Plant development, as in other multicelluarsystems, involves coordinately producing distinctive cell types with the specification programs. Asymmetric cell division (ACD), in which the generated daughter cells differ in size, cellular components and fate, is a universal mechanism in multicellularorganisms to generate cellular diversity and maintain stem cell populations. In animal systems, ACD is generally achieved by establishment of an internal cell polarity. However, notable structure differences in plant cells and the absence of conserved homologsof animal or fungal ACD regulators may require plants utilize new molecules and mechanisms to create asymmetries. By studying stomataldevelopment, we have identified BASL (Breaking of Asymmetry in the StomatalLineage), a novel regulator of asymmetric division in Arabidopsis. BASL accumulates in a polarized crescent before the division, which mimics the behavior of animal polarity regulators, and then is only inherited to the larger daughter cell. Presence of BASL at the cell peripheral functions to promote a regional outgrowth and is crucial for its function, and we propose that BASL represents a plant-unique solution to the challenge of ACD. The BASL protein encodes an unknown protein without obvious predictable function domains, fine-scale analysis of the BASL protein is carried out for diagnosis of how this novel protein functions in the stomatalACD. Recent studies on BASL partners, genetic and physical, integrate new components in this polarity module in plants.

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