View source for Conidium

{{short description|Asexual, non-motile spore of a fungus}}
[[File:Conidium.png|thumb|Conidia on conidiophores]]
[[File:Chain of conidia of an Alternaria sp. fungus PHIL 3963 lores.jpg|thumb|Chain of conidia of ''[[Alternaria]]'']]
[[File:Seiridium canker 100814w.JPG|thumb|Conidiomata of [[Cypress canker]] (probably ''[[Seiridium]] cardinale'') erupting on a ''[[Thuja]]'' twig]]

A '''conidium''' ({{IPAc-en|k|ə|ˈ|n|ɪ|d|i|ə|m|,_|k|oʊ|-}} {{respell|kə|NID|ee|əm|,_|koh|-}}; {{plural form}}: '''conidia'''), sometimes termed an '''asexual [[chlamydospore]]''' or '''chlamydoconidium''' ({{plural form}}: '''chlamydoconidia'''),<ref>Jansonius, D.C., Gregor, Me., 1996. Palynology: principles and applications. American association of stratigraphic palynologists foundation.{{page needed|date=February 2015}}</ref> is an [[Asexual reproduction|asexual]],<ref name="pmid11377860">{{cite journal |doi=10.1111/j.1574-6968.2001.tb10667.x|pmid=11377860|title=The molecular mechanisms of conidial germination|journal=FEMS Microbiology Letters|volume=199|issue=2|pages=153–60|year=2001|last1=Osherov|first1=Nir|last2=May|first2=Gregory S|doi-access=free}}</ref> non-[[motility|motile]] [[spore]] of a [[fungus]]. The word ''conidium'' comes from the [[Ancient Greek]] word for [[dust]], {{wikt-lang|grc|κόνις}} (''{{grc-transl|κόνις}}'').<ref>{{cite Collins Dictionary|conidium}}</ref> They are also called '''mitospores''' due to the way they are generated through the cellular process of [[mitosis]].{{citation needed|date=May 2018}} They are produced exogenously. The two new [[haploid]] cells are genetically identical to the haploid parent, and can develop into new organisms if conditions are favorable, and serve in [[biological dispersal]].

Asexual reproduction in [[ascomycete]]s (the phylum [[Ascomycota]]) is by the formation of conidia, which are borne on specialized stalks called '''conidiophores'''. The [[Morphology (biology)|morphology]] of these specialized conidiophores is often distinctive between species and, before the development of molecular techniques at the end of the 20th century, was widely used for identification of (''e.g.'' ''[[Metarhizium#Species|Metarhizium]]'') species.

The terms '''microconidia''' and '''macroconidia''' are sometimes used.<ref name="pmid15020411">{{cite journal |doi=10.1534/genetics.166.1.113|pmid=15020411|pmc=1470687|title=REN1 is Required for Development of Microconidia and Macroconidia, but Not of Chlamydospores, in the Plant Pathogenic Fungus Fusarium oxysporum|journal=Genetics|volume=166|issue=1|pages=113–24|year=2004|last1=Ohara|first1=T.|last2=Inoue|first2=I|last3=Namiki|first3=F|last4=Kunoh|first4=H|last5=Tsuge|first5=T}}</ref>

==Conidiogenesis==
There are two main types of conidium development:<ref name="pmid2682248">{{cite journal |doi=10.1007/BF00443056|pmid=2682248|title=Problems in application of the terms 'blastic' And 'thallic' To modes of conidiogenesis in some onygenalean fungi|journal=Mycopathologia|volume=106|issue=3|pages=155–61|year=1989|last1=Sigler|first1=Lynne|s2cid=8218393}}</ref>
* ''Blastic'' conidiogenesis, where the spore is already evident before it separates from the conidiogenic [[hypha]] which is giving rise to it, and
* ''Thallic'' conidiogenesis, where first a cross-wall appears and thus the created cell develops into a spore.

