We modulated the level of a hormone gene expression in poplars using either 35S promoter (p35S) of cauliflower mosaic virus
(CaMV) or
aux promoter (pAUX) of
A. rhizogenes. The transgenic poplars (
Populus alba ×
P.
tremula var. glandulosa), in which the bacterial trans-zeatin secretion (
tzs) gene was attached either to the 35S promoter or to the
aux promoter, were compared for their performance in tissue culture as well as in nursery. Northern blot analysis of total RNA
probed with
tzs coding region showed that the total
tzs mRNA expression by p35S was approximately 200–300-fold higher than that driven by pAUX. In contrast, the cellular zeatin
content of p35S-
tzs transgenic poplars was merely 13-fold of those found in pAUX
-tzs plants. Due to different levels of cellular zeatin levels, the two types of transgenic poplars showed different morphogenetic
as well as growth responses. The p35S-
tzs transgenic plants showed morphological characteristics typical of those treated with cytokinin in culture. These include
multiple axillary shoot formation, thick stems, narrow leaves and absence of roots. In contrast, the pAUX
-tzs plants had slightly higher cellular cytokinin levels than did control plants and showed a lower degree of cytokinin-related
phenotypes, including a few axillary shoots in root-inducing media. Since p35S-
tzs did not develop roots, only pAUX
-tzs transgenic poplars could be transplanted to the nursery where they resumed a close-to-normal growth. Nevertheless, pAUX
-tzs plants transferred to the nursery developed cytokinin-related phenotypes, including greater number of shoots, smaller leaves
and slightly retarded growth in height, but with a high total biomass.
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