Role of Bio-Fertilizers in Sustainable Agriculture

Muhammad Salman Akram 1 , Muhammad Abubakar Cheema 1 , Faisal Nadeem 1 , Muhammad Waqas 2 , Muhammad Bilal 3 and Muhammad Saeed 1 * *Centre of Agricultural Biochemistry and Biotechnology, University of Agriculture, Faisalabad, Pakistan. Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan. Basic Health Unit Sarki district Muzaffargarh Pakistan. Correspondence Author Email: muhammadsaeed7360@gmail.com*


Introduction
By increasing population of the world the demand for the food is increasing rapidly (1,2). The food demand is increasing in those developed countries where the land resources are not contributing part in crop production as the population need in daily bases needs the food requirements.to increase this capacity of land to produce more production the people first move to the application of chemical fertilizers which increase crop production. But the long application of chemical fertilizers shows adverse effects on the environment (1,3,4). These chemicals accumulate in the water and also can the cause of eutrophication (2,3). These chemicals also harm the soil fertility, decrease Holding capacity of soil, increases the salinity and disparity in soil nutrients (1). So the intention goes towards the bio fertilizers. These were used to decrease the bad impacts of low fertility on the soil , the impact of environmental stress and the effect of biotic stress such as pathogens and other microorganisms (5)(6)(7). In this aspect many work has done in this field so many organic fertilizers has been introduced in these years. These work as the natural stimulators of plant growth and the development of the plants (8)(9)(10). Important group of organic fertilizers were made on the basis of plant growth promoting microorganisms (PGPM). There are many groups of the microorganism which are beneficial for the plants but the major contribution are of the following three which considered as important for plants growth: arbuscular mycorrhizal fungi (10,11). Plant growth promoting rhizobacteria, and nitrogen fixing bacteria that are not considered as PGPR (4,10) the bio fertilizers are the material that contain living microbes that applied on seed, surfaces of plants or the soil, formation of colonies the rhizosphere or the interior of the plant, and promote the growth of the plant by increasing the supply of primary 9 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com availability of the nutrients which can be assimilated easily by the plants (1,14,15) The organic fertilizers are promoted to get the natural means of getting nutrients by the plants (16) and increase the mobilization (17) which increase the fertility of the soil and increase the crop yield (18,19).

Why we need bio-fertilizers
General use of chemicals as a fertilizers can full fell the demand of fast growth and high amount of food supply but obviously those leads to damage the environment (damaging microbial biota, killing friendly insects and increase in pH). However the use of chemicals fertilizers make the crops susceptible to disease and also results in decease soil fertility (5,20).
It has been estimated the in 2020 the world population will we more than 8 billon and to feed them we need 28.8 million tons of food and nutrients and we estimated the only 21.6 million tons will be available to community of the world) (21).
To feed the world population we have to increase the amount of agriculture and increase the productivity in such a way the it is useful and also show no bad effect on either environment, Human, beneficial insects etc. for this purpose we have to increase agriculture productions using chemical fertilizers, pesticides, herbicides, fungicides, and insecticides (22).
As already discussed due to hazardous effect we are moving towards other sources rather the chemicals which have to be more safe and more effective and ecologically safe and cost effective as well, all these properties are present in bio-fertilizers (23).
The bio-fertilizers like other man-made chemicals are commercial products having beneficial microbes, substances important for growth of microbes and other additives which can probably increase the growth of microbes. It was reported that bio-fertilizer increase the contents of food protein, amino acids, vitamins, and nitrogen and other essential elements from renege 10%-40% (24).
The bio-fertilizer provides and help the plant getting both macro and micro essential elements for growth and also help in supply of hormones and other organic element for plants (25).
The application of bio fertilizer is most effective and natural way to keep the bio system active and working, the microbes in soil provides nutrients to plants and helping the biodiversity maintains by improving the quality of soil (26).

