''Nigrospora sphaerica'' is an airborne filamentous fungus in the phylum
Ascomycota
Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...
. It is found in soil, air, and plants as a leaf
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
. It can occur as an
endophyte
An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; h ...
where it produces antiviral and antifungal secondary metabolites. Sporulation of ''N. sphaerica'' causes its initial white coloured colonies to rapidly turn black. ''N. sphaerica'' is often confused with the closely related species ''N. oryzae'' due to their morphological similarities.
History
''N. sphaerica'' was first identified by E. W. Mason in 1927. In 1913, S. F. Ashby and E. F. Shepherd isolated fungal cultures from banana plants and
sugarcane
Sugarcane or sugar cane is a species of (often hybrid) tall, Perennial plant, perennial grass (in the genus ''Saccharum'', tribe Andropogoneae) that is used for sugar Sugar industry, production. The plants are 2–6 m (6–20 ft) tall with ...
, respectively, which were classified under the genus ''Nigrospora'' due to its morphology. Mason studied these cultures and noticed the persistent appearance of two distinct mean spore sizes. The persistence of the division in spore size led to the classification, by Mason, of the larger spore isolates as ''N. sphaerica'', and the smaller isolates as ''N. oryzae''. Since its classification in 1927, it has been under the class
Sordariomycetes
Sordariomycetes is a class of fungi in the subdivision Pezizomycotina (Ascomycota), consisting of 28 orders, 90 families, 1344 genera. Sordariomycetes is from the Latin sordes (filth) because some species grow in animal feces, though growth habit ...
.
Growth and morphology
''N. sphaerica'' colonies grow rapidly and appear hairy or woolly. The conidiophores are short and clustered surfacing from
mycelium
Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrate (biology), substrates. A typical single ...
. They appear translucent in colour and have an average range of 8-11μm in diameter. The conidiophores are often straight stalks or slightly curved.
Conidia
A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...
grow from the tips of the translucent conidiophores. The conidia are brownish black, oblate spheroid, and single celled. On average they range from 16-18μm in diameter. The initial white translucent looking colony of ''N. sphaerica'' turns brown/black due to mass sporulation of conidia from the conidiophores. In laboratories, ''N. sphaerica'' is grown on
''N. sphaerica'' is commonly found in air, soil, various plants, and some cereal grains. It is rarely found in indoor environments. ''N. sphaerica'' has been identified in many areas around the world, however it is most prevalent in tropical and subtropical countries.
A study shows ''N. sphaerica'' to be the most abundant airborne fungal species found in various urban sites in
Singapore
Singapore (), officially the Republic of Singapore, is a sovereign island country and city-state in maritime Southeast Asia. It lies about one degree of latitude () north of the equator, off the southern tip of the Malay Peninsula, borde ...
. Air samples were collected using an RCS microbial air sampler. Fungal spores trapped on the agar strips were developed and counted. They were then cultured into isolates allowing for identification by morphology. Results showed ''N. sphaerica'' with the highest spore counts at ground levels and low altitudes around 40m.
During asexual reproduction ''N. sphaerica'' releases spores known as conidia. The conidia are ejected out forcefully at maximum horizontal distances of 6.7 cm, and 2 cm vertically. Discharge of spores occurs in all directions. The mechanism for projection relies on the conidiophore consisting of a flask-shaped support cell that bears the conidium. Liquid from the support cell squirts through the supporting cell projecting the spore outwards. This characteristic of forcible spore discharge is rarely seen in
hyphomycetes
Hyphomycetes are a form classification of fungi, part of what has often been referred to as fungi imperfecti, Deuteromycota, or anamorphic fungi. Hyphomycetes lack closed fruit bodies, and are often referred to as moulds (or molds). Most hyphom ...
. ''N. sphaerica'' requires moisture to release spores into the air, therefore accumulation begins around 2:00 a.m. with peak time of abundance occurring around 10:00 a.m. Spore count rapidly decreases after 10:00 a.m. and remains low throughout the day.
Plant pathogenicity
Decaying plants is one of the most common places where ''N. sphaerica'' is found. Many studies around the world found ''N. sphaerica'' as a leaf pathogen. ''N. sphaerica'' was isolated from various plants displaying leaf spots. These reported cases reveal newly identified plant hosts for the pathogen ''N. sphaerica'' that have been validated through
Vaccinium corymbosum
''Vaccinium corymbosum'', the northern highbush blueberry, is a North American species of blueberry which has become a food crop of significant economic importance. It is native to eastern Canada and the eastern and southern United States, from ...
''), licorice (''
Glycyrrhiza glabra
Liquorice (British English) or licorice (American English) ( ; also ) is the common name of ''Glycyrrhiza glabra'', a flowering plant of the bean family Fabaceae, from the root of which a sweet, aromatic flavouring can be extracted.
The liqu ...
''), and ''
Wisteria sinensis
''Wisteria sinensis'', commonly known as the Chinese wisteria, is a species of flowering plant in the pea family, native to China, in the provinces of Guangxi, Guizhou, Hebei, Henan, Hubei, Shaanxi, and Yunnan. Growing tall, it is a deciduous v ...
'' (Chinese Wisteria). Initial lesions resemble small red spots around 2–5 mm particularly near the tips and edges of leaves, eventually resulting in complete defoliation. The fungus also causes a blight disease of the commercial tea plant, ''
Camellia sinensis
''Camellia sinensis'' is a species of evergreen shrub or small tree in the flowering plant family Theaceae. Its leaves and leaf buds are used to produce the popular beverage, tea. Common names include tea plant, tea shrub, and tea tree (not to ...
