Genetic mechanisms of petal size regulation in Arabidopsis thaliana
Final Report Abstract
How the size of plant organs is controlled by the species-specific genetic blueprint is a fundmental problem of developmental biology. In this project, we have isolated two important regulators of plant organ size from the model plant Arabidopsis and characterized their function in detail. One of the genes acts as a negative regulator of organ growth, ensuring that cell proliferation in growing floral organs stops at the correct time; in mutants lacking this gene, cells proliferate for too long, leading to the formation of larger flowers, while in plants with increased activity of this gene proliferation stops too early, causing strongly reduced organ size. The precise activity level of the gene is closely correlated with the final size of the organs, indicating that it represents an important controlling element. The encoded protein acts to mark other cellular proteins for degradation, suggesting that its targets function as stimulators of growth, which accumulate to excessive levels in the mutant flowers. Identifying these targets and also the factors that fine-tune the activity level of the gene are important objectives for future research. The second gene characterized in this project acts as a stimulator of organ growth; mutants lacking it have smaller leaves and flowers, while excess activity of the gene leads to organ enlargement. Again, the basis for these changes in final organ size is an alteration in the timing of when proliferation stops, confirming the importance of this step for controlling final size. The gene encodes a protein that is most likely involved in the generation of a small mobile signalling factor. This protein is only present in the cells at the edge of growing organs, but its activity affects the proliferation of cells throughout the organ, suggesting that the produced signal moves from the edge into the centre of the organs. This in turn suggests a simple geometrical model for how plants can ensure that proliferation in growing organs stops once they have reached a certain size. The source of the signal (the edge) depends only linearly on the organ diameter, while the overall area grows as a square of the diameter. Thus, the ratio between the signal source and the area it has to serve decreases, as the organ grows. This in turn is predicted to lead to a dilution of the signal in the organ, up to a point where its concentration is no longer sufficient to support further growth. Interestingly, such an underlying principle of a localized signal source that grows more slowly than the area it serves is also found in the control of wing size in flies, suggesting that despite different molecules involved the basic principle may be the same in plants and animals. The knowledge gained from studying the control of leaf and flower size in the model Arabidopsis may be used for manipulating organ size in related economically important species, such as oilseed rape. For example, the growth inhibitor could be used to reduce the size of petals, which after shedding act as a growth substrate for fungal pathogens, causing significant yield loss. An unexpected outcome of the project was that the characterization of the described growth stimulator led to a simple and potentially general model of size measurement in plant organs. The growth stimulator and the resulting model for size control was featured for example in The Times of India (http://timesofindia.indiatimes.com/articleshow/2619742.cms), BBSRC Business (http://www.bbsrc.ac.uk/publications/corporate/magazine/2008/0804_business.pdf, p. 7), and was picked up in several online science blogs, for example: http://www.innovationsreport.de/html/berichte/agrar_forstwissenschaften/bericht-100021.html http://www.sciencedaily.com/releases/2007/12/071212201420.htm
Publications
- (2006) The E3 ubiquitin ligase BIG BROTHER controls Arabidopsis organ size in a dosagedependent manner. Current Biology 16: 272-9
Disch, S., Anastasiou, E., Sharma, V.K., Laux, T., Fletcher, J.C., and Lenhard, M.
- (2007) Control of plant organ size by KLUH/CYP78A5-dependent intercellular signaling. Developmental Cell 13: 843-56
Anastasiou, E., Kenz, S., Gerstung, M., MacLean, D., Timmer, J., Fleck, C., and Lenhard, M.
- (2007) Growing up to one’s standard. Current Opinion in Plant Biology 10: 63-9
Anastasiou, E., and Lenhard, M.
- (2008). Control of plant organ size. In Plant Growth Signalling, L. Bögre & G. Beemster, eds., Springer Press
Anastasiou, E., and Lenhard, M.
