[Infectiology] Plantae

Taxonomy 

• Plantae

 

Classification

Domain : Eukaryota 

Kingdom : Plantae 

Phylum : Chlorophyta

• Prototheca wickerhamii : protothecosis
• Prototheca zopfii : protothecosis

 

Characteristics

• Most plants are autotrophic. This means that they use external energy and simple molecules to  produce complex molecules.
• Most plants are photoautotrophic. This means that they use photosynthesis to trap energy from electromagnetic radiation (from sunlight). 
• Most plants live in terrestrial enviroments (on land or on top of water). This is partially because efficiency of photoautotrophism increases in direct proportion with proximity to the Sun.
• Many plants live in aquatic enviroments (underwater). There are key differences:
• Energy from sunlight is attenuated by the water. 
• Essential nutrients are diffused throughout the water.
• At a certain sea depth there is almost no visible light radiation. This makes photosynthesis impossible. However, the sea floor features hydrothermal vents which provide thermal energy and useful molecules. In this enviroment, chemoautotrophic bacteria and archaea thrive and form the base of the food chain. Plants are not known to live there.
• Many plant species are saprophytes. This means they can release the resources of dead or dying organisms and ingest these resources. Saprophytes are heterotrophs. This means that they consume other organisms to obtain complex molecules.

 

Pathogenesis

• Plants cause many human diseases through the poisons and carcinogens they produce. 
• Many substances are synthesised by plants for the function of poisoning. This allows their species to reduce the population size of the species which consume them.
• Many plant-derived substances are only poisonous to animals because their biochemical composition is very different and incompatible.
• There are very few examples of parasitism of human cells by organisms in the kingdom Plantae. 

 

Explaining the low prevalence of pathogenic plants

One reason for this may be the autotrophic capabilities of plants.
Autotrophic species are specialised to live independently. They are the very first organisms and they occupy the original ecological niche in every enviroment. They are the foundation of every food chain in every ecosystem. The evolution of heterotrophs follows the evolution of autotrophs. Instead of competing with autotrophs, heterotrophs steal their resources by consumption or parasitism.

The development of parasitic capabilities in an autotroph is not worth the energy investment unless they are replacing photosynthesis. This is only necessary in certain niches where conditions do not favour photosynthesis (e.g.when there is intense competition from other plant species).

Another reason may be the size and complexity of most plant species.
Most parasites share a number of properties:

• Microscopic size
• Unicellular
• Simple genomes 
• Simple physiology
• Rapid generation times 
• High rates of successful reproduction
• Short lives

They can efficiently convert resources into offspring. They can quickly evolve to fill new parasitic niches. They can disseminate themselves around multiple organ systems, and fight against immune systems.

In comparison, most plant species share these properties:

• Macroscopic, large size (large root networks and large body size)
• Multicellular
• Complex genomes
• Complex physiology
• Long generation times
• Low rates of successful reproduction
• Long lives

Their large size facilitates efficient energy capture and high-capacity resource storage. These resource stores provide a defence against temporary enviromental changes.

The characteristics of large plants facilitate a stable, independent life. However, they also necessitate a life of immobility or, at most, passive movement.