Structural innovation under environmental pressure: the evolutionary origins and regulatory mechanisms of thorns

Plant thorns represent a prominent evolutionary innovation formed through millions of years of natural selection. However, thorn structures greatly restrict the cultivation and management of many economically important crops, making the breeding of thornless varieties a key objective in modern agricultural breeding. Although different plant species possess apparently similar sharp appendages, these structures originate from different plant organs or tissues. Thorns are not merely mechanical defense structures but are the result of evolutionary adaptations by plants to cope with herbivore pressure and abiotic stresses over evolutionary time. Thorn morphogenesis integrates multiple pathways involved in developmental regulation, defense signaling, and environmental perception, reflecting the resource allocation trade-offs between growth investment and defense efficiency in plants. This review provides a comprehensive synthesis of the developmental origins, anatomical diversity, and ecological defense functions of plant thorns. By integrating paleobotanical evidence, this review traces the evolutionary emergence of thorns during the Paleogene and Neogene periods and reveals the adaptive associations between thorn diversification and the radiation of large herbivorous mammals. At the molecular level, this review focuses on recent advances in developmental genetics, with an emphasis on the regulatory networks underlying thorns' development, as well as the regulatory roles and crosstalk mechanisms of phytohormone signaling pathways involved in defense, including jasmonic acid and gibberellin, in thorns' development. Through a multidisciplinary perspective, this review provides a synthesis of current knowledge on the origin, diversification, and environmental adaptive significance of thorns, providing new insights into the developmental plasticity of plant defense structures.