Summary Wood production in fast‐growing Eucalyptus grandis trees is highly dependent on both potassium (K) fertilization and water availability but the molecular processes underlying wood formation in response to the combined effects of these two limiting factors remain unknown. E. grandis trees were submitted to four combinations of K‐fertilization and water supply. Weighted gene co‐expression network analysis and MixOmics‐based co‐regulation networks were used to integrate xylem transcriptome, metabolome and complex wood traits. Functional characterization of a candidate gene was performed in transgenic E. grandis hairy roots. This integrated network‐based approach enabled us to identify meaningful biological processes and regulators impacted by K‐fertilization and/or water limitation. It revealed that modules of co‐regulated genes and metabolites strongly correlated to wood complex traits are in the heart of a complex trade‐off between biomass production and stress responses. Nested in these modules, potential new cell‐wall regulators were identified, as further confirmed by the functional characterization of EgMYB137. These findings provide new insights into the regulatory mechanisms of wood formation under stressful conditions, pointing out both known and new regulators co‐opted by K‐fertilization and/or water limitation that may potentially promote adaptive wood traits.

Although eucalypts are the most planted hardwood trees worldwide, the majority of them are frost sensitive. The recent creation of frost-tolerant hybrids such as Eucalyptus gundal plants (E. gunnii × E. dalrympleana hybrids), now enables the development of industrial plantations in northern countries. Our objective was to evaluate the impact of cold on the wood structure and composition of these hybrids, and on the biosynthetic and regulatory processes controlling their secondary cell-wall (SCW) formation. We used an integrated approach combining histology, biochemical characterization and transcriptomic profiling as well as gene co-expression analyses to investigate xylem tissues from Eucalyptus hybrids exposed to cold conditions. Chilling temperatures triggered the deposition of thicker and more lignified xylem cell walls as well as regulation at the transcriptional level of SCW genes. Most genes involved in lignin biosynthesis, except those specifically dedicated to syringyl unit biosynthesis, were up-regulated. The construction of a co-expression network enabled the identification of both known and potential new SCW transcription factors, induced by cold stress. These regulators at the crossroads between cold signalling and SCW formation are promising candidates for functional studies since they may contribute to the tolerance of E. gunnii × E. dalrympleana hybrids to cold.