• Disease on morel
Pileus rot disease was first observed on cultivated Morchella importuna in 2013 in China (He et al., 2018a). The causal agent was identified as Diploocpora longispora based on both morphology and the ribosomal DNA internal transcribed spacer (rDNA-ITS). White, velvety mycelia could be observed on the infected ascomata and eventually resulted in malformed fruiting bodies.
Pileus rot disease of Morchella importuna and its causal fungus, Diploöspora longispora. aNaturally infected morel. b Colony of pathogenic isolate grown on PDA at 25 °C for 14 days. cConidia and conidiophores observed by light microscope. d Conidia (arrows) and conidiophores observed by scanning electron microscope (SEM). e Chlamydospores and hyphae with chlamydospores in situ observed by light microscope. f Chlamydospores (arrow) observed by SEM. Symptoms observed 2, 4 and 6 days after wound-inoculation of healthy morels with fungal suspensions are shown in g, h and i, respectively. j Symptoms observed 6 days after inoculation without wounding. No symptoms developed on the ascomata treated with sterile water after wound-inoculation (k) and nonwound-inoculation (l)
• Cytological and molecular studies
There are very few studies look into the molecular level of Morchella, which may be related to the difficulties in completing the whole life cycle of Morchella under laboratory conditions. Gene expression in sclerotia formation of M. conica (not legitimate nomination though) has been investigated (Chen et al., 2014). Zhang et al. (2018) provided several ideal reference genes for future qRT-PCR gene expression analysis of Morchella spp. In their study, thirty-two genes of M. importuna were selected and designed according to transcriptome data and the expression level of these genes were tested under different experimental conditions. Several genes with most stability were selected and evaluated for quantitative real-time PCR (qRT-PCR) normalization in ten different Morchella spp. They also found that the stability of reference genes varied under different experimental conditions, indicating the necessity of identifying specific reference genes for specific conditions.
The nuclear behavior during meiosis and ascosporogenesis on M. importuna was studied using confocal laser scanning microscopy (He et al., 2017). The study suggested that a total of six nuclear divisions typically took place during ascosporogenesis. First two meiotic divisions and subsequent one mitosis gave rise to eight nuclei. Then the nucleus in each ascospore underwent three mitosis and finally six to eight nuclei were formed in each ascospore, which were homokaryons. Two idiomorphs, MAT1-1 and MAT1-2 were identified on M. importuna and population genetic investigation suggested the heterothallic characteristics of three Morchella strains, M. importuna, M. sextelata and Mel-20 (Chai et al., 2017). Relative direct evidence for hybridization fruiting nature of M. importuna was provided by De Novo sequencing and comparative genome analysis of two monospores with a different mating type isolated from M. importuna (Liu et al., 2018). Results indicated independent mating type structures in the two monospores and M. importuna might be a heterothallic fungus.
Recently, He et al. (2018b) first highlighted the involvement of autophagy and apoptosis and lipid accumulation in sclerotia morphogenesis of Morchella importuna (slcerotia initial [SI], sclerotia development [SD], sclerotial maturation [SM]). They compared the untrastructural features of the undifferentiated mycelial stage and three main sclerotial stages by using transmission electron microscopy. The characteristics of autophagy was observed during the SI phase and apoptotic characteristics were found in some cells during the SD phase. Moreover, they found lipid was the energy-rich substance in both hyphae and sclerotia of M. importuna. And Sclerotia had a significantly higher content of lipid than that in hyphae, which is consistent with the hypothesis that sclerotia serve as nutrient storage organs.
• Aging (senescence)
Spawn aging has been a prominent problem in morel cultivation industry. Senescence of M. elata, characterized by slow mycelial growth, pigment production ahead of time and death of hyphal tips, was observed by successive subculturing (He et al., 2015). Microscopy studies revealed the involvement of autophagy, apoptosis and necrosis in senescence of M. elata (He et al., 2015). Nevertheless, the molecular mechanism of these behaviors need further study.
Citations:
Chai H, Chen L, Chen W, et al., 2017. Characterization of mating-type idiomorphs suggests that M. importuna, Mel-20 and M. sextelata are heterothallic. Mycological Progress 16, 743-52
Chen LJ, Chai HM, Chen WM, Huang XQ, Zhao YC, 2014. Gene Expressing Difference in Sclerotial Formation of Morchella conica. Indian J Microbiol 54, 274-83.
He P, Cai Y, Liu S, Han L, Huang L, Liu W, 2015. Morphological and ultrastructural examination of senescence in Morchella elata. Micron 78, 79-84.
He P, Li C, Cai Y, Zhang Y, Bian Y, Liu W, 2018a. First report of pileus rot disease on cultivated Morchella importuna caused by Diploöspora longispora in China. Journal of General Plant Pathology 84, 65-9.
He P, Wang K, Cai Y, et al., 2018b. Involvement of autophagy and apoptosis and lipid accumulation in sclerotial morphogenesis of Morchella importuna. Micron 109, 34-40
He P, Wang K, Cai Y, Liu W, 2017. Live cell confocal laser imaging studies on the nuclear behavior during meiosis and ascosporogenesis in Morchella importuna under artificial cultivation. Micron 101, 108-13.
Liu W, Chen L, Cai Y, Zhang Q, Bian Y, 2018. Opposite polarity monospore genome de Novo sequencing and comparative analysis reveal the possible heterothallic life cycle of Morchella importuna. International journal of molecular sciences 19, 2525.
Zhang Q, Liu W, Cai Y, Lan A, Bian Y, 2018. Validation of Internal Control Genes for Quantitative Real-Time PCR Gene Expression Analysis in Morchella. Molecules 23, 2331.