Genomic and Transcriptomic Insight of Giant Sclerotium Formation of Wood-Decay Fungi

Many fungi form persistent and dormant sclerotia with compact hardened mycelia during unfavorable circumstances. While most of these sclerotia are small in size, Wolfiporia cocos, a wood-decay fungus, grows into giant sclerotia, which are mainly composed of polysaccharides of linear (1→3)-β-D-glucans. To explore the underlying mechanism of converting sophisticated wood polysaccharides for biosynthesis of highly homogenized glucans in W. cocos, we sequenced and assembled the genome of a cultivated W. cocos strain (WCLT) in China. The 62-Mb haploid genome contains 44.2% repeat sequences, of which, 48.0% are transposable elements (TEs). Contrary to the genome of W. cocos from North America, WCLT has independently undergone a partial genome duplication (PGD) event. The large-scale TE insertion and PGD occurrence overlapped with an archeological Pleistocene stage of low oxygen and high temperature, and these stresses might have induced the differences in sclerotium due to geographical distribution. The wood decomposition enzymes, as well as sclerotium-regulator kinases, aquaporins, and highly expanded gene families such as NAD-related families, together with actively expressed 1,3-β-glucan synthase for sclerotium polysaccharides, all have contributed to the sclerotium formation and expansion. This study shall inspire further exploration on how fungi convert wood into simple glucans in the sclerotium of W. cocos.

A Novel Effector Protein SsERP1 Inhibits Plant Ethylene Signaling to Promote Sclerotinia sclerotiorum Infection

Sclerotinia sclerotiorum is one of the most devastating pathogens in Brassica napus and causes huge economic loss worldwide. Though around one hundred putative effectors have been predicted in Sclerotinia sclerotiorum genome, their functions are largely unknown. In this study, we cloned and characterized a novel effector, SsERP1 (ethylene pathway repressor protein 1), in Sclerotinia sclerotiorum. SsERP1 is a secretory protein highly expressed at the early stages of Sclerotinia sclerotiorum infection. Ectopic overexpression of SsERP1 in plant leaves promoted Sclerotinia sclerotiorum infection, and the knockout mutants of SsERP1 showed reduced pathogenicity but retained normal mycelial growth and sclerotium formation, suggesting that SsERP1 specifically contributes to the pathogenesis of Sclerotinia sclerotiorum. Transcriptome analysis indicated that SsERP1 promotes Sclerotinia sclerotiorum infection by inhibiting plant ethylene signaling pathway. Moreover, we showed that knocking down SsERP1 by in vitro synthesized double-strand RNAs was able to effectively inhibit Sclerotinia sclerotiorum infection, which verifies the function of SsERP1 in Sclerotinia sclerotiorum pathogenesis and further suggests a potential strategy for Sclerotinia disease control.

Sclerotia Formation of Phlebopus Portentosus in Wild and Artificial Cultivation

Phlebopus portentosus is a favorite wild edible mushroom in the Xishuangbanna region, Yunnan, China and in northern Thailand. This bolete has a unique biotrophy. It can be saprobic but also form a tripartite association with root mealy bugs and plants. P. portentosus is the only edible fungus of Boletales in the world that can be cultivated artificially and anniversary produced at present. Sclerotium is often found at its natural environment and cultivated media, but the regularity and growth characteristics of the sclerotium are unknown. In this study the whole process of birth, growth, death and rebirth of the sclerotium of P. portentosus at the national and lab conditions was reported for the first time. The sclerotium formation in the nature is related to adversity, such as reduced rainfall and low temperature. The more rainfall, the less sclerotia. It seems that the lower temperature increased the sclerotium formation, however the relationship of the sclerotium formation to temperature was not obvious as the rainfall. Under artificial conditions the sclerotium formation of P. portentosus is related to the fungus physiological maturation, and the sclerotium occurrence always accompanied by appearance of the water drops on the colony. The result will set up a platform for research on importance of the sclerotium in life circle of P. portentosus.

Timing of Occurrence of Claviceps purpurea Ascospores in Northeast Oregon

Ergot, caused by Claviceps purpurea, is an important floral disease of grasses, characterized by sclerotium formation within the host flowers. To determine whether annual variation in ergot severity in Kentucky bluegrass is a result of ascospore density and/or timing of ascospore occurrence, Burkard 7-day volumetric spores traps were used to monitor ascospores of C. purpurea in each of two Kentucky bluegrass fields in the Grand Ronde Valley in northeastern Oregon between mid-May and late June, 2008-2010. Ascospores were typically trapped between midnight and 6:00 a.m. In 2008 and 2010, most ascospores were released prior to flowering in Kentucky bluegrass, corresponding to no observed ergot in 2008 and a low level of ergot in 2010. In 2009, ascospore release and pollination coincided, but few airborne ascospores were present, resulting in a low level of ergot. Similar ergot levels were observed in fungicide trials, suggesting that fungicides for ergot control were unnecessary. In years when there are few ascospores during flowering in Kentucky bluegrass, a reduction of up to two fungicide applications may be possible. Accepted for publication 2 November 2010. Published 23 November 2010.

Morphological Transitions Governed by Density Dependence and Lipoxygenase Activity in Aspergillus flavus

ABSTRACT Aspergillus flavus differentiates to produce asexual dispersing spores (conidia) or overwintering survival structures called sclerotia. Results described here show that these two processes are oppositely regulated by density-dependent mechanisms and that increasing the cell density (from 101 to 107 cells/plate) results in the lowest numbers of sclerotial and the highest numbers of conidial. Extract from spent medium of low-cell-density cultures induced a high-sclerotium-number phenotype, whereas high-cell-density extract increased conidiation. Density-dependent development is also modified by changes in lipid availability. Exogenous linoleic acid increased sclerotial production at intermediate cell densities (104 and 105 cells/plate), whereas oleic and linolenic acids inhibited sclerotium formation. Deletion of Aflox encoding a lipoxygenase (LOX) greatly diminished density-dependent development of both sclerotia and conidia, resulting in an overall increase in the number of sclerotia and a decrease in the number of conidia at high cell densities (>105 cells/plate). Aflox mutants showed decreased linoleic acid LOX activity. Taken together, these results suggest that there is a quorum-sensing mechanism in which a factor(s) produced in dense cultures, perhaps a LOX-derived metabolite, activates conidium formation, while a factor(s) produced in low-density cultures stimulates sclerotium formation.