Dissecting drought tolerance for Syria rehabilitation

Abstract

Wheat Triticum spp. is the most widely grown cereal crop in the world and one of the central pillars of global food security (Rosegrant et al., 2001), about 728 million metric tons of wheat was produced on 208 million hectares in 2013 (FAO, 2015). By the year 2050, the world population is estimated to be 9 billion and the demand for wheat will exceed 900 million tons. Fulfilling this demand is very challenging in the face of climate change, increasing drought, heat stress, and emergence of new virulent diseases and pests. Offsetting these challenges requires designing an effective wheat breeding strategy with the application of new technologies and tools in order to develop varieties with high yield potential and resistance/tolerance to abiotic and biotic stresses, and with acceptable end-use qualities. (Tadesse et al., 2013). Durum wheat (Triticum turgidum L. var. durum) is a self-pollinated tetraploid cereal crop (Soiano et al.. 2018) grown in a range of climatic zones varying from warm and dry to cool and wet environments (Giraldo et al., 2016) .The Mediterranean Basin is the largest durum producing area worldwide (Bonjean et al., 2016). Drought is the main environmental stress that determines its productivity (Mardeha et al., 2006), and often exacerbated by the incidence of extreme temperature during the grain filling period causing high losses in production (Rajaram et al., 2006). Drought is arguably the most important abiotic stress that affects wheat productivity in the world and affects both source and sink strengths, leading to yield reduction up to 92% in wheat, depending on the crop growth stage, duration and intensity of drought stress (Semenov et al., 2014). The drought stress particularly during reproductive development reduces grain number and grain size in wheat (Dolferus et al., 2011; Dong et al., 2017; Ma et al., 2017). Developing wheat cultivar tolerant to drought stress occurring during reproductive phase is currently a big challenge to wheat breeders (Cattivelli et al., 2008; Mwadzingeni et al., 2016). Drought tolerance is not a qualitative trait, but a complex quantitative plant trait, that is controlled by numerous genes and plant traits, with minor individual contributions (Blum, 2010; Dolferus et al., 2013; Hu and Xiong, 2014; Serba and Yadav, 2016). Breeding for drought tolerant wheat cultivar is made especially challenging due to the network of traits involved and their polygenic control. This in turn results in high genotype by environment (G×E) interactions, low heritability, and difficulty to conduct mass screening of plant traits and genes (Cattivelli et al., 2008; Fleury et al., 2010; Hu and Xiong 2014). On the other hand, the narrow genetic base of many durum wheat varieties ((Makai et al., 2016) selected under strong breeding pressure for identical target objectives, does not seem to provide the amount of plasticity necessary to target such a complex trait (Jing et al., 2013). Compared with domesticated varieties, crop wild relatives and primitive wheat have been challenged in natural environments for thousands of years and maintain a much higher level of diversity (Zhang et al., 2016). Hence, interspecific hybridization between durum elite lines and wild relatives of the Gramineae family is a promising method to restore variability to the modern breeding germplasm (Rajaram and Hettel, 1994). For instance, Sall et al (2018) tested 24 durum wheat genotypes for tolerance to extreme heat along the Senegal River, to reveal that the top and stable yielders were derived from interspecific hybridizations with Triticum dicoccum and Aegilops speltoides. Similarly, Zaim et al (2017) was able to confirm that crop-wild relatives crosses (CWR) out-yielded the best elites and cultivars when tested across drought-prone environments in North Africa, and even showed better quality characteristics.