Breeding Rice for Sustainable Bioenergy Production

*Manasi Dash, Abinash Mishra and Mahendra Kumar Mohanty*

### **Abstract**

Bioenergy including biofuels from lignocellulosic biomass has immense potential to meet growing energy demand of the ever-growing world population. Bioenergy will help to mitigate the environmental problems arising due to burning of fossil fuels. Rice is the staple food for more than half of the world population and is grown in more than 100 countries. Rice straw is rich in lignocellulose and several technologies are available for efficient extraction and conversion of cellulose to ethanol. Thus, the surplus rice straw can be utilised to produce biofuel, so as to replace conventional fossil fuel sources. But it is reported that the present-day rice varieties showing high lignocellulosic straw biomass have low grain yield potential. Hence, it is important to re orient the breeding strategies for developing dual purpose rice varieties that are bioenergy efficient without compromising grain yield.

**Keywords:** Rice, Bioenergy, Cellulose, Lignin, Cell wall architecture, Genomics, QTLs

#### **1. Introduction**

After the Paris climate change agreement in 2016, its signatories are making considerable efforts towards reducing carbon emissions into the atmosphere. Production of biofuel also called as 'green energy' will be a key target to achieve this by reducing the use of petrochemicals. Now focus is to harness ethanol from the existing ample quantity of lignocellulosic feedstocks such as rice and wheat straws, which are usually burnt in the fields thereby causing air pollution and health hazards [1]. The bioenergy crops have tremendous potential to address the twin issues of climate change and energy security by eliminating the 'food verses fuel' disputes.

Of the various crops grown worldwide, rice has an immense potential to be used as a dual-purpose crop, due to it's wide geographical distribution, covering entire tropical, subtropical and Mediterranean region of the globe [2]. High amount of cellulose (32–47%) and hemicellulose (19–27%) in rice straw, which can be converted to biofuel, has made it a potential future bioenergy crop [3–6]. But the cell wall polymers (cellulose, hemicellulose and lignin) form a complex network by crosslinking with each other. Hence, various pre-treatments are employed in order to break this complex to ensure higher amount of cellulose availability for the activity of cellulosic enzymes to yield considerable sugars. These pre-treatments are costly and environment unfriendly, so various genetic approaches can be utilised to enhance cellulose availability. Lignin, comprising three main types of monolignols, serves as a promising target to alter the cell wall architecture in different ways in rice [7–9]. Cellulose synthases genes particularly, OsCesA4, OsCesA7 and OsCesA9, associating with specific phenotypes, can also be suitably engineered to enhance cellulose content without changing lignin and other polymers in cell wall [10]. So, breeding approaches that can alter plant cell wall architecture can be used to develop bioenergy efficient rice variety but, subjected to one condition that it should not affect yield contributing traits negatively.

Usually, a negative correlation has been observed between grain yield and biomass traits. Breeding for high grain yield is associated with developing cultivars with reduced plant height and short leaves and thereby, reducing the plant biomass as a whole. The plant breeding strategies have to be reoriented towards selection of higher yielding plants with moderate biomass traits including lesser ash & potassium content in vegetative biomass. Also, the role of stay green traits, fostering greater decomposition of vegetative biomass as well as rewarding higher yield, can never be underestimated in this regard. This chapter will deal with the above said issues and measures, with the prime focus on methods for developing rice genotypes for higher yield and greater biofuel production, in subsequent heads.
