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Doctoral thesis2000Open access

The role of phytochrome A and gibberellins in growth under long and short day conditions : studies in hybrid aspen

Eriksson, Maria Elenor


This thesis addresses questions concerning the regulation of growth and, specifically, the cessation of growth in response to short days in deciduous tree species. The model tree used in the studies was hybrid aspen (Populus tremula L. x P. tremuloides Michx.). We have exploited the possibility of transforming this species to modulate the level of expression of target genes using over-expression and antisense techniques.
The target genes in the studies were the photoreceptor phytochrome A (phyA) and gibberellin 20-oxidase (GA 20-oxidase), the latter being a highly regulated enzyme involved in the biosynthesis of gibberellins (GAs). The photoreceptor phyA has been implicated in photoperiodic regulation of growth, while GAs may regulate the physiological response further downstream. The endogenous expression of these genes has been investigated in parallel with studies of various plants with ectopic and reduced levels of expression. The main focus has been on the early stages of induction of growth cessation and its physiological and molecular mechanisms.
Studies of hybrid aspen plants with an increased or reduced expression of phyA, show this receptor to mediate the photoperiodic regulation of growth. Plants with ectopic expression could not stop growing despite drastically shortened photoperiods, while the antisense plants showed the reverse phenotype, with a higher sensitivity resulting in earlier cessation of growth. The role of GAs in growth inhibition was also addressed using plants with a reduction in GA levels. These plants showed early cessation of growth and dormancy, and thus an increased sensitivity toward daylength. Conversely, plants with increased rates of GA biosynthesis showed increased growth and stopped growing much later. Furthermore, increases in GA biosynthesis, resulting in high levels of GAs have a major impact on growth. Plants with high GA levels have increased elongation and diameter growth, due to higher rates of cell production in the apical meristem and cambium, respectively. Also, these plants have altered wood properties showing more numerous (71%) and longer (8%) fibres as compared to the control plants.
GA levels were modulated by altering the expression of the multifunctional enzyme GA 20-oxidase. This enzyme was shown by the over-expression studies to be a limiting factor in the biosynthesis of GAs. This enzyme was also shown to be regulated at the transcriptional level, both by photoperiod and active GA4. Our studies indicate that GA 20-oxidase is very likely to be one of the most important factors in the GA-regulation of growth and growth cessation.
​​​​​​​In conclusion, these studies have shed light on the early stages of growth cessation in deciduous trees, especially with respect to the role of phyA and GAs. It has also given new information on the importance of GAs in growth as such, with important implications for wood production.


gibberellin; GA 20-oxidase; phytochrome A; photoperiodism; growth cessation,growth; fibre; Populus; hybrid aspen; transgenic trees

Published in

Acta Universitatis Agriculturae Sueciae. Silvestria
2000, number: 164ISBN: 91-576-5898-6
Publisher: Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences