Suppression of the Barleyuroporphyrinogen III synthaseGene by aDsActivation Tagging Element Generates Developmental Photosensitivity

Abstract

Chlorophyll production involves the synthesis of photoreactive intermediates that, when in excess, are toxic due to the production of reactive oxygen species (ROS). A novel, activation-tagged barley (Hordeum vulgare) mutant is described that results from antisense suppression of a uroporphyrinogen III synthase (Uros) gene, the product of which catalyzes the sixth step in the synthesis of chlorophyll and heme. In homozygous mutant plants, uroporphyrin(ogen) I accumulates by spontaneous cyclization of hydroxyl methylbilane, the substrate of Uros. Accumulation of this tetrapyrrole intermediate results in photosensitive cell death due to the production of ROS. The efficiency of Uros gene suppression is developmentally regulated, being most effective in mature seedling leaves compared with newly emergent leaves. Reduced transcript accumulation of a number of nuclear-encoded photosynthesis genes occurs in the mutant, even under 3% light conditions, consistent with a retrograde plastid-nuclear signaling mechanism arising from Uros gene suppression. A similar set of nuclear genes was repressed in wild-type barley following treatment with a singlet oxygen-generating herbicide, but not by a superoxide generating herbicide, suggesting that the retrograde signaling apparent in the mutant is specific to singlet oxygen.

Reactive oxygen species generation and signaling in plants

“…86 The accumulation of uroporphyrin I in the uroporphyrinogen III cosynthase antisense barley plants results in necrotic leaves and ultimately cell death because of accumulation of ROS. 87 Like uroporphyrin I, uroporphyrin III, an oxidized derivative of uroporphyrinogen III, an intermediate of the chlorophyll biosynthesis pathway, also acts as a photosensitizer. Accumulation of uroporphyrin III leads to light-dependent necrosis in tobacco 30,88 and in maize.…”

Genetic analyses of cell death in maize (Zea mays, Poaceae) leaves reveal a distinct pathway operating in the camouflage1 mutant

“…The c f1 mutant tissues do not accumulate H 2 O 2 , whereas Les22 lesions do. Previous studies have found that plants with alterations at different steps in tetrapyrrole biosynthesis experience oxidative stress, and, like Les22 , the mutant plants exhibited leaf lesions that resemble defense responses to pathogens (Hu et al, 1998; Mock et al, 1998; Molina et al, 1999; Ishikawa et al, 2001; Williams et al, 2006; Ayliffe et al, 2009). In these cases, the accumulation of phototoxic porphyrin intermediates is thought to give rise to detrimental levels of ROS that cause cell death.…”

“…Mutants in the tetrapyrrole synthesis pathway in plants display leaf lesions similar in appearance to those seen in the HR during attack by pathogens (Hu et al, 1998; Ishikawa et al, 2001; Williams et al, 2006; Ayliffe et al, 2009). For example, Lesion mimic22 ( Les22 ) is a maize mutant that displays small necrotic leaf lesions that require light for induction (Hu et al, 1998).…”

RLIN1, encoding a putative coproporphyrinogen III oxidase, is involved in lesion initiation in rice

“…In the lesion formation mutants as described above, the function of UROD, CPOX, PPOX, plastidic ferrochelatase and uroporphyrinogen III synthase are partly deficient rather than knockout (Ayliffe et al, 2009). Tetrapyrrole metabolism genes involved in lesion mimics are likely to cause lethal if completely deleted.…”

“…It has been supposed that the phenomenon was correlated with peak flux through the chlorophyll biosynthetic pathway, and this hypothesis could also explain why fully expanded mature leaves of dark-grown plants did not show lesion mimic phenotype when grown under light condition (Hu et al, 1998;Ishikawa et al, 2001;Ayliffe et al, 2009).…”