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Chapter 4
Phytotoxin
- Huang’s chapter 6
- e book
Phytotoxin
 Plant-pathogenic toxins
 Produced by bacteria or fungi
 The toxins may be glycosides, amino acid
derivatives, peptides, terpenoids, sterols,
pyridines, and quinones, etc.
Phytotoxin
 Divided into two categories
Host-specific toxins
Host-nonspecific toxins
Toxins produced by bacteria
 Most of the toxins are nonhost specific.
 Chlorosis-inducing toxins
Tabtoxin – Pseudomonas syringae pv. tabaci (煙草野
火病菌) (pv. stands for pathovar.)
Tabtoxinine-b-lactam
Inhibit glutamine synthetase which converts
glutamic acid to glutamine.
Phaseolotoxin – Pseudomonas syringae pv.
phaseolicola (葉燒病)
A tripeptide toxin
Rhizobitoxine – Rhizobium japonicum
Block methionine biosynthesis pathway
(Continued)
Tabtoxin
 Non-host-specific toxin that inhibit glutamine synthetase.
 Pseudomonas syringae pv. tabaci vs. tobacco, bean, soybean
(tobacco wildfire or halo blight disease, 煙草野火病)
 Dipeptide, tabtoxinine-b-lactam moiety linked to either Lthreonine or serine
 Tabtoxin itself is biologically inactive, but is readily cleaved
by amino-peptidase of bacteria or plants origin to yield the
active moiety tabtoxinine-b-lactam (TbL).
 Accumulation of the TbL in plants results in chlorotic halos
surrounding the bacterial lesions.
 TbL is not essential for pathogenicity.
 The gene (tabA) responsible for tabtoxin production has been
identified by Tn5 mutagenesis. It encode a enzyme involved
in tabtoxin biosynthesis.
Soybean wildfire caused by Pseudomonas syringae
pv. tabaci
Phaseolotoxin
 Produced by Pseudomonas syringae pv. phaseolicola, which
causes halo blight on bean (菜豆葉燒病)
 Phaseolotoxin is an phosphosulfamylornithine-alanine-arginine
tripeptide.
 Soon after the toxin is excreted by the bacterium into the plant,
plant enzymes cleave the tripeptide and release alanine,
arginine, and phosphosulfamylornithine.
Phosphosulfamylornithine is the biologically functional moiety
of phaseolotoxin.
 The toxin affects cells by binding to the active site of and
inactivating the enzyme ornithine carbamoyltransferase, which
normally converts ornithine to citrulline, a precursor of
arginine.
 Phaseolotoxin may act as a disease-resistance suppressor.
Halo blight (Pseudomonas phaseolicola) of beans
Toxins produced by bacteria
 Wilt-inducing bacterial polysacchrides
Extracellular polysaccharidal slime (EPS)
Erwinia sp., Pseudomonas sp. and
Xanthomonas sp.
Lipopolysaccharide (LPS) in Salmonella
typhimurium and Shigella dysenteriae
Amylovorin – Erwinia amylovora (梨火傷病菌)
 Glycopeptide toxins
Corynebacterium sp. , G (+)
 Peptide toxins
Syringomycin – P. syringae
Hexapeptide
Bacterial wilt caused by Erwinia tracheiphila
Sticky strand test on cut stems, with bacterial slime streaming
from xylem tissues of cucurbit (葫蘆).
Relative viscosity and wilt-inducing effect of
culture filtrate of P. solanacearum
Culture filtrate
Highly virulent
Weakly pathogenic
Avirulent
relative viscosity
83
50
51
wilting index
5
0
0
Fire blight (火傷病, Erwinia amylovora) of apple
Devastation by fire blight of a 2-year-old high-density Gala
apple orchard in Michigan after a violent storm. Other orchards
in the path of the storm had similar losses.