''. Symptoms of
blight
Blight refers to a specific symptom affecting plants in response to infection by a pathogenic organism.
Description
Blight is a rapid and complete chlorosis, browning, then death of plant tissues such as leaves, branches, twigs, or floral organ ...
was observed in commercial tea estates in
Darjeeling
Darjeeling (, , ) is a town and municipality in the northernmost region of the Indian state of West Bengal. Located in the Eastern Himalayas, it has an average elevation of . To the west of Darjeeling lies the easternmost province of Nepal, ...
, India. The disease affected plants of all ages, being especially pronounced in younger plants. Fungal colonies displayed an initial white colour that eventually turned gray/brown. Based on these morphological characteristics, ''N. sphaerica'' was identified as the fungal pathogen. Inoculation of the pathogen using conidial suspension spray, and re-isolation of ''N. sphaerica'' satisfied Koch’s postulates.
rRNA
Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosoma ...
sequence comparison of the ITS region confirmed ''N. sphaerica'' identity. Cases of leaf spot disease of kiwi fruit (''
Actinidia deliciosa
''Actinidia deliciosa'', the fuzzy kiwifruit, is a fruiting vine native to Southern China. Other species of '' Actinidia'' are also found in China and range east to Japan and north into southern areas of Russian Far East. This species grows nat ...
'') have been reported from orchards in
Huangshan
Huangshan (),Bernstein, pp. 125–127. literally meaning the Yellow Mountain(s), is a mountain range in southern Anhui Province in eastern China. It was originally called “Yishan”, and it was renamed because of a legend that Emperor Xuanyu ...
, Anhui Provence, China. Infected leaves browned and defoliated. Conidia morphology and culture properties suggested ''N. sphaerica'' as the etiological agent, later confirmed by Koch’s postulates and ITS identification.
Human pathogenicity
Often the common response to ''N. sphaerica'' in humans is hay fever or asthma. ''N. sphaerica'' is not widely considered a true human pathogen, however there are various reported cases of ''Nigrospora'' species in human eye and skin infections. Of those, there have only been a handful of reported cases of ''N. sphaerica'' infection in human.
One specific case study identified ''N. sphaerica'' as the cause of an
onychomycosis
Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. Symptoms may include white or yellow nail discoloration, thickening of the nail, and separation of the nail from the nail bed. Toenails or fingernails may be affected ...
case in a 21-year-old man. Onychomycosis is a fungal infection of the nail. Fungal spores found in the body of the nail resembled the characteristic morphology of ''N. sphaerica''. DNA sequence analysis further confirmed the identity.
Another case found ''N. sphaerica'' isolated from a corneal ulcer. A woman in south India was diagnosed with a fungal corneal ulcer after being hit in the eye from a cow’s tail. Analysis of corneal scrapings showed presence of hyphae elements suggesting cause of ulcer from a fungal pathogen. Isolated cultures were grown and examined. Conidia and colony characteristics of the culture led to identification of ''N. sphaerica'' as the fungal pathogen. It was hypothesized that this special case of fungal corneal ulcer was caused by transfer of spores to the patients eye from contamination with soil (a common habitat of the fungus) or other matter from the cow’s tail.
Secondary metabolites
Although ''N. sphaerica'' is often considered as a pathogen, it can also act as an endophyte depending on its host. Various studies have identified novel metabolites isolated from ''N. sphaerica''. Some of these metabolites act as
phytotoxin
Phytotoxins are substances that are poisonous or toxic to the growth of plants. Phytotoxic substances may result from human activity, as with herbicides, or they may be produced by plants, by microorganisms, or by naturally occurring chemical react ...
s, while others contain antiviral or antifungal properties. The purpose of the production of many of these metabolites by the fungus are not fully understood or still unknown and is an area that needs to be further studied.
Aphidicolin
Aphidicolin is a tetracyclic diterpene antibiotic isolated from the fungus '' Cephalosporum aphidicola'' with antiviral and antimitotic properties. Aphidicolin is a reversible inhibitor of eukaryotic nuclear DNA replication. It blocks the cell ...
is a mycotoxin originally known to be produced by the fungus, ''Cephalosporium aphidicola''. This antiviral compound was isolated in mycelium culture filtrate of ''N. sphaerica''.
Epoxyexserophilone is a metabolite similar to the phytotoxin, exserohilone. Fermentation of ''N. sphaerica'' led to the production of epoxyexserophilone. Etiolated wheat coleoptile bioassay indicated that the compound is biologically inactive, and ineffective against both
gram-positive
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
Gram-positive bacte ...
and
gram-negative bacteria
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
.
Nigrosporolide is a 14-membered lactone produced by ''N. sphaerica''. It is structurally related to the phytotoxic metabolite, seiricuprolide, which is produced by the fungus, ''Seiridium cupressi''. The compound is shown to fully inhibit growth of etiolated wheat coleoptiles, at concentrations of 10−3M.
Phomalactone (5,6-dihydro-5-hydroxy-6-prop-2-enyl-2H-pyran-2-one) is found to be produced by ''N. sphaerica''. It inhibits mycelial growth of plant pathogenic fungi, ''Phytophthora infestans''. The metabolite also inhibits sporangium and
zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves.
Diversity Flagella types
Zoospores may possess one or mo ...
germination of both ''P. infestans'' and ''Phytophthora capsici''. The study also shows that the metabolite reduces progression of late blight disease in tomatoes caused by ''P. infestans''.