Distinct dynamics of the nucleolus in response to nutrient availability and during development in the rice blast fungus

The nucleolus has emerged as a central hub for nuclear functions in eukaryotes. Studies in mammals and the model yeast Saccharomyces cerevisiae showed that the nucleolus is pivotal to nuclear functions other than ribosome biogenesis. Importantly, rDNA transcription and ribosome assembly were shown to positively correlate with the size of the nucleolus. However, little is known about the nucleolus of filamentous fungi. Here, we investigated how the size and shape of nucleolus in the plant pathogenic filamentous fungus, Magnaporthe oryzae, respond to nutrient conditions and infection-related development by monitoring the nucleolar marker protein, MoNOP1 tagged with red fluorescent protein (MoNOP1-RFP). Our work revealed that in the hyphae of M. oryzae, the nucleolar size is decoupled from the nucleolar activity under low nutrient condition. A complete lack of carbon or nitrogen did not cause such decoupling. In conidia, no or faint RFP signals were initially observed, hinting at the absence of functional nucleoli. RFP signals only started to become visible in conidia within 2 hours of germination when the germ tube tip began to differentiate a specialized infection structure, the appressorium. RFP signals diminished thereafter. In contrast, in the appressorium, signals became concentrated after 6 hours as the appressorium matured. We propose that such nucleolar dynamics may reflect the strategy of filamentous fungi under low nutrient availability to forage for food resources as well as the roles of the nucleolus during fungal development. IMPORTANCE: The nucleolus is a dynamic subnuclear structure that is involved in many fundamental processes of the nucleus. In higher eukaryotic cells, the size and shape of nucleoli correlate with nucleolar activities. For fungi, knowledge of the nucleolus and its functions is primarily gleaned from budding yeast. Whether such correlation is conserved and how nucleolar functions are regulated in filamentous fungi including important human and crop pathogens are largely unknown. Our observations reveal that the dynamics of nucleolus in a model plant pathogenic fungus, Magnaporthe oryzae, is distinct from those of animal and yeast nucleoli under low nutrient availability and during pathogenic development. Our data not only provide new insight into the nucleoli in filamentous fungi but also highlight the need for investigating how nucleolar dynamics is regulated in comparison to other eukaryotes.