Summary

Institutt for plante- og miljøvitenskap

Abscission is the process where leaves, flowers, fruits and seeds are disposed of, most often when redundant. This is an economically important process because it is often the product of interest that is shed, e.g. seeds in cereals, fruits, berries or ornamental flowers.

In greenhouse production of ornamentals susceptible for pre-mature flower abscission, plants are routinely sprayed prior to sale with chemicals containing silver-thiosulphate (STS) that block the action of ethylene. The overall aim in the Norwegian greenhouse industry is to reduce the use of chemicals. Research into this topic is therefore of a potential benefit to the environment and will promote a more sustainable greenhouse industry.

There are many factors involved in the abscission process. Environmental factors such as temperature and day length determine the time for leaves to fall in the autumn. Natural aging induce processes as senescence and abscission, and can be accelerated by chemicals such as ethylene, known to be the plant-aging hormone. Stress caused by drought or drowning
can also induce plant survival processes and lead to abscission of flowers and leaves.

For a plant organ to detach from the stem, active steps have to be taken. Signalling molecules, for example hormones, have to be produced to start a cascade of gene expressions. Nutrients have to be retrieved from the senescing plant part and cell wall dissolving enzymes must be allocated to the right place to reduce adhesion, breakdown of parts of the cell wall and
facilitate separation. Subsequently, a new seal has to be made to close the ‘wound’ in order to prevent infections from entering before internal forces push the redundant organ off.

In poinsettia, the model for our studies, we are able to induce the abscission in a controlled manner whereby the flower abscises after one week. This enables us to study the differences in gene expression and other events from day to day, until Day 7 when the flower falls off. The changes in the abscission process were studied at a molecular level using
Differential Display and by investigating the effect of applying the plant hormones ethylene, auxin, abscisic acid and gibberellin, all important signal molecules for plant growth and development. We also discovered that the position of the decapitation was of great importance; if the cut was too high (some of the floral organs left) or too low (none of the
floral bottom left), the induction of abscission was not as good as when the cut was precisely in the right place (almost 100 % induction).

In the studies applying hormones, gibberellin (GA3) delayed abscission more than 20 days compared to control. Abscisic acid seems to accelerate the abscission process so much 4 that the distal part dies and dries up before abscission can be finalised through a proper separation zone.

Ethylene had the special effect of initiating an additional abscission zone
high above the one normally initiated. We show here that the position of the abscission zone is not necessarily predetermined but can be induced.
In the molecular study, flower abscission zones were harvested every 24h from the time of induction (Day 0), mRNA extracted and cDNA produced for Differential Display.

Differential Display is a PCR based tool and the 36 random primers we used should statistically cover at least 75 % of the gene expression in the plant. The sequences found differentially expressed during abscission were cloned, sequenced, compared with sequences in published databases and confirmed as differentially expressed with Real-Time RT-PCR. 11 gene sequences were identified as homologs to other proteins, like immunophilin, UV-Brepressible protein, pol protein and an integral membrane protein, and were either up- or down-regulated. We also isolated and cloned two genes from Euphorbia pulcherrima, 18S and alfa tubulin. Additional 17 sequences have been confirmed as differentially displayed, but are putatively unknown genes with insignificant homology, or no homology to published sequences so far.

These studies have also given material for several future research projects. We hope to see that some of the sequences found have the key to regulate flower abscission in poinsettia and, possibly for other plants and organs as well. The findings of known and unknown genes together with our hormonal studies will help us to further elucidate the process of flower abscission with the overall practical goal to improve the quality of flowers and loss of crops in the field in the future.

Munster, C. 2006. On the flower abscission of poinsettia (Euphorbia pulcherrima Willd. Ex Klotzsch) - a molecular and plant hormonal study. Norwegian University of Life Sciences (UMB), Ås, Norway, Doctor Scientiarum Thesis 2006: 2, ISBN: 82-575-0690-7, ISSN: 0802-3220




Oppdatert: 02.02.06
Utskriftsvennlig versjon : Arkiv : Tips en venn