Significance and backdrop:
Snail paced approval systems and widespread misinformation campaigns have been a hurdle in India for genetically modified food crops. As such only a cash crop, i.e. Bt cotton has been approved for agricultural production in India which does not pose any risk to the consumer (since it is not consumed in any diet) or the land on which it is cultivated. The approval testing for Bt-brinjal developed by MaHyCo (Maharashtra Hybrid Seeds Corporation) has been going on for well over a decade with no end in sight. Many other GM crops were banned right away. This attitude has been a huge put-off for global investors and native genetic laboratories alike.
During this time it so happened that the adverse effects of synthetic chemical fertilizers in the form of soil quality loss, ground water contamination, nitrate leaching, greenhouse gas release and eutrophication started coming to light and the governments and scientists across the world turned to biofertilizers. Biofertilizers contain specific chosen microbes, organic products and carrier substances. The most efficient type mix of biofertilizers majorly work in three path, i.e. Enhancing soil fertility, enhancing availability of nutrients from soil to crop by solubilizing them, enhancing external defense mechanisms of the crop (biocontrol action) against harmful fungus.
However, to express their growth-promoting effect, sufficient number of individual microbes introduced (of the biofertilizer mix) have to survive in both soil and rhizosphere, which does not always happen in fields. Consequently, the efficiency of initial mixes were not always sufficient for commercial production and marketing, so there was a need to improve their performance. Their shelf-life was also low when compared to chemical fertilizers available in the market. Other than that dry spells, presence of competing microorganisms, predation by protozoans, and lysis by infecting bacteriophages adversely affect the number of introduced microbes.
Currently, a variety of biofertilizers are
obtainable in the market that employ a variety of mechanisms to guarantee maximum viability of the microbes used in such formulations. The mechanisms include using drought-resistant strains and liquid versions or carrier substances like clay, charcoal, rice bran, etc. Most effective biofertilizers employ liquid or multiple carriers to carry symbiotic or free-living microbes. An ideal biofertilizer should be cost-effective, resistant against
Fig.1: Azospirillum macro-beads made of drought, should enrich soil by fixing alginate, to facilitate slow release. nutrients, accelerate mineral uptake, boost crop growth and yield.
The race to be the first to introduce second generation of biofertilizers began more than half a decade ago. This led to the idea of using recombinant DNA technology to genetically engineer the microbes used in biofertilizers to one up their wild types of the first generation.
Genetically engineered microbes: Ideas and examples:
The foreign genes used for transforming microbes could be integrated into host genome. For this,the gene should be placed in a promoter sequence which would optimize the expression of particular gene which can give an ability to microbess lacking such genes. To understand this idea, we can look at how a plant growth promoting rhizobium (PGPR) called Rhizobium melliloti with excellent root colonizing efficiency can be engineered to produce chitinases by taking chitinases genes from other PGPR and incorporating it into our host effectively can give it an additional biocontrol feature to help the crop against fungal invasion.
GM Sinorhizobium melliloti with improved nodulation capability coincidentally also happened to increase beneficial arbuscular mycorrhiza symbiotic association with fungus like Glomus mosseae.
Alcaligenes faecalis, a non-nodule forming nitrogen fixer bacteria was genetically modified by insertion of a nifA regulatory gene. Nitrogen fixation rate went up 15–20% higher and the yield of the rice crop was 5–12% higher compared to the non-treated rice fields in China.
Genetically modified Azotobacter vinelandii excretes great amounts of compounds like Urea and ammonia which can be solubilized and absorbed by the plants. It’s major plus point over many other bacteria is that it functions almost equally well in both aerobic and anaerobic conditions, making it ideal on field.
Overall, most of the genetically modified microbes will be nifL deletion mutants because nifL acts as a neutralizer control over nifA, so to increase expression of nifA, it’s a good choice.
In many other GM microbes the urease gene complex ureABC has been deleted, Urea transport gene amtB has been silenced or disrupted, and glutamine synthase gene highly regulated so that the Urea or ammonia excretion is enhanced.
Essentially speaking, nitrogen fixation rate, root colonization efficiency, nodule size, promoting other associations while defending against harmful microbes, etc. are different features that can be brought into one host PGPR/microbe using genetic engineering.
Nitrogen-fixing bacteria can be considered as a self spreading source of nitrogen for plants. Unfortunately all plants are not able to perform interactions with nitrogen fixing bacteria. The introduction of symbiotic biological nitrogen fixation into cereals and other major non-legume crops would be regarded as one of the most significant contributions that recombinant DNA technology could make to the agro-based sectors.
Many pseudomonads in the soil make siderophores that are used to chelate Fe ions, and so increase Fe uptake by the plants. Genes for this function can be used to genetic engineer other host microbes that have more features now can be used in fortifying food crops with Fe upto some extent. If the host did not already have chitinases production genes, those can be taken from pseudomonads that have the genes for chitinase.
Fig.2 Nitrogenase enzyme.
The more our microbe from the biofertilizer produces of it, the better.
Survivability and impact of GM microbes on field ecosystem:
The survival rates of these GM strain can be studied by following the wild strain. The
expression of the inserted gene needs a extra energy that could decrease their environmental fitness. Besides, the addition would have a good chance of insertional disruption in some functions declining the competiveness of the strains. The genetically modified microbes can acclimatize to the prevailing ecological conditions through natural selection among themselves. Genetically modified microbes have been observed to survive better than the wild strains.
Symbiotic bacteria like Rhizobium and Sinorhizobium form root nodules in leguminous plants and fix free atmospheric nitrogen to soil. These bacteria have been reported to survive in soil for years, in some cases even without a definite host. The host-free survival was significantly longer in case of GM bacteria even under sub-optimal pH and temperature conditions in short dynamic seasonal timeframes.
Conclusion:
Even with limited field trials, the preliminary results of usage of genetically modified microbes in biofertilizers so far have been very promising all around the world. Judicial use of genetic engineering technology both in plants and these soil microbes is believed to be the key for food security of the future for an ever growing human population. Genetically modified biofertilizers have been introduced with great success in terms of their activity and survival rates. Till now non-target effects of genetically modified biofertilizers that have been reported are almost Fig.2 Nitrogenase enzyme. The more our microbe from the biofertilizer produces of it, the better. insignificant and negligible in comparison to natural variants. These GM biofertilizers do a better work on maintaining soil quality over a long term because it is derived from a natural process, and since agricultural land is getting scarcer by the day all around the world, the onus will be on producers to produce the most from the smallest tracts of lands, and that will hopefully be partly because of these GM biofertilizers.
By - Athul Nair
Department of Biochemistry and Biotechnology
St. Xavier’s College, Ahmedabad
References:
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Image sources:
Fig.1 taken from Handbook for Azospirillum: Technical issues and protocols, Cassan et al. , 2015 on researchgate.com
Fig.2 taken from Nitrogenase structure and genes required for its biosynthesis, on researchgate.com