Metabolomics and transcriptomics analyses reveal the synthetic pathway of sweet substances in the bamboo shoots of Dendrocalamus brandisii

Most bamboo shoots taste bitter, yet Dendrocalamus brandisii shoots are sweet and directly edible. Such sweetness is desirable for bamboo breeding; however, its physiological mechanism is unclear. This study integrated physiological, metabolomic, and transcriptomic analyses, revealing that the primary factor underlying the sweetness of D. brandisii shoots is the differential accumulation of soluble sugars, particularly fructose. Enzyme activity assays revealed no significant differences in fructose synthesis between sweet and bitter shoots of D. brandisii. However, the activity of hexokinase, which is involved in fructose catabolism, was found to be significantly lower in the sweet shoots. This reduced activity inhibits fructose phosphorylation, thus promoting fructose accumulation and potentially explaining the source of the sweetness. Transcriptome analysis identified the down-regulation of two key genes in fructose metabolism, DbHXK4 and DbFRK2. Functional validation via RNAi and overexpression experiments in Oryza sativa demonstrated that overexpression of bamboo-derived DbHXK4 decreased fructose content in rice, whereas RNAi-mediated silencing of rice homologous genes OsHXK4 and OsFRK2 resulted in increased fructose accumulation. Accordingly, perturbation of the key fructose-metabolizing genes HXK4 and FRK2 changes fructose accumulation. Therefore, we conclude that reduced hexokinase activity is the primary mechanism behind fructose accumulation and consequent sweetness in D. brandisii. These findings elucidate the molecular basis of sweetness in bamboo shoots, offering valuable insights for future quality improvement and germplasm selection.