Modular cytokinin evolution: Integrating heat stress and development during terrestrialization

Cytokinin signaling, long regarded primarily as a developmental regulator, has emerged as a central integrator of plant terrestrial adaptation. Comparative genomic and structural phylogenetics indicate a stepwise assembly of the phosphorelay, where receptor domains diversified alongside lineage-specific shifts in cytokinin usage, from cis-zeatin-enriched systems in early-diverging lineages toward trans-zeatin-dominant signaling in vascular plants, while downstream executors remained evolutionarily constrained. Cross-lineage transcriptomic comparisons under heat stress suggest a rewiring of physiological outputs. Although growth repression is deeply conserved, broader metabolic responses differ across clades: trehalose-associated osmoprotection in early land plants, whereas angiosperms transitioned to redox- and transport-based strategies. Overall, cytokinin signaling forms a modular framework that enabled early environmental buffering and was subsequently refined to support the complex architectures of vascular plants.