Institut für Pflanzengenetik und Kulturpflanzenforschung, Corrensstrasse 3, D-06466 Gatersleben, Germany

Background and objectives
Transgenic tobacco plants with reduced activity of either uroporphyrinogen decarboxylase (UROD) or coproporphyrinogen oxidase (CPO), two enzymes of the tetrapyrrole biosynthetic pathway, are characterized by necrotic leaf lesions resulting from the accumulation of photosensitizing tetrapyrrolic intermediates [1]. These molecules exert their photodamaging effects via the formation of reactive oxygen species, e.g. singlet oxygen or hydrogen peroxide. We are interested in describing the cellular responses upon tetrapyrrole-induced oxidative stress, including anti-oxidative and pathogen defence reactions.

Results and conclusions
Compared with control plants, the transformants have increased levels of anti-oxidant mRNA species, particularly those encoding superoxide dismutase, catalase and glutathione peroxidase [2]. These elevated transcript levels correlate with increased activities of cytosolic and mitochondrial superoxide dismutase isoforms and of most enzymes of the Halliwell-Asada pathway. The total activity of catalase decreases in older leaves of transformants to lower levels than in wild-type plants, reflecting an increased turnover of this photosensitive enzyme. However, formation of leaf necrosis reflects limited capacity of the anti-oxidative defence system which is apparent from decreased levels of low-molecular-weight anti-oxidants such as ascorbate, glutathione and tocopherol.

In parallel to the induction of anti-oxidative defence reactions, accumulation of tetrapyrroles induces cellular responses normally associated with pathogen defence. UROD and CPO antisense plants accumulate the highly fluorescent coumarin scopolin, a compound which is also formed after TMV infection of tobacco. In addition, plants with leaf lesions display increased pathogenesis-related protein expression and higher levels of free and conjugated salicylic acid. Plants with the necrotic phenotype of several transgenic lines show increased resistance to TMV, relative to wild-type controls or transformants without leaf lesions. Our results demonstrate that deregulation of tetrapyrrole biosynthesis can induce a set of defence responses resembling the hypersensitive reaction observed after pathogen attack. We hypothesize that reactive oxygen species derived from excessive tetrapyrroles interfere with signalling events inducing pathogen defence responses. Experiments are currently being performed to dissect the molecular events leading to anti-oxidative and pathogen defence responses. In conclusion, our model system offers the future possibility to improve pathogen defence responses in plants by genetically modifying the tetrapyrrole biosynthetic pathway.

1. Mock H-P, Grimm B, 1997. Plant Physiology 113, 1101-1112.
2. Mock H-P, Keetman U, Kruse E, Rank B, Grimm B, 1998. Plant Physiology 116, 107-116.