View source for Rhodococcus fascians

{{Short description|Species of bacterium}}
{{Italic title}}
{{Speciesbox
| image=Rhodococcus fascians.jpg
| taxon = Rhodococcus fascians
| authority = (Tilford 1936) Goodfellow 1984
}}

'''''Rhodococcus fascians''''' (known as '''''Corynebacterium fascians''''' until 1984) is a [[Gram positive]] [[bacterium|bacteria]]l  [[Phytopathology|phytopathogen]] that causes [[gall|leafy gall disease]].<ref name="goethals">{{cite journal |vauthors=Goethals K, Vereecke D, Jaziri M, Van Montagu M, Holsters M |title=Leafy gall formation by ''Rhodococcus fascians'' |journal=Annu Rev Phytopathol |volume=39 |pages=27–52 |year=2001 |pmid=11701858 |doi=10.1146/annurev.phyto.39.1.27 }}</ref> ''R. fascians'' is the only phytopathogenic member of the genus ''[[Rhodococcus]]''; its host range includes both [[dicotyledonous]] and [[monocotyledonous]] hosts.  Because it commonly afflicts [[tobacco]] (''[[Nicotiana]]'') plants, it is an agriculturally significant pathogen.

== Physiology and morphology ==
''R. fascians'' is an [[Aerobic organism|aerobic]], [[pleomorphism (microbiology)|pleiomorphic]], [[actinomycete]] that is nonmotile and does not form [[spores]].<ref name="vereeke">{{cite journal  |vauthors=Vereecke D, Cornelis K, Temmerman W, etal |title=Chromosomal Locus That Affects Pathogenicity of Rhodococcus fascians |journal=J. Bacteriol. |volume=184 |issue=4 |pages=1112–20 |date=February 2002 |pmid=11807072 |pmc=134788 |doi=10.1128/jb.184.4.1112-1120.2002 |hdl=1854/LU-322105 }}</ref>  When grown on the surface of an agar plate, colonies are orange in color and appear both smooth or rough.{{cn|date=November 2022}}

==Virulence==
''R. fascians'' can be a pathogen of plants, both angiosperm or gymnosperm. Infected plants show typical symptoms, such as leaf deformation, [[witches broom]] and leaf [[gall]], which development depends on the plant's [[cultivar]], plant's age, and the bacterial strain.<ref name="goethals"/>

Leaf deformation consists of widening of [[parenchyma]] and growth of vascular system, resulting in wrinkling of [[Leaf#Divisions of the lamina (blade)|laminae]] and widening of veins. Leafy gall is a gall originated from a [[bud]] which would not develop under normal conditions.
All effects coming from the infection of ''R. fascians'' do not depend on plant cells [[transformation (genetics)|transformation]] (as for [[Agrobacterium tumefaciens]] or [[Agrobacterium rhizogenes]]), but on expression of virulence-related [[genes]] of bacterium and on the production of compounds that can interfere with normal plant growth and development. During the infection, ''R. fascians'' usually stays outside vegetal tissues, near a junction or cavity of a plant's [[cell walls]], maybe to avoid environmental stresses. Presence of ''R. fascians'' was also observed in intercellular spaces inside tissues (in leaf or galls) and even inside cell walls. Presence of ''R. fascians'' on the infected plant is necessary, not only for the initiation of infection, but also for its maintenance.{{cn|date=November 2022}}

===Virulence genes===
[[Virulence]] in ''R. fascians'' is controlled by genes on a [[plasmid]] (strains lacking that plasmid are not virulent) and on the [[chromosome]]. Using deletion [[mutations]], it was possible to identify three [[locus (genetics)|loci]] on the plasmid: ''fas'', ''att'', and ''hyp'', and one locus on the chromosome, ''vic''.<ref name="vereeke"/><ref name="temmermann"/><ref name="maels">{{cite journal  |vauthors=Maes T, Vereecke D, Ritsema T, etal |title=The att locus of Rhodococcus fascians strain D188 is essential for full virulence on tobacco through the production of an autoregulatory compound |journal=Mol. Microbiol. |volume=42 |issue=1 |pages=13–28 |date=October 2001 |pmid=11679063 |doi=10.1046/j.1365-2958.2001.02615.x |s2cid=11681436 |doi-access=free }}</ref>