How biofertilizers work?
Bacteria, fungi and other microbes an important and unequally distributed micro-organism present everywhere in the world with different amount of concentration present in more concentration near the root and perform very important works.
The Bacteria which are present near the roots (rhizosphere) have the capability to promote plant growth accordingly are called as plant growth promoting microbes, the bacteria which play its role as a plant growth promoter are called as plant promoting bacteria or (PGPB). Today many of the PGPB are available on commercial scale as a bio fertilizers (27) PGPB have very important role in the fertility of soil and plant growth. Its paly role in two ways:  PGPBs supply important hormones like auxins, cytokinin, gibberellin, which play role in plant growth directly; PGPBs help in providing the essential elements like nitrogen by nitrogen fixation, phosphorous by method of phosphorous solubilization, potassium intake, which is called as direct way.
 Helping the genes against the pathogens in many ways which are called as indirect way.(28)

Auxin
Auxin is a hormone which work in geo and phototropism, tissue differentiation, cell division, and stem root elongation (29).
PGPBs show positive effect on plant growth as they have the ability to produce the auxin, 80% microbes present in rhizosphere produce the auxin and their secondary metabolite (30).
Rhizobacteria change the level of auxin concentration in in the form of indole-3-acetic acid (IAA) which can increase the surface area of roots and large amount of uptake of auxin by plants and remember that going thought research is has been noted that auxin and IAA are interchangeable (31).
Different cells of different plants are sensitive at different level to auxin (32). Where the PGPB produce the IAA work as auxin when concentration of IAA is suboptimal roots will be develop and when optimal shoots will be (28,33).

Ethylene
Ethylene is very important hormone in plant in which at play its role in both cell division and normal growth as well as also have very significant role in the helping the plant against different types of biotic and abiotic stress (34).
Almost all plant at any stage of their life are effected by ethylene which is influenced by hormones of plant, temperature, gravity, light, nutrition and various biotic and abiotic stress (28).
High concentration of ethylene help the plant to response to various stresses either temperature, chemicals like salts, or drought and light, pathogens (35,36).
The PGPB make bond with the roots of plants and take some of the tryptophan released by plant and change it to IAA which is then released by bacteria and taken by the plant (37).

Cytokinins
Cytokinins are found in algae, bacteria, fungi and higher plants. We didn"t know much about bacteria producing cytokinins and their role. Cytokinins are produced in the tip of roots and from there they are transfer to shoot though xylem. (38) It control the cell division and differentiation in meristem cells (39) In many PGPBs the cytokinin genes are evidenced and when they were added to plant surrounding the growth of the plant increases and as well other Phyto-hormone configuration. (38) When the bacteria Bacillus subtilis was used as cytokinin producing agent the results shows that the growth of plant were altered and , when the sinorhizobium melioti over produce cytokinin and which were used as tester for alfalfa plants against the drought stress. (39)

Gibberellin (GA)
Gibberellin have a group of teracyclic diterpenoid carboxylic acids with C 20 or C 19 (40) Gibberellins work in the activation and stimulation of plant growth and other important process like elongation of stem, seed germination, sitting of fruit, plant flowering (41) And also inhabit the growth of plant though GAs signaling pathway. (38) The biochemistry of both plant produce GAs and bacteria GAs are similar and we readily absolved the absence of GAs as a reduction of side roots number and length. (42) The pathway though which GA is produce is known in plants and fungi but in bacteria it is still unknown. (38) Bacteria Bacillus pumilus and Bacillus licheniformis are isolated which have the ability to produce GA. (43)

Nitrogen fixation
Nitrogen is an essential macroelement which is an important element required for plant growth. It is also required for the formation of nucleic acid, chlorophyll, and proteins. The nitrogen is present in the atmosphere of about 78%. (44,45) This is present in gaseous form so is not available for the plants. Different nitrogen fertilizers made by industries are available which increase agricultural productivity (38,46,47).
Biological microbial fixation involves the conversion of the nitrogen present in the atmosphere into the form that is readily uptaken by the plants such as ammonia (46,48).
The conversion of nitrogen into ammonia Need high amount of energy because of the presence of triple bond in the N2. Many nitrogen-fixing bacteria have been identified which fix the nitrogen symbiotically with the plants such as legumes. Examples are Rhizobium, sinorhizombium, azorhizombium etc. (7,26). Rhizobial bacteria form colonies on the root cells and start the root nodule formulation. (38). In this the bacteria act as bacteroid (having no cell wall)  (50) It is not a simple process either complex of enzyme are involved in it i.e. Nitrogenase which contains Di Nitrogenase reductase containing (Fe) iron and molybdenum (Mo) as a cofactor (51).
It convert N 2 into to NH 3 by using electrons and also oxygen is essential for respiration of Rhizobium sp.(4).
There are three different kinds of Nitrogenase complexes depending upon changes in cofactor of di Nitrogenase Mo-Nitrogenase, V-Nitrogenase and Fe-Nitrogenase (52).
Some genes are involved in the N 2 Fixation which are called "Nf"genes they are essential for nitrogen fixation in free-living & nitrogen-fixing organism (53).
The Nif genes are involved in the activation of Fe/Mo cofactor and also for the electron donation, biosynthesis of cofactors, and functioning and regulatory of enzymes (54).
Experiments show that when plants were inoculated with the strain of Rhizobium wild type strain Retile 68% more nirtogenase activity was seen, 25-30% increasing in leaf substances and seeds resultant was increase up to 16%. .
Abeles et al 2012 also showed that when rhizobium sp was applied so the ethylene level was also inease which is one of the major plant regulators and also help in inhibiting of rhizobia infection.

