Cucurbitacin B (CuB) is the primary compound responsible for bitterness in melon (Cucumis melo L.) and negatively affects fruit quality and consumer acceptance. Although CuB biosynthesis is regulated by multiple transcription factors, the regulatory mechanisms underlying CuB biosynthesis across different tissues remain unclear. Here, we generated two independent knockout lines (Cmbt-18 and Cmbt-19) of the basic helix-loop-helix (bHLH) transcription factor gene CmBt (Bitter fruit) using CRISPR-Cas9. Loss of CmBt reduces CuB levels in fruits and roots and produces non-bitter leaves, indicating that CmBt predominantly regulates CuB accumulation in leaves. By contrast, its paralog CmBr (Bitter root) predominantly controls CuB accumulation in fruits and roots. These results redefine CmBt as a leaf-predominant regulator and CmBr as a fruit/root-predominant regulator of CuB biosynthesis, revising previous findings. Furthermore, we demonstrate that CmBt physically interacts with CmBr. The CmbtCmbr double mutants exhibit a more severe reduction in CuB content in fruits, leaves, and roots, compared with either single mutant, confirming a synergistic interaction between CmBt and CmBr. Reverse transcription quantitative PCR (RT-qPCR) and tobacco dual-luciferase assays further show that CmBt and CmBr synergistically activate the CuB biosynthetic gene CmBi. Additionally, the CmbtCmbr double mutant completely prevents CPPU-induced fruit bitterness. Our findings reveal that CmBt and CmBr exert tissue-specific, partially redundant, and synergistic roles in activating CmBi and promoting CuB biosynthesis. These results identify the CmBt-CmBr locus as a promising target for breeding non-bitter melon cultivars.

The Cucurbitaceae family comprises economically valuable vegetables such as cucumber, melon, and pumpkin. GRAS proteins, which are crucial transcription factors, play diverse roles in plant growth and development. However, comparative investigations of GRAS proteins across Cucurbitaceae species are limited. Here, we identified 241 GRAS family genes in six cucurbit crops. The number of GRAS genes in cucumber, melon, wax gourd, watermelon, and bottle gourd ranged from 36 to 37, while the pumpkin genome contained 57 GRAS genes, possibly due to a recent whole-genome duplication. We classified cucurbit GRAS genes into 16 subfamilies and identified species-specific motifs and specific-expression patterns in the SCLB and RAD1 subfamilies. Notably, we identified 38 tissue-specific expressed genes, particularly fruit-specific genes potentially involved in fruit development. Additionally, we predicted the role of GRAS genes in regulating hypocotyl elongation under weak or dark light conditions in cucurbit plants. These findings enhance our understanding of the characteristics, evolution, and potential functions of GRAS genes in six cucurbit crops, providing valuable resources for genetic research in the Cucurbitaceae family as well as important agronomic traits.

In this study, we present the complete plastid genome of Cryptophyceae sp. CCMP2293. The circular genome is 139,208 bp in length and contains 142 protein-coding genes (PCGs), 30 transfer RNA (tRNA) genes, 6 ribosome RNA (rRNA) genes, and 1 transfer-messenger RNA (tmRNA) gene. The overall nucleotide composition is: 33.6% A, 32.5% T, 16.8% C, and 17.1% G with a total A + T content of 66.1%. The phylogenetic tree was constructed to explore the taxonomic status of Cryptophyceae sp. CCMP2293, which is closely related to G. theta and R. salina.

The complete plastid genome of Scytosiphon lomentaria was determined in this study. The circular genome was 139,208 bp in length with the GC content of 31.3%. It contained 137 protein-coding genes (PCGs), 27 transfer RNA (tRNA) genes, and 8 ribosome RNA (rRNA) genes. A phylogenetic analysis based on the plastid genomes of Phaeophyceae indicated that S. lomentaria is located in the Ectocarpales lineage, being closely related to Endarachne binghamiae.