Plant organ growth and development are determined by a subtle balance between growth stimulation and inhibition. Fruit size and shape are important quality traits influencing yield and market value; however, the underlying mechanism regulating the balance of fruit growth to achieve final size and shape is not well understood. Here, we report a mechanistic model that governs cucumber (Cucumis sativus) fruit elongation through fine-tuning of ethylene homeostasis. We identified a cucumber mutant that bears short fruits owing to repressed cell division. SF1 (Short Fruit 1) encodes a cucurbit-specific RING-type E3 ligase, and the mutation resulted in its enhanced self-ubiquitination and degradation, but accumulation of ACS2 (1-aminocyclopropane-1-carboxylate synthase 2), a rate-limiting enzyme for ethylene biosynthesis. The overproduction of ethylene contributes to the short-fruit phenotype of sf1. Dysfunction of ACS2 resulted in reduced ethylene production, but still repressed cell division and shorter fruit, suggesting that ethylene is still required for basal fruit elongation. SF1 ubiquitinates and degrades both itself and ACS2 to control ethylene synthesis for dose-dependent effect on cell division and fruit elongation. Our findings reveal the mechanism by which ethylene dosage is regulated for the control of cell division in developing fruit.

Unisexual flowers provide a useful system for studying plant sex determination. In cucumber (Cucumis sativus L.), three major Mendelian loci control unisexual flower development, Female (F), androecious [a; 1-aminocyclopropane-1-carboxylate {ACC} synthase 11, acs11], and Monoecious (M; ACS2), referred to here as the Female, Androecious, Monoecious (FAM) model, in combination with two genes, gynoecious (g, the WIP family C2H2 zinc finger transcription factor gene WIP1) and the ethylene biosynthetic gene ACC oxidase 2 (ACO2). The F locus, conferring gynoecy and the potential for increasing fruit yield, is defined by a 30.2-kb tandem duplication containing three genes. However, the gene that determines the Female phenotype, and its mechanism, remains unknown. Here, we created a set of mutants and revealed that ACS1G is responsible for gynoecy conferred by the F locus. The duplication resulted in ACS1G acquiring a new promoter and expression pattern; in plants carrying the F locus duplication, ACS1G is expressed early in floral bud development, where it functions with ACO2 to generate an ethylene burst. The resulting ethylene represses WIP1 and activates ACS2 to initiate gynoecy. This early ACS1G expression bypasses the need for ACS11 to produce ethylene, thereby establishing a dominant pathway for female floral development. Based on these findings, we propose a model for how these ethylene biosynthesis genes cooperate to control unisexual flower development in cucumber.

Salt stress, as a significant adverse consequence of global climate change, severely restricts the yield and quality of watermelon. In this study, salt-tolerant cultivar T23 and salt-sensitive cultivar B2 were subjected to a 200 mM NaCl treatment (0 h, 6 h, 24 h, 48 h, and 168 h) at the three-leaf stage, and the adaptation mechanisms of the watermelon roots to salt stress were systematically investigated at the phenotypic, physiological, and gene transcription levels. Phenotypic observations revealed that salt stress inhibited seedling growth, caused leaf curling, and induced root yellowing, with the damage being significantly more severe in B2 than in T23. Compared with B2, the activities of superoxide dismutase (SOD) were increased by −7.13%, 169.15%, 34.95%, 84.87%, and 39.87% under NaCl treatment at 0 h, 6 h, 24 h, 48 h, and 168 h, respectively. Compared to the 0 h NaCl treatment, the proline content in B2 increased by 4.25%, 14.39%, and 110.00% at 24 h, 48 h, and 168 h of NaCl treatment, respectively, while T23 showed increases of 93.74%, 177.55%, and 380.56% at the corresponding time points. The provided physiological data demonstrate that T23 exhibits superior antioxidant and osmoregulatory abilities relative to B2. The transcriptome analysis identified differentially expressed genes (DEGs) between the two cultivars under salt stress, with T23 showing the highest number of DEGs at 6 h, while B2 exhibited a significant increase in DEGs at 168 h. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that metabolic pathways such as plant hormone signal transduction, terpenoid biosynthesis, mitogen-activated protein kinase (MAPK) signaling pathways, transporter activity, and transcription regulator activity play important roles in the salt stress response. Furthermore, yeast overexpression experiments preliminarily validated the critical roles of the tonoplast dicarboxylate transporter gene ClCG01G010280 and the NAC transcription factor gene ClCG05G024110 in salt stress tolerance. This study provides new molecular insights into the salt tolerance mechanism of watermelon and offers potential genetic resources for breeding salt-tolerant varieties.

Leaf color mutants are key resources for uncovering the molecular mechanisms of chloroplast development and photosynthesis. Here, we identified a novel yellow-green melon mutant, ‘ygp2’, which displays yellow-green leaves and dwarfism throughout development. Genetic analysis indicated that the trait is controlled by a single recessive nuclear gene. Map-based cloning delimited the candidate region to an 805 kb interval on chromosome 11, within which only one missense mutation was identified in MELO13C_11G242690, encoding a triosephosphate isomerase (CmpdTPI). Phylogenetic analysis suggested its plastid localization, which was confirmed by transient expression of CmpdTPI-GFP in tobacco. The ‘ygp2’ mutant exhibited significantly reduced TPI enzyme activity and net photosynthetic rate. Transcriptome analysis revealed downregulation of genes related to light-harvesting complexes, cell division, and the cell cycle. These results demonstrate that the point mutation in CmpdTPI impairs chloroplast function and photosynthesis, leading to the yellow-green phenotype in melon. This study provides insight into the role of plastidial TPI in chlorophyll metabolism and chloroplast development.

To identify microRNAs (miRNAs) responsive to powdery mildew (PM) infection and elucidate their regulatory roles in melon PM resistance, thereby laying a foundation for deciphering the underlying molecular mechanisms, we combined high-throughput sequencing with bioinformatics analysis to screen PM-responsive miRNAs and their target genes using PM-resistant and PM-susceptible melon genotypes. In total, 113 non-redundant miRNAs were identified in both genotypes, including 70 known and 43 novel miRNAs. Subsequent differential expression analysis revealed distinct miRNA responses to PM infection between resistant and susceptible genotypes. Upon PM inoculation, 13 miRNAs showed significant differential expression in both susceptible and resistant genotypes. In addition, four miRNAs, including miR164c, miR396a, miR398a and miRn39, displayed differential expression specifically in the susceptible genotype. Conversely, five miRNAs (miR167c, miR398b, miR399g, miR530a and miRn8) showed differential expression exclusively in the resistant genotype upon PM infection. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses demonstrated that these miRNAs mediate melon’s susceptibility or resistance to PM by modulating plant immune homeostasis, antioxidant metabolism, and pathogen-triggered cell death. Quantitative real-time PCR (qRT-PCR) validation confirmed a negative regulatory relationship between the expression of PM-responsive miRNAs and their predicted target genes. Collectively, our findings provide novel insights and candidate targets for further investigations into miRNA functions and regulatory mechanisms underlying melon PM resistance.