Integrated analysis of metabolome and transcriptome provides insights into the metabolic adjustments of heteroblastic foliage in Pinus massoniana seedlings

Pine needle extract is rich in bioactive compounds. Pinus massoniana exhibits heteroblastic foliage in the first growing season, forming primary needle seedling (PNS) and secondary needle seedling (SNS). Significant differences in leaf morphology, photosynthetic physiology, and growth between PNS and SNS have been previously documented; however, the metabolic adjustments associated with heteroblastic foliage remain unclear. In this study, metabolomic and transcriptomic analyses revealed that elevated levels of auxin and cytokinin in PNS, mediated by high expression of IAA and ARR-A genes, hinder the germination and growth of axillary buds. In contrast, abscisic acid, jasmonic acid, and gibberellin acid levels increased in SNS, promoting the accumulation of phenylpropanoids, flavanones, and flavones. PNS also exhibited higher concentrations of amino acids and derivatives, along with flavonols, isoflavones, and anthocyanins, which could inhibit auxin polar transport and prevent auxin/cytokinin expulsion. Key genes such as PAL and CHS were up-regulated in SNS, while DFR, FLS, COMT, and PRDX6 were down-regulated, influencing flavonoid subclasses biosynthesis. Our findings demonstrate that phytohormones play a vital role in the metabolic adjustments of heteroblastic foliage through crosstalk between phytohormones and metabolites. These insights advance our understanding of the regulatory mechanisms and pharmaceutical potential associated with the heteroblastic foliage of conifers.