Submitted to: Current Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/10/2005
Publication Date: 2/7/2006
Citation: Disch, S., Anastasiou, E., Sharma, V.K., Laux, T., Fletcher, J.C., Lenhard, M. 2006. The E3 Ubiquitin Ligase BIG BROTHER Controls Arabidopsis Organ Size in a Dosage-Dependent Manner. Current Biology. 16(3):272-9. Interpretive Summary: This article reports the cloning, expression pattern and biochemical function of the Arabidopsis BIG BROTHER (BB) gene in controlling Arabidopsis organ size. Plants lacking BB form larger petals, sepals and stems, and accumulate more flower biomass than normal plants. Conversely, plants that make more BB protein than normal have smaller organs and make less biomass. We cloned the BB gene and found that it encodes a RING-finger protein that limits the amount of time that plant organs grow. We find that BB is expressed in all growing tissues, but that the protein is rapidly turned over by a specific protein degradation pathway. Biochemical studies show that BB is itself part of the protein degradation pathway, targeting proteins for turnover when they are no longer needed. Our results demonstrate that BB is a central negative regulator of Arabidopsis floral organ size that restricts the period of proliferative growth.
Technical Abstract: Organ growth up to a species-specific size is tightly regulated in plants and animals 1; 2. Final organ size is remarkably constant within a given species, suggesting that a species-specific size checkpoint terminates organ growth in a coordinated and timely manner. Phytohormones influence plant organ size 3; 4; 5; 6; 7, but their precise functions in size control are unclear because of their pleiotropic and complex developmental roles 8; 9; 10. The Arabidopsis transcription factors AINTEGUMENTA and JAGGED promote organ growth by maintaining cellular proliferation potential 11; 12; 13; 14; 15; 16. Loss of the Antirrhinum transcription factor CINCINNATA causes leaf overgrowth, yet also leads to a highly abnormal leaf shape 17. Thus, no dedicated factor that limits the final size of plant organs has been isolated. Here, we identify the novel RING-finger protein BIG BROTHER (BB) as a repressor of plant organ growth. Small changes in BB expression levels substantially alter organ size, indicating a central regulatory role for BB in growth control. Recombinant BB protein has E3 ubiquitin-ligase activity that is essential for its in vivo function, suggesting that BB acts by marking cellular proteins for degradation. Our data indicate that plants limit the duration of organ growth and ultimately organ size by actively degrading critical growth stimulators.