Tansley Review No. 75 Arabidopsis- its use in the genetic and molecular analysis of plant morphogenesis.

In the last decade, the weed Arabidopsis thaliana has come to prominence as a major new model system for investigating genetic and molecular aspects of developmental plant morphology. Extensive genetic and molecular information about the Arabidopsis genome, facilitated by international collaborations and the production of novel mutagenic systems, has enabled a vast array of mutants to be identified, most of which reveal nuclear genes that control different aspects of plant developmental processes. An ever increasing number of these newly identified genes have been isolated and within the next few years an overall view of the molecular control of plant development is likely to emerge. Particularly prevalent amongst these Arabidopsis mutants are those which alter morphogenic processes cither by changes in differentiation patterns of specific cell types, homeotic conversion of entire structures or abnormal patterns of cell division. Mutants in the control of morphogenesis of most parts of the Arabidopsis plant have been identified and characterized. The most abundant classes are in embryogenesis including seedling pattern formation, root morphogenesis, floral morphology (including pollen and anther formation) and mutunts affecting shoot apical inenstern morphology. The first genes to be isolated from morphological mutants have been identified as transcription factors capable of controlling expression of other gene classes as part of a hierarchy of gene control. The relative ease with which many interesting and potentially important genes in morphogenesis have been revealed by identificatic.n of mutants makes it highly likely that with the aid of Arabidopsis thatiana, an understanding of the extremely complex molecular basis of plant morphogenesis may at last be Hirbin reach. Contents Summary 19 I. Introduction 19 II. Why Arabidopsis? 20 III. The developing seed 22 IV. The root 25 V. The shoot apex 27 VI. The Leaf 28 VII. Whole plant morphology 30 VIII. Floral development 31 IX. Sub-cellular tnorphology 32 X. Discussion and future possihihties 32 XI. Acknowledgements 32 XII. References 34.