Dwarf or semi-dwarf plant architecture characterized by reduced vine or internode length is a desirable trait for winter squash (Cucurbita maxima) production, but the regulatory mechanisms and specific genes responsible for this trait remain unidentified. In this study, we conducted map-based QTL cloning and functional characterization of a major-effect QTL, underlying the dwarf phenotype (short vine length) in the inbred line Agol. A near-isogenic line, SV015 carrying the wildtype long-stem allele in Agol background was developed. Comparative histological observations in the two NILs indicated that the dwarf phenotype is due to reduced cell elongation, and thus internode length during stem elongation. Genetic analysis in NIL-derived segregating populations confirmed that a single dominant gene, CmaDw-1, controlled the dwarf phenotype. Bulked segregant analysis and fine genetic mapping identified CmaCh03G013990 as the sole candidate gene for CmaDw-1 that encodes gibberellin 2-oxidase 2 (CmaGA2ox2), a key enzyme in gibberellin (GA) metabolism. Sequence analysis revealed multiple single nucleotide polymorphisms (SNPs) or Insert/Deletions (InDels) within the promoter and coding region of CmaGA2ox2 that are associated with reduced stem elongation in Agol. Spatiotemporal expression analysis of CmaGA2ox2 revealed its differential expression in the hypocotyls between NILs in the early development stage. Quantitation of endogenous GAs and exogenous phytohormone treatments in NILs and ectopic expression of CmaDw-1 in Arabidopsis further support the critical role of CmaGA2ox2 in stem elongation. This work provides the first molecular insights into the regulation of dwarf plant architecture in C. maxima.

High temperature-induced hypocotyl elongation is a typical thermomorphogenesis trait that may significantly affect early seedling growth and subsequent crop yield. The ambient temperature and endogenous auxin are two critical factors that regulate hypocotyl growth. However, the mechanism of temperature and auxin integration in horticultural plants remains poorly understood. In this study, the roles of the basic helix-loop-helix transcription factor CsPIF4 in regulating auxin biosynthesis genes and the auxin content in the hypocotyl of cucumber (Cucumis sativus L.) seedlings under high temperature were investigated. qRT-PCR and in situ hybridization analysis revealed that expression of CsPIF4 was enhanced in the epidermis and vascular bundles in the hypocotyl of cucumber seedlings in response to high temperature. qRT-PCR and HPLC analysis showed that CsPIF4 positively regulated transcription of the auxin biosynthesis gene CsYUC8 and the auxin content in the hypocotyl under high temperature (35 °C). The CRISPR/Cas9-mediated knockout of CsPIF4 resulted in a shorter hypocotyl compared with that of the wild type, together with decreased expression of CsYUC8 and lower auxin content in response to high temperature. Furthermore, biochemical assays showed that CsPIF4 could bind directly to the G-box motif of the CsYUC8 promoter and thereby activate CsYUC8 expression. These findings provide insight into the molecular mechanism of high temperature-mediated hypocotyl elongation in cucumber.