Phytoplasmas

Source: ncbi.nlm.nih.gov

Phytoplasmas are obligate intracellular parasites of plant phloem tissues & of the insect vectors that are involved in their plant-to-plant transmission.

Taxonomy :

Super Kingdom: Prokaryote

Kingdom: Monera

Domain: Bacteria

Phylum: Firmicutes ( low-G+ C, Gram-Positive Eubacteria)

Class: Mollicutes

Genus: Candidatus Phytoplasmas

Morphology :

• Phytoplasmas are Mollicutes, bound by a triple-layered membrane.
• Lack of cell wall.
• Pleomorphic or filamentous shape with a diameter of less than 1micro meter and a small genome.
• Cell membrane contains a single immunodominant protein of unknown function that constitutes most of the protein in the membrane.
• Phytoplasmic DNA is distributed throughout the cytoplasm like other prokaryotes.

Host Range :

• Phytoplasmas comprise approximately 30 distinct Claude’s based on 16S rRNA gene sequence analyses of 200 Phytoplasmas.
• They are mostly dependent on insect transmission for their spread and survival.
• Their lifecycle involves replication in insects & plants.
• They infect the insect but are phloem-limited in plants.
• Members of Phytoplasmas asterisk are found in 80 monocot & dicot plant species.

Transmission :

• Phytoplasmas are spread principally by insects of the families Cicadellidae(leaf hoppers), Fulgoridae(planthoppers), and Psyllidae(jumping plant lice).
• These insects feed on the phloem of infected plants ingesting Phytoplasmas & transmitting them to the next plant.
• Phytoplasmas enter the insect the body through stylet, pass through the intestine and then to hemolymph, colonizing in salivary glands.
• Phytoplasmas can also spread through dodders or by vegetative propagation like the grafting of infected plant tissue onto healthy plants.
• Phytoplasmas move within phloem from a source to a sink, passing through the sieve tube element.

Disease symptoms :

 In plants, Phytoplasmas induce symptoms that suggest interference with plant development.

Typical symptoms include :

• witches’ broom (clustering of branches) of developing tissues.
• Phyllody (retrograde metamorphosis of the floral organs to the condition of leaves).
• Virescence (green colouration of non-green flower parts).
• Bolting(growth of elongated stalks).
• Formation of bunchy fibrous secondary roots
• Reddening of leaves and stems.
• Generalized yellowing, decline and stunting of plants.
• Phloem necrosis.

Phytoplasmas can be pathogenic to some insect hosts, but generally do not negatively affect the fitness of their major insect vectors.

Phytoplasmas can increase fecundity & survival of insect vectors and may influence flight behaviour and plant host preference of their insect hosts.

Disease control :

The most common practices are the spraying of various insecticides to control insect vectors and the removal of symptomatic plants. Phytoplasma-resistant cultivars are not available for the vast majority of affected crops.