Background: fire blight, caused by the bacterium Erwinia
amylovora, are becoming increasingly common in Michigan’s
high-value apple orchards. Orchardists in the west-central
Michigan fruit belt experienced devastating losses after a
severe rainstorm on 31 May 1998 that had winds estimated up
to 140 miles per hour. Two weeks later, as predicted by a
program for forecasting fire blight (MARYBLYT), fire blight
symptoms began to show up on trees that had survived the
storm. Then a hailstorm on 16 June further compounded the
problem by spreading bacteria to fresh injuries. Young trees in
high-density plantings are particularly vulnerable to infection.
Cultivars exhibiting the greatest damage include Gingergold,
Gala, and Jonathan. In early July, E. amylovora began to ooze
from the rootstock of trees on Malling 9 and 26 rootstocks. The
presence of E. amylovora in the ooze was confirmed by
isolation of the bacteria and identification by PCR. In August,
many apparently healthy trees will exhibit discoloration of the
foliage and then collapse and die as the pathogen girdles the
tree.
Toxins produced by fungi
Host-specific toxins
Host-nonspecific toxins
Host-specific toxins
 Microbial metabolites have similar host specificity.
e.g., a plant susceptible to a pathogen is sensitive to
its toxin, whereas a plant resistant to a pathogen is
insensitive to its toxin.
 The virulence of the pathogenic strains is positively
correlated to their capacity to produce the toxin. e.g.,
a highly virulent strains produce more toxin.
 The toxin is able to produce symptom characteristic
of the disease caused by the pathogen.
Host-specific toxins
 Victorin (HV-toxin) – Cochliobolus victoriae
(Helminthosporium victoriae) vs. oat (Leaf blotch, 燕
麥葉枯病)
 T-toxin (HMT toxin) - Cochliobolus heterostrophus
(Helminthosporium maydis) race T vs. corn (southern
corn leaf blight, 玉米葉枯病)
 HS-toxin – Cochliobolus sacchari (Helminthosporium
sacchari) vs. sugarcane (甘蔗眼點病)
 HC-toxin – Cochliobolus carbonum (玉米葉斑病)
(Helminthosporium carbonum) race 1 vs. corn
 AK-toxin – Alternaria kikuchiana vs. pears (梨黑斑病)
 AM-toxin – Alternaria mali vs. apple (蘋果黑點病)
p. 54, Table 3.1 of e book
Leaf blotch (燕麥葉枯病)
Helminthosporium victoriae
(Perfect stage Cochliobolus victoriae)
Victorin (HV-toxin)
 Host specific toxin
 Victorin (HV-toxin) – Helminthosporium victoriae
(Perfect stage Cochliobolus victoriae 燕麥葉枯病) vs.
oat variety Victoria
 A cyclic pentapeptide
 Victorin A, B, C, D (major), E
 Victoria was introduced in 1945. Victoria and its
derivatives contained the Vb gene for resistance to oat
crown rust.
 Oats contain Vb gene are susceptible to victorin.
 Resistance to oat crown rust and susceptibility to the
Helminthosporium victoriae are controlled by the same
pair of alleles.
Victorin (HV-toxin)
 Victorin is an unusual chlorinated peptide
compound that target the Vb gene product on the
host cells.
 Vb gene encodes a victorin receptor (100kD
membrane protein), which is a subunit of the
glycine decarboxylase enzyme that catalyzes the
conversion of two glycine molecules into one serine
molecule.
 This enzyme is essential for photorespiration and
plant cells appear to go through an induced
senescence and programmed cell death with
cleavage of RUBISCO.
(p. 55 of e book, continued)
Victorin (HV-toxin)
 Increase permeability of the plasma membrane to
electrolytes.
 Dramatic changes in transmembrane potential,
increasing respiration and ethylene production
 Victorin is active against sensitive oats at 10pg/ml,
but does not affect resistant oats at concentration
one million fold (1mg/ml) higher.
T-toxin (HM-toxin)
 In the 1960s, corns with the Texas male sterility
cytoplasm trait (T-cms) was bred into much of the
US maize for ease of hybrid production.
 This trait confers male sterility on the maize.