Azospirillum
Azospirillum are gram negative bacteria with ability of nitrogen fixation belong to spirilaceae family (2).
They make associated symbiosis with the plants especially with those which have C4 dicarboxylic pathway of photosynthesis (56). They produce plant regulators like IAA, GA and cytokinin with work as root development due to which plants are able to uptake large amount of nutrients (57).
The major advantage of these bacteria is that they full, fell the requirement of nitrogen in paddy, sorghum and sunflower up to 25%.

Azotobacter
They are the photoautotrophic, aerobic, non-symbiotic free-living bacteria belong to the family azotobacteriaceae, playing very important role in nitrogen cycle of plant. They are present where soil is alkaline and most commonly arable soil (58). Azotobacter vinelandii, Azotobacter beijerinckii, Azotobacter insignis,and Azotobacter macrocytogenes are the common sp of Azotobacor (21). 30 Reported that Different type of hormones i.e.
gibberellins, naphthalene acetic acid is produced by these bacteria which prevent the formation of root pathogen but enhance the root growth by minerals and nutrients uptake by plant. Vitamins such as vitamin B complex is also produced by these bacteria.
These type of bacteria are found in different type of crops such as rice, maize, sugarcane, vegetables (Azobactor play important role in production of some other vitamins such as riboflavin and thiamin (59)
These are present in large amount in paddy field belonging to eight different types of families and enhance the growth of plant by producing different type of hormones such as Indole acetic acid, Gibberellic acid (21).
To increase the productivity Nitrogen is essential mineral for that and that nitrogen is provided to rice plant by means of biological nitrogen fixation (4,60).
4-5% of nitrogen is present in Azolla on dry basis and 0.2-0.4% is present on wet basis. Azolla is very important means of organic fertilizer as it provide nitrogen to rice plant (21).
It also has the ability to decompose the soil fastly and also provide the plant important minerals like phosphorus, potassium iron, zinc and other micro elements (61).

Phosphate solubilization
The bacteria which solubilize phosphorus are known as phosphate solubilizing bacteria. These bacteria (38,62). This process cause the acidification of the microbial cell and its surroundings.

Iron uptake
Iron play different important roles in plants as a photosynthetic system, chlorophyll for light absorbance and take part in many different biosynthetic mechanisms. the iron present in soil is not sufficient for the plants. Although the iron is abundant on the earth it cannot be utilized by the plants and microbes due to its presence in insoluble form.
The bacteria convert them into a soluble form by following method. They excrete molecules that bind with iron called siderophores. These molecules make bind with high affinity to iron. These soluble complexes then taken by bacteria or plants and convert it into ferrous or iron. Siderophores have low molecular weight having iron-binding groups. (38,69,70)

Indirect methods
Many plant diseases are causing the loss of yield of many plants of about 10% in many developed countries (38).
The scientists are promoting the PGPB in case of decrease of the chemicals.

