1. Chapter 4 Phytotoxin
- Huang’s chapter 6 - e book

2. Phytotoxin Plant-pathogenic toxins Produced by bacteria or fungi
The toxins may be glycosides, amino acid derivatives, peptides, terpenoids, sterols, pyridines, and quinones, etc.

3. Phytotoxin Divided into two categories Host-specific toxins
Host-nonspecific toxins

4. 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)

5. 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 L-threonine 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.

6. Soybean wildfire caused by Pseudomonas syringae pv. tabaci

7. 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.

8. Halo blight (Pseudomonas phaseolicola) of beans

9. 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

10. Bacterial wilt caused by Erwinia tracheiphila
Sticky strand test on cut stems, with bacterial slime streaming from xylem tissues of cucurbit (葫蘆).

11. Relative viscosity and wilt-inducing effect of culture filtrate of P
Relative viscosity and wilt-inducing effect of culture filtrate of P. solanacearum
Culture filtrate relative viscosity wilting index
Highly virulent
Weakly pathogenic
Avirulent

12. 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.

13. 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.

14. Toxins produced by fungi
Host-specific toxins
Host-nonspecific toxins

15. 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.

16. 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

17. Leaf blotch (燕麥葉枯病) Helminthosporium victoriae
(Perfect stage Cochliobolus victoriae)

18. 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 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.

19. 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)

20. 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.

21. 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.

22. 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 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)

23. 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 T-urf13 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

24. Southern corn leaf blight

25. 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 disease-susceptibility and cytoplasmic male sterility.
HM-toxin binds to the URF13 protein in Tms mitochondria and inhibits ATP synthesis.

26. 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.

27. Alternaria blotch (蘋果黑點病) on apples

28. 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 D-amino acids and inhibits histone deacetylase in the plant nucleus, causing hyperacylation of histone and changes in gene expression.
(p. 55 of e book)

29. 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

30. 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

31. 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.)

32. 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)

33. Coronatine
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 defense-related 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)

34. 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.

35. 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

36. Phytotoxins Amino acid-derived and peptide phytotoxins
Derived from the acetate-mevalonate pathway
Derived from the acetate-polymavalonate route
Heterocycles

37. 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.

38. 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)

39. Phytotoxins derived from the Acetate-mavalonate 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.

40. Phytotoxins derived from the Acetate-polymavalonate pathway
T-toxin – Helminthosporium maydis
PM-toxin – Phyllosticta maydis
AK-toxin – Altenaria kikuchiana
Cercosporin – Cercospora sp.

41. 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

42. END