Botrytis cinerea

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1. Classification

a. Higher order taxa

Eukarya (Domain); Fungi (Kingdom); Ascomycota (Phylum); Leotiomycetes (Class); Helotiales (order); Scelerotiniaceae (family) [1]

b. Species

Botrytis Cinerea [1]

c. Other names

Sclerotinia fuckeliana or Botryotinia fuckeliana [1]

2. Description and significance

Botrytis Cinerea is a fungus that has a primary impact on vine grapes, strawberries and tomatoes, but has the ability to infect over 1400 host plant species [2, 3]. A key aspect of B. cinerea is its ability to present as a pathogenic Grey Rot and a non-pathogenic Noble Rot, used in the development of sweeter wines or berries [4]. B. cinerea is an influential fungus with applications in pathology, farming, viticulture, and culinary arts. Current research is focused on differentiating the two outcomes of B. cinerea infection to be able to have greater confidence in preventing Grey Rot or fostering Noble Rot [4].

3. Genome structure

B. cinerea has 18 chromosomes with an average guanine-cytosine (GC) content of 42.75% and an estimated genome size of 41.2 Mb. The B. cinerea strain B05.10 genome, originally isolated from grapes in California, has 10,701 protein-coding genes and the B. cinerea strain T4 has 10,427 protein-coding genes [5]. Other B. cinerea strains have not yet been sequenced, creating a large gap in knowledge of the strain-specific effects and overall genome properties of B. cinerea. For now, the B. cinerea strain B05.10 is used as the standard genome because it has been repeatedly sequenced with improved accuracy and completion compared to other strains [5]. Many of protein-coding genes found in B. cinerea strain B05.10 genome are pathogenicity-related genes, such as genes involved in reactive oxygen species (ROS) production and peptidase secretion. It is predicted that around 34% of protein-coding genes are extracellular proteins secreted by the endoplasmic reticulum-Golgi apparatus secretion pathways [5]. These pathways include proteases, protein activating host plant hypersensitive response, and oxidative burst enzymes. Environmental conditions, such as nutrients and pH, influence transcription, translation, and secretion levels of protein-coding genes [5].

4. Cell structure

B. cinerea is characterized by a gray/brown mycelium made of stem-like conidiophore filaments that branch off into elliptical seed-like conidia cells [6]. To endure tough conditions like extreme temperatures, moisture, and strong sunlight, B. cinerea creates hardy, black structures called sclerotia, allowing this fungus to survive extended periods of time [6, 7]. These sclerotia are larger in B. cinerea compared to other Botrytis species, measuring about 4 millimeters. When the environment becomes more favorable for growth, these sclerotia can start growing again, forming mycelia and more conidia [8]. In the wild, B. cinerea rarely goes through the sexual part of its life cycle, but this has been achieved in a laboratory setting [9]. This sexual process involves microconidia [10], which when joined with structures in the fungus, triggers the creation of sexual structures called apothecia [9]. Inside these apothecia, tiny sacs called asci with ascospores are formed and situated on the upper surface [8]. The ascospores carry genetic material, similar to a gamete in plants and animals [11]. B. cinerea's ability to do this is controlled by a specific genetic factor called the mating type locus, which has two versions, MAT-1 and MAT-2. In nature, both versions are common, and most of the time, B. cinerea needs to work with the opposite version to make these ascospores [12]. Photograph of the fuzzy gray mycelium of B. cinerea on infected common fig branches. [13]

5. Metabolic processes

B. cinerea is a necrotrophic fungi, such that its metabolic pathways rely on degrading the host cells and using the host’s nutrients for survival [12]. B. cinerea metabolizes organic acids and carbohydrates for carbon sources and amino acids and peptides for nitrogen sources, because these substrates are abundantly found in plant tissue and other hosts. B. cinerea adapts to the acidic host environment through increasing decarboxylase activity [12]. B. cinerea has the ability to cause Grey Rot leading to economic loss of crops or Noble Rot in grape berries leading to highly valued botrytized wine. Noble Rot and Grey Rot involve different metabolic processes due to different upregulated genes [4]. For example, Noble Rot upregulates BcCBH-b gene and Bcsod1 gene that code for proteins involved in colonizing and degrading the berry skin. Other upregulated genes in the Noble Rot assist in producing botrytized wine favorable metabolites such as acetaldehyde, increasing sugar and protein content, and synthesizing antimicrobials. Grey Rot upregulates genes expressing oxidoreductase, such as Bclcc7 gene and Bcbot1 gene, to provide protection against host defense mechanisms [4].

6. Ecology

The environment where B. cinerea is located plays an important role in determining its relationship to a host [4,14]. When the B. cinerea is found on plants in a higher humidity environment, the fungus is more likely to take on the pathogenic form, Grey Rot, which creates a parasitic relationship that benefits the fungus while damaging the host plant or berry [14]. When B. cinerea is found on plants in its non-pathogenic form, Noble Rot, it has a mutualistic relationship with its host by using the berry or plant as a nutrient source and sweetening it at the same time. B. cinerea is found world-wide and the microclimate of the vineyard or farm it is found on determines the presence of Noble Rot or Grey Rot symptoms [4].

