MOLLICUTES

Fred Gildow and Padmini Herath contributed to this lecture

Cell wall

Cytoplasm ofsieve tube element

Phytoplasma

1. Mycoplasmas

- humans & animal pathogens

2. Spiroplasmas

- spiral shape

- culturable

- plant pathogens

(corn stunt)

3. Phytoplasmas

- circular, oval, tubular

- plant pathogens

KINGDOM: PROKARYOTECLASS: MOLLICUTES

PHYTOPLASMA & SPIROPLASMA

Saskia Hogenhout : http://www.oardc.ohio-state.edu/phytoplasma/

Plant Pathogens!

Derived from gram + bacteria

No cell wall:

Mollis = soft

Cutis = skin

No flagella

Plant pathogenic: phloem specific

Vectored in Circulative propagative manner

Phytoplasma

•Circular, oval, tubular•Unculturable•Obligate biotroph•Koch’s postulates not completed

Spiroplasma

•Helical,spiral•Culturalable•Facultative saprophyte•Koch’s postulates performed. •Motile – corkscrew action.

SPIROPLASMA EXAMPLE

Corn Stunt Disease

Southern US, Central and

South America

Yellow streaks on young

leaves

Older leaves turn purple

Stunting – reduced

distance between nodes

Sterile tassels

Many small, seedless ears

Movement

Colonies on agar

CORN STUNT

PHYTOPLASMAS: THE DISCOVERY

Studied Mulberry dwarf disease

Years of work – no biological agent discovered

EM found no viruses as expected

Veterinarian colleague noted structures similar to mycoplasma

Accidental discovery - cross disciplinary advantage

Concluded mulberry dwarf caused by:

Mycoplasma-Like Organism (MLO)

Similar structures then seen associated with Aster

yellows disease

Doi, Y, et al., 1967. Annals of the Phytopathological Society of Japan.

Koch’s Postulates Never Completed. Phytoplasma association proven by…

Pleomorphic, membrane-bound cells occurred only in phloem sieve elements of diseased plants, not in healthy plants

Morphology was similar to mycoplasma infecting animals

Tetracycline causes symptom remission and disappearance of phytoplasma cells (MLO)

Penicillin had no effect Penicillin inhibits bacterial wall formation (outer layer)

PHYTOPLASMA CHARACTERISTICS 75+ distinct phytoplasmas

Cause over 600 diseases in 700 plant species

Amorphous shape

Ranging from 70-1000 nm in diameter.

Similar to chloroplast

Asexual reproduction: budding

In plant and insect (vector) cytoplasm

General disease symptoms…

Chlorosis and reddening

Shortening of internodes (stunting)

Loss of apical dominance (witches’ broom)

Peach X

Lethal Palm Yellowing

IMPORTANT PHYTOPLASMAS

Aster YellowsOrnamentals and vegetable crops

Elm Yellows (Elm Phloem Necrosis)Eastern US - finishing off the elms (DED)

Branch-inducing phytoplasmaPoinsettia

Ash Yellows (Ash decline)USA- forest and nursery epidemics

ASTER YELLOWS Host range: 200+ dicots, over 40 plant

families Vegetables, Flowers, Ornamentals, Weeds

Overwinter in dandelion, thistle

Symptoms:

Witches’ brooms, woolly 2o roots,

stunted and tapered.Bad taste!

Periwinkle:Chlorosis, dwarfing

Phylloidy: flowers develop as vegetative tissue (Horomone disruption)

Carrot

ASTER YELLOWS IN VIVO (PLANT HOST)

Sieve plate

S.T.E. cell 1

S.T.E. cell 2

Phytoplasma

Phytoplasma in phloem sieve tube element at sieve plate passing between adjacent cells

Phloem necrosis - sieve elements eventually die, blocking carbohydrate translocation in plant.

Phloem

StyletEpidermis

Mesophyll

ASTER YELLOWS

Transmitted by: Grafting, budding Aster Leafhopper

CirculativePropagative

!

Phytoplasmas hit home…ELM Yellows

ELM YELLOWS (ELM PHLOEM NECROSIS)

Candidatus Phytoplasma ulmi

Symptoms:

Fine, fibrous roots die 1st

Chlorotic leaves (as roots die)

Brownish-yellow discoloration of phloem

Phloem sieve cells partially filled w/ callose (hard, gummy carbohydrate)

Wintergreen odor when cut (American Elm)

Maple syrup odor for red elm

Defoliation and death (within one year)

ELM YELLOWS

Under bark: Necrosis of the phloem

Vector: white-banded leafhopper Circulative (persistent) -Propagative

Latent period (10-45 days) Phytoplasma must infect salivary

glands before transmission

Lay eggs in Elm bark (overwinters)

EY not transmitted to eggs

Through root grafts Phloem connections

Pollen/seed transmission unknown

EY TRANSMISSION

www.na.fs.fed.us

Nymph

Adult

Control of Elm Yellows: Slow epidemic rate (r)

Tetracycline Injections

Antibiotic puts disease in remission

But EY Elms will die, cannot be “saved” Tetracycline is produced by Streptomyces sp.

(Brand names: Achromycin, Sumycin)

Removal of infected trees:

Reliable ID system developed at Penn State

Continuous scouting

Break up root grafts

Insecticides to control vector… efficiency?

European elms more resistant to EY

But susceptible to DEDDouble-edged

sword

Old Main

Elm yellows on PSU campus

Padmini Herath26/342 Elms infected

WB leafhopper not found

Poinsettia

Euphorbia polcherrima

POINSETTIA

Native to tropical Central Am. And Mexico Introduced to US by Joel Robert Poisette in

1825. 1st US Ambassador to Mexico.

In the wild – a 10ft tree.

Extensive breeding: 2 commercially grown types: Free-branching

Developed in 1923 by Paul Ecke in CA

Deemed more desirablePerfect bracts

Foliage retention

Restricted (strong apical dominance)

WHAT CAUSES BRANCH INDUCTION?

Free-branching (FB) characteristic disappeared following heat treatment/ meristem tissue culture…

These treatments are used in breeding to eliminate pathogens (such as viruses)

Branching restored when treated plants were grafted onto FB rootstock.

Originally thought poinsettia mosiac virus was cause…

No, it was found in both restricted and FB types

ELISA confirmed no virus in FB plants

FREE BRANCHING… LOOK FAMILIAR?

Restricted Free Branching Aster Yellows in carrot

Proof of pathogenicity

Transmitted phytoplasma from FB poinsettia to periwinkle. Connected vascular tissues using dodder as a bridge.

Then, transmitted phytoplasma into to Restricted poinsettia from periwinkle. Observed branching-induction!

Lee et al. 1995. Nature Biotechnology. 15: 178-182

POINSETTIA BRANCH-INDUCING PHYTOPLASMA

Free-branching is a symptom!

Not economically detrimental

PBIP not found in other plants in nature.

Related to phytoplasma that causes Peach X

disease.

Insect vector not known.

Poinsettias can also be infected

with Peach X and Aster Yellows

Undesirable!

FOOD FOR THOUGHT… Phytoplasma in poinsettia is desirable Color-breaking tulips

Potyvirus Are these poinsettias/tulips diseased? Are these organisms beneficial?

Symbionts or pathogens? Friend or foe?

IN-CLASS EXERCISE:PHYTOPLASMAS IN THE NEWS

Read your article.

Discuss with your group:

Was article factual?

Info on phytoplasmas correct/informative?

Did you find any scientific errors?

What info should be added to make the piece more helpful to the public, informative or correct?

If you were contacted by a media outlet, as a phytoplasma expert, what would you add to the article?

Present your ideas to the class.