12.3Breeding Approaches to Improve

12.3Breeding Approaches to Improve RSA and Abiotic Stress Tolerance12.3.1Conventional Breeding ApproachUsing conventional breeding approaches, RSA traits of crop plants can beimproved, which would lead to the better utilization of nutrient resources andwater sequestration, leading to better production and yield [18]. Some of theimportant criteria used for studying RSA traits are the length, thickness, anatomy,branch number, distribution in soil, and penetrating ability [19]. During thedomestication process, people selected plants mainly based on their aerial characterand yield, ignoring the root system. Recently, plant breeders have realized theimportance of RSA in crop productivity and turned their attention to breed varietiesconsidering their RSA traits [7,8]. Improved RSA traits in wheat, using conven?tional breeding, have provided tolerance to abiotic stresses [20]. Several stresstolerance and RSA-related traits of interest from selected wheat cultivars are beingintrogressed into high-yielding cultivars to develop cultivars with tolerance toabiotic stresses [7,8]. Through improvement of stem, leaf patterning, and rootarchitecture, an increase in yield of 68% has been achieved in foxtail millet [1]. Ithas been shown in barley (Hordeum vulgare) that the root system is significantlyimproved using a breeding approach [21]. Similar to other crops, huge geneticdiversity in RSA traits exists among different rice cultivars that also vary in terms ofyield and stress tolerance [22]. Drought-resistant rice cultivars with better RSAtraits have been developed using conventional breeding [23]. The RSA traits (suchas deeper and thicker roots with more xylem vessels, increased root length anddensity, etc.) have been shown to play very important roles in providing droughttolerance in rice [24,25]. IET1444 and GZ5121-5-2 rice cultivars with improved RSAtraits have been shown to produce high yields under stress condition [24].Although conventional breeding based on RSA traits has made some progress,its efficiency is limited by the lack of huge breeding fields, trained breeders, and theextremely laborious process of studying less-accessible roots systems of a largenumber of plants. Moreover, it is time-consuming and the phenotypic observationsare often not reproducible due the variation of RSA traits caused by the complexinteraction between RSA and environmental factors as well as the soil condition [21,25]. In an effort to overcome these limitations, the conventional breedingapproach has gradually been replaced by the modern QTL-based approach, whichincludes identification of genetic loci (QTLs) related to RSA traits and the use ofMAS-based breeding [25].12.3.2Identification of QTLs Associated with Specific RSA Traits and Stress ToleranceA QTL is a region or portion of the genome responsible for a particular trait orphenotype. Several studies have identified QTLs associated with the root traits thatenable the plants to sustain abiotic stresses like drought, nutrient deficiency,extreme heat/cold, flooding, and so on [26]. QTLs have been identified for severaltraits like deep-root systems, root lodging, stele, and xylem structure in rice, maize,and wheat [27–29].It has been reported in rice that the genotypes classified as drought resistantshowed consistently higher cell membrane stability, more stable hydraulicconductivity, more responsiveness of root anatomy to drought, and higher levels oftemporal expression of aquaporin genes [30]. This range of traits allows efficientcontrol of the plant water status under drought [30]. Water uptake by roots isinfluenced by the size and number of xylem vessels; roots with thicker xylemvessels have a higher potential to uptake water than thinner ones [31,32]. Uga et al.have identified QTLs for rice root thickness, root stele transversal area and numberof late meta xylem vessels using a recombinant inbred line population derived fromthe cross between a lowland cultivar IR64 (with shallow RSA) and upland cultivarKinandang Patong (with deep RSA) [28]. Using the same population they furtheridentified a QTL Dro1 (deeper rooting 1) that is linked with deep RSA traits andinvolved in drought avoidance under natural conditions with limited water supply[27]. Recently, Dro1 has been shown to improve RSA and yield under droughtcondition, when introduced in a shallow-rooting rice [33]. Steele et al. have reporteda QTL for root length (between RN242 and RM201), which significantly increasedthe root length under stressed (and non-stressed) conditions and improved thepenetration ability of a rice cultivar [25,34]. By introgressing this QTL to Kalinga IIIlandrace, they developed a drought-tolerant rice cultivar that was released as “BirsaVikas Dhan 111” [25,35]. Using the double haploid population derived from a crossbetween IR64 and Azucena (upland rice cultivar), 15 QTLs for RSA and other traitshave been identified [36]. The chromosomal s