===Conidia germination===

A conidium may form [[germ tube]]s (germination tubes) and/or [[conidial anastomosis tubes]] (CATs) in specific conditions. These two are some of the specialized hyphae that are formed by fungal conidia. The germ tubes will grow to form the hyphae and fungal [[mycelia]]. The conidial anastomosis tubes are morphologically and physiologically distinct from germ tubes. After conidia are induced to form conidial anastomosis tubes, they grow homing toward each other, and they fuse. Once fusion happens, the nuclei can pass through fused CATs. These are events of fungal vegetative growth and not sexual reproduction. Fusion between these cells seems to be important for some fungi during early stages of colony establishment. The production of these cells has been suggested to occur in 73 different species of fungi.<ref>{{cite journal |doi=10.1038/ng1839|pmid=16832356|title=Emergence of a new disease as a result of interspecific virulence gene transfer|journal=Nature Genetics|volume=38|issue=8|pages=953–6|year=2006|last1=Friesen|first1=Timothy L|last2=Stukenbrock|first2=Eva H|last3=Liu|first3=Zhaohui|last4=Meinhardt|first4=Steven|last5=Ling|first5=Hua|last6=Faris|first6=Justin D|last7=Rasmussen|first7=Jack B|last8=Solomon|first8=Peter S|last9=McDonald|first9=Bruce A|last10=Oliver|first10=Richard P|s2cid=6349264}}</ref><ref>{{cite journal |doi=10.1016/j.femsle.2005.06.048|pmid=16040203|title=Conidial anastomosis tubes in filamentous fungi|journal=FEMS Microbiology Letters|volume=249|issue=2|pages=191–8|year=2005|last1=Gabriela Roca|first1=M.|last2=Read|first2=Nick D.|last3=Wheals|first3=Alan E.|doi-access=free}}</ref>

=== Germination in ''Aspergillus'' ===
As evidenced by recent literature, conidia germination of ''Aspergillus'', a common mold, specifically is of interest. ''Aspergillus'' is not only a familiar fungus found across various different settings in the world, but it poses a danger for immunocompromised individuals, as inhaled ''Aspergillus'' conidia could germinate inside the respiratory tract and cause aspergillosis, a form of pulmonary infection, and continual developments of aspergillosis such as new risk groups and the resistance against antifungal drugs.

==== Stages of Germination: Dormancy ====
Germination in ''Aspergillus'' follows a sequence of three different stages: dormancy, isotropic growth, and polarized growth. The dormant conidia are able to germinate even after an year of remaining at room temperature, due to their resilient intracellular and extracellular characteristics, which enable them to undergo harsh conditions like dehydration, variation in osmotic pressure, oxidation, and temperature, and change in UV exposure and acidity levels. These abilities of the dormant conidia are dictated by a few central regulatory proteins, which are the main drivers of the conidia and conidiophore formation. One of these proteins, the developmental regulatory protein wetA, has been found to be particularly essential; in wetA-defective mutants have reduced tolerance to external factors mentioned above, and exhibit weak synthesization of the conidial cell wall. In addition to these central regulators, some notable groups of genes/proteins include other regulatory proteins like the velvet regulator proteins, which contribute to fungal growth, and other molecules that target specific unfavorable intra and extracellular conditions, like heat shock proteins. <ref name=":0">{{Cite journal |last1=Baltussen |first1=Tim J. H. |last2=Zoll |first2=Jan |last3=Verweij |first3=Paul E. |last4=Melchers |first4=Willem J. G. |date=2020-02-19 |title=Molecular Mechanisms of Conidial Germination in Aspergillus spp |journal=Microbiology and Molecular Biology Reviews |language=en |volume=84 |issue=1 |doi=10.1128/MMBR.00049-19 |issn=1092-2172 |pmc=6903801 |pmid=31801804}}</ref><ref name=":1">{{Citation |last=Osherov |first=Nir |title=Conidial Germination in Aspergillus fumigatus |date=2014-04-09 |work=Aspergillus fumigatus and Aspergillosis |pages=131–142 |editor-last=Latgé |editor-first=Jean-Paul |url=http://doi.wiley.com/10.1128/9781555815523.ch10 |access-date=2024-05-11 |place=Washington, DC, USA |publisher=ASM Press |language=en |doi=10.1128/9781555815523.ch10 |isbn=978-1-68367-138-1 |editor2-last=Steinbach |editor2-first=William J.|url-access=subscription }}</ref>

