- Slides: 28
Cell wall Phytoplasma Cytoplasm of sieve tube element MOLLICUTES Fred Gildow and Padmini Herath contributed to this lecture
KINGDOM: PROKARYOTE CLASS: MOLLICUTES 1. Mycoplasmas - humans & animal pathogens 2. Spiroplasmas - spiral shape - culturable - plant pathogens (corn stunt) 3. Phytoplasmas - circular, oval, tubular - plant pathogens
PHYTOPLASMA & SPIROPLASMA Plant Pathogens! Derived from gram + bacteria No cell wall: Mollis Cutis = soft = skin No flagella Plant pathogenic: phloem specific Vectored in Circulative propagative manner Saskia Hogenhout : http: //www. oardc. ohio-state. edu/phytoplasma/
Spiroplasma Phytoplasma • Circular, oval, tubular • Unculturable • Obligate biotroph • Koch’s postulates not completed • • 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 Many Movement tassels small, seedless ears Colonies on agar
PHYTOPLASMAS: THE DISCOVERY Doi, Y, et al. , 1967. Annals of the Phytopathological Society of Japan. 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
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 Amorphous shape Ranging from 70 -1000 nm in diameter. Similar to chloroplast Asexual reproduction: budding Cause over 600 diseases in 700 plant species Peach X In plant and insect (vector) cytoplasm General disease symptoms… Chlorosis and reddening Shortening of internodes (stunting) Loss of apical dominance (witches’ broom) Lethal Palm Yellowing
IMPORTANT PHYTOPLASMAS Aster Yellows Ornamentals and vegetable crops Elm Yellows (Elm Phloem Necrosis) Eastern US - finishing off the elms (DED) Branch-inducing phytoplasma Poinsettia 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: Carrot Periwinkle: Chlorosis, dwarfing Phylloidy: flowers develop as vegetative tissue (Horomone disruption) Witches’ brooms, woolly 2 o roots, stunted and tapered. Bad taste!
ASTER YELLOWS IN VIVO (PLANT HOST) S. T. E. cell 1 Phytoplasma Sieve plate S. T. E. cell 2 Phytoplasma in phloem sieve tube element at sieve plate passing between adjacent cells Phloem necrosis - sieve elements eventually die, blocking carbohydrate translocation in plant.
ASTER YELLOWS Transmitted by: Grafting, budding Aster Leafhopper Circulative Propagative! Stylet Phloem Epidermis Mesophyll
Phytoplasmas hit home… ELM Yellows
ELM YELLOWS (ELM PHLOEM NECROSIS) Candidatus Phytoplasma ulmi Symptoms: Fine, fibrous roots die 1 st Chlorotic leaves (as roots die) Brownish-yellow Phloem sieve cells partially filled w/ callose (hard, gummy carbohydrate) Wintergreen discoloration of phloem odor when cut (American Elm) Maple syrup odor for red elm Defoliation and death (within one year)
Under bark: Necrosis of the phloem
EY TRANSMISSION Vector: white-banded leafhopper Circulative Latent (persistent) -Propagative period (10 -45 days) Phytoplasma must infect salivary glands before transmission Lay EY Nymph eggs in Elm bark (overwinters) not transmitted to eggs Adult Through root grafts Phloem connections www. na. fs. fed. us Pollen/seed transmission unknown
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 DED Double-edged sword
Padmini Herath 26/342 Elms infected WB leafhopper not found Old Main Elm yellows on PSU campus
Poinsettia Euphorbia polcherrima
POINSETTIA Native to tropical Central Am. And Mexico Introduced to US by Joel Robert Poisette in 1825. 1 st US Ambassador to Mexico. In the wild – a 10 ft tree. Extensive breeding: 2 commercially grown types: Free-branching Restricted (strong apical dominance) Developed in 1923 by Paul Ecke in CA Deemed more desirable Perfect bracts Foliage retention
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 Lee et al. 1995. Nature Biotechnology. 15: 178 -182 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!
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.