Abstract
Poinsettia (Euphorbia pulcherrima Willd ex Klotzsch) is a non-food and non-feed,
vegetatively propagated ornamental plant which is among the economically most important
ornamentals worldwide. Desirable plant height is one of the most important traits in such species.
To obtain compact plants, growers are regularly using chemical growth retardants such as CCC
(chlormequat chloride) or alar/daminozide (dimethylaminosuccinamic acid), which inhibit the
gibberellin (GA) biosynthesis, resulting in compact plants. However, growth retardants have
negative impacts on the environment and human health among others by potentially being
carcinogenic. Thus, it is highly desirable to further restrict their use.
This PhD project has explored the use of a plant genetic engineering approach and
regulation of light quality by using light emitting diodes (LEDs) to control shoot elongation in
poinsettia. The SHORT INTERNODES gene from Arabidopsis thaliana (AtSHI) was introduced
into poinsettia by the use of an Agrobacterium-based transformation system (paper I). Light
quality effects on plant morphology was investigated in greenhouse compartments and growth
chambers by comparing use of traditional high pressure sodium (HPS) lamps as supplementary
light with a combination of 80% red (R) and 20% blue (B) light from light emitting diodes
(LED) (paper II). Also, the effect a 30 min end-of-day (EOD) treatment provided by R LED light
was investigated in both light regimes (Paper II). To investigate the effect of manipulation of the
phytochrome system on hormone physiology, and since the knowledge on hormone physiology
in poinsettia was limited, the effects of EOD-R and EOD-FR on hormone levels were compared
(paper III).
Three independent transgenic lines (TL1, TL2, TL3) harbouring AtSHI were identified by
PCR, and stable integration was confirmed by Southern blot analysis (paper I). When grown
under short (SD; 10 h) or long day (LD; 16 h) conditions all three transgenic lines showed
reduced shoot elongation compared to untransformed wild type (WT) control plants. TL1 showed the shortest stems and internodes under SD with 52% and 49% reduction, respectively,
compared to the WT. This correlated with the highest AISHI expression, and a trend of 31%
lower levels of indole-3 acetic acid (IAA) in TL1 compared to the WT.
All three cultivars tested ('Christmas Spirit', 'Christmas Eve' and 'Advent Red') showed
reduced plant height (20-34%) under 20% B and 80% R provided by LED light compared to the
traditionally used HPS lamps (5% B) (paper II). The phytochrome photostationary state (PPS)
under the LED and HPS was very similar, indicating that B light receptors such as
cryptochromes are important to control stem elongation of poinsettia. Furthermore, in 'Advent
Red exposure to EOD-R resulted in reduced stem extension by 13% when HPS was used as
supplementary light (paper II). By contrast, under the B-R supplementary LED light, EOD-R did
not reduce shoot elongation, suggesting that the light-receptor dependent inhibition of shoot
elongation had already been saturated due to the high content of B and R. In the other cultivar
tested, 'Christmas Eve', no effect of the EOD-R-treatment was observed under any of the two
light regimes. This might be due to differences in phytochrome light receptors or their action.
Lower plants under EOD-R compared to EOD-FR correlated with lower levels of IAA,
gibberellin (GA) and abscisic acid (21%, 28% and 19%, respectively) in shoot tips (paper III).
The GA analyses revealed that the 13-hydroxylation pathway of GA biosynthesis is probably
dominating over the non-13-hydroxylation pathway.
In conclusion, these results demonstrates the potential for using genetic engineering and
exploiting light quality responses in order to reduce the use of plant growth retardants in
greenhouse production of poinsettia. However, the difference in response to EOD-R in the tested
cultivars emphasizes the importance of investigating light quality responses in different
commercially grown cultivars.