Integrated Metabolomic and Transcriptomic Analyses Reveal the Potential Molecular Mechanism Underlying Callus Browning in Paeonia ostii

Callus browning is a significant problem that hinders plant tissue regeneration <i>in Paeonia ostii</i> "Fengdan" by causing cell death and inhibiting growth. However, the molecular mechanism underlying callus browning in <i>P. ostii</i> remains unclear. In this study, we investigated the metabolites and potential regulatory genes involved in callus browning of <i>P. ostii</i> using metabolomic and transcriptomic analyses. We found a significant accumulation of phenolic compounds in the browned callus, represented by flavonoid compounds. Notably, the accumulations of luteotin and disomentin were higher in browning calli compared to non-browning calli. Transcriptomic analysis identified that candidate genes associated with flavonoid biosynthesis, including <i>flavonoid 3-hydroxylase</i> (<i>PoF3H</i>) and <i>flavone synthase II</i> (<i>PoFNSII</i>), were highly expressed in the browned callus of <i>P. ostii</i> "Fengdan". Weighted gene co-expression network analysis (WGCNA) further highlighted that <i>polyphenol oxidase</i> (<i>PoPPO</i>) which encoded polyphenol oxidase, together with flavonoid biosynthesis-related genes such as <i>flavanone 3-hydroxylase</i> (<i>PoF3H</i>) and <i>flavonone Synthase II</i> (<i>PoFNSII</i>), as well as cellular totipotency-related genes <i>wuschel-related homeobox 4</i> (<i>PoWOX4</i>), were involved in callus browning. Based on these findings, we proposed the molecular mechanism by which flavonoid accumulation, polyphenol oxidation, and cellular totipotency pathways contribute to callus browning in <i>P. ostii.</i> Our study provides new insights into the molecular mechanism underlying callus browning and offers the foundations to facilitate the establishment of an efficient plant tissue regeneration system in <i>P. ostii</i>.