7. Pathology

Botrytis fungi are able to form sclerotia that can withstand extreme temperatures and environmental conditions, as well as sexually reproduce once conditions are more stable, making them a persistent pathogen [7]. Chlamydospores are also produced by B. cinerea, with this mechanism being more helpful for surviving short bursts of extreme weather [8]. Microconidia may also help the fungi survive extreme conditions [9,15]. In order to inoculate its host, B. cinerea disperses as conidia via wind or rain, transportation by insects, pollen, and flower petals [16]. Research has shown that high amounts of conidia are the most likely to induce an aggressive infection in a given host, as well as change host symptoms [17]. For B. cinerea, the fungi’s germ tubes form photoappressoria and simple appressoria to adhere to their host [18]. While this is the common infection mechanism for B. cinerea, it may also infect by dispersing its conidia through air, in what is called a dry inoculation [7]. B. cinerea is known for infecting numerous host areas, including styles, carpels, and stamen. B. cinerea also utilizes the fluids of hosts in order to defend themselves from insects [7]. B. cinerea has been shown to successfully penetrate necrotic or wound tissue, epidermal tissue, and natural tissue openings [7]. Botrytis as a genus is able to infect hosts before and after host death, by either forming sclerotia or via a sporulation method on necrotic tissue [5]. B. cinerea has the ability to infect numerous host plants at a time [7]. Most other Botrytis pathogen inoculum is from the crop of interest or remains from a previous crop. B. cinerea is also able to use numerous other hosts as inoculum due to its ability to infect a wide range of plants [16].

8. Pathogenicity

B. cinerea is a model generalist pathogen, attracting scientific interest due to its unique ability to affect a wide range of plant hosts [2]. Individual genetic isolates of B. cinerea still have the ability to infect a broad range of hosts, unlike other generalist pathogens that evolve into host-specific isolates. This adaptability is a key aspect of the pathogen's biology, making it particularly challenging to combat [2]. The pathogenic form of B. cinerea, Grey Rot, is often treated with fungicides, but this does not always prove to be effective due to the prevalence of fungicide resistance. The infection period and effectiveness of treatment of B. cinerea can vary due to its ability to remain latent and seem absent on a host but appear during or after harvest of the infected plant [17].

9. Current research

B. cinerea has been the subject of extensive research to unravel its complex pathology and discover novel control strategies.

a. Genetic Variation and Pathogenesis

Recent research has delved into the genetic variation within B. cinerea, specifically focusing on genes associated with its ability to infect different plant genotypes. Among these, BcPG1 and BcPG2, two genes encoding proteins that degrade the plant cell wall, have been identified as around 10 times more polymorphic than other loci not linked to pathogenesis. This genetic diversity enhances B. cinerea's interactions with plant defense proteins that specifically target polygalacturonases (PGs) [19]. Additionally, certain genetic variations in B. cinerea, especially in the beta-tubulin gene, contribute to its resistance to multiple fungicides and prolific sporulation. This genetic diversity is a driving force behind B. cinerea 's ability to infect over 235 host plants worldwide [2, 20]. In addition, a subset of B. cinerea genes where allelic variation was linked to a difference in the ability to infect wild vs. domesticated tomato genotypes has been discovered [2].

b. Noble Rot's Impact on Grapevines

Noble rot, a unique form of the B. cinerea fungus, has been investigated for its role in reprogramming genes within grapevine berries. Researchers have discovered that noble rot specifically targets genes high in transcription factors, leading to a significant transcriptional reprogramming of the berries [3]. Moreover, the genes responsible for abscisic acid production are repressed during noble rot infection. This repression accelerates the dehydration and withering of grapevine berries, as abscisic acid plays a crucial role in fruit acid production. These findings shed light on the fascinating interplay between noble rot and grapevine physiology [3]. B. cinerea also has a wide range of harvest times, such as early versus late fall, and current research focuses on observing differences in transcriptomic and metabolic profiles for the various harvest times, which will help winemakers identify the ideal harvest times for botrytized wine [21]

c. Chemical Inhibition of B. cinerea

Thymol and carvacrol, two natural compounds, have been explored for their effectiveness in inhibiting B. cinerea growth. Thymol, in particular, demonstrated greater efficacy in damaging the mycelia of the fungus [22]. Through scanning electron microscopy and comprehensive analyses of cellular components, specific cellular damage inflicted on B. cinerea was identified, providing insights into potential strategies for controlling the fungus [22].

10. References

[1] Schoch, C.L., et al. (2020). NCBI Taxonomy: a comprehensive update on curation, resources and tools. Database (Oxford).

[2] Soltis, N.E., Atwell, S., Shi, G., Fordyce, R., Gwinner, R., Gao, D., Shafi, A., & Kliebenstein, D.J. (2019). Interactions of Tomato and Botrytis Cinerea Genetic Diversity: Parsing the Contributions of Host Differentiation, Domestication, and Pathogen Variation. The Plant Cell 31(2), 502–519.

