Lecture - 38 Phytoplasma: General characteristics and Role in causing plant diseases
Phytoplasma:-
Introduction of Mycoplasma:-
> Mycoplasmas are the smallest among the known aerobic prokaryotes.
> They were first discovered by Pasteur in 1843, during his work on the possible causal agent of pleuropneumonia of cattle. Thus they were called pleuro- pneumonia-like organism (PPLO). Pasteur was unable to isolate them in pure culture.
> Later, Nocard and Roux (1898), the French microbiologists, were successful in growing them in pure culture-medium containing serum and confirmed by inoculation and subsequent expression of disease in healthy cattle.
> Mycoplasmas are commonly found in soil, hot spring, sewage water and also in plants and animals including man. Borrel (1910) named these organisms Asterococcus mycoides. Later, in 1929, Nowak placed them under the genus Mycoplasma.
General Characteristics of Mycoplasma:-
> They are unicellular, smallest, non-motile and prokaryotic organisms forming fried egg shaped colonies.
> They are pleomorphic i.e., able to change their shape depending upon culture media.
> They may be rod like, ring like, globoid or filamentous. The filaments are of uniform diameter (100-300 nm) and vary in length from 3 nm to 150 nm.
> Some mycoplasma predominantly assume spherical shape (300-800 nm in diameter).
> They are ultra-filterable i.e., they can pass through bacteria-proof filters.
> They do not possess rigid cell wall.
> The cells are delimited by soft tripple layered lipo-proteinaceous membrane. It is unit membrane about 10 nm thick.
> Within the cytoplasm ribosomes are found scattered in the peripheral zone. These are 14 nm in diameter and resemble with bacteria in sedimentation characteristic of both the nucleoprotein and nucleic acid.
> The ribosomes are 72S type.
> Within the cytoplasm fine fibrillar DNA is present. It is double stranded helix.
> Mycoplasma generally grow more slowly than bacteria.
> They require sterol for their nutrition.
> They are usually resistant to antibiotics like penicillin, cephaloridine, vencomycin etc. which action cell wall.
> They are sensitive to tetracycline.
> They are also killed by temperature of 40-55°C in 15 minutes.
> They do not produce spores.
> Like other prokaryotes, they usually divide by binary fission.
Structure of Mycoplasma:-
> The cell is devoid of cell wall which makes them readily deformable showing irregular and variable shapes.
> They may be ring-like, granular, coccoid, pear-shaped, filamentous, etc.
> The filaments are of two types: unbranched or branched.
> The cells are very small and measure 0.3-0.9 µm in diameter.
> The cells are covered by cytoplasmic (lipoprotein) membrane.
> Cell membrane covers the cytoplasm which contains nucleoplasm like structure and ribosomes.
> The genetic material is composed of DNA and RNA. It is about less than 50%, the amount present in other prokaryotic organisms. The amount of RNA (8%) is more than DNA (4%).
> They are usually non-motile, but some forms show gliding movements.
> They reproduce by vegetative means i.e., by binary fission and budding.
> They are sensitive to antibiotics like chloramphenicol, streptomycin, erythromycin etc., but are insensitive to penicillin, ampicillin etc., due to the absence of cell wall.
Economic importance of Mycoplasma:-
Diseases Caused by Mycoplasma:- Mycoplasmas cause different serious diseases in plants and animals including man.
a. Plant Diseases:-
i. Little leaf disease of brinjal
ii. Bunchy top of papaya
iii. Big bud of tomato
iv. Witches broom of legumes
v. Yellow dwarf of tobacco
vi. Strip disease of sugarcane
vii. Clover dwarf
viii. Cotton vires- cence
b. Human Diseases:-
i. Primary atypical pneumonia (PAP) by Mycoplasma pneumoniae.
ii. Mycoplasma hominis causes pleuropneumonia, prostatitis, inflammations of genitals etc.
iii. Mycoplasma fermentants causes infertility in man.
c. Animal Diseases:-
i. Mycoplasma agalactia causes agalactia of goat and sheep.
ii. Mycoplasma mycoides causes pleuropneumonia of cattle.
iii. M. bovigenitalium causes inflammation of genitals of different animals.
Role in causing plant diseases:-
Phytoplasmas as harmful plant pathogens:-
> Phytoplasmas infect numerous plant species, also including many ornamental plants.
> Examples of ornamental plants in which phytoplasmas have been reported to cause disease include:
i. Aster (yellowing)
ii. Gladiolus (virescence and other symptoms)
iii. Lily (virescence and stunting)
iv. Hydrangea (virescence)
> As a plant pathogen, it is without the scope of this report to fully review the diseases caused by phytoplasmas in ornamentals.
> Plant diseases associated with the presence of phytoplasmas typically exhibit a number of symptoms that are suggestive of disturbances in the normal balance of plant hormones. These symptoms include:
i. Virescence (loss of normal flower colour, green flowers).
ii. Phyllody (development of floral parts into leaf-like structures).
iii. Proliferation of auxilary shoots (side shoots) resulting in ’witches broom’.
iv. Sterility of flowers.
v. Compact growth at the end of stems.
vi. Yellowing.
vii. Phloem necrosis.
viii. Dieback of branches in woody plants.
Transmission of phytoplasmas:-
> The plant to plant spread of phytoplasmas in nature is mediated by phloem-feeding leafhoppers and psyllids (a group of insects related to aphids and leafhoppers).
> Therefore, control of phytoplasma in agriculture or horticulture is highly dependent on the control of vectors, that is of leafhoppers and psyllids.
> In the laboratory, phytoplasmas may also be transmitted from plant to plant by grafting.
> The phytoplasmas will move from infected plants into healthy plant.
> Another possibility when grafting is not possible (for example between two incompatible plant species) is the use of Cuscuta (dodder) which is a plant that is itself a plant parasite. This plant can form an ’infection bridge’ between plants, and the phytoplasma can move through dodder into healthy plants.
> Phytoplasmas are not considered to be seed-transmissible, and are lost during for instance conventional breeding.
Elimination of phytoplasmas:-
> Phytoplasmas can be eliminated from their plant hosts, as they are generally heat labile and are not present in the shoot meristem.
> Furthermore they are sensitive to some antibiotics such as tetracycline.
> Several metods have been applied to clean plant material for phytoplasmas, these include in vitro tissue culture such as shoot tip or micropropagation sometimes in combination with heat or antibiotic treatment.