The ''fas'' is an [[operon]] made of six genes (orf 1-6) and a regulatory gene, ''fasR''. Because deletions of some ''fas'' genes give a non-virulent [[phenotype]], for fas a main role in virulence was proposed .<ref name="temmermann">{{cite journal |vauthors=Temmerman W, Vereecke D, Dreesen R, Van Montagu M, Holsters M, Goethals K |title=Leafy Gall Formation Is Controlled by fasR, an AraC-Type Regulatory Gene in Rhodococcus fascians |journal=J. Bacteriol. |volume=182 |issue=20 |pages=5832–40 |date=October 2000 |pmid=11004184 |pmc=94707 |doi=10.1128/JB.182.20.5832-5840.2000 }}</ref>  Gene ''fasR'' is an araC-like [[Transcriptional regulators|transcriptional regulator]]. Its transcription can be induced ''in vitro'' in cultures containing certain [[carbon]] sources (such as [[glucose]], [[sucrose]], [[arabinose]], [[glycerol]], [[pyruvate]], [[mannitol]], [[mannose]]) or [[nitrogen]] sources (such as [[histidine]]), and is influenced by culture [[pH]] and [[optical density]].<ref name="temmermann"/> Also, fasR can be induced by gall extract created by virulent strain.
The operon codifies for genes involved in [[cytokinin]] synthesis and degradation (orf 4,5,6), in particular for an isopentenyl transferase, a cytokinin oxidase and a glutation-s transferase.
The orf1,2,3 transcribe for a [[cytochrome]] 450, a ferridoxine containing also a [[pyruvate dehydrogenase]] alfa-like [[structural domain|domain]] and a pyruvate dehydrogenase beta subunit. It was supposed that the first three genes supply energy for the synthesis and degradation of cytokinin, performed by the last three genes of the operon: ''R. fascians'' can actually produce and degrade [[zeatin]] and isopentenil [[adenine]].<ref name="temmermann"/>
The compound cytokin oxidase (orf4) can also create adenine with a reactive nitrogen in position 6, which can react with other lateral chains, to form cytokininn-like compounds, more efficient in inducing plant tissue growth.<ref name="temmermann"/>

The ''att'' is an operon composed of nine genes: ''{{not a typo|attR}}'', a transcriptional regulator, ''{{not a typo|attX}}'', a gene including domains for transmembrane localization (perhaps needed for exportation of compounds made by other ''att'' genes), and several genes ''attA''-''{{not a typo|attH}}''.<ref name="maels"/> Many point and Δ''att'' mutants show an attenuated virulence.<ref name="maels"/>

Gene ''{{not a typo|attR}}'' is a transcriptional factor including a [[helix-turn-helix]] motif. Its transcription is regulated by the same factors that regulate ''fasR'' transcription, but with a higher intensity, suggesting, with the attenuation of virulence in ''att'' mutants, that ''att'' may regulate fas transcription. Transcription of ''att'' operon is regulated with a [[quorum sensing|quorum-sensing]] mechanism: indeed, density of cultures can influence transcription of ''{{not a typo|attR}}'', and leafy gall extracts coming from galls made by ''att'' mutant strains are less effective on transcription of ''{{not a typo|attR}}''.<ref name="maels"/>

Genes ''attA''-''{{not a typo|attH}}'' may be involved in synthesis of compounds needed for transcription of ''{{not a typo|attR}}'' and ''{{not a typo|attX}}''. In fact, ''attA'', ''attD'' & ''{{not a typo|attH}}'' are involved in [[betalactamase]] synthesis, but no traces of those compounds were found in culture supernatants.<ref name="maels"/>

The ''hyp'' codifies for an RNA-helicase; mutants for this gene are hypervirulent. Also, ''hyp'' is involved in post transcriptional control of virulence-related genes, maybe on ''fas'' products.

