Adaptive strategies and transcriptional characteristics of Populus × euramericana in response to shading intensity based on phenotypic and bioinformatic analyses

Light is the primary energy source for plant growth, and shading can significantly inhibit the normal development of plant tissues and organs. Populus × euramericana is a fast-growing, high-yielding tree species with strong adaptability, which is highly dependent on light conditions; however, the molecular mechanisms underlying its responses to light stress remain poorly elucidated. In this study, five P. × euramericana clones were subjected to three light treatments (CK: natural light, 3Z: 55% shading, 6Z: 80% shading) to investigate shading effects on their growth and development, and transcriptome sequencing was employed to identify key genes and regulatory mechanisms involved in shading responses. The results indicated that shading significantly reduced leaf traits, net photosynthetic rate, and seedling growth, while increasing chlorophyll content and decreasing the chlorophyll a/b ratio. Under shading stress, all 56 differentially expressed genes (DEGs) related to the MAPK signaling pathway were upregulated, whereas 86 cold stress-responsive DEGs induced by shading were downregulated. We identified key light signaling homologous genes (including bZIP8/bZIP16 , bHLH family members, and COP1-1 / COP1-2 ) as well as genes involved in light-induced hormone responses, chlorophyll metabolism, and photosynthesis. Furthermore, three major molecular regulatory mechanisms of poplar shading responses were proposed: (1) shading likely induces the integration of multiple hormone signaling pathways, which coordinately regulate shading responses via bZIP, bHLH, and COP1; (2) Shading may suppress photosynthetic performance. This suppression could be mediated by the downregulation of PSII- and PSI-related genes, as well as genes involved in ATP/ADP transmembrane transport and ATP hydrolysis, which might ultimately reduce photosynthetic rate.; (3) Shading may activate the MAPK signaling pathway and potentially trigger low-temperature signaling to regulate cold stress-responsive genes. These findings provide valuable insights into the molecular mechanisms of light responses in poplar and a theoretical basis for future studies on tree adaptation to complex light environments.