[3] Lovato, A., Zenoni, S., Tornielli, G.B., Colombo, T., Vandelle, E.,& Polverari, A. (2019). Specific molecular interactions between Vitis vinifera and Botrytis Cinerea are required for noble rot development in grape berries. Postharvest Biology and Technology 156(25), 104-150.

[4] Fournier, E., Gladieux, P., & Giraud, T. (2013). The ‘Dr Jekyll and Mr Hyde fungus’: noble rot versus Grey mold symptoms of Botrytis Cinerea on grapes. Evolutionary Applications 6(6), 960-969.

[5] Cheung, N., Tian, L., Liu, X., & Li, X. (2020). The Destructive Fungal Pathogen Botrytis Cinerea—Insights from Genes Studied with Mutant Analysis. Pathogens 9(11):923.

[6] Uysal-Morca , A., Kinay-Teksür, P. & Egercï, Y. (2021).Morphological and phylogenetic identification of Botrytis Cinerea causing blossom blight and fruit rot of sweet cherries in Aegean region, Turkey. J Plant Dis Prot 128, 1051–1060.

[7] Holtz, G., Coertze, S., & Williamson, B. (2004). The ecology of Botrytis on plant surfaces. Botrytis: biology, pathology and control (pp 9–24).

[8] Dewey, F. M., & Grant-Downton, R. (2015). Botrytis: The Fungus, the Pathogen and its Management in Agricultural Systems. Springer.

[9] Urbasch, I. (1983). On the genesis and germination of chlamydospores of Botrytis Cinerea. Journal of Phytopathology 108:54–60.

[10] Fukumori, Y., Nakajima, M., & Akuts, K. (2004). Microconidia act the role as spermatia in the sexual reproduction of Botrytis Cinerea. Journal of General Plant Pathology 70:256–260.

[11] Neiman AM. (2005) Ascospore formation in the yeast Saccharomyces cerevisiae. Microbiol Mol Biol Rev. Dec;69(4):565-84

[12] Faretra, F., Antonacci, E., & Pollastro, S. (1988). Sexual behaviour and mating system of Botryotinia fuckeliana, teleomorph of Botrytis Cinerea. Microbiology 134:2543–2550.

[13] Nelson, S. (2017). Common fig (Ficus carica): Gray mold caused by Botrytis Cinerea. photograph.

[14] Otto, M., Geml, J., Hegyi, A.I., Hegyi-Kaló, J., Pierneef, R., Pogány, M., Kun, J., Gyenesei, A., & Váczy, K.Z. (2022). Botrytis Cinerea expression profile and metabolism differs between noble and grey rot of grapes, Food Microbiology Vol 106.

[15] Zheng, L., Campbell, M., Murphy, J., Lam, S., & Xu, J.R. (2000). The BMP1 gene is essential for pathogenicity in the Grey mold fungus Botrytis Cinerea. Molecular Plant-Microbe Interactions 13(7), 724-732.

[16] Pearson, M.N. & Bailey, A.M. (2013). Chapter Nine - Viruses of Botrytis. Advances in Virus Research, Academic Press 86(249-272).

[17] Gonzalez-Dominguez, E., Caffi, T., Ciliberti, N., & Rossi, V. 2015. A Mechanistic Model of Botrytis Cinerea on Grapevines That Includes Weather, Vine Growth Stage, and the Main Infection Pathways. PLoS One 10(10): e0140444.

[18] Barnes, S., & Shaw, M. (2002). Factors affecting symptom production by latent Botrytis Cinerea in Primula × polyantha. Plant Pathology 51(6), 746–754.

[19] Rowe, H. C., & Kliebenstein, D. J. (2007). Elevated genetic variation within virulence-associated Botrytis Cinerea polygalacturonase loci. Molecular Plant-Microbe Interactions 20(9),1126–1137.

[20] Polat, I., Baysal, Ö., Mercati, F., Gümrükcü, E., Sülü, G., Kitapci, A., Aranti, F., & Carimi, F. (2018). Characterization of Botrytis Cinerea isolates collected on pepper in Southern Turkey using molecular markers, fungicide resistance genes, and virulence assay. Infection, Genetics, and Evolution 60,151–159.

[21] Wang, H.C., Li, L.C., Cai, B., Cai, L.T., Chen, X.J., Yu, Z.H., & Zhang, C.Q. (2018). Metabolic Phenotype Characterization of Botrytis Cinerea, the Causal Agent of Grey Mold. Frontiers Microbiology 9:470.

[22] Zhang, J., Ma, S., Du, S., Chen, S., & Helong, S. (2019). Antifungal activity of thymol and carvacrol against postharvest pathogens Botrytis Cinerea. Journal of Food Science and Technology 56(5), 2611-2620.

Edited by Madison Marano, Swastika Sharma, Victoria Kennedy, Zoe Ireland, and Grace Sewell, students of Jennifer Bhatnagar for BI 311 General Microbiology, 2023, Boston University.