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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]

Summary 1I. Introduction 1II. The shoot apical meristem: the creation of a field of cells from which leaves can be formed 2III. Patterning: the selection of leaf initiation site 2IV. Morphogenesis: the initial stages of leaf formation 4V. Differentiation and determination: transcriptional networks controlling determinancy 6VI. The elaboration of leaf form 6VII. Early steps in leaf histogenesis 7VIII. Later steps in leaf differentiation: epidermal cell fate 8IX. Compound and simple leaves: variation on a theme? 9X. Changes in leaf form: developmental and environmental influences 9XI. A model of leaf formation 9 Acknowledgements 10 References 10The formation of a leaf is a basic aspect of plant development. This review provides an overview of our present understanding of the process from initiation to the final form of the leaf. Molecular genetic and cell biology approaches have yielded significant advances in this area, adding not only to our knowledge of leaf development but also to fundamental principles in plant biology. These principles will be highlighted, as well as areas where our understanding is still incomplete, in particular the problem of coordinating the multifaceted steps involved in the generation of the leaf structure.New Phytologist(2004)doi: 10.1111/j.1469-8137.2004.01292.x© New Phytologist(2004) [ABSTRACT FROM AUTHOR]

Summary To identify genes expressed at the earliest stages of leaf development, we have performed a differential display analysis using portions of meristems destined to form leaves. Our analysis led to the identification of five genes showing an asymmetric pattern of gene expression within the meristem associated with leaf formation. Surprisingly, three of these genes encoded enzymes involved in carbohydrate metabolism (ADPglucose pyrophosphorylase, sucrose synthase and an SNF1-like kinase). Furthermore, specific transcript patterns were responsive to specific sugar and hormonal treatments. The other two genes identified encoded a Phantastica-like myb transcription factor (associated with the acquisition of leaf dorsiventrality) and CYP85 (a cytochrome P450, which plays a pivotal role in brassinolide metabolism). These data, firstly, identify a novel set of marker genes for the analysis of the earliest stages of leaf formation. Secondly, the function of the proteins encoded by these genes and their expression patterns within the meristem indicate that carbohydrate metabolism is spatially regulated within a tissue involved in key developmental processes. Finally, our data provide the first indication of an asymmetry in gene expression related to hormone biosynthesis in the meristem. [ABSTRACT FROM AUTHOR]