Cytoplasmic retention and degradation of a mitotic inducer enable plant infection by a pathogenic fungus

Transcriptome Comparison between Two Strains of Ustilago esculenta during the Mating

Ustilago esculenta is a smut fungus that obligately infects Zizania latifolia and stimulates tissue swelling to form galls. Unlike T-type, MT-type U. esculenta can only proliferate within plant tissues and infect the offspring of their host. Production of telispores, haploid life, and plant cuticle penetration are not essential for it, which may lead to the degeneration in these processes. Transcriptome changes during the mating of T- and MT-type U. esculenta were studied. The functions of several secreted proteins were further confirmed by knock-out mutants. Our results showed that MT-type U. esculenta can receive environmental signals in mating and circumstance sensing as T-type does. However, MT-type U. esculenta takes a longer time for conjunction tube formation and cytoplasmic fusion. A large number of genes encoding secreted proteins are enriched in the purple co-expression module. They are significantly up-regulated in the late stage of mating in T-type U. esculenta, indicating their relationship with infecting. The knock-out of g6161 (xylanase) resulted in an attenuated symptom. The knock-out of g943 or g4344 (function unidentified) completely blocked the infection at an early stage. This study provides a comprehensive comparison between T- and MT-type during mating and identifies two candidate effectors for further study.

Incompatibility between proliferation and plant invasion is mediated by a regulator of appressorium formation in the corn smut fungus Ustilago maydis

Plant pathogenic fungi often developed specialized infection structures to breach the outer surface of a host plant. These structures, called appressoria, lead the invasion of the plant by the fungal hyphae. Studies in different phytopathogenic fungi showed that appressorium formation seems to be subordinated to the cell cycle. This subordination ensures the loading in the invading hypha of the correct genetic information to proceed with plant infection. However, how the cell cycle transmits its condition to the genetic program controlling appressorium formation and promoting the plant's invasion is unknown. Our results have uncovered how this process occurs for the appressorium of Ustilago maydis, the agent responsible for corn smut disease. Here, we described that the complex Clb2-cyclin-dependent kinase (Cdk)1, one of the master regulators of G2/M cell cycle progression in U. maydis, interacts and controls the subcellular localization of Biz1, a transcriptional factor required for the activation of the appressorium formation. Besides, Biz1 can arrest the cell cycle by down-regulation of the gene encoding a second b-cyclin Clb1 also required for the G2/M transition. These results revealed a negative feedback loop between appressorium formation and cell cycle progression in U. maydis, which serves as a "toggle switch" to control the fungal decision between infecting the plant or proliferating out of the plant.

A cyclin protein governs the infectious and sexual life cycles of Cryptococcus neoformans

Cell cycle is a fundamental process underlying growth and development in evolutionarily diverse organisms, including fungi. In human fungal pathogens, cell cycle control generally determines their life cycles, either in the environment or during infections. Thus, cell cycle components can potentially serve as important targets for the development of antifungal strategy against fungal infections. Here, in Cryptococcus neoformans, the most common cause of fatal fungal meningitis, we show that a previously uncharacterized B-type cyclin named Cbc1 is essential for both its infectious and sexual cycles. We reveal that Cbc1 coordinates various sexual differentiation and molecular processes, including meiosis. Especially, the absence of Cbc1 abolishes formation of sexual spores in C. neoformans, which are presumed infectious particles. Cbc1 is also required for the major Cryptococcus pathogenic attributes. Virulence assessment using the murine model of cryptococcosis revealed that the cbc1 mutant is avirulent. Together, our results provide an important insight into how C. neoformans employs shared cell cycle regulation to coordinate its infectious and sexual cycles, which are considered crucial for virulence evolution and the production of infectious spores.

Cycling in synchrony

The corn smut fungus uses two different mechanisms to control its cell cycle when it is infecting plants.