Sugar beet is a significant sugar crop in China, primarily cultivated in arid regions of the north. However, drought often affects sugar beet cultivation, leading to reduced yield and quality. Therefore, understanding the impact of drought on sugar beets and studying their drought tolerance is crucial. Previous research has examined the role of SPL (SQUAMOSA promoter-binding protein-like) transcription factors in plant stress response; however, the precise contribution of SPLs to the drought stress response in sugar beets has yet to be elucidated. In this study, we identified and examined the BvSPL6, BvSPL7, and BvSPL9 genes in sugar beets, investigating their performance during the seedling stage under drought stress. We explored their drought resistance characteristics using bioinformatics, quantitative analysis, physiological experiments, and molecular biology experiments. Drought stress and rehydration treatments were applied to sugar beet seedlings, and the expression levels of BvSPL6, BvSPL7, and BvSPL9 genes in leaves were quantitatively analyzed at 11 different time points to evaluate sugar beets’ response and tolerance to drought stress. Results indicated that the expression level of the BvSPL6/9 genes in leaves was upregulated during the mid-stage of drought stress and downregulated during the early and late stages. Additionally, the expression level of the BvSPL7 gene gradually increased with the duration of drought stress. Through analyzing changes in physiological indicators during different time periods of drought stress and rehydration treatment, we speculated that the regulation of BvSPL6/7/9 genes is associated with sugar beet drought resistance and their participation in drought stress response. Furthermore, we cloned the CDS sequences of BvSPL6, BvSPL7, and BvSPL9 genes from sugar beets and conducted sequence alignment with the database to validate the results. Subsequently, we constructed overexpression vectors, named 35S::BvSPL6, 35S::BvSPL7, and 35S::BvSPL9, and introduced them into sugar beets using Agrobacterium-mediated methods. Real-time fluorescence quantitative analysis revealed that the expression levels of BvSPL6/7/9 genes in transgenic sugar beets increased by 40% to 80%. The drought resistance of transgenic sugar beets was significantly enhanced compared with the control group.

Betalains are natural antioxidant pigments valued as food colorants (E162), yet their combined responses to light quality and methyl jasmonate (MeJA) during long-term growth remain poorly understood. In this study, the interactive effects of blue light and MeJA on growth, betacyanin, betaxanthin, antioxidant enzyme activities [peroxidase (POD), superoxide dismutase (SOD), catalase (CAT)], malondialdehyde (MDA), and proline (PRO) were investigated at weeks 3, 10, and 13 of sugar beet seedling growth. Four treatments were established: white light (W, control), white light + MeJA (WM), white light + blue light (WB), and white light + blue light + MeJA (WBM). The results showed that WB rapidly induced betaxanthin accumulation and enhanced SOD and POD activities while reducing MDA at week 3. MeJA alone triggered an explosive increase in betacyanin (45.74 mg·g⁻¹ FW) at week 10, accompanied by elevated activities of POD, SOD, and CAT. The combined treatment (WBM) maintained the highest betacyanin (36.48 mg·g⁻¹ FW) and betaxanthin (8.97 mg·g⁻¹ FW) contents, the lowest MDA level (17.17 nmol·g⁻¹ FW), and a high proline level (528.39 μg·g⁻¹ FW) at week 13, providing sustained pigment maintenance at the late stage. The three antioxidant enzymes exhibited a temporal division of labor: high SOD activity at the early stage, while POD and CAT activities continuously increased during the middle and late stages. Notably, the correlation between proline and MDA shifted from positive at week 10 to negative at week 13, suggesting a temporal transition in the protective role of proline against membrane lipid peroxidation during late-stage development. In conclusion, blue light and MeJA enhance the antioxidant capacity of sugar beet seedlings through a temporally synergistic pattern of “priming by blue light, burst by MeJA, and maintenance by combined treatment,” offering a potential preharvest regulation strategy for the production of natural pigments and functional food ingredients.