RD29A promoter constitutively drives a rice Hd3a expression to promote early-flowering in Saussurea involucrate Kar. et Kir. ex Maxim

S. involucratae, an endemic and endangered plant, is a valuable and traditional Chinese medicinal herb. In order to control the flowering time of S. involucratae, we used the well-known stress inducible RD29A promoter to drive Hd3a (a FT ortholog from rice) expression in S. involucratae. Unexpectedly, the majority of regenerated buds in RD29A::Hd3a transgenic lines (S-RH) produced flowers in tissue culture stage under normal growth (25 ± 2 °C) condition. Their flowering time was not further influenced by salt treatment. Hd3a in S-RH was strongly expressed in MS media supplemented with or without 50 mM NaCl. RD29A::GUS transgenic experiments further revealed that RD29A constitutively promoted GUS expression in both S. involucrate and halophyte Thellungiella halophile, in contrast to glycophic plants Oryza sativa L. 'Zhonghua 11', in which its expression was up-regulated by cold, salinity, and drought stress. The results supported the hypothesis that RD29A promoter activity is inducible in stress-sensitive plants, but constitutive in stress-tolerant ones. Importantly, S-RH plants produced pollen grains and seeds under normal conditions. Additionally, we found that OsLEA3-1::Hd3a and HSP18.2::Hd3a could not promote S. involucrate to flower under either normal conditions or abiotic stresses. Taken together, we demonstrated the potential of RD29A::Hd3a might be served as a feasible approach in breeding S. involucrate under normal condition.