Unraveling the Formation Mechanism of Wax Powder on Broccoli Curds: An Integrated Physiological, Transcriptomic and Targeted Metabolomic Approach

As a vital appearance quality trait of broccoli, curd-surface wax powder not only affects its commercial value but also plays a key role in plant resistance to abiotic stresses. However, its formation mechanism remains unclear. Using low-wax variety CK (‘QH18’) and high-wax variety T1 (‘QHMS4’) as materials, this study systematically elucidated the molecular mechanism of wax powder formation via physiological indexes, scanning electron microscopy (SEM), targeted metabolomics, and transcriptomics. Determination of fatty acid (FA) content in broccoli flower bud tissue showed a close association between FA content and wax deposition. SEM observation revealed that T1 had significantly denser wax crystals, mainly granular, than CK. Targeted metabolomics identified 25 fatty acids in the two varieties. And the linolenic and palmitic acids, with high content and significant differences, may be key metabolites regulating wax synthesis. Integrated transcriptomics and metabolomics indicated that BolfabG, BolLACS, BolKCS1, BolKCS2 and BolMAH1 genes are involved in wax biosynthesis. Moreover, AP2/ERF-ERF transcription factor (TF)-encoding genes (BolERF018, BolERF1F.1, BolERF1F.2 and BolERF1C) played the primary role in regulating wax biosynthesis, followed by NAC (BolNAC62.1), MYB (BolMYB44), and MADS-MIKC(BolPISTILLATA). These TFs may regulate BolfabG, BolLACS, BolKCS1, BolACOX2 and BolACAA1 to affect linolenic and palmitic acid balance, altering wax precursor synthesis and accumulation, and finally leading to differences in wax morphology and content. This study reveals a “Transcription Factors–Differentially Expressed Genes–Differentially Accumulated Metabolites–Fatty Acids” (TFs-DEGs-DAMs-FA) network, providing a basis for understanding broccoli wax formation.