Spermidine is required for morphogenesis in the human pathogenic fungus, Penicillium marneffei

Penicillium marneffei is a thermally dimorphic fungus that is a highly significant pathogen of immune compromised persons living or having traveled in Southeast Asia. When cultured at 25°C, the wild-type strain of P. marneffei exhibits a mycelial morphology that is marked by the development of specialized structures bearing conidia. Incubation of the wild type at 37°C, however, promotes the development of a yeast form that divides by fission. Development of the yeast morphology in vivo appears to be requisite for pathogenesis. In a prior study using Agrobacterium-mediated transformation for random mutagenesis via T-DNA integration, we generated a morphological mutant (strain I6) defective in conidiation. The T-DNA insertion site in strain I6 was determined to be within the gene encoding S-adenosylmethionine decarboxylase (sadA), an enzyme critical to spermidine biosynthesis. In the present study, we demonstrated that strain I6 was able to grow on rich media in either the mold or yeast forms at 25°C and 37°C, respectively. However, reduced growth of strain I6 was observed on minimal medium at either temperature. In addition, strain I6 produced mycelia with impaired conidiation on minimal medium at 25°C. Supplementation of minimal medium with spermidine restored the ability of strain I6 to produce conidia at 25°C and promoted yeast development at 37°C. Moreover, conidia of strain I6 exhibited poor germination frequencies in the absence of this polyamine. All three of these processes (conidiogenesis, germination, and growth) were reinstated in strain I6 by complementation of the partially deleted of sadA gene by ectopic insertion of an intact wild-type copy. These results augment prior observations that spermidine biosynthesis is essential to normal growth, conidiogenesis, spore germination, and dimorphism in a variety of fungi. Given the presumption that P. marneffei infections are initiated following inhalation of conidia, and that pathogenesis is dependent upon yeast development, this study further suggests that the spermidine biosynthetic pathway may serve as a potential target for combating infections by this medically important fungus.