fungi as plant pathogens - youngstown state universitycrcooper01.people.ysu.edu/fungi ch14...

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Fungi As Plant Pathogens

Anubhav Vinayak Lisa Tofil

Heather Rice

Introduction

Definitions:

•  Parasite – organism that gains all or part of its nutritional requirements from the living tissues of another organism – the host.

•  Pathogen – organism that causes disease.

A pathogen almost always a parasite but not all parasites cause diseases.

Pathogens

•  Pathogens – Necrotrophic – (kill the host tissues usually by

directly invading them or by producing toxins or degradative enzymes and then feed on the tissues that they kill)

– Biotrophic – (feed on living host tissues by producing special nutrient-absorbing structures that tap into host’s tissues)

Pathogens

Pathogen

Necrotroph Biotroph

Host-specialized Non-specialized Host-specialized

Seed rots and seedling pathogens

•  Athelia rolfsii – One of the most devastating seedling pathogens

in the warmer parts of the world where rainfall is seasonal

Source: http://www.agroatlas.ru/content/diseases/Olee/Olee_Athelia_rolfsii/Olee_Athelia_rolfsii.jpg

Seedling pathogens (cont)

•  Rhizoctonia solani – Common seedling pathogen that attacks the

basal stem tissues of plants – Examples

Source:http://www.ctahr.hawaii.edu/nelsons/Misc/1_potato_sore_shin_rhizoctonia_1.jpg

Source: http://www.turffiles.ncsu.edu/diseaseID/diseasedetail.aspx?disease=RHISL

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Pectic Enzymes

•  Degrade the middle lamella (the cementing layer)

•  Three major types – Pectin methyl esterase (PME) – Pectic lyase (PL) – Polygalacturonase (PG)

Source: http://www.ccrc.uga.edu

Host-Specialized Necrotrophic Pathogens

•  Necrotrophic pathogens: –  Invade normal healthy tissues of plants – Overcome the defense mechanisms of their hosts

•  Physical barriers •  Fungitoxic chemicals

Botrytis cinerea Source: www.php.wur.nI/UK/Research/

Tissue Degrading Enzymes

•  Necrotrophic fungi and nongrass plants: F. oxysporum and V. albo-atrum –  Pectic enzymes

•  Gramineae (grass family): R. cerealis, F. culmorum, and T. yallundae –  Arabinase, xylanase, and glucanase enzymes –  Small amounts of pectic enzymes

•  Able to degrade typical wall components of their hosts

Plant Defenses Against Necrotrophic Pathogens

•  Hypersensitive response: –  Upon attack the infected cells die and accumulate

fungitoxic chemicals

•  Physical barriers: –  Leaf cuticle

•  Cutinase enzyme allows the fungus to penetrate through the cuticle

–  Papilla •  Thickening of the cell wall

at the point of invasion

Source: staff.jccc.net/PDECELL/plants/transpiration.html


•  Physical barriers: – Oxidative burst

•  Produces hydrogen peroxide at or near plant cell surface •  Hydrogen peroxide strengthens cell wall against attack

–  Cross-links plant cell wall proteins


•  Chemical defenses: – Phytoanticipins

•  Converted to an active compound in response to infection

•  S. lycopersici: –  Can overcome phytoanticipins –  Produce enzymes:

  Avenacinase and α-tomatinase

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•  Chemical Defenses: – Phytoalexins

•  Low molecular weight •  Protect plants against general attack by fungi •  Produced in response to infection

–  Activated by an oxidative burst   Hydrogen peroxide is initial signal

Systemic Acquired and Induced Resistance

•  Systemic acquired resistance (SAR): –  Increased resistance following infection

•  Mediated by salicylic acid

•  Induced systemic resistance (ISR): –  Increased resistance following colonization of

plants roots by rhizosphere bacteria •  P. fluorescens •  Mediated by jasmonic acid and ethylene

Vascular Wilt Pathogens

•  Three main fungi: 1.  V. albo-atrum 2.  V. dahliae 3.  F. oxysporum

•  More than 80 strains (“special forms”) •  Strains survive by producing

chlamydospores •  Symptoms:

