Conditional repression of AUXIN BINDING PROTEIN1 reveals that it coordinates cell division and cell expansion during postembryonic shoot development in Arabidopsis and tobacco.

AUXIN BINDING PROTEIN1 (ABP1) has long been characterized as a potentially important mediator of auxin action in plants. Analysis of the functional requirement for ABP1 during development was hampered because of embryo lethality of the null mutant in Arabidopsis thaliana. Here, we used conditional repression of ABP1 to investigate its function during vegetative shoot development. Using an inducible cellular immunization approach and an inducible antisense construct, we showed that decreased ABP1 activity leads to a severe retardation of leaf growth involving an alteration in cell division frequency, an altered pattern of endocycle induction, a decrease in cell expansion, and a change in expression of early auxin responsive genes. In addition, local repression of ABP1 activity in the shoot apical meristem revealed an additional role for ABP1 in cell plate formation and cell shape. Moreover, cells at the site of presumptive leaf initiation were more sensitive to ABP1 repression than other regions of the meristem. This spatial context-dependent response of the meristem to ABP1 inactivation and the other data presented here are consistent with a model in which ABP1 acts as a coordinator of cell division and expansion, with local auxin levels influencing ABP1 effectiveness.

Restoration of DWF4 expression to the leaf margin of a dwf4 mutant is sufficient to restore leaf shape but not size: the role of the margin in leaf development.

The role of the margin in leaf development has been debated over a number of years. To investigate the molecular basis of events in the margin, we performed an enhancer trap screen to identify genes specifically expressed in this tissue. Analysis of one of these lines revealed abnormal differentiation in the margin, accompanied by an abnormal leaf size and shape. Further analysis revealed that this phenotype was due to insertion of the trap into DWF4, which encodes a key enzyme in brassinolide biosynthesis. Transcripts for this gene accumulated in a specific and dynamic pattern in the epidermis of young leaf primordia. Targeted expression of DWF4 to a subset of these cells (the leaf margin) in a dwf4 mutant background led to both restoration of differentiation of a specific group of leaf cells (margin cells) and restoration of wild-type leaf shape (but not leaf size). Ablation of these cells led to abrogation of leaf development and the formation of small round leaves. These data support the hypothesis that events in the margin play an essential role in leaf morphogenesis, and implicate brassinolide in the margin as a key mediator in the control of leaf shape, separable from a general function of this growth factor in the control of organ size.

Induction of differentiation in the shoot apical meristem by transient overexpression of a retinoblastoma-related protein.

The shoot apical meristem contains cells that undergo continual growth and division to generate the building blocks for the aerial portion of the plant. As cells leave the meristem, they undergo differentiation to form specific cell types. Most notably, heterotrophic cells of the meristem rapidly gain autotrophic capability by synthesis and assembly of components of the chloroplast. At the same time, cells undergo enlargement via vacuolation. Despite significant advances in the characterization of transcriptional networks involved in meristem maintenance and leaf determination, our understanding of the actual mechanism of meristem cell differentiation remains very limited. Using a microinduction technique, we show that local, transient overexpression of a retinoblastoma-related (RBR) protein in the shoot apical meristem is sufficient to trigger cells in the meristem to undergo the initial stages of differentiation. Taken together with recent data showing that RBR protein plays a key role in restricting stem cell differentiation in the root apical meristem, our data contribute to an emerging picture of RBR proteins as a central part of the mechanism controlling meristem cell differentiation.

Cell division pattern influences gene expression in the shoot apical meristem.

The shoot apical meristem of angiosperms shows a highly conserved cellular architecture in which a change of cell division orientation correlates with early events of leaf initiation. However, the causal role of this altered cellular parameter in leaf formation is debatable. We have used the dynamin-like protein phragmoplastin as a tool to modify the pattern of cell division within the apical meristem. Taking a microinduction approach, we show that local alteration in cell division orientation is not sufficient to induce morphogenesis in the meristem. Surprisingly, an altered cell division pattern did lead to an altered pattern of expression of genes implicated in aspects of leaf formation. Our data identify inducible expression of phragmoplastin as a tool to manipulate cell division pattern. Furthermore, they indicate that a mechanism exists by which cells in the meristem can respond at the level of gene expression to altered parameters of cell division. These data are discussed in the context of a model linking leaf morphogenesis and differentiation.

Manipulation of leaf shape by modulation of cell division.

The role of cell division as a causal element in plant morphogenesis is debatable, with accumulating evidence supporting the action of cell division-independent mechanisms. To directly test the morphogenic function of cell division, we have utilised a microinduction technique to locally and transiently manipulate the expression in transgenic plants of two genes encoding putative effectors of the cell cycle, a tobacco A-type cyclin and a yeast cdc25. The results show that local expression of these genes leads to modulation of cell division patterns. Moreover, whereas altered cell division in the apical meristem had no influence on organogenesis, local induction of cell proliferation on the flanks of young leaf primordia led to a dramatic change in lamina development and, thus, leaf shape. These data indicate that the role of cell division in plant morphogenesis is context dependent and identify cell division in the leaf primordium as a potential target for factors regulating leaf shape.