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Summary: The quantitative and spatial coordination of stomatal pores in the epidermis and airspaces in the underlying mesophyll tissue is vital for efficient gas exchange in the leaf. The mechanisms that determine the distribution of stomata in the epidermis have been studied extensively, but how this relates to the regulation of mesophyll airspace configuration is poorly understood. Recent studies have investigated how development is coordinated between these tissue layers. The evidence suggests that multiple mechanisms are likely to work concurrently to coordinate stomatal and mesophyll development for optimal leaf gas exchange, and that both genetic and physiological factors contribute to this regulation. Such advances in our understanding of leaf development have important implications for potential improvement of crop water use efficiency. [ABSTRACT FROM AUTHOR]

The article provides a quantitative analysis of the process of capitulum growth and the gradual but controlled process by which elements within the capitulum become destined to form individual flowers. The data provide a foundation for the modelling of flower formation and for linking the observed parameters of floral morphogenesis with the wealth of molecular data on organogenesis in plants. An analysis of auxin flux and distribution during leaf initiation is also presented. The situation in the capitulum may be linked with the spatial and temporal control of factors specific to flower formation.

Plasmodiophora brassicae (clubroot) infection leads to reprogramming of host development resulting in the formation of characteristic galls. In this work we explored the cellular events that underly gall formation in Arabidopsis thaliana with the help of molecular markers of cell division ( CYCB1:GUS) and meristematic activity ( ANT:GUS). Our results show that gall development involved the amplification of existing meristematic activities within the vascular cambium (VC) and phloem parenchyma (PP) cells in the region of the hypocotyl. Additionally we found that the increase in VC activity and prolonged maintenance of cambial-derived cells in a meristematic state was crucial for gall formation; disruption of the VC activity significantly decreased the gall size. Gall formation also perturbed vascular development with a significant reduction in xylem and increase in PP in infected plants. This situation was reflected in a decrease in transcripts of key factors promoting xylogenesis (VND6, VND7 and MYB46) and an increase in those promoting phloem formation and function (APL, SUC2). Finally we show, using the cell cycle inhibitor ICK1/KRP1 and a cle41 mutant with altered regulation of cambial stem cell maintenance and differentiation, that a decrease in gall formation did not prevent pathogen development. This finding demonstrates that although gall formation is a typical symptom of the disease and influences numbers of spores produced, it is not required for completion of the pathogen life cycle. Together, these results provide an insight into the relationship of the cellular events that accompany Plasmodiophora infection and their role in disease progression. [ABSTRACT FROM AUTHOR]

The causal relationship between cell division and growth in plants is complex. Although altered expression of cell-cycle genes frequently leads to altered organ growth, there are many examples where manipulation of the division machinery leads to a limited outcome at the level of organ form, despite changes in constituent cell size. One possibility, which has been under-explored, is that altered division patterns resulting from manipulation of cell-cycle gene expression alter the physiology of the organ, and that this has an effect on growth. We performed a series of experiments on retinoblastoma-related protein ( RBR), a well characterized regulator of the cell cycle, to investigate the outcome of altered cell division on leaf physiology. Our approach involved combination of high-resolution micro CT imaging and physiological analysis with a transient gene induction system, providing a powerful approach for the study of developmental physiology. Our investigation identifies a new role for RBR in mesophyll differentiation that affects tissue porosity and the distribution of air space within the leaf. The data demonstrate the importance of RBR in early leaf development and the extent to which physiology adapts to modified cellular architecture resulting from altered cell-cycle gene expression. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]