A chitinase CsChi23 promoter polymorphism underlies cucumber resistance against Fusarium oxysporum f. sp. cucumerinum

Fusarium wilt disease, caused by Fusarium oxysporum f. sp. cucumerinum (Foc), leads to widespread yield loss and quality decline in cucumber. However, the molecular mechanisms underlying Foc resistance remain poorly understood. We report the mapping and functional characterisation of CsChi23, encoding a cucumber class I chitinase with antifungal properties. We assessed sequence variations at CsChi23 and the associated defence response against Foc. We functionally characterised CsChi23 using transgenic assay and expression analysis. The mechanism regulating CsChi23 expression was assessed by genetic and molecular approaches. CsChi23 was induced by Foc infection, which led to rapid upregulation in resistant cucumber lines. Overexpressing CsChi23 enhanced fusarium wilt resistance and reduced fungal biomass accumulation, whereas silencing CsChi23 causes loss of resistance. CsHB15, a homeodomain leucine zipper (HD-Zip) III transcription factor, was found to bind to the CsChi23 promoter region and activate its expression. Furthermore, silencing of CsHB15 reduces CsChi23 expression. A single-nucleotide polymorphism variation -400 bp upstream of CsChi23 abolished the HD-Zip III binding site in a susceptible cucumber line. Collectively, our study indicates that CsChi23 is sufficient to enhance fusarium wilt resistance and reveals a novel function of an HD-Zip III transcription factor in regulating chitinase expression in cucumber defence against fusarium wilt.

Transcriptomic and functional analysis provides molecular insights into multicellular trichome development

Trichomes, the hair-like structures located on aerial parts of most vascular plants, are associated with a wide array of biological processes and affect the economic value of certain species. The processes involved in unicellular trichome formation have been well-studied in Arabidopsis (Arabidopsis thaliana). However, our understanding of the morphological changes and the underlying molecular processes involved in multicellular trichome development is limited. Here, we studied the dynamic developmental processes involved in glandular and nonglandular multicellular trichome formation in cucumber (Cucumis sativus L.) and divided these processes into five sequential stages. To gain insights into the underlying mechanisms of multicellular trichome formation, we performed a time-course transcriptome analysis using RNA-sequencing analysis. A total of 711 multicellular trichome-related genes were screened and a model for multicellular trichome formation was developed. The transcriptome and co-expression datasets were validated by reverse transcription-quantitative PCR and in situ hybridization. In addition, virus-induced gene silencing analysis revealed that CsHOMEOBOX3 (CsHOX3) and CsbHLH1 are involved in nonglandular trichome elongation and glandular trichome formation, respectively, which corresponds with the transcriptome data. This study presents a transcriptome atlas that provides insights into the molecular processes involved in multicellular trichome formation in cucumber and can be an important resource for future functional studies.

Glandular trichomes: new focus on horticultural crops

Plant glandular trichomes (GTs) are epidermal outgrowths with the capacity to biosynthesize and secrete specialized metabolites, that are of great scientific and practical significance. Our understanding of the developmental process of GTs is limited, and no single plant species serves as a unique model. Here, we review the genetic mechanisms of GT initiation and development and provide a summary of the biosynthetic pathways of GT-specialized metabolites in nonmodel plant species, especially horticultural crops. We discuss the morphology and classification of GT types. Moreover, we highlight technological advancements in methods employed for investigating GTs. Understanding the molecular basis of GT development and specialized metabolites not only offers useful avenues for research in plant breeding that will lead to the improved production of desirable metabolites, but also provides insights for plant epidermal development research.

TINY BRANCHED HAIR functions in multicellular trichome development through an ethylene pathway in Cucumis sativus L

The fruit trichomes of Cucurbitaceae are widely desired in many Asian countries and have been a key determinant of cucumber (Cucumis sativus L.) cultivar selection for commercial production and breeding. However, our understanding of the initiation and development of cucumber trichomes is still limited. Here, we found that the cucumber TINY BRANCHED HAIR (TBH) gene is preferentially expressed in multicellular trichomes. Overexpression of CsTBH in tbh mutants restored the trichome phenotype and increased the percentage of female flowers, whereas silencing of CsTBH in wild-type plants resulted in stunted trichomes with a lower rate of female flowers. Furthermore, we provide evidence that CsTBH can directly bind to the promoters of cucumber 1-Aminocyclopropane-1-Carboxylate Synthase (CsACS) genes and regulate their expression, which affects multicellular trichome development, ethylene accumulation, and sex expression. Two cucumber acs mutants with different trichome morphology and sex morphs compared with their near-isogenic line further support our findings. Collectively, our study provides new information on the molecular mechanism of CsTBH in regulating multicellular trichome development and sex expression through an ethylene pathway.

