The fasciclin-like arabinogalactan protein FLA11 of Ostrya rehderiana impacts wood formation and salt stress in Populus

Wood is an abundant source of biomass and an important biomaterial for renewable resources and industrial products worldwide. Fasciclin-like arabinogalactan proteins (FLAs) are essential for plant growth, development, and responses to various abiotic stresses and are also closely associated with wood formation. However, the biological roles of FLAs in trees remain largely unclear. Here, we identified the FLA family in the ironwood tree (Ostrya rehderiana) and elucidated its possible function in poplar. Our results showed that OreFLAs underwent significant gene expansion due to multiple duplication events, leading to apparent functional redundancy. Among these genes, OreFLA11 was highly expressed in the xylem and roots. Overexpression of OreFLA11 in Populus alba var. pyramidalis altered xylem structure and cell wall composition, such as thickening the xylem and secondary cell wall (SCW), decreasing lignin contents and increasing cellulose contents, resulting in increased stem tensile strength but decreased salt tolerance, suggesting that OreFLA11 plays a negative role in the salt stress response. The mutant Palfla11 generated using CRIPSR-Cas9 methods showed the opposite phenotype compared to that of OreFLA11-OE poplars, indicating that the orthologues function similarly in wood formation. Furthermore, the transcriptome analysis showed that genes such as PME, BGAL and BGLU, related to cell wall biosynthesis, were up-regulated while the genes associated with salt stress resistance (CYSB, TSPO, RCI2B and CIPK6) were down-regulated in OreFLA11-OE poplars. These results indicated that OreFLA11 promotes wood formation and modulates stem mechanical properties but decreases tolerance to salt stress by decreasing the transcript abundance of salt-responsive genes in trees.