Evolutionary origin of cross-laminated cell walls predates lignification in vascular plant supporting tissues

Vertical elongation in land plants is a primary adaptive strategy driven by competitive pressures for increased height, conferring significant advantages for photosynthesis and long-distance dispersal. In these lineages, structural integrity is primarily mediated by the deposition of thick extracellular cell walls. In seed plants, these walls often exhibit a cross-laminated architecture, defined by cellulose microfibrils arranged at alternating angles, and lignin reinforcement, leveraging structural anisotropy to enhance mechanical resilience. However, the evolutionary chronology of the acquisition of these traits in basal vascular plant lineages is poorly understood. To investigate this question, we performed comprehensive histological and histochemical profiling of 35 basal vascular plant species, comprising eight lycophytes and 27 monilophytes, to elucidate the emergence of cross-laminated cell walls and their subsequent lignification in supporting tissues. In all examined taxa, sterome tissue, which is typically located in subepidermal regions, exhibited thick cell walls with a distinct cross-laminated architecture. Although lignification was ubiquitous in tracheary elements, its presence in sterome tissue showed significant interspecific variation. Most examined taxa possessed lignin-reinforced steromes, while specific lineages relied on nonlignified structural reinforcements. Quantitative morphometric analysis of sterome tissues and cells demonstrated that the mechanical contribution of steromes was depended on species-specific body plans. The findings suggest that the ability to form cross-laminated cell walls is a fundamental innovation that likely emerged before or independently of lignification in the supporting tissues of vascular plants. This study further highlights the functional significance of cell wall patterning in the successful colonization of terrestrial environments by basal plant lineages.