A PORPHYRIN PATHWAY IMPAIRMENT IS RESPONSIBLE FOR THE PHENOTYPE OF A DOMINANT DISEASE LESION MIMIC MUTANT OF MAIZE

The maize lesion mimic gene Les22 is defined by dominant mutations and characterized by the production of minute necrotic spots on leaves in a developmentally specified and light-dependent manner. Phenotypicall y, Les22 lesions resemble those that are triggered during a hypersensi tive disease resistance response of plants to pathogens. We have clone d Les22 by using a Mutator-tagging technique. It encodes uroporphyrino gen decarboxylase (UROD), a key enzyme in the biosynthetic pathway of chlorophyll and heme in plants. Urod mutations in humans are also domi nant and cause the metabolic disorder porphyria, which manifests itsel f as light-induced skin morbidity resulting from an excessive accumula tion of photoexcitable uroporphyrin. The phenotypic and genetic simila rities between porphyria and Les22 along with our observation that Les 22 is also associated with an accumulation of uroporphyrin revealed wh at appears to be a case of natural porphyria in plants.