The HD-ZIP IV transcription factor GL2-LIKE regulates male flowering time and fertility in cucumber

Cucumber is dioecious by nature, having both male and female flowers, and is a model system for unisexual flower development. Knowledge related to male flowering is limited, but it is reported to be regulated by transcription factors and hormone signals. Here, we report functional characterization of the cucumber (Cucumis sativus) GL2-LIKE gene, which encodes a homeodomain leucine zipper (HD-ZIP) IV transcription factor that plays an important role in regulating male flower development. Spatial-temporal expression analyses revealed high-level expression of CsGL2-LIKE in the male flower buds and anthers. CsGL2-LIKE is closely related to AtGL2, which is known to play a key role in trichome development. However, ectopic expression of CsGL2-LIKE in Arabidopsis gl2-8 mutant was unable to rescue the gl2-8 phenotype. Interestingly, the silencing of CsGL2-LIKE delayed male flowering by inhibiting the expression of the florigen gene FT and reduced pollen vigor and seed viability. Protein-protein interaction assays showed that CsGL2-LIKE interacts with the jasmonate ZIM domain protein CsJAZ1 to form a HD-ZIP IV-CsJAZ1 complex. Collectively, our study indicates that CsGL2-LIKE regulates male flowering in cucumber, and reveals a novel function of a HD-ZIP IV transcription factor in regulating male flower development of cucumber.

Comprehensive Analysis of the Chitinase Gene Family in Cucumber (Cucumis sativus L.): From Gene Identification and Evolution to Expression in Response to Fusarium oxysporum

Chitinases, a subgroup of pathogenesis-related proteins, are responsible for catalyzing the hydrolysis of chitin. Accumulating reports indicate that chitinases play a key role in plant defense against chitin-containing pathogens and are therefore good targets for defense response studies. Here, we undertook an integrated bioinformatic and expression analysis of the cucumber chitinases gene family to identify its role in defense against Fusarium oxysporum f. sp. cucumerinum. A total of 28 putative chitinase genes were identified in the cucumber genome and classified into five classes based on their conserved catalytic and binding domains. The expansion of the chitinase gene family was due mainly to tandem duplication events. The expression pattern of chitinase genes was organ-specific and 14 genes were differentially expressed in response to F. oxysporum challenge of fusarium wilt-susceptible and resistant lines. Furthermore, a class I chitinase, CsChi23, was constitutively expressed at high levels in the resistant line and may play a crucial role in building a basal defense and activating a rapid immune response against F. oxysporum. Whole-genome re-sequencing of both lines provided clues for the diverse expression patterns observed. Collectively, these results provide useful genetic resource and offer insights into the role of chitinases in cucumber-F. oxysporum interaction.

A CsTu-TS1 regulatory module promotes fruit tubercule formation in cucumber

The fruit epidermal features such as the size of tubercules are important fruit quality traits for cucumber production. But the mechanisms underlying tubercule formation remain elusive. Here, tubercule size locus CsTS1 was identified by map-based cloning and was found to encode an oleosin protein. Allelic variation was identified in the promoter region of CsTS1, resulting in low expression of CsTS1 in all 22 different small-warty or nonwarty cucumber lines. High CsTS1 expression levels were closely correlated with increased fruit tubercule size among 44 different cucumber lines. Transgenic complementation and RNAi-mediated gene silencing of CsTS1 in transgenic cucumber plants demonstrated that CsTS1 positively regulates the development of tubercules. CsTS1 is highly expressed in the peel at fruit tubercule forming and enlargement stage. Auxin content and expression of three auxin signalling pathway genes were altered in the 35S:CsTS1 and CsTS1-RNAi fruit tubercules, a result that was supported by comparing the cell size of the control and transgenic fruit tubercules. CsTu, a C₂ H₂ zinc finger domain transcription factor that regulates tubercule initiation, binds directly to the CsTS1 promoter and promotes its expression. Taken together, our results reveal a novel mechanism in which the CsTu-TS1 complex promotes fruit tubercule formation in cucumber.