''Rhodococcus fascians'' (known as ''Corynebacterium fascians'' until 1984) is a
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 bact ...
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
l
phytopathogen
Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...
that causes
leafy gall disease.
''R. fascians'' is the only phytopathogenic member of the genus ''
Rhodococcus
''Rhodococcus'' is a genus of aerobic, nonsporulating, nonmotile Gram-positive bacteria closely related to ''Mycobacterium'' and ''Corynebacterium''. While a few species are pathogenic, most are benign, and have been found to thrive in a broad ...
''; its host range includes both
dicotyledonous
The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, t ...
and
monocotyledonous
Monocotyledons (), commonly referred to as monocots, ( Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of ...
hosts. Because it commonly afflicts
tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
(''
Nicotiana
''Nicotiana'' () is a genus of herbaceous plants and shrubs in the Family (biology), family Solanaceae, that is Native plant, indigenous to the Americas, Australia, Southwestern Africa and the South Pacific. Various ''Nicotiana'' species, common ...
'') plants, it is an agriculturally significant pathogen.
Physiology and morphology
''R. fascians'' is an
aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cellu ...
,
pleiomorphic,
actinomycete
The Actinomycetales is an order of Actinomycetota. A member of the order is often called an actinomycete. Actinomycetales are generally gram-positive and anaerobic and have mycelia in a filamentous and branching growth pattern. Some actinomycete ...
that is nonmotile and does not form
spores
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
.
When grown on the surface of an agar plate, colonies are orange in color and appear both smooth or rough.
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
Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants, fungi, or animals. Plant galls are abnormal outgrowths of plant tissues, similar to be ...
, which development depends on the plant's
cultivar
A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture, ...
, plant's age, and the bacterial strain.
Leaf deformation consists of widening of
parenchyma
Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms.
Etymology
The term ''parenchyma'' is New Latin from the word π ...
and growth of vascular system, resulting in wrinkling of
laminae and widening of veins. Leafy gall is a gall originated from a
bud
In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be spec ...
which would not develop under normal conditions.
All effects coming from the infection of ''R. fascians'' do not depend on plant cells
transformation
Transformation may refer to:
Science and mathematics
In biology and medicine
* Metamorphosis, the biological process of changing physical form after birth or hatching
* Malignant transformation, the process of cells becoming cancerous
* Trans ...
(as for
Agrobacterium tumefaciens
''Agrobacterium radiobacter'' (more commonly known as ''Agrobacterium tumefaciens'') is the causal agent of crown gall disease (the formation of tumours) in over 140 species of eudicots. It is a rod-shaped, Gram-negative soil bacterium. Sympto ...
or
Agrobacterium rhizogenes
''Rhizobium rhizogenes'' (formerly ''Agrobacterium rhizogenes'') is a Gram-negative soil bacterium that produces hairy root disease in dicotyledonous plants. ''R. rhizogenes'' induces the formation of proliferative multiple-branched adventitious ...
), but on expression of virulence-related
genes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
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.
Virulence genes
Virulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...
in ''R. fascians'' is controlled by genes on a
plasmid
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...
(strains lacking that plasmid are not virulent) and on the
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
. Using deletion
mutations
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
, it was possible to identify three
loci on the plasmid: ''fas'', ''att'', and ''hyp'', and one locus on the chromosome, ''vic''.
The ''fas'' is an
operon
In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...
made of six genes (orf 1-6) and a regulatory gene, ''fasR''. Because deletions of some ''fas'' genes give a non-virulent
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
, for fas a main role in virulence was proposed .
Gene ''fasR'' is an araC-like
transcriptional regulator
In molecular biology and genetics, transcriptional regulation is the means by which a cell regulates the conversion of DNA to RNA ( transcription), thereby orchestrating gene activity. A single gene can be regulated in a range of ways, from a ...
. Its transcription can be induced ''in vitro'' in cultures containing certain
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
sources (such as
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
,
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
,
arabinose
Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group.
For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structurally ...
,
glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
,
pyruvate
Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.
Pyruvic aci ...
,
mannitol
Mannitol is a type of sugar alcohol used as a sweetener and medication. It is used as a low calorie sweetener as it is poorly absorbed by the intestines. As a medication, it is used to decrease pressure in the eyes, as in glaucoma, and to lower ...
,
mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylation ...
) or
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
sources (such as
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...
), and is influenced by culture
pH and
optical density.
Also, fasR can be induced by gall extract created by virulent strain.
The operon codifies for genes involved in
cytokinin
Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and lea ...
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
Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bin ...
450, a ferridoxine containing also a
pyruvate dehydrogenase
Pyruvate dehydrogenase is an enzyme that catalyzes the reaction of pyruvate and a lipoamide to give the acetylated dihydrolipoamide and carbon dioxide. The conversion requires the coenzyme thiamine pyrophosphate.
Pyruvate dehydrogenase is us ...
alfa-like
domain
Domain may refer to:
Mathematics
*Domain of a function, the set of input values for which the (total) function is defined
**Domain of definition of a partial function
**Natural domain of a partial function
**Domain of holomorphy of a function
* Do ...
