The Molecular Mechanism of Relatively Low-Temperature-Induced Broccoli Flower Bud Differentiation Revealed by Transcriptomic Profiling

Broccoli (Brassica oleracea L. var. italica) has a large, edible green flower head, which is one of its critical economic traits. A prerequisite of broccoli flower head formation is flower bud differentiation (FBD). Low-temperature treatment is an effective way to induce FBD in broccoli. However, the molecular mechanism underlying low-temperature-induced broccoli FBD remains largely unclear. In this study, using broccoli cultivar Zhongqing 10 as the experimental material, we investigated the effects of low-temperature treatment on FBD by comparing the plants grown at low temperatures (17 °C/9 °C, 16 h/8 h) with the control plants grown under normal temperature conditions (25 °C/17 °C, 16 h/8 h). After 15 days of different temperature treatments, the flower buds of the plants growing under the low-temperature condition started to differentiate. However, the control plants remained in the vegetative growth stage, indicating that low temperature successfully induced flower bud formation. Subsequently, a global transcriptomic analysis was conducted to detect the differentially expressed genes (DEGs) during low-temperature-induced FBD in broccoli. A total of 14 DEGs in five phytohormone signaling pathways, 42 DEGs in nine transcription factor families, and 16 DEGs associated with the floral development pathways were identified. More DEGs were present in the auxin signaling pathway than in other phytohormone signaling pathways, which indicated that the auxin signaling pathway played a critical role in modulating low-temperature-induced FBD in broccoli. Furthermore, four TF classes, including bZIP, GCM domain factors, MADS-box factors, and C2H2 zinc finger factors, possessed enriched motifs, indicating that their closely related DETFs ABI5, HY5L, WRKY11, WRKY15, WRKY22, SOC1, AGL8, FLC, SPL8, and SPL15 may be directly involved in the transcription regulation of broccoli FBD. This study provides an important basis for further investigation of the molecular regulatory mechanism of broccoli flower development under low temperatures.