Endofungal bacterium controls its host by an hrp type III secretion system

Area of Science:

• Microbiology
• Mycology
• Symbiotic interactions

Background:

• Burkholderia rhizoxinica and Rhizopus microsporus form a unique symbiosis.
• Intracellular bacteria produce the phytopathogenic fungus's virulence factor.
• The fungus requires endobacteria for sporulation.

Purpose of the Study:

• Investigate the role of the type III secretion system (T3SS) in the B. rhizoxinica-R. microsporus symbiosis.
• Determine the T3SS's function in maintaining the bacterial-fungal alliance.
• Characterize the T3SS within the context of bacterial-fungal interactions.

Main Methods:

• Genetic analysis of T3SS mutants in B. rhizoxinica.
• Microscopic observation of bacterial survival and fungal sporulation.
• Gene expression analysis of T3SS components during co-cultivation.
• Phylogenetic analysis of T3SS genes.

Main Results:

• B. rhizoxinica possesses a functional T3SS essential for symbiosis.
• T3SS-deficient mutants exhibit reduced intracellular survival and impaired fungal sporulation.
• T3SS gene expression is upregulated during co-cultivation.
• The identified T3SS is a novel prototype within the hrp superfamily.

Conclusions:

• The T3SS plays a critical role in the maintenance of the B. rhizoxinica-R. microsporus symbiotic relationship.
• This study reports the first instance of a T3SS involved in bacterial-fungal symbiosis.
• Rhizoxin production is not essential for the establishment of this symbiosis.