The Elusive Plant Mitochondrion as a Genetic System.

The mitochondrion, as the site of energy metabolism, plays a fundamental role within the eukaryotic cell. In plants, the function of this important organelle is made more fascinating by the presence of a second energy-generating system in the chloroplast, with which biochemical and genetic activities must be coordinated. The majority of components necessary for mitochondrial and chloroplast functions are supplied by genes encoded within the nucleus. The presence of genetic information within the mitochondrion and chloroplast, as well as within the nucleus, requires that some form of coordinate gene expression must occur. This intracellular cooperation is necessary not only to assure production of essential components for respiratory and photosynthetic processes, but also for the synthesis of transcription, transcript processing, translation, replication, and organellar transmission machinery requisite to organellar maintenance. Mechanisms regulating the cellular genetic network are poorly understood, as is the evolution of this interorganellar dependence. Mitochondria are currently viewed as integrated endosymbionts, originating from a large group of eubacteria (reviewed by Gray, 1993). The organellar genetic systems are, therefore, somewhat independent of the nucleus insofar as they obey many of their own unique rules of genetics, including uniparental inheritance, somatic recombination, vegetative segregation, gene expression, and genome organization. Many of the nuclear genes required for mitochondrial function are believed to be the result of continuing gene transfer from the mitochondrion during the course of evolution. Within the plant kingdom there can be found a number of presumptive evolutionary intermediates. The legume family, for example, provides convincing evidence that many functions that were originally encoded within the mitochondrion are now gradually being transferred to the nucleus (Covello and Gray, 1992; Nugent and Palmer, 1992). Whereas a particular mitochondrial Cyt oxidase subunit (coxII) is encoded and expressed within the mitochondrion in pea, the same subunit is encoded but not expressed within the nucleus. In soybean and common bean, the same gene duplication exists within the mitochondrion and nucleus, but only the nuclear gene is expressed. In mung bean and cowpea, the mitochondrial form of the gene is no longer present and the nuclear gene is the only functional form. This interorganellar transfer has apparently occurred via an RNA intermediate. Other mitochondrial genes are now being identified that have apparently entered this gene-transfer process. To compound the complexity already inherent in this cellular arrangement, respiratory demands vary greatly among different plant tissues, with the highest respiratory rates occurring during seed germination, pollen development, and fruit ripening. This variation in respiratory activity is presumably reflected in altered regulatory signals at the cell level. It is also apparent that mitochondrial numbers and mitochondrial DNA concentration vary greatly at different stages of plant development, presenting yet another form of regulation required within the cell (Bendich, 1987).