==== Stages of Germination: Isotropic and Polarized Growth ====
The phases following dormancy include isotropic growth, in which increased intracellular osmotic pressure and water uptake causes swelling of the conidia and increased cellular diameter, and polarized growth, in which the swelling from isotropic growth directs the growth to one side of the cell, and leads to the formation of a germ tube. First, however, the conidia must go through the stage of breaking dormancy. In some species of Aspergillus, dormancy is broken when the dormant conidia is introduced to a carbon source in the presence of water and air, while in other species, the mere presence of glucose is enough to trigger it. The dense outer layer of the dormant conidia is shed and the growth of the hyphae cells begins, which has a significantly different composition compared to the dormant conidia cell. Breaking of dormancy involves transcription, but not translation; protein synthesis inhibitors prevent isotropic growth, while DNA and RNA synthesis inhibitors do not, and the start of breaking of dormancy is accompanied by and increase in transcripts for genes for biosynthesis of proteins, and immediate protein synthesis. Following the expansion of the cell via isotropic growth, studies have observed many new proteins emerging from the processes in the breaking of dormancy and transcripts associated with remodeling of the cell wall, suggesting that remodeling of the cell wall is a central process during isotropic growth. In the polarized growth stage, upregulated and overexpressed proteins and transcripts included ones involved in synthesis of chitin (a major component of the fungal cell wall), mitosis and DNA processing, remodeling of cell morphology, and ones in germ tube formation pertaining to infection and virulence factors. <ref name=":0" /><ref name=":1" /> 

===Structures for release of conidia===
Conidiogenesis is an important mechanism of spread of plant pathogens.  In some cases, specialized macroscopic fruiting structures perhaps 1 mm or so in diameter containing masses of conidia are formed under the skin of the host plant and then erupt through the surface, allowing the spores to be distributed by wind and rain.  One of these structures is called a '''conidioma''' (plural: '''conidiomata''').<ref>{{cite web|url=http://www.forestpathology.org/fungi.html#Asexual%20forms%20of%20Ascomycota |access-date=20 February 2023|year=2023|website=Forest Pathology|title=Fungi|author=James J. Worrall}}</ref><ref>{{cite journal |doi=10.1094/PHI-I-2001-0219-01|title=Illustrated Glossary of Plant Pathology|journal=The Plant Health Instructor|year=2001|last1=d'Arcy|first1=C.J.|last2=Eastburn|first2=D.M.|last3=Schumann|first3=G.L.|doi-access=}}</ref>

Two important types of conidiomata, distinguished by their form, are:
*'''pycnidia''' (singular: '''[[pycnidium]]'''), which are flask-shaped, and
*'''acervuli''' (singular: '''acervulus'''), which have a simpler cushion-like form.

Pycnidial conidiomata or ''pycnidia'' form in the fungal tissue itself, and are shaped like a bulging vase. The conidia are released through a small opening at the apex, the '''ostiole'''.

Acervular conidiomata, or ''acervuli'', are cushion-like structures that form within the tissues of a host organism:
*''subcuticular'', lying under the outer layer of the plant (the [[Plant cuticle|cuticle]]),
*''intraepidermal'', inside the outer cell layer (the [[Epidermis (botany)|epidermis]]),
*''subepidermal'', under the epidermis, or deeper inside the host.

Mostly they develop a flat layer of relatively short conidiophores which then produce masses of spores.  The increasing pressure leads to the splitting of the epidermis and cuticle and allows release of the conidia from the tissue.