–  Yellowing and death of leaves –  Collapse of leaf stalks

•  Epinasty

Source: www.anbg.gov.au/cpbr/program/sc/path_vari.htm

Vascular Wilt Pathogens

•  Mode of infection: –  Spores enter water-conducting xylem vessels

•  Vectors •  Wounds •  Young root tips

–  Carried upwards in the water flow –  Become trapped on perforated end walls

•  Produce additional spores –  Remain restricted to the xylem vessels until the

plant dies

Plant Response to Vascular Wilt Fungi

•  Tyloses- (balloon-like swelling)

•  Pectic gels •  Release of phenolic

compounds •  Accumulation of

phytoalexins

Source: Figure 14.14, Deacon (2006) Fungal Biology, 4th ed.

Smut Fungi

•  Members of the Basidiomycota •  Over 100 species:

– Loose smut of wheat: U. nuda – Maize smut: U. maydis – Stinking smut of wheat: T. caries

Source: www.uky.edu/.../wheat/disease/Ismut/Ismut1.htm

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Smut Fungi •  Mode of infection:

– Germinating spores penetrate developing ovary

– Resulting seeds germinate, colonize, and enter flowering spike

– At maturity flowering spike produces a mass of black spores

Source: www.uky.edu/.../wheat/disease/Ismut/Ismut1.htm

Fungal Endophytes and Their Toxins

•  Significance –  produce secondary metabolites useful for

commercial use

–  induce abiotic tolerance

–  insect antifeedant Source: http://cc.oulu.fi/~ampir/kotisivu-3.html

Fungal Endophytes and Their Toxins Cont.

•  “Clavicipitaceous endophytes” – Common species is called Claviceps purpurea – These type of endophytes are found in several

pasteur grasses in US, Europe and New Zealand

•  Lolium (ryegrass) •  Festuca (fescue) •  Dactylis glomerata (cocksfoot)


•  Dilemmas of endophytes – Produce mycotoxins – Mycotoxins harm grazing animals –  endophytes increase stress tolerance –  endophytes prevent insect damage over other

grasses


•  Types of mycotoxins and effects –  loliterm B- saturated aminopyrrolizidine alkaloids--

“ryegrass staggers” disease of grazing sheep and cattle. Staggers are loss of coordination and/or tremors occur at times

–  Egrovaline- causes “fesuce foot” and “fescue toxicosis” where the animal can experience hyperthermia, weight loss, reduced pregnancy rates, decreased milk production, and for horses birth defects or abortions

–  Peramine- tripeptide that is repellent and toxic to insects, but not mammals


Sources:

http://www.biology.ed.ac.uk/research/groups/jdeacon/FungalBiology/chap14_3.htm#Fig14.18

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Phytophthora Diseases

•  They are not fungi but are part of Oomycota •  Phytophthora contains more than 50 species •  Common known Phytophthora disease was the potatoe

blight •  Causes

–  Root rot –  Cankers-develop near base of trees –  Collar rots and crown rots- develop near base of steams of woody

herbaceous plants –  Aerial blights –  Fruit rots

Phytophthora Diseases Cont.

Sources:

http://www.ent.iastate.edu/images/plantpath/soybean/rootrot/root_rot.jpg

http://hyg.ipm.illinois.edu/photos/canker.jpg

Sources Cont. http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=1434

http://aggie-horticulture.tamu.edu/publications/tomatoproblemsolver/ripe/images/photos/sour_rot.jpg


•  Phytophthora infestans –  Heterothallic fungi –  Originated in Central Mexico, both mating types

present there (A1 and A2) –  Moved over to Europe by 1840’s causing Potatoe

Blight –  Only A1 was dominantly present outside Central

Mexico, but in 1980 began to spread


•  Phytophthora infestans Cont. –  Symptoms

•  Black spreading lesions on foliage •  Cool humid conditions allow black lesions to produce

sporangiophores that emerge from leaf stomata •  Potato tubers become infected later (spread of motile zoospores) •  Tubers rot then destroyed by secondary bacteria invaders •  May also be spread by wind born sporangia or splashed sporangia

onto leaves

•  Sources: –  http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/blight.htm ( fig 14.19a )


