We humans have inherited a turquoise globe that bears an infinite number of life forms, animals, plants and microbes, which are dependent and consequently susceptible to a dynamic atmosphere that is fuelling global warming and climate change. Amidst such forecasts of turmoil within this spherical greenhouse of life, there is an emerging need to lower the collective strength of heat-retaining emissions (greenhouse gases), by tinkering with biological systems that possess the intrinsic capacity to attenuate greenhouse gas emissions and to mitigate the ripple effects stemming from an orbiting furnace that traps heat in the form of inward-reflected solar irradiation. A potent contributor to climate change is nitrous oxide, a gas emitted from the actions of nitrifying and denitrifying bacteria on natural pools of soil nitrogen and synthetic nitrogen fertilizers, which possesses a 298 fold higher capacity to warm our planet compared to the global warming footprint of carbon dioxide. Furthermore, nitrous oxide is a gas capable of stratospheric O-zone depletion and consequently can be labeled as ‘dangerous’ for human health. One instrument of biology earmarked as a solution to the widespread application of chemical fertilizers is nitrogen fixation, nature’s way of trapping the gaseous elemental form of nitrogen in to building blocks of biomolecules.
Nitrogen fixation can be easily termed as a biological trick that utilizes elemental nitrogen to build up biochemical and genetic bricks of nature. This ingenious mechanism of milking the vast pools of nitrogen gas has slowly made inroads in to mainstream agriculture as well as its offshoot, organic agriculture, as more and more farmers are tapping the promise of nitrogen biofertilizers (biological organisms that can convert diatomic nitrogen in to usable sources of nitrogen, namely ammonia) to replenish plants with a continuous supply of nitrogen, which has a myriad of downstream synthetic applications, since without nitrogen, the backbone of proteins, the bases of nucleic acids and the rings of chlorophyll would simply not exist.
One of the most commonly used nitrogen fertilizer in contemporary agriculture is a slightly curved ‘comma’-shaped flagellated bacterium (Figure 1) of the class alpha-proteobacteria designated by the genus Azospirillum which is used for the infusion of nitrogen in to crops belonging to the family Poaceae, many of which contain the C4 photosynthesis toolkit. Where this comma-shaped bacterium really bends and defies convention is that it forms an associative symbiotic relationship with a wide variety of plants. By using a process of adsorption and anchorage, the bacterium is able to move towards the root using its propelling flagella and use its innumerable pili to live in the exudate-rich localities of the rhizosphere. The association with the roots is facilitated by a glycoprotein that is able to claw and hold on to the surface of the plant root, like a alpine mountaineer using ice screws to fasten to a steep icy mountain surface. This remarkable trick gives this bacterium the best of both worlds – free-living and symbiotic, as host specificity is lost due to the non ‘true-symbiotic’ nature of the association, i.e there is no special compartment such as a nodule formed by the association of plant and specific bacterium, and therefore the bacterium is able to colonize a range of plants and prosper in the nutrient-rich broth found on the proximities of plant roots.
The name Azospirillum stems from the fusion of the terms, ‘azo’ pertaining to nitrogen and ‘spirilla’ defining a small spiral or twirl (although not strictly in the case of Azospirillum), and together they form this genus which contains six species of nitrogen fixers. Azospirillum was first isolated and identified by the father of nitrogen fixation, Martinus Beijerinck, as early as 1925. Like other diazotrophs, this genus of bacteria possesses the catalytic machinery in the form of the nitrogenase enzyme that possesses the remarkable aptitude to cleave the symmetry of diatomic nitrogen for the formation of asymmetrical ammonia. Nitrogen fixation is favored by this bacterium in nitrogen-deficient or limiting soil conditions and like any other form or subtype of the nitrogenase enzyme, this heteromeric catalytic engine requires microaerophilic conditions to transform a relatively inert pool of gas into to mobile building blocks of ammonia.
Outside of nitrogen fixation, Azospirillum is also the best known plant growth-promoting bacterium used in contemporary agriculture beneficial to plants, which has intrinsic capacities to secrete a broad spectrum of growth promoting substances such as phytohormones which are able to alter metabolic pathways and boost root architecture, to supply energy requirements and to better absorb minerals from the soil. Indole acetic acids, indole lactic acids, gibberellins and cytokines are examples of the phytochemical arsenal synthesized and released by this plant growth enhancing bacterium. Therefore, although earmarked for its nitrogen-fixing potential, this remarkable elixir for plant growth and development, can be termed as a true miracle capable of sustaining the agriculture of Asia, which is largely held on the herculean shoulders of grass staples such as rice and wheat.
It is noteworthy that many strains of Azospirillum have been sequenced and the data sets have unveiled a strong ancestral contingent of genes and a near 25% non-ancestral component of the genome, where genes encoding for plant root colonization, plant growth promotion and rhizosphere adaptation are strictly strain specific hinting at nice-specific adaptability of different strains. Therefore, it appears that Azospirillum does not simply grow in a narrow range of environments but is driven by evolution to adapt or acclimatize to a wide variety of niches.
Asia is universally known for its fanatical followers of the game cricket. Cricket in Asia, is not just a adrenaline-charged game, but a religion where people build shrines of cricketing deity, hold effigies (even burn them in despair), follow sermons of the bat and the ball and take to the streets for the celebration of holy days of nail-biting victories and days of holding aloft a gold-plated chalice. What we should not forget, even for a fleeting second, in these manic episodes of cricket is that the cricketing outfield is a turf of grass that can be easily be fed with Azospirillum to replenish its nitrogen needs. Azospirillum is environmentally friendly, economically-sustainable and unlike its chemical surrogate, urea, will not contribute towards global warming through the release of greenhouse gases such as nitrous oxide. Therefore, all in all, the future is rosy and ripe for Azospirillum, a true wonder of nature that will be forever an accomplice to the grasses, and bosom friend and not staunch foe to the future well-being of the climate-change threatened environment.
With the current crisis of population growth, which in turn has a strain on contemporary agriculture, it is important to find solutions that not only increase the yield parameters but also preserve environmental integrity. This can be partially achieved by the adoption and supplementation of Azospirillum to sustain agricultural production systems. Even the wave of organic agriculture that is flowing fast and furious, will only serve to fortify the beneficial claims of Azospirillum and other biofertilizers.
Some marriages are made above the canopy of clouds in the elysian fields of heaven, and some, on the way to the fires of hell, buried deep in the soils of the earth. Azospirillum belongs to the latter. Whether it is rice terraces or even wheat horizons, Azospirillum would be there hidden beneath the top soil, to spur on the lush grains of precious gold. I guess, it is more than fair to say, that just like cricket, Azospirillum is here to stay, in the golden basins of Asia.