Application in Phosphorous Solubilization

Phosphorous in soil

Phosphorus (P) deficiency in soil is a major constraint for agricultural production worldwide. Many soils throughout the world are P-deficient because the free phosphorus concentration (the available form to plants) even in fertile soils is generally not higher than 10 mM, even at pH 6.5 when it is most soluble. Low levels of P are due to the high reactivity of soluble P with calcium (Ca), iron (Fe) or aluminum (Al), which leads to P precipitation. Inorganic P in acidic soils is associated with Al and Fe compounds, whereas calcium phosphates are the predominant form in calcareous soils.

Despite this, most soils contain significant amounts of total soil P that occurs in inorganic and organic fractions and accumulates with phosphorus fertilization. Organic phosphorus inputs to soil from plants and microbes are mainly phosphodiesters, which must be hydrolysed by phosphodiesterase and phosphomonoesterase prior to the release of free phosphate for biological uptake.

The ability of soil microorganisms to solubilize various forms of insoluble P fractions is well documented. Bacillus spp. and Rhizobium spp. are the most powerful phosphate solubilizers among bacteria.

Role of Engineered B. subtilis

The engineered B. subtilis will perform two functions:

1. Detecting the plant
2. Amplifying its own phosphodiesterase production

Upon detecting the plant, the engineered bacteria will secrete enzymes that convert the phosphorus in the soil into a form that is more easily assimilated by the plants. The production of the enzyme will be amplified by incorporating a gene amplification circuit within the chassis bacteria. Since the circuit will be triggered only in the presence of specific exudates, depending upon the plant of interest, weeds will effectively be prevented from using up most of the phosphate in the soil.

Potential application: Systemic acquired resistance (SAR)

What is SAR?

Systemic acquired resistance is an induced defense mechanism in plants that inherently protects the plant against a variety of diseases. It is analogous to the immune system in animals, as it primes the plant to defend itself against pest attacks and diseases following earlier localized exposure to the pathogen. The limitation of this system is that it is activated only after exposure to the pathogen, i.e., after the pest has attacked the plant once. Further, the effect of SAR is not perpetual and wears off with time.

Synthetic activation of SAR

A number of commercially available chemical activators induce SAR in plants, the most popular of which is salicylic acid (SA). However, a number of problems are reported with the continuous and prolonged usage of such activators, such as low yields and reduced seed sizes. This is mainly due to the large energy deficit caused to plants when they are under constant influence of the activator.

A Solution

A way around this problem is to design a dosage system in such a way that the plant is periodically exposed to the activator such that it has intermediate periods in which it is free of the activators’ influence. For this purpose, an oscillatory circuit can be incorporated within the chassis organism, B. subtilis, such that it releases salicylic acid (which can be synthesized from a metabolic intermediate), in optimized doses.

How will this project benefit Society?

As a part of Integrated Human Practices, the team spoke to multiple experts in the field of agriculture, fertilizers and bio-fertilizers, including farmers and innovators. From the surveys and farm visits, a lot of information was gained which indicated the need for the solutions that this project promises.

The efficiency of phosphate solubilization can be increased with the use of the engineered bacteria, which may improve the popularity of bio-fertilizers among farmers, who currently prefer chemical fertilizers. Thus the larger effect would be to reduce the usage of chemical fertilizers in Indian agriculture. The project also has potential to reduce the usage of weedicides.

Pesticides are currently being used as a curative measure for plants affected by pathogens and pests. However, pesticides have a large number of well-known ill-effects such as persistence in groundwater, neurological disorders, cancer, respiratory illnesses in farmers, and contamination of fruits and vegetables.

Moreover, the use of pesticides is not a preventative measure. If SAR can be continuously primed in the plants, it would eliminate the need for pesticides and ensure that the disease does not affect the plant in the first place.