==Health issues==
Conidia are always present in the air, but levels fluctuate from day to day and with the seasons. An average person inhales at least 40 conidia per hour.<ref>Humans inhale ~10<sup>3</sup> to 10<sup>10</sup> mold conidia (i.e., vegetative spores) daily. - {{cite journal |last1=Shlezinger |first1=Neta |last2=Irmer |first2=Henriette |last3=Dhingra |first3=Sourabh |last4=Beattie |first4=Sarah R. |last5=Cramer |first5=Robert A. |last6=Braus |first6=Gerhard H. |last7=Sharon |first7=Amir |last8=Hohl |first8=Tobias M. |title=Sterilizing immunity in the lung relies on targeting fungal apoptosis-like programmed cell death |journal=Science |date=8 Sep 2017 |volume=357 |issue=6355 |pages=1037–1041 |doi=10.1126/science.aan0365|pmid=28883073 |pmc=5628051 |bibcode=2017Sci...357.1037S |doi-access=free }}</ref>
Exposure to conidia from certain species, such as those of ''[[Cryptostroma corticale]]'', is known to cause [[hypersensitivity pneumonitis]], an occupational hazard for forest workers and paper mill employees.<ref>{{cite web|url=https://forestpathology.org/canker/sooty-bark-maple/|title=Sooty-Bark Disease of Maple|website=Forest Pathology|last=Worrall|first=James J.|year=2023|access-date=18 February 2023}}</ref><ref>{{cite journal|last1=Braun|first1=Markus|last2=Klingelhöfer|first2=Doris|last3=Groneberg|first3=David A.|title=Sooty bark disease of maples: the risk for hypersensitivity pneumonitis by fungal spores not only for woodman|journal=Journal of Occupational Medicine and Toxicology|volume=16|year=2021|issue=1 |page=2 |id=2|doi=10.1186/s12995-021-00292-5|pmid=33478566 |pmc=7819180 |doi-access=free}}</ref>

Conidia are often the method by which some normally harmless but heat-tolerating (thermotolerant), common fungi establish infection in certain types of severely [[immunocompromised]] patients (usually acute [[leukemia]] patients on induction chemotherapy, [[AIDS]] patients with superimposed B-cell lymphoma, [[bone marrow transplantation]] patients (taking immunosuppressants), or major organ transplant patients with [[graft versus host disease]]). Their immune system is not strong enough to fight off the fungus, and it may, for example, colonise the lung, resulting in a [[pulmonary]] infection.<ref>Of particular concern is the high rate of mortality associated with invasive fungal infections, which often exceeds 50% despite the availability of several antifungal drugs. - {{cite journal |last1=Brown |first1=Gordon D. |last2=Denning |first2=David W. |last3=Gow |first3=Neil A. R. |last4=Levitz |first4=Stuart M. |last5=Netea |first5=Mihai G. |last6=White |first6=Theodore C. |title=Hidden Killers: Human Fungal Infections |journal=Sci Transl Med |date=19 December 2012 |volume=4 |issue=165 165rv13 |pages=165rv13 |doi=10.1126/scitranslmed.3004404|pmid=23253612 |s2cid=3157271 }}</ref> Especially with species of the ''Aspergillus'' genus, germination in the respiratory tract can lead to aspergillosis, which is quite common, can vary in severity, and has shown signs of developing new risk groups and antifungal drug resistance. <ref name=":0" />

==See also==
{{Portal|Fungi}}
*[[Arthroconidium]]
*[[Ascocarp]]
*[[Basidiocarp]]
*[[Budding]]
*[[Gemma (botany)|Gemma]]
*[[Phialide]]

==References==
{{Reflist|2}}

==External links==
* {{Cite NSRW|short=x|wstitle=Conidia}}

{{Fungus structure}}

[[Category:Fungal morphology and anatomy]]