Temperature-dependent sex expression in cucurbits and beyond: mechanisms, reproductive plasticity, and breeding implications.

Global warming and increasingly frequent heat extremes threaten plant reproductive success and yield stability. Beyond causing general reproductive heat injury, such as pollen sterility, stigma dysfunction, and fertilization failure, temperature can also modify floral sex expression during early developmental windows. Temperature-dependent sex expression encompasses temperature-responsive changes in floral sex fate, floral sex ratios, and sex-expression patterns, and represents an underexplored dimension of reproductive plasticity under thermal stress. Here, we clarify the distinctions among sex determination, sex differentiation, sex expression, and sexual plasticity, and distinguish temperature-dependent sex expression from general reproductive heat injury. We focus on cucumber and melon as the most informative model systems and synthesize evidence for temperature-sensitive phenotypes, ethylene-centered ACS/ACO-WIP-ERF networks, GA-associated pathways, and epigenetic regulation involving DNA methylation and small RNAs. Importantly, we emphasize that direct causal links between defined temperature cues and individual sex-regulatory components remain incompletely resolved. Evidence from dioecious and non-model plants, including spinach and Cannabis, is critically evaluated to distinguish direct temperature-induced changes in floral sex fate from hormone-responsive sexual plasticity and sex-specific stress responses. We further assess the context-dependent adaptive potential and constraints of sexual plasticity and identify priorities involving causal validation, single-cell and spatial omics, natural variation, and field-relevant temperature regimes. This review provides a critical framework for understanding temperature-responsive sex expression and for translating mechanistic advances into temperature-stable breeding, hybrid seed production, and controlled-environment cultivation.