Tomato (Solanum lycopersicum) is a model plant for studying fruit development and ripening. In this study, we found that 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. Analysis of chlorophyll content and TEM observations was consistent with these phenotypes, indicating that SlBL4 was involved in chlorophyll accumulation and chloroplast formation. Ripened fruit of SlBL4-RNAi plants had noticeably decreased firmness, larger intercellular spaces, and thinner cell walls than the wild-type. RNA-seq identified differentially expressed 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 the expression of protoporphyrinogen oxidase (SlPPO), magnesium chelatase H subunit (SlCHLD), pectinesterase (SlPE), protochlorophyllide reductase (SlPOR), chlorophyll a/b binding protein 3B (SlCAB-3B), and homeobox protein knotted 2 (TKN2). In contrast, it positively regulated the expression of squamosa promoter binding protein-like colorless non-ripening (LeSPL-CNR). Our results indicate that SlBL4 is involved in chlorophyll accumulation, chloroplast development, cell wall metabolism, and the accumulation of carotenoids during tomato fruit ripening, and provide new insights for the transcriptional regulation mechanism of BELL-mediated fruit growth and ripening.

中文翻译：

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BEL1-Like HOMEODOMAIN4调节番茄果实中的叶绿素积累，叶绿体发育和细胞壁代谢。

番茄（Solanum lycopersicum）是研究果实发育和成熟的典范植物。在这项研究中，我们发现番茄钟状同源结构域4（SlBL4）的下调导致水果的表型略深绿色，并且淀粉，果糖和葡萄糖的积累增加。叶绿素含量的分析和TEM观察与这些表型一致，表明SlBL4参与叶绿素积累和叶绿体形成。SlBL4的成熟果实-RNAi植物比野生型植物的硬度明显降低，细胞间空间更大，细胞壁更薄。RNA-seq鉴定了参与叶绿素代谢，叶绿体发育，细胞壁代谢和类胡萝卜素代谢的差异表达基因。ChIP-seq识别（G / A）GCCCA（A / T / C）和（C / A / T）（C / A / T）AAAAA（G / A / T）（G / A）主题。SlBL4直接抑制的表达原卟啉原氧化酶（SlPPO），镁螯合H亚基（SlCHLD），果胶酯酶（SLPE），原叶绿素还原酶（SlPOR），叶绿素a / b结合蛋白3B（SlCAB-3B）和同源盒蛋白打结2（TKN2）。相反，它正调控鳞状启动子结合蛋白样无色非成熟（LeSPL-CNR）的表达。我们的结果表明，SlBL4参与番茄果实成熟过程中的叶绿素积累，叶绿体发育，细胞壁代谢和类胡萝卜素的积累，并为BELL介导的果实生长和成熟的转录调控机制提供了新的见识。