Shining new lights on chytrid cell biology: quantitative live cell imaging of rhizoid development in an early-diverging fungus

Chytridiomycota (Chytrids) are the most basal lineage within the true fungi, however they have largely remained in the dark in terms of their fundamental cell biology. In aquatic ecosystems, chytrids can dominate ‘dark matter’ surveys and are important saprotrophs of recalcitrant organic carbon. They therefore play an integral biogeochemical role in carbon cycling. Unlike ‘higher’ dikaryan fungi that feed via multicellular hyphae, chytrids are unicellular and develop an anucleate rhizoid system that acts as the trophic interface of the cell. Understanding the functions of the rhizoid has the potential to shed light on the trophic biology of ‘dark matter’ chytrids. We applied 3D and 4D live-cell confocal microscopy to morphometrically quantify rhizoid development in the model saprotrophic chytrid Rhizoclosmatium globosum under different nutrient treatments. Rhizoid branching was highest under carbon-rich conditions, whereas under carbon-starved treatments, rhizoids grew significantly longer and were less branched, in what we interpret to be a ‘search strategy’ for nutrient sources. F-actin and the cell wall were identified throughout the rhizoid system. Chemical inhibition of actin and cell wall glucan synthesis induced the development of hyperbranched paramorphs, suggesting that these components underpin rhizoid branching and organising cell polarity at the rhizoid tip. Previous studies have shown that inhibition of these components induces an identical phenotype in dikaryan fungi. These findings represent an important step in understanding the trophic biology of a biogeochemically important microbe and unveil striking similarities in cell development between early-diverging and ‘higher’ dikaryan fungi.