 In 1970, it was found that the corns with this trait
was susceptible to Bipolaris maydis
(Helminthosporium maydis) race T.
 It was then found that the T-cms and T-urf13 is
the same trait.
T-toxin (HM-toxin)
 Host-specific toxin
 Bipolaris maydis (Helminthosporium maydis) race T vs.
corn (southern corn leaf blight,玉米葉枯病)
 Race T appeared in the “corn belt” in 1969. By 1970,
it had spread throughout the corn belt, attacking only
corn that had the Texas male sterile cytoplasm. That is,
corn with the Texas male sterile (Tms) cytoplasm is
susceptible to the T-toxin, whereas corn with normal
cytoplasm is resistant to the toxin.
 The T race contains two genes, a polyketide synthase
(PKS1) and a decarboxylase (DEC1) that are required
for toxin biosynthesis.
(p. 55 of e book, continued)
T-toxin (HM-toxin)
 T-toxins are linear polyketols varying from C35 to
C45 in length.
 T-toxin binds to a 13 kD inner mitochondrial
membrane protein (URF-13), the product of the Turf13 gene, to create a pore on the membrane, cause
leakage of small molecules, and subsequently inhibit
ATP synthesis, resulting in cell death.
 T-toxin does not seem to be necessary for
pathogenicity of H. maydis race T, but it increase the
virulence of the pathogen.
See Figure 3.4 on p.56 of e book
Southern corn leaf blight
Texas male sterile (Tms)
 T-urf13 is a gene in the Tms mitochondrial
genome.
 The gene encodes a membrane bound 13 kD
polypeptide, URF13, which forms a pore in the
membrane.
 URF13 is associated with both diseasesusceptibility and cytoplasmic male sterility.
 HM-toxin binds to the URF13 protein in Tms
mitochondria and inhibits ATP synthesis.
AM-toxin
 Alternaira mali causes Alternaria blotch disease (蘋
果黑點病) on apples.
 Characterized by necrotic spots on leaves, shoots,
and fruits of susceptible apples.
 Cyclic tetradepsipeptide
 0.1 ~ 0.2ng/mL of AM-toxin are toxic to susceptible
apples, whereas resistant apples are affected at a
concentration of 104 ~ 105-fold higher.
Alternaria blotch (蘋果黑點病) on apples
HC-toxin (玉米葉斑病)
 Produced by C. carbonum (northern leaf spot)
 Does not directly cause plant cell death, instead it
suppress plant defense responses.
 The toxin is a cyclic tetrapeptide containing Damino acids and inhibits histone deacetylase in the
plant nucleus, causing hyperacylation of histone
and changes in gene expression.
(p. 55 of e book)
Nonhost-specific toxins
 Toxic substances produced by phytopathogenic
microorganisms can cause disease symptoms on the host plant
as well as on other plants that are not normally attacked by
the pathogen in nature.
 The toxins do not have a role in the establishment of the
pathogen in the host. It is able to induce same characteristics
of the disease symptom.
 Tentoxin – Alternaria tenuis
 Fusicoccin – Fusicoccum amygdali vs. almond and peach
Tabtoxin – Pseudomonas syringae pv. tabaci vs. tobacco,
bean, soybean
 Coronatine – Pseudomonas syringae pv. glycinea,
maculicola, tomato vs. soybean, crucifer, and tomato
 Phaseolotoxin – Pseudomonas syrinage pv. phaseolicola vs.
bean and some legumes
Phytotoxin
 Scheffer and Pringle (1967) group toxins as the
pathogen-produced determinants of disease and
group them into:
Primary determinants of pathogenicity
Secondary determinants of pathogenicity
Primary determinants of pathogenicity
 Primary determinants are those essential for
pathogenicity, including
Victorin (HV-toxin)
HC-toxin
AK-toxin
AM-toxin
(They are all host-specific toxins.)
