Histone deacetylase gene SlHDT1 regulates tomato fruit ripening by affecting carotenoid accumulation and ethylene biosynthesis.

Fruit development and ripening is a complicated biological process, that is not only regulated by plant hormones and transcription factors, but also affected by epigenetic modifications. Histone deacetylation is an important way of epigenetic modification, and little information about it is available. In this study, an RNAi vector was constructed and transferred successfully into wild-type tomato for further research on the detailed functions of the histone deacetylation gene SlHDT1. The expression level of PSY1 was upregulated, and the transcription levels of LCY-B, LCY-E and CYC-B were downregulated, which was consistent with the increased accumulation of carotenoids. In addition, the expression levels of ethylene biosynthetic genes (ACS2, ACS4 and ACO1, ACO3), ripening-associated genes (RIN, E4, E8, PG, Pti4 and LOXB) and fruit cell wall metabolism genes (HEX, MAN, TBG4, XTH5 and XYL) were significantly upregulated further strengthening the results, including an increased ethylene content, advanced fruit ripening time and a shortened shelf life of tomato fruits. In addition, the increased total histone H3 acetylation level also provides evidence of a connection between epigenetic regulation by histone deacetylation and fruit development and ripening. Hence, SlHDT1 is a negative regulator and plays an essential role in regulating ethylene and carotenoid biosynthesis during fruit ripening through influences on the acetylation level.

Silencing SlMED18, tomato Mediator subunit 18 gene, restricts internode elongation and leaf expansion.

Mediator complex, a conserved multi-protein, is necessary for controlling RNA polymerase II (Pol II) transcription in eukaryotes. Given little is known about them in tomato, a tomato Mediator subunit 18 gene was isolated and named SlMED18. To further explore the function of SlMED18, the transgenic tomato plants targeting SlMED18 by RNAi-mediated gene silencing were generated. The SlMED18-RNAi lines exhibited multiple developmental defects, including smaller size and slower growth rate of plant and significantly smaller compound leaves. The contents of endogenous bioactive GA3 in SlMED18 silenced lines were slightly less than that in wild type. Furthermore, qRT-PCR analysis indicated that expression of gibberellins biosynthesis genes such as SlGACPS and SlGA20x2, auxin transport genes (PIN1, PIN4, LAX1 and LAX2) and several key regulators, KNOX1, KNOX2, PHAN and LANCEOLATE(LA), which involved in the leaf morphogenesis were significantly down-regulated in SlMED18-RNAi lines. These results illustrated that SlMED18 plays an essential role in regulating plant internode elongation and leaf expansion in tomato plants and it acts as a key positive regulator of gibberellins biosynthesis and signal transduction as well as auxin proper transport signalling. These findings are the basis for understanding the function of the individual Mediator subunits in tomato.

A histone deacetylase gene, SlHDA3, acts as a negative regulator of fruit ripening and carotenoid accumulation.

KEY MESSAGE: SlHDA3 functions as an inhibitor and regulates tomato fruit ripening and carotenoid accumulation. Post-translational modifications, including histones acetylation, play a pivotal role in the changes of chromatin structure dynamic modulation and gene activity. The regulation of histone acetylation is achieved by the action of histone acetyltransferases and deacetylases, which play crucial roles in the regulation of transcription activation. There is an increasing research focus on histone deacetylation in crops, but the role of histone deacetylase genes (HDACs) in tomato has not been elucidated. With the aim of characterizing the tomato RPD3/HDA1 family histone deacetylase genes, SlHDA3 was isolated and its RNA interference (RNAi) lines was obtained. The fruit of SlHDA3 RNAi lines exhibited accelerated ripening process along with short shelf life characteristics. The accumulation of carotenoid was increased due to the alteration of the carotenoid pathway flux. Climacteric ethylene production also stimulated along with significantly up-regulated expression of ethylene biosynthetic genes (ACS2, ACS4, ACO1 and ACO3) and fruit ripening-associated genes (RIN, E4, E8, PG, Pti4, LOXB, Cnr and TAGL1) in SlHDA3 RNAi lines. Besides, fruit cell wall metabolism-associated genes (HEX, MAN, TBG4, XTH5 and XYL) were enhanced in transgenic lines. Relative to wild type (WT) plants, SlHDA3 RNAi seedlings displayed shorter hypocotyls and more sensitivity to ACC (1-aminocyclopropane-1-carboxylate). These results indicated that SlHDA3 is involved in the regulation of fruit ripening by affecting ethylene biosynthesis and carotenoid accumulation.

Silencing of histone deacetylase SlHDT3 delays fruit ripening and suppresses carotenoid accumulation in tomato.

The acetylation levels of histones on lysine residues are regulated by histone acetyltransferases and histone deacetylases, which play an important but understudied role in the control of gene expression in plants. There is an increasing research focus on histone deacetylation in crops, but to date, there is little information regarding tomato. With the aim of characterizing the tomato HD2 family of histone deacetylases, an RNA interference (RNAi) expression vector of SlHDT3 was constructed and transformed into tomato plants. The time of fruit ripening was delayed and the shelf life of the fruit was prolonged in SlHDT3 RNAi lines. The accumulation of carotenoid was decreased by altering of the carotenoid pathway flux. Ethylene content was also reduced and expression of ethylene biosynthetic genes (ACS2, ACS4 and ACO1, ACO3) and ripening-associated genes (RIN, E4, E8, PG, Pti4 and LOXB) was significantly down-regulated in SlHDT3 RNAi lines. The expression of genes involved in fruit cell wall metabolism (HEX, MAN, TBG4, XTH5 and XYL) was inhibited compared with wild type. These results indicate that SlHDT3 functions as a positive regulator of fruit ripening by affecting ethylene synthesis and carotenoid accumulation and that SlHDT3 lies upstream of SlMADS-RIN in the fruit ripening regulatory network.

The tomato histone deacetylase SlHDA1 contributes to the repression of fruit ripening and carotenoid accumulation.

Histone deacetylation is one of the well characterized post-translational modifications related to transcriptional repression in eukaryotes. The process of histone deacetylation is achieved by histone deacetylases (HDACs). Over the last decade, substantial advances in our understanding of the mechanism of fruit ripening have been achieved, but the role of HDACs in this process has not been elucidated. In our study, an RNA interference (RNAi) expression vector targeting SlHDA1 was constructed and transformed into tomato plants. Shorter fruit ripening time and decreased storability were observed in SlHDA1 RNAi lines. The accumulation of carotenoid was increased through an alteration of the carotenoid pathway flux. Ethylene content, ethylene biosynthesis genes (ACS2, ACS4 and ACO1, ACO3) and ripening-associated genes (RIN, E4, E8, Cnr, TAGL1, PG, Pti4 and LOXB) were significantly up-regulated in SlHDA1 RNAi lines. In addition, the expression of fruit cell wall metabolism genes (HEX, MAN, TBG4, XTH5 and XYL) was enhanced compared with wild type. Furthermore, SlHDA1 RNAi seedlings displayed shorter hypocotyls and were more sensitive to ACC (1-aminocyclopropane-1-carboxylate) than the wild type. The results of our study indicate that SlHDA1 functions as a negative regulator of fruit ripening by affecting ethylene synthesis and carotenoid accumulation.