quantitative polymerase chain reaction (PCR) (ChIP-qPCR) assays were employed to verify that SlBEL11 protein could bind to the promoters for TKN2, CAB and POR. Taken together, our findings demonstrated that SlBEL11 plays an important role in chloroplast development and Chl synthesis in tomato fruit.

The zinc‐finger transcription factor CcLOL1 controls chloroplast development and immature pepper fruit color in Capsicum chinense and its function is conserved in tomato

“…A deletion mutation in the cucumber ortholog of APRR2-LIKE was associated with reduced chlorophyll content and the white skin color of immature fruit in this species (Jiao et al, 2017). More recently, a BEL-LIKE HOMEODO-MAIN 11 transcription factor was identified as a new negative regulator of chloroplast development and chlorophyll biosynthesis in tomato fruit (Meng et al, 2018). The auxin response factor SlARF10 was found to be a positive regulator of chlorophyll content in the tomato fruit (Yuan et al, 2018).…”

Optimization of a virus-induced gene silencing system and functional elucidation of MaBAM9b in postharvest banana fruits

“…For Lycium plants, the optimized conditions included an OD600 of 1.0, with the cells incubated at room temperature for 3-4 h. The pTRV1 and pTRV2 solutions were mixed at a ratio of 1:1 for injection [30]. In tomato, the OD600 was adjusted to 0.5 or 1.0 for all of the vectors, and the resuspended cells with pTRV1 and pTRV2 were then combined at a ratio of 1:1 and kept for more than 10 d in a plastic tray [8,31]. In loquat, the OD600 was adjusted to 1.2, and the resuspended cells with pTRV1 and pTRV2 were combined at a ratio of 1:1 and co-cultivated for a week [18].…”

Progress on Understanding Transcriptional Regulation of Chloroplast Development in Fleshy Fruit

“…More recently, seven homeodomain-containing transcription factors with specific ripening-associated expression patterns were investigated, and three BEL1-like (BELL) members showed fruit-specific expression in tomato [22]. BEL1-LIKE HOMEODOMAIN11 (SlBEL11) was shown to regulate chloroplast development and chlorophyll synthesis in tomato fruits.…”

“…In general, chloroplast development in fruits shares similar regulatory mechanisms with those found in leaves [17,20]. However, it was discovered that some transcription factors have special contribution on the regulation of fruit chloroplast development, while they only have little effect on leaf chloroplast development [17,21,22]. In the past few years, understanding the molecular mechanism underlying the fine manipulation of chloroplast development in fleshy fruits has been intensively investigated.…”