Challenges ahead of the use of biofuels and their environmental impacts

The limited availability, high cost and the adverse environmental impacts of fossil fuels, persuaded the researchers toward the use of renewable energies. Biofuels are considered as one of the main sources of renewable energies. Woody debris, agricultural waste, sugar cane, vegetables as well as oils of cereals and vegetables are among the primary sources for biofuels production. Biofuels are produced from living organisms or their metabolic byproducts (organic waste products or food). Lignocelluloses of plant cell wall are  interesting sources for production of sustainable biofuels. Theconversion of lignocellulose to biofuels using existing technologies is too expensive mainly because of the need for  chemical pretreatments and enzymatic hydrolysis required forcell wall deconstruction. Modification of cell wall composition by using biotechnological approaches resulted in the reduction of cell wall hardness in various plant species. These results are obtained by reducing the amount of lignin as well as changing the composition and structure of final polymer. In addition, in some cases the reduction of cell wall hardness has been achieved by altering the hemicellulose biosynthesis genes as well as the ectopic expression of microbial genes in the plants with the aim of breaking the connections between lignin and carbohydrates residues. So far, genetic engineering has been used to improve the enzymes involved in digestion of cellulose and lignocellulose sources and engineering metabolic pathways involved in the production of biofuels. Also, these transgenic plants often produced higher sugar yield and ethanol. Modification of the cell wall in transgenic plants can reduce the stiffness of the cell wall without negative impact on performance. This shows that cell wall modification can be resulted without affecting the cell wall integrity or plant performance. To better understanding of the cell walls structure, we need to modify the lignocellulosic components as well as to reduce the price of biofuels production. In this review we tried to give a comprehensive introduction of biofuels and to introduce different potent methods of genetic engineering for the efficient production of biofuels. We also tried to point out all the important points on the pathway that can be modify and improved by biotechnology engineering. Finally, by evaluation the ahead challenges, the appropriate methods are suggested.