Operon ''vic'' is an operon made of five genes, located on the bacterial chromosome.<ref name="vereeke"/> The only known gene is ''vicA'', the fourth gene in the operon, whose product is a Mas homologue, a protein needed for the switch from [[citric acid cycle]] to [[glyoxylate cycle]], both for metabolic reasons and to avoid glyoxylate accumulation, which is toxic for the bacteria. Mutations in ''vicA'' reduce virulence due to incapacity of ''R. fascians'' to resist glyoxylate accumulation.<ref name="vereeke"/>

===Induction of transcription in infected plant===
In [[tobacco]], infection of ''R. fascians'' leads to hyperexpression of a cytochrome P450, homologue to a gene involved in inactivation of [[abscisic acid]] in ''[[Arabidopsis thaliana]]'', of a [[gibberellic acid]] oxidase, which inactivates this hormone and its precursors, a [[proline]] dehydrogenase, which has its transcription induced by cytokinin and turns proline into [[glutamic acid]], and a factor involved in [[molybdenum]] cofactor, needed for [[sulfur]], carbon and nitrogen metabolism control and for abscisic acid synthesis.<ref name="simon-mateo">{{cite journal |vauthors=Simon-Mateo C, Depuydt S, De Oliveira Manes CL, Cnudde F, Holsters M, Goethals K, Vereeke D |title=The phytopathogen Rhodococcus fascians breaks apical dominance and activate auxiliary meristems by inducing plant genes involved in hormones metabolism |journal=Molecular Plant Pathology |volume=7 |issue=2 |pages=103–12 |year=2006 |doi= 10.1111/j.1364-3703.2006.00322.x|pmid=20507431}}</ref>

===Role of phytohormones during infection===
All the effects of ''R. fascians'' infection can be attributed to hormone hyperdosage. In particular, most of the effects are connected to [[auxin]] and cytokinin, such as: formation of green islands on leaves, wrinkling of [[Leaf#Divisions of the lamina (blade)|laminae]], bud proliferation, delay of senescence, and inhibition of lateral roots. In fact, ''R. fascians'' can produce itself cytokinin, or cytokinin-like compounds: using ''orf4'' and ''orf5'' in the ''fas'' operon, it can stimulate infected plants to produce cytokinin, and it can produce [[indole-3-acetic acid]] itself, using a pathway starting from tryptophan and passing through production of 3-indol-piruvic acid and 3-indol-acetaldeid.<ref name="vandeputte">{{cite journal  |vauthors=Vandeputte O, Oden S, Mol A, etal |title=Biosynthesis of Auxin by the Gram-Positive Phytopathogen Rhodococcus fascians Is Controlled by Compounds Specific to Infected Plant Tissues |journal=Appl. Environ. Microbiol. |volume=71 |issue=3 |pages=1169–77 |date=March 2005 |pmid=15746315 |pmc=1065166 |doi=10.1128/AEM.71.3.1169-1177.2005 }}</ref> ''R.fascians'' can also degrade cytokinin to influence the cytokinin/auxin ratio.{{cn|date=November 2022}}

Beside cytokinin and auxin, ''R. fascians'' acts on other hormones: in particular, it can block [[abscisic acid]] and [[gibberellic acid]] synthesis in infected plants. Abscisic acid represses growth, so a block of production is needed to allow proliferation of cells in leafy galls. Gibberellic acid controls [[cellular differentiation]], so its block is needed for maintenance of [[meristem]]atic cells and for their proliferation.<ref name="simon-mateo"/>

== Plant diseases ==
''R. fascians'' causes diseases in several host plants including [[List of tobacco diseases|tobacco]], small fruits ([[List of caneberries diseases|caneberries]], [[List of strawberry diseases|strawberries]]) and ornamental plants ([[List of butterfly flower diseases|butterfly flowers]], [[List of primula diseases|''Primula'']], [[List of kalanchoe diseases|kalanchoes]], [[List of impatiens diseases|''Impatiens'']], [[List of geranium diseases|geraniums]], [[List of carnation diseases|carnations]]){{cn|date=November 2022}}

== References ==
{{Reflist|30em}}

==External links==
* [http://bacdive.dsmz.de/index.php?search=10883&submit=Search Type strain of ''Rhodococcus fascians'' at Bac''Dive'' -  the Bacterial Diversity Metadatabase]

{{Taxonbar|from=Q309787}}

[[Category:Mycobacteriales]]
[[Category:Bacterial plant pathogens and diseases]]
[[Category:Tobacco diseases]]
[[Category:Small fruit diseases]]
[[Category:Bacterial strawberry diseases]]
[[Category:Ornamental plant pathogens and diseases]]