Establishment of a High-Frequency Plant Regeneration Protocol for the Multipurpose Handroanthus chrysanthus

Handroanthus chrysanthus (Jacq.) S.O. Grose is a Neotropical tree species highly valued for its ornamental beauty, durable timber, and medicinal properties. However, overexploitation and the recalcitrant nature of its seeds have constrained propagation and conservation efforts, and the species has been listed as Vulnerable on the IUCN Red List since 2020. In this study, a direct adventitious shoot regeneration system was developed for H. chrysanthus by systematically evaluating explant type, basal medium, plant growth regulator combination, and light quality. Hypocotyls were identified as the most responsive explants for shoot induction, and the highest adventitious shoot induction frequency, 51.79%, was obtained on Murashige and Skoog medium (MS) supplemented with 5 mg·L^-1 6-benzylaminopurine, 0.5 mg·L^-1 indole-3-butyric acid, and 0.2 mg·L^-1 thidiazuron under white fluorescent light. The highest shoot multiplication coefficient, 3.27, was obtained on MS medium containing 4 mg·L^-1 6-benzylaminopurine and 0.3 mg·L^-1 gibberellic acid. The maximum rooting frequency, 80%, was obtained on R14 medium after 30 days of culture. After acclimatization, 95% of the regenerated plantlets survived and grew vigorously under greenhouse conditions. To our knowledge, no direct adventitious shoot regeneration system has been reported for the Tabebuia alliance. This efficient regeneration protocol provides a practical platform for clonal propagation, germplasm conservation, and genetic improvement of H. chrysanthus and may also support regeneration and conservation in related endangered taxa within the Tabebuia alliance.