Question

A farmer in Japan recently discovered that his rice seedlings were dying as a result of “rice seedling blight,” caused by Rhizopus microsporus.

In addressing this problem, the farmer did not want to harm mycorrhizal fungi, so he sprayed his fields with a dilute solution of ciprofloxacin.

Explain how exactly this approach halted the disease’s progression on infected seedlings and prevented its subsequent spread to other seedlings.

Answer 1

Rice Seedling blight:

• Rice Seedling blight or damping off is a disease complex caused by seed-borne and soil-borne fungi caused by Rhizopus microsporus along with a bacteria called Burkholderia sp
• Cold and damp weather is most convenient for the disease development.
• The severity of seedling blight depends on three factors:

1. percentage of the seed affected by seed-borne fungi
2. soil temperature
3. soil moisture content

Endosymbiotic relationship between fungi(Rhizopus microsporus) and bacteria (Burkholderia sp):

• The main cause of this disease is Rhizopus Microsporus associated with the endosymbiotic relationship with Burkholderia sp
• Rhizopus microsporus is a fungal plant pathogen which infects rice, maize, sunflower etc that harbour bacterial endosymbionts
• Endosymbiosis is a type of symbiosis in which one organism exists inside the other and the two together behaves as a single organism
• The bacteria deliver antimitotic macrolide rhizoxin to its host (Rhizopus microsporus) and also provides the essential substance for vegetative spore formation of the fungus
• The generation of rhizoxin by the bacteria inhibits the ability of rice plant cells to perform mitosis and due to which weakens or outright killing young rice seedlings.

Mycorrhizal relationship between fungi (Rhizopus microsporus)and plant (Rice):

• It indicates a symbiotic/mycorrhizal relationship with the plant.
• Plants are autotrophic and will produce its food through the process of photosynthesis. But plants often have difficulty obtaining and absorbing many of the essential nutrients particularly nitrogen and phosphorus.
• Fungi are heterotrophic organisms. Fungi have the ability to easily absorb elements such as phosphorus and nitrogen which are essential for the plant. So the fungi will provide these nutrients to the plant and the fungi in return will absorb the food from the plant
• In order to maximize both organisms expertise to flourish, most plants allow and indeed require mycorrhizal fungi to colonize their roots.

Treatment with a dilute solution of ciprofloxacin:

• Ciprofloxacin is an antibacterial drug
• Treatment with dilute solution of ciprofloxacin kills the bacteria Burkholderia which releases Rhizoxin to the fungi Rhizopus microsporus and thereby halts the disease's progression on infected seedlings and prevented its subsequent spread to other seedlings.
• This treatment will not harm the fungi Rhizopus microsporus
• The symbiotic/mycorrhizal relationship between the rice plant and fungi (Rhizopus microsporus) will not be affected.