The identification of Cucumis sativus Glabrous 1 (CsGL1) required for the formation of trichomes uncovers a novel function for the homeodomain-leucine zipper I gene

Abstract

The spines and bloom of cucumber (Cucumis sativus L.) fruit are two important quality traits related to fruit market value. However, until now, none of the genes involved in the formation of cucumber fruit spines and bloom trichomes has been identified. Here, the characterization of trichome development in wild-type (WT) cucumber and a spontaneous mutant, glabrous 1 (csgl1) controlled by a single recessive nuclear gene, with glabrous aerial organs, is reported. Via map-based cloning, CsGL1 was isolated and it was found that it encoded a member of the homeodomain-leucine zipper I (HD-Zip I) proteins previously identified to function mainly in the abiotic stress responses of plants. Tissue-specific expression analysis indicated that CsGL1 was strongly expressed in trichomes and fruit spines. In addition, CsGL1 was a nuclear protein with weak transcriptional activation activity in yeast. A comparative analysis of the digital gene expression (DGE) profile between csgl1 and WT leaves revealed that CsGL1 had a significant influence on the gene expression profile in cucumber, especially on genes related to cellular process, which is consistent with the phenotypic difference between csgl1 and the WT. Moreover, two genes, CsMYB6 and CsGA20ox1, possibly involved in the formation of cucumber trichomes and fruit spines, were characterized. Overall, the findings reveal a new function for the HD-Zip I gene subfamily, and provide some candidate genes for genetic engineering approaches to improve cucumber fruit external quality.

CsTTG1 Encodes a WD-repeat Protein That Regulates Fruit Wart Formation in Cucumis sativus through Interaction with the Homeodomain-leucine Zipper I Protein Mict

“…Mict is known to localize to the cell nucleus in onion (Allium cepa) epidermal cells when expressed heterologously (Li et al, 2015), and we also observed here that CsTTG1-GFP was localized mainly to the nucleus and plasma membrane of onion epidermal cells (Fig. 9A).…”

“…In cucumber, CsGA20ox1, a gene putatively encoding GA 20-oxidase, has been found to be a negative regulator of fruit spine growth, and transgenic lines with significantly increased expression of CsGA20ox1 were reported to have shorter fruit spines than the wild type (Li et al, 2015). In this study, we found that 35S:CsTTG1 transgenic lines had longer fruit spines ( Fig.…”

“…To explore the regulatory mechanism of CsTTG1 in fruit bloom trichome and wart formation, the genetic relationship between CsTTG1 and Mict/CsGL1, which is required for further differentiation of cucumber trichomes (Li et al, 2015), was examined. The relative expression level of Mict in the 35S:CsTTG1 plants was lower than in wild-type plants (Supplemental Fig.…”

“…When spines are combined with tubercules, typically formed at the base of the spine, a mound-like joint connected to the fruit, cucumber fruits have a characteristic warty (Wty) trait. Smooth fruits that lack spines and tubercules, are more convenient to clean, transport, pack and store (Zhang et al, 2010;Yang et al, 2014;Li et al, 2015), and there is therefore great interest in identifying the molecular mechanism that controls the formation of cucumber bloom trichomes and warts (composed of spines and tubercules).…”

“…It has been shown that the phenotype of the csgl cucumber mutant is controlled by a single recessive nuclear gene and that csgl1 is epistatic to the Tuberculate fruit (Tu) gene (Cao et al, 2001;Yang et al, 2014). Tu and Mict/CsGL1 were both identified, and Mict/CsGL1 was shown to be required for further differentiation of cucumber trichomes in all aerial parts of the plant, including leaves, stems, tendrils, floral organs, and fruits, but not for their initiation (Li et al, 2015;Zhao et al, 2015). Recently, a homeodomainleucine zipper gene, Tril, involved in multicellular trichome initiation in cucumber, was identified (Wang et al, 2016).…”