Secondary determinants of pathogenicity
 Secondary determinants contribute to virulence of
pathogen but do not control its pathogenecity,
including
Tabtoxin
Strains of Pseudomonas syringae pv. tabaci
produce tabtoxin cause the distinctive halo
surrounding a necrotic lesion. Strains that fail
to produce tabtoxin cause necrotic lesions
without halos.
Coronatine
T-toxins (a host-specific toxin)
Coronatine
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Coronatine is produced by a number of P. syringae
pv. glycinea, maculicola, tomato vs. soybean, crucifer,
and tomato
Various pathovars with Tn5 insertion mutation are
still pathogenic.
Coronatine consists of two structural components,
the polyketide coronafacic acid and the amino acid
derivative coronamic acid.
Coronatine causes structure change of chloroplasts
and chlorosis.
Coronatine may suppress the induction of defenserelated genes by plants, allowing greater pathogen
ingress and multiplication.
The biosynthetic genes are clustered in a 32 kb
region of a 90 kb plasmid.
(p.93 of e book)
Syringomycin
 Produced by P. syringae pv. syringae
 Cyclic lipodepsipeptides consisting of a b-hydroxy
acid and nine amino acids
 Four genes are involved in syringomycin
production: syrA for regulation, syrB and syrC for
biosynthesis, and syrD for transport.
Syringomycin
 Elicits Ca+2-dependent callose synthesis in
suspension-cultured cells.
 Inhibit plasma membrane H+-ATPase activity in
maize and storage tissue of sugar beet.
 Increase respiration of succinate and NADH and
the hydrolysis of ATP in isolated maize and pea
mitochondria.
 Causes K+ efflux, stomatal closure, and reduction
in stomatal aperture of plants. => similar to ABA
 Stimulates opening of Ca+2 channels of plasma
membrane
Phytotoxins
 Amino acid-derived and peptide phytotoxins
 Derived from the acetate-mevalonate pathway
 Derived from the acetate-polymavalonate route
 Heterocycles
Amino acid-derived and peptide phytotoxins
 Fusaric acid – Fusarium and Gibberella
 Tenuazonic acid – Alternaria tenuis
 Rhizobitoxine – Bradyrhizobium japonicum and
Pseudomonas andropogonis
 Coronatine – Pseudomonas syringae
 Fumonisins – Fusarium sp.
 AAL-toxin – Alternaria alternata f.sp. lycopersici
 Triticones – Pyrenophora tritici-repentis
 Cytochalasins and pyrichalasins – Phomopsis sp.
Peptide phytotoxins
 Maculosin – Alternaria alternata
 Sirodesmins – Sirodesmium diversum
 Taxtomins – Streptomyces sabies
 Tabtoxins – Pseudomonas syringae pv. tobaci
 Phaseolotoxin – Pseudomonas syringae pv.
phaseolicola
 Tentoxin – Alternaria sp.
 AM-toxin – Alternaria mali
 HC-toxin – Helminthosporium carbonum
 HV-toxin – Helminthosporium victoriae
(p.55, 93 of e book)
Phytotoxins derived from the Acetatemavalonate pathway
 Phytotoxins with structures of terpenoid origin
Foeniculoxin – Phomopsis foeniculi
Eremophilanes – Macrophomina phaseolina
HS-toxin – Bipolaris sacchari (=
Helminthosporium sacchari)
Fusicoccin – Fusarium amygdali
Colletotrichin – Colletotrichum sp.
Phytotoxins derived from the Acetatepolymavalonate pathway
 T-toxin – Helminthosporium maydis
 PM-toxin – Phyllosticta maydis
 AK-toxin – Altenaria kikuchiana
 Cercosporin – Cercospora sp.
Roles of phytotoxins in plant pathogenesis
 As pathogenicity factors in plant-pathogen interactions
 Victorin (HV-toxin)
 HC-toxin
 AK-toxin
 AM-toxin
 As virulence factors in plant-pathogen interactions
 Tabtoxin
 T-toxin
 As disease-resistance suppressors
 Phaseolotoxin
 Coronatine
END