The use of modern molecular biological techniques has played a significant role in elucidating various aspects of the molecular organization of plant genes. Most of the information gained concerns characters and phenomena that are of relevance to plant breeding. It has been shown at the molecular level that cereal and legume seed storage proteins are controlled by complex multigenic nuclear loci. The analysis of movable nuclear genes, exemplified by the Ds-Ac transposable elements in maize, has given insight into the nature of these elements and the genetic changes that occur at their insertion and deletion sites. The characterization of the heterogeneous mitochondria1 DNA and the uniform chloroplast DNA has helped to elucidate the molecular basis of cytoplasmic male sterility, RuBP carboxylase enzyme synthesis and herbicide resistance, In N, fixation, the emphasis has been put on the molecular dissection of N,-fixing genes in bacteria and the interactions between symbiotic and host DNAs. It has been possible to develop vectors of Agrobacterium rurnefaciens Ti-plasmids which lack oncogenes for DNA transfer. Cauliflower and gemini viruses as well as liposomes are also potential systems for DNA transport to protoplasts which can be regenerated into plants. Furthermore, it has been shown that certain molecular biological techniques are useful for screening purposes in genetic analysis and plant breeding work. During the last few years a great deal of work has been initiated on the molecular biology of cultivated plants. The information acquired on the molecular genetics of microorganisms and mammalian cells together with the practical implications of this knowledge have stimulated this research. The molecular characterization of the constitution, expression and regulation of the genetic material in crop plants is a prerequisite for molecular genetic engineering and directed plant modification. Also, the elucidation of the molecular basis of naturally occurring phenomena that lead to genetic variability, e.g. gene transposition and interactions between crop plants and symbiotic or infectious microorganisms, is of utmost importance. The recent developments in molecular biology offer a battery of techniques for work on defined DNA segments of either natural or synthetic origin, and their transfer into genetic complements.Use of existing and induced genetic variability is the working strategy of plant breeding. Molecular genetic engineering adds a new dimension to the arsenal of established breeding strategies. Conventional plant breeding methods have proven to be efficient means of crop improvement. "Chromosome engineering" in wheat (SEARS 1972;LAW 1983) is a classical example of the methods applied in genetic characterization and modification of a crop and should constitute a firm base that provides guidance when molecular genetic engineering is to be challenged.The aim of this review is to summarize, in a simple way, the recent achievements in molecular biology which are pertinent to crop plants and breeding. It is ...
