In 1960, when first green revolution occurred; there was a substantial increase in crop yield. That revolution was dependent on the use of chemical fertilizers, introduction of new methods of cultivation and some new crop varieties.
But from that time to now; crop yield is stagnant. Reason is tremendous increase in world population. There is a need to integrate all possible means to feed the increasing population.
Scientists are trying to introduce different ways to increase Agricultural production with little environmental negative impact. They are also thinking of making nitrogen fixation possible in crops that can’t fix it biologically.
New symbiosis means to introduce a new route to make biological nitrogen fixation possible in crops that are not able to fix atmospheric nitrogen, to make developments in plant physiology and somatic hybridization of plant cells.
The main reason for the interest of soil microbiologists in this symbiosis is to reduce excessive use of nitrogen fertilizers which negatively affect human and environmental health through ozone layer depletion, eutrophication, green house gas emission etc.
Nitrogen fixation in Cereals
Cereals (Maize, Rice, Wheat and Sorghum) are our major food. They make almost 78% of our calorific uptake. Cereal production is highly dependent on the use of chemical fertilizers.
Biological nitrogen fixation is a potentially alternative source to reduce the use of chemical fertilizers. Biological nitrogen fixation accounts for 30-50% of total nitrogen in crop yield.
Prospects to engineer nitrogen symbiosis in Cereals
Cereals can be engineered by
- Transferring active nitrogenase into crop plants
- Development of root nodular symbiosis in cereals
- Development of diazoplasts
Assembling of an active nitrogenase in cereals
Active nitrogenase can be assembled by incorporating bacterial genetic machinery to encode functional nitrogenase system. Nitrogen fixation is highly energy demanding process. So Chloroplasts and mitochondria are considered suitable sites for nitrogen fixation because they fulfill energy requirements for nitrogen fixation.
Nif genes can be targeted to mitochondria because maximum ATP production and oxygen consumption is here. Nif genes are genes encoding enzymes involved in fixation of atmospheric nitrogen into a form available to crops. Regulation of Nif gene transcription is by NifA protein.
It becomes activated when there is no sufficient nitrogen. When there is sufficient reduced nitrogen, NifL becomes activated. It stops nitrogen fixation process. This method to engineer cereals is quite difficult and involves genetic machinery but there will be no need of any bacterial interaction.
Root nodular symbiosis in Cereals
Another way to engineer cereals is to make them capable of sensing nitrogen fixing bacteria and to perceive nod factors. We have to make cereals capable of talking to bacteria.
All crops have specific mechanism to activate nodule formation. What cereals lack; is that specific nod factor receptors to identify nod factors. So another possibility can be to engineer nod factor receptors to make cereals capable of fixing atmospheric nitrogen by nodule formation.
These nod factor receptors belong to LysM receptor family. As nod factor receptors are specific for a specific nod factor so engineering of that specific nod factor receptor is not an easy task.
Development of diazoplasts
Diazoplast is a new organelle similar to chloroplast that help plant to overcome genetic and physiological bacterial-dependent nitrogen fixation problems. These diazoplasts are formed by Gluconacetobacter diazotrophicus (Gd).
According to scientists thinking, it can be easiest and the best one method. Because there is no need to genetically engineer cereals. These specific bacteria produce cellulases, hemicellulases and pectinases that facilitate their penetration through cell wall.
This bacterium develops diazoplast and colonize cereals. Gd has many characteristics that make it attractive for the establishment of intracellular symbiotic nitrogen fixation without the need for nodulation.
Our agriculture is mainly dependent on cereals. If cereals are engineered, they will bring a new revolution by different means. This engineering will save energy because the process of nitrogen fixation requires 16 ATP which is much less than that of Haber-Bosch process-an energy intensive process. This new insight will save money, environment and give more production.
Development of diazoplasts can be the easiest and most efficient method to make cereals capable of fixing atmospheric nitrogen. Soil microbiologists are not only concerned with increasing production but also introducing safe means which may not pollute our environment. Population is increasing year by year but agricultural production is stagnant.
Soil contamination and many other soil problems are arising. There are chances of deterioration of soil in future. Scientists are trying to make engineering of cereals possible.
If it happens; not only dependence of cereals on nitrogen fertilizers will decrease, soil will also become healthier to increase production in future. When production will increase by biological means then the new revolution’ll occur.
Student of MSc (Hons) Soil Science