Mollicutes
Bacteria that lack cell walls
Features
Lack cell walls around their cell membrane
Makes them resistant to many antibiotics which target cell wall synthesis
Very small (0.1-0.3 micrometers)
Smallest known cell in biology
Parasites of plants and animals; some are saprotrophic
Phytoplasma, Spiroplasma, and Mycoplasma are common mollicute plant pathogens
Phytoplasmas
Very small bacterial cells (less than 0.1 µm) with small genomes
Obligate parasites on plant phloem cells, affecting many important agriculture plants
A common symptom is "phyllody", or the creation of compacted leafy shoots in place of flowers, causing sterility.
Above: Phyllody in Echinacea purpurea
Above: Aster-yellows in carrots (and marigolds) caused by Candidatus phytoplasma asteris
Below: Prunus X-disease caused by Candidatus phytoplasma pruni
Left: Pear decline caused by Candidatus phytoplasma pyri
Right: Lethal yellowing of coconuts caused by Candidatus phytoplasma palmae
Spiroplasmas
Spiral-shaped
Usually plant pathogens
Infect the phloem sieve elements
Above: Spiroplasmas in a phloem cells
Left: Citrus stubborn disease caused by Spiroplasma citri (on left)
Mycoplasmas
Very small bacterial cells (0.1 µm)
Can survive without oxygen
Additional Resources
How Plants turn into Zombies - Phytoplasmas effect of the lifecycle of a plant (NY Times 1Oct2021)