Abstract
The higher-plant shoot apical meristem is a dynamic structure continuously producing cells that become incorporated into new leaves, stems and flowers. The maintenance of a constant flow of cells through the meristem depends on coordination of two antagonistic processes: self-renewal of the stem cell population and initiation of the lateral organs. This coordination is stringently controlled by gene networks that contain both positive and negative components. We have previously defined the ULTRAPETALA1 (ULT1) gene as a key negative regulator of cell accumulation in Arabidopsis shoot and floral meristems, because mutations in ULT1 cause the enlargement of inflorescence and floral meristems, the production of supernumerary flowers and floral organs, and a delay in floral meristem termination. Here, we show that ULT1 negatively regulates the size of the WUSCHEL (WUS)-expressing organizing center in inflorescence meristems. We have cloned the ULT1 gene and find that it encodes a small protein containing a B-box-like motif and a SAND domain, a DNA-binding motif previously reported only in animal transcription factors. ULT1 and its Arabidopsis paralog ULT2 define a novel small gene family in plants. ULT1 and ULT2 are expressed coordinately in embryonic shoot apical meristems, in inflorescence and floral meristems, and in developing stamens, carpels and ovules. Additionally, ULT1 is expressed in vegetative meristems and leaf primordia. ULT2 protein can compensate for mutant ULT1 protein when overexpressed in an ult1 background, indicating that the two genes may regulate a common set of targets during plant development. Downregulation of both ULT genes can lead to shoot apical meristem arrest shortly after germination, revealing a requirement for ULT activity in early development.
Highlights
Meristems, as centers of cell proliferation and organ initiation, are the foundation of all plant structures
We have previously identified ULTRAPETALA1 (ULT1) as an additional factor that negatively regulates Arabidopsis shoot and floral meristem activity, as ult1 mutations cause the enlargement of inflorescence and floral meristems, leading to the production of supernumerary flowers and floral organs (Fletcher, 2001). ult1 mutants have reduced floral meristem determinacy, and ULT1 has been shown to negatively regulate WUS expression in order for floral meristem termination to occur at the correct stage of flower development (Carles et al, 2004)
We discuss the functions of the ULT factors throughout plant development, and in light of what is known about SAND domain-containing factors in animals, we propose that the ULT proteins may act as direct regulators of developmental gene expression
Summary
As centers of cell proliferation and organ initiation, are the foundation of all plant structures. The constant formation of new organs and tissues throughout plant life relies on precise mechanisms that maintain overall meristem integrity for upwards of hundreds of years in some species. Cells in the outermost layer (L1) produce epidermal tissues, whereas cells of the sub-epidermal layer (L2) and the internal layers (L3) differentiate into vascular and internal tissues. Superimposed across these cell layers are distinct zones of differential meristem activity. Maintenance of SAM integrity requires a precise coordination between the flow of cells leaving the PZ and their replacement by cells from the CZ, which implies that the different regions of the meristem are in communication with one another. In Arabidopsis, one key component of the meristem communication system is the CLAVATA (CLV) extracellular signaling pathway
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
