Photorespiratory Carbon and Nitrogen Cycling: Evidence from Studies of Mutant and Transgenic Plants

Abstract

SummaryPhotorespiratory mutants represent the most complete set of mutants for any metabolic pathway in plants. The photorespiratory pathway is also a prime example of the integration and co-ordination of carbon and nitrogen (N) metabolism. Studies of mutant and transgenic plants with lesions in photorespiratory metabolism have confirmed its cyclic nature, its origin in the reductive pentose phosphate pathway, and the associated N cycling. They have led to new insights into the nature of these processes and aspects of their regulation and control. Unlike most pathways in plants, the specific isozymes in chloroplasts, mitochondria and peroxisomes involved in photorespiratory metabolism have been unequivocally identified. In the case of some mutations, isozymes not specifically involved in photorespiratory metabolism provide an alternative route and so by-pass the lesion. We discuss how, in mutant plants in which the recycling of N is defective, the normal photorespiratory pathway involving glycine decarboxylation may be partially by-passed by a modified form of photorespiration involving glyoxylate decarboxylation. Apart from metabolic feedback, we also discuss how mutants have been used to study the regulation of gene expression and the role of photorespiration during light and drought stress.