Natural antisense transcripts are linked to the modulation of mitochondrial function and teliospore dormancy in Ustilago maydis.

Abstract

The basidiomycete smut fungus Ustilago maydis causes common smut of corn. This disease is spread through the production of teliospores, which are thick-walled dormant structures characterized by low rates of respiration and metabolism. Teliospores are formed when the fungus grows within the plant, and the morphological steps involved in their formation have been described, but the molecular events leading to dormancy are not known. In U. maydis, natural antisense transcripts (NATs) can function to alter gene expression and many NATs have increased levels in the teliospore. One such NAT is as-ssm1 which is complementary to the gene for the mitochondrial seryl-tRNA synthetase (ssm1), an enzyme important to mitochondrial function. The disruption of ssm1 leads to cell lysis, indicating it is also essential for cellular viability. To assess the function of as-ssm1, it was ectopically expressed in haploid cells, where it is not normally present. This expression led to reductions in growth rate, virulence, mitochondrial membrane potential and oxygen consumption. It also resulted in the formation of as-ssm1/ssm1 double-stranded RNA and increased ssm1 transcript levels, but no change in Ssm1 protein levels was detected. Together, these findings suggest a role for as-ssm1 in facilitating teliospore dormancy through dsRNA formation and reduction of mitochondrial function.