Molecular mechanisms controlling cell division and differentiation during maize development

Elaborate genetic mechanisms are involved in controlling cell division and differentiation during plant development. The maize Ex&barbelow;tra c&barbelow;ell l&barbelow;ayers1 (Xcl1) mutation provides insight into these developmental pathways since it causes aberrant oblique, periclinal divisions to occur in the protoderm layer. These periclinal divisions occur at the expense of normal anticlinal divisions in the protoderm and cause the production of extra cell layers with epidermal characteristics, indicating that cells are differentiating according to lineage instead of position. Mutant kernels have several aleurone layers instead of one, indicating that Xcl1 alters cell division orientation in cells that divide predominantly in the anticlinal plane. Dosage analysis of Xcl1 reveals that the mutant phenotype is caused by overproduction of a normal gene product. This allows cells that have already received differentiation signals to continue to divide in aberrant planes and suggests that the timing of cell division determines differentiation. Cells that divide early and in the absence of differentiation signals use positional information, while cells that divide late after perceiving differentiation signals use lineage information instead of position.; Double mutant analyses indicate that XCL1 may interact with several other genetic pathways during normal shoot development. Among these are TANGLED1, a microtubule-binding protein, and CRINKLY4, a receptor kinase involved in epidermal differentiation. XCL1 also displays genetic interactions with KNOTTED1-like homeobox (KNOX) proteins involved in maintaining pools of undifferentiated cells in the shoot apical meristem. The double mutant interactions of Xcl1 with dominant KNOX mutants such as Knotted1, Gnarly1 , and Rough sheath1 indicate that XCL1 may play a role in linking KNOX proteins with auxin-mediated developmental pathways. Thus, XCL1 is an important regulator of both cell division orientation and differentiation signal transduction pathways during plant development.