Carotenoids are the main pigments in watermelon (Citrullus lanatus L.) fruit and contribute to its aesthetic and nutritional value. Phytoene synthase (PSY) is reported to be the first rate-limiting enzyme in carotenogenesis and controls the carotenoid flux. This study aimed to identify PSY genes responsible for carotenoid biosynthesis in the red-fleshed watermelon cultivar LSW-177. The PSY gene members ClPSY1, ClPSY2 and ClPSY3 were characterized and their catalytic activities were displayed in the heterologous complementation assay. The transcript levels of ClPSY genes at the different developmental stages of LSW-177 fruit and the promoter sequence of ClPSY1 were also analyzed. Transcription factors involved in regulating the ClPSY1 expression were scanned with previous RNA-seq data of the different stages during fruit ripening. Results showed that the PSY proteins from watermelon LSW-177 contained the conserved PSY domains and exhibited the ability to condense GGPP into phytoene in E. coli. ClPSY1 is the dominant carotenogenic gene during fruit ripening; and can be induced by light and hormones. Furthermore, Cla013914 and Cla007950 that, respectively encode the transcription factors WD40-like protein and bZIP, likely upregulate ClPSY1 during fruit ripening. In conclusion, ClPSY1 play a dominant role in carotenoid biosynthesis during watermelon fruit ripening and is regulated by complex light and hormone-responsive networks.

Maize (Zea mays L.) is a global staple crop for food, feed and industrial use in China. As a thermophilic crop, its seedlings suffer from cold stress. Heilongjiang, China’s major maize producer, faces worsening early cold that impedes germination and seedling growth, causing heavy yield losses. Improving germination cold tolerance stabilizes yields. Ubiquitous proline (Pro) works as a key osmoprotectant and core stress-resistance regulator in plants. In this field experiment, the maize cultivar Damin 3307 was used to explore the effects and regulatory mechanisms of Pro seed soaking on low-temperature-stressed maize. We analyzed seedling emergence, plant growth, photosynthetic performance, nitrogen metabolism, dry matter accumulation, ear traits and grain yield of treated maize under low temperature. The results showed that seed soaking with 15 mmol·L⁻¹ Pro effectively improved the seedling emergence rate of early-sown maize under low temperature and compensated for cold-induced population decline. It significantly increased plant height at the seedling and jointing stages, which enhanced vegetative growth. Exogenous Pro regulated the activities of key nitrogen metabolism enzymes, including NR, GS, GOGAT, GDH, GOT and GPT, alleviating low-temperature-induced nitrogen metabolism inhibition and accelerating ammonia assimilation as well as protein and amino acid synthesis to promote maize growth. Additionally, Pro soaking mitigated the cold-induced suppression of leaf photosynthetic indices (Pn, Gs, Tr, Ci) and key photosynthetic enzymes (RuBPCase, PEPCase). It maintained higher leaf chlorophyll content and green leaf area, delayed leaf senescence, and sustained stable carbon and energy supply for nitrogen metabolism. These improvements strengthened maize cold resistance, promoted dry matter accumulation, optimized ear and kernel traits by increasing ear number and kernels per ear and reducing empty kernels, and ultimately improved the grain yield of early-sown maize under low-temperature stress.