Roots: The route to crop improvement
Authors: Monika Dalal^*, Rohini Sreevathsa, Subodh Kumar Sinha, Basavaprabhu L. Patil
National Research Centre on Plant Biotechnology, Pusa, New Delhi-110012
^*Corresponding author e-mail: monika@nrcpb.org

The root system architecture (RSA) of a plant describes the spatial configuration of root system in soil with respect to shape and structure of the root system (de Dorlodot et al. 2007). RSA determines the amount of water and nutrient uptake from soil, and hence, is a major determinant of yield specifically under input deficient and abiotic stress environment. Therefore, one approach to maximize the performance of a plant under nutrient and water deficient soil would be to modify its RSA. Genetic improvement in root system would require an understanding of the mechanisms of constitutive and adaptive development of RSA and identification of genes controlling RSA.

In the recent past, owing to advance in technology and resources, considerable progress has been made in understanding the physiology and molecular biology of RSA in plants. Reverse and forward genetics approach has been instrumental in deciphering many aspects of root development in Arabidopsis (de Lucas and Brady 2013). Extensive root transcriptomics and proteomic studies have been carried out in different cell types in roots, and gene networks have been analyzed in Arabidopsis (Birnbaum et al., 2003, Brady et al., 2007).

The root architecture and anatomy of Arabidopsis, a dicot plant, differs from monocots such as wheat, rice and maize. Arabidopsis has a tap root system, while cereals root system is composed of fibrous root system. Even among cereals, regulation of RSA in rice that is grown in flooded conditions would be different from that of wheat or maize. In case of rice and maize, several mutants related to root development have been characterized. Studies on these root specific mutants from rice and maize have also revealed that certain core components of root development are conserved between dicots and monocots (Hochholdinger and Zimmermann, 2008). By using LASER micro-dissection and microarray profiling, a rice transcriptome atlas was created for 40 different cell types including 13 from roots (Jiao et al., 2009). Comparative analysis of the data with that of Arabidopsis, revealed similar as well as unique genes and gene networks that can be useful for further research. In maize and wheat, transcriptome and proteome analysis of roots under control and abiotic stress conditions has also been reported (Mohammadi et al., 2008, Zou et al., 2010).

The underground nature and plasticity of the roots creates difficulty in phenotyping. Hence, efforts have been made to identify the quantitative trait loci (QTL) for various root parameters such as root length, thickness, penetration etc. Once QTLs are tagged with molecular markers, these markers can be used to screen and select the desired plant type (with desired QTLs) from the segregating population even in the seedling stage in the lab conditions. Though there are many QTLs for root traits, these QTLs have not been used in marker-assisted selection (MAS) breeding because either they were minor QTLs or have not been validated in different genetic background and target environments. Recently two major QTLs, DRO1 (Deeper Rooting 1) and Pup1 (phosphorus uptake 1) which regulate root growth under drought stress and phosphorous deficiency stress respectively (Uga et al. 2011, Wissuwa et al. 2002) have been identified. Fine mapping of these QTLs have led to identification of DRO1 (Deeper Rooting 1) and PSTOL1 (Phosphorus Starvation Tolerance 1) genes in rice (Uga et al. 2013, Gamuyao et al. 2012). DRO1 is negatively regulated by auxin and is involved in cell elongation. Higher expression of DRO1 increases the root growth angle, making roots grow deeper. The study revealed that under drought conditions in field, DRO1 facilitates better grain filling thus resulting in higher yield. Moreover there is no yield penalty under normal or no drought conditions. PSTOL1 encodes a protein kinase which enhances early root growth, and enables plants to acquire more phosphorus and other nutrients. In field conditions, rice with the PSTOL1 gene has been found to produce about 20% more grain than rice varieties lacking this gene. These studies reveal that there is enormous scope for crop improvement through modification of root system architecture in crops.

Until recently, the progress and success in RSA studies especially in crop plants had been rather slow due to the difficulty in phenotyping root traits. Advent of High-resolution and dynamic 3D imaging tools, software and highthroughput phenotyping plateforms would aid in unraveling this hidden better half of the plant. This in turn would hasten the progress in understanding RSA for agricultural benefit.

References:
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