Abstract text: Secondary cell wall (SCW) formation in woody plants is a coordinated spatiotemporal process linking transcriptional control with progressive changes in SCW chemistry. The poplar transcription factor MYB156 has been characterized as a repressor of lignification. Notably, MYB156 over-expressing (OE) lines exhibit hypo-lignified fibres, while the lignification of vessels and rays remains unaffected. Using an integrative single-cell transcriptomic-spectroscopic framework, we mapped gene expression with SCW composition across the successive stages of xylogenesis in Populus, both in WT and in MYB156-OE. Cross-modal analyses revealed that in the WT, stage-resolved gene-spectral correlations captured coordinated molecular and chemical transitions, whereas these associations were reorganized in MYB156-OE. Within stages of fibre differentiation, MYB156 expression showed positive correlations with crystalline cellulose regions and negative correlations with lignin-associated FTIR wavenumbers. Co-expression network analysis further indicated that MYB156 overexpression altered the temporal organization of regulatory modules. Integrative analysis highlighted 32 genes associated with MYB156-mediated remodelling of SCW. Together, these results indicate that MYB156 influences the temporal coordination between gene regulatory activity and cell wall deposition, and that its constitutive expression disrupts this balance during fibre differentiation. Our study also establishes a cross-modal single-cell framework to connect transcriptional and chemical information in the dynamics of fibre SCW formation.