Cambium reactivation of temperate trees in spring is important for adaptation and growth. In some trees, the resumption of cambium cell division occurs before bud flush. However, knowledge about the source of the internal stimuli that promote cambium cell division during reactivation in these trees is limited. Here, the anatomical changes of cambium during reactivation along with the unfolding of vegetative buds in Populus tomentosa were examined under natural and water-culture conditions. Meanwhile, the levels of free indole-3-acetic acid (IAA) and transcripts of enzymes involved in its production were measured during cambium reactivation. The results showed that cambium cell division occurred before bud unfolding during cambium reactivation in P. tomentosa. Interestingly, the levels of IAA and the transcripts of poplar homologs of aldehyde oxidase 2 and nitrilases 2 encoding enzymes functioning in de novo IAA biosynthesis increased before bud unfolding, while the transcript levels of IAA conjugate hydrolases almost showed no increase. These results suggest that free IAA from de novo biosynthesis in cambium regions promotes cambium reactivation before bud unfolding, providing new insights into the regulatory mechanism of cambium reactivation.
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