Antibiotics
The PGPBs control the effect of many pathogen attacks by producing antibiotics that will act against the pathogen and prevent the plant from the disease. The PGPBs produce different antibiotic at different conditions. we can alter the PGPBs producing antibiotics in laboratories conditions.(71)

Cell wall degrading enzymes
Some plants produce cell wall degrading enzymes which act against fungus. for example, chitinase produced by the plants used to degrade the chitin of the fungus. The PGPBs also producing many cell wall degrading enzymes when they were studying in the lab to become an active biocontrol agent. So the PGPBs give protection to the plants by

Increasing crop productivity
Biofertilizers can be used as an economic input to enhance the productivity of crop; doses of fertilizers could be reduced and nutrients in high quality from soil can be harvested. The technology of production in case of biological fertilizers is simple and cost-effective in comparison to synthetic fertilizer plants.
When biofertilizers are employed as a soil inoculants they proliferate and play a role in ecological cycle of nutrients and proved helpful for the productivity of crops (75).
Growth and yield of plant have been upgraded by frequent inoculation of very potent rhizobacteria (26) and inoculation with(1).
It is reported that microorganisms such as phosphate solubilizing bacteria which we use as biofertilizer can increase the yield up to 200-500kg/ha and therefore superphosphate of about 30-50kg can be stored.
We conducted experiments on Bray-1P and cabbage by applying chemical fertilizers and organic biofertilizers to evaluate their effects on the yield of cabbage and bray-1P.

Role of biofertilizer in photosynthesis
Higher photosynthesis show improve growth rate of plant as about 90% of plant body is achieved through carbon dioxide assimilation by photosynthesis (76). It was announced that some strains of Rhizobia sufficiently increase the plant leaves surface area, plant rate of photosynthesis, stomatal opening and closing & efficiency of water availability showing that the photosynthetic ability of plant can be increased by Rhizobial administration (77).
Leaves are major part of plant having photosynthetic organs (chlorophyll). Therefore number of leaves are very important. Increase in number of leaves may activate the high growth of roots and also improve the transportation of water taken by plant roots and also improve the accumulation of minerals and yield (38)

Cost Effective
Biofertilizers on action increase the soil fertility without leaving any harmful effect in the soil and they are much more cost effective. There is a need for that agriculture system which is sustainable, in which the fertility of soil is maintained by using cost-effective resources which should be renewable such as biological fertilizers.
The need for decreasing the costs of fertilizing crops with reconvert able energy forms has revitalized the importance of biofertilizers all around the world.
Chicken manure as a biofertilizer containing a higher nitrogen content in comparison to expensive inorganic or synthetic fertilizers, easily available, hence it is easily affordable for the farmers. (2)

Mode of action of bio-fertilizers
Bio fertilizers can be applied in many ways into the soil  Dry biofertilizers are heterogeneous with the seeds  Biofertilizers applied in powdered form in soil.
 A little amount of water is added into the seed hopper and then biofertilizers are add this is called ass sprinkle method.
 Biofertilizers are used in suspended form in water after that added to seeds this is called slurry method.
 Method of seed pelleting (coating the seed with biofertilizer)  In a slurry the seed and biofertilizers are mixed and adhesive are applied and coat the seed with lime like materials.
 During the sowing the peat suspension in water sprayed into the furrow.
 Inoculation seed treatment.
 Apply in soil  Apply in seed links or root dip.(78)

Future perspective
For the decades the development and growth of the plants were depended on the conventional methods like chemical fertilizers (62,79,80) Which increase the production of yield up to a limit but they also make the environment pollute and the use of chemicals were hazardous to both human and beneficial microbes.
A review clearly brings us to the point that bio fertilizers are the sources of increasing the growth as well as nutritional amount in crops. Different types of bio fertilizer are in practice and the field of bio fertilizer is an emerging one in which many microbes are used to increase the yield of crop as well as protect it from different types of pathogens. The use of bio fertilizers are very useful and it believe that it will growth with time (81) But in future few issues regarding bio fertilizers will need to be addressed and serious action have to be taken to solve those (28). And also require research on the performance of different efficient stains and also their effect on other beneficial microbes and if the conditions change what will be there effect and performance.

Conclusion
At last of our review we concluded that today microbial based fertilizers are very significant for the improvements crop. There are two means due to which we have to use the bio fertilizers. First one that they provide the unavoidable amount of yield and nutrition to human food, they are very safe to use for environment, plants and animals and human and highly eco-friendly. Second one is that they ensure the sustainable growth of agriculture by providing the nutrition to plant in it rhizosphere such as N, P, and K and other minerals and vitamins. Different types microbes are used for this purpose like PGPB and fungi which work exceptional well.