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
Zeatin is a cytokinin derived from adenine, which occurs in the form of a ''cis''- and a ''trans''- isomer and conjugates. Zeatin was discovered in immature corn kernels from the genus '' Zea''. It promotes growth of lateral buds and when sprayed ...
and isopentenil
adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...
.
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.
The ''att'' is an operon composed of nine genes: ', a transcriptional regulator, ', a gene including domains for transmembrane localization (perhaps needed for exportation of compounds made by other ''att'' genes), and several genes ''attA''-'.
Many point and Δ''att'' mutants show an attenuated virulence.
Gene ' is a transcriptional factor including a
helix-turn-helix
Helix-turn-helix is a DNA-binding protein (DBP). The helix-turn-helix (HTH) is a major structural motif capable of binding DNA. Each monomer incorporates two α helices, joined by a short strand of amino acids, that bind to the major groove of D ...
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
In biology, quorum sensing or quorum signalling (QS) is the ability to detect and respond to cell population density by gene regulation. As one example, QS enables bacteria to restrict the expression of specific genes to the high cell densities at ...
mechanism: indeed, density of cultures can influence transcription of ', and leafy gall extracts coming from galls made by ''att'' mutant strains are less effective on transcription of '.
Genes ''attA''-' may be involved in synthesis of compounds needed for transcription of ' and '. In fact, ''attA'', ''attD'' & ' are involved in
betalactamase synthesis, but no traces of those compounds were found in culture supernatants.
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.
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
The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins ...
to
glyoxylate cycle
The glyoxylate cycle, a variation of the tricarboxylic acid cycle, is an anabolic pathway occurring in plants, bacteria, protists, and fungi. The glyoxylate cycle centers on the conversion of acetyl-CoA to succinate for the synthesis of carbohydrat ...
, 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.
Induction of transcription in infected plant
In
tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
, infection of ''R. fascians'' leads to hyperexpression of a cytochrome P450, homologue to a gene involved in inactivation of
abscisic acid
Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental s ...
in ''
Arabidopsis thaliana
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land.
A winter a ...
'', of a
gibberellic acid
Gibberellic acid (also called gibberellin A3, GA, and GA3) is a hormone found in plants and fungi. Its chemical formula is C19H22O6. When purified, it is a white to pale-yellow solid.
Plants in their normal state produce large amounts of GA3. It ...
oxidase, which inactivates this hormone and its precursors, a
proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
dehydrogenase, which has its transcription induced by cytokinin and turns proline into
glutamic acid
Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can synt ...
, and a factor involved in
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
cofactor, needed for
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
, carbon and nitrogen metabolism control and for abscisic acid synthesis.
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
Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essenti ...
and cytokinin, such as: formation of green islands on leaves, wrinkling of
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
Indole-3-acetic acid (IAA, 3-IAA) is the most common naturally occurring plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, con ...
itself, using a pathway starting from tryptophan and passing through production of 3-indol-piruvic acid and 3-indol-acetaldeid.
''R.fascians'' can also degrade cytokinin to influence the cytokinin/auxin ratio.
Beside cytokinin and auxin, ''R. fascians'' acts on other hormones: in particular, it can block
abscisic acid
Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental s ...
and
gibberellic acid
Gibberellic acid (also called gibberellin A3, GA, and GA3) is a hormone found in plants and fungi. Its chemical formula is C19H22O6. When purified, it is a white to pale-yellow solid.
Plants in their normal state produce large amounts of GA3. It ...
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
Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular ...
, so its block is needed for maintenance of
meristem
The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
atic cells and for their proliferation.
Plant diseases
''R. fascians'' causes diseases in several host plants including
tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
, small fruits (
caneberries
''Rubus'' is a large and diverse genus of flowering plants in the rose family, Rosaceae, subfamily Rosoideae, with over 1,350 species.
Raspberries, blackberries, and dewberries are common, widely distributed members of the genus. Most of thes ...
,
strawberries
The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown hybrid species of the genus '' Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The fruit is widely ap ...
) and ornamental plants (
butterfly flowers,
''Primula'',
kalanchoes,
''Impatiens'',
geraniums
''Pelargonium'' () is a genus of flowering plants that includes about 280 species of perennials, succulents, and shrubs, commonly called geraniums, pelargoniums, or storksbills. '' Geranium'' is also the botanical name and common name of a separ ...
,
carnations
''Dianthus caryophyllus'' (), commonly known as the carnation or clove pink, is a species of ''Dianthus''. It is likely native to the Mediterranean region but its exact range is unknown due to extensive cultivation for the last 2,000 years.Med ...
)
References
External links
Type strain of ''Rhodococcus fascians'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Taxonbar, from=Q309787
Mycobacteriales
Bacterial plant pathogens and diseases
Tobacco diseases
Small fruit diseases
Bacterial strawberry diseases
Ornamental plant pathogens and diseases