•  Phytophthora ramorum: sudden oak death –  First found in Germany and Netherlands in 1993 –  Described as P. ramorum in 2001 –  Now found in coastal fog belt of northern California

and southern Oregon –  European mating type is A1 –  North American mating type is A2 –  Separate origin of sources

Phytophthora Diseases Cont. •  Phytophthora ramorum: sudden oak death cont.

–  Plants affected and symptoms •  Rhododendron branches, Vibrurnum bushes, ornamental

Rhododendron and Vibrurnum (Europe), and oaks-- “live and tan oaks” (US)

•  Produces lesions on leaves and terminal shoots of shrubs (shoot dieback)

•  Cankers at base of trees (destroys cambium) •  Cracked bark •  Seepage of dark, viscous sap from bark near base of tree

(gummosis) •  Sudden death •  Slow progressive building of infection under bark

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Phytophthora Diseases Cont. •  Phytophthora ramorum: sudden oak death

cont.

Sources:

http://www.biology.ed.ac.uk/research/groups/jdeacon/FungalBiology/pramorum.htm (Fig 14.20 a-e)

Biotrophic Plant Pathogens

•  Characterized by an extended nutritional relationship with living host cells as opposed to necrotrophic pathogens which kills plant tissues

•  Success depends upon –  ability to avoid eliciting host cell death –  securing a continuous nutrient supply from

living host tissues

Biotrophic Plant Pathogens Cont.

•  Example Fulvia (Cladosporium) fulvum – One strain only feeds from the host if it avoids

inducing hypersensitive response – Governed by avirulence genes of the pathogen

and corresponding resistance genes in host •  gene for gene relationship is common for biotrophic

plant pathogens

– Specific type of biotroph covered is Haustorial biotrophs


Source:



•  Rust Fungi – Black stem rust of wheat (Puccinia graminis)

•  Life cycle fig 14.22

Sources:


Biotrophic Plant Pathogens Cont. •  Powdery Mildew Fungi

–  Part of Ascomycota cause problems throughout all seasons, especially when dry and hot and if not treated with fungicides

– Common types •  powdery mildew of roses (Sphaerotheca pannosa) •  powdery mildew of gooseberries (S. mors-uvae) •  powdery mildew of hawthorn bushes (Podosphaera

clandestina)

– This species undergoes multiple cycles of infection in one season like rust fungi

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Biotrophic Plant Pathogens Cont. •  Downy mildews

– Part of Oomycota –  haustorial biotroph act like rust fungi and

powdery mildew fungi, but need humid conditions for infection

– Examples •  Blemia lactucae of lettuce •  Plasmopara viticola on grape vines •  Pseudoperonospora humuli on hops

Sources:

http://www.hort.cornell.edu/department/facilities/lihrec/vegpath/photos/downymildew_lettuce.htm

Question 1

•  What structure do pectic enzymes degrade in plant cells? A.  Primary wall B.  Middle lamella C.  Secondary wall D.  Plasmodesmata

Question 2

•  What is the name of the signal molecule that mediates systemic acquired resistance? A.  Jasmonic acid B.  Xylanase C.  Ethylene D.  Salicylic acid

Question 3

•  Which of the following is not a defense mechanism used by plants to prevent invasion by necrotrophic pathogens? A.  Phytoanticipins B.  Cutinase enzyme C.  Oxidative burst D.  Phytoalexins

Question 4

•  What type of fungi has multiple life cycles including uredospore, teliospores, basidiospores, and aeciospores? A.  Powdery Mildew Fungi B.  Rust Fungi C. Blemia lactucae D. Phytophthora ramorum

Question 5

•  Which type of fungi has two types found in the US (A1 and A2) A. Phytophthora infestans B. Phytophthora ramorum C. Blemia lactucae D. Pseudoperonospora humuli

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