In the preface to the first edition ofthis book, we expressed a conviction that there was a need for a short book that highlighted important advances in the new discipline of plant molecular biology. The rapid development of this topic has been brought about by the recognition of the unique properties of plants in the study of growth and development together with the application of recombinant DNA techniques to tackle these problems. Plant cells contain DNA in nuclei, plastids and mitochondria, and so ofTer the unique challenge of studying the interaction of three separate genetic systems in a single organism. The molecular approach has provided, in recent years, a wealth of important information about how plants function, and how they interact with bacteria, fungi and viruses. Furthermore, plant development involves the regulation of gene expression in response to internal and external signals, and plant molecular biology has provided a fundamental insight into how this development is regulated. This is not only of considerable scientific interest, but also has important implications for the production of plants and plant products in agriculture, horticulture and the food industries.