Drought and salinity are two major abiotic stresses that severely limit agricultural production worldwide. The severity and occurrence of both drought and salinity stresses is going to increase as a result of global environmental changes, with a major implication for food supply (Shabala 2013; Tester and Langridge 2010). On the other hand, increasing world population requires increase in food production by more than 70% by 2050 (FAO 2011). One of the sustainable and economical solutions to achieve this goal is developing drought and salt-tolerant crops (Ashraf 2009). However, very slow progress has been made in improving tolerance or developing tolerant cultivars due physiological and genetic (quantitative inheritance) complexity of tolerance traits. Also, high variability of the field environments and low efficiency of selection methods further handicapped the progress. Most researchers agree that it is highly unlikely that tolerance to these stresses may be improved by a manipulating with expression level (function) of merely one gene. More likely, we should brace ourselves for a painstakingly slow pyramiding of useful traits. Taking salinity stress tolerance as an example, vacuolar Na+ sequestration mediated by NHX Na+/H+ exchanger
(Apse et al. 1999) could be not possible without sufficient activity of tonoplast H+-pump to
energize this process (Shabala 2013). Moreover, this sequestration will become a futile cycle if Na+ back-leak from vacuole via Na+-permeable fast (FV)- and slow (SV)- vacuolar channels is not prevented (Bonales-Alatorre et al. 2013a; Bonales-Alatorre et al. 2013b). Given that the molecular identity of some of this transport systems (e.g. FV channels) is yet to be revealed, transgenic approach to such pyramiding remains highly challenging.
In addition to the “biological” complexity of the salinity- and drought- tolerance issue, a social aspect of the problem should be not ignored. The generally negative public perception of GMO world-wide casts serious doubt over the prospects of tackling this issue by a broad use of transgenic crops. The recently imposed total ban on the use of GM-crops in Tasmania
47
is a good illustration of this fact. During a recent Barley Technical Symposium in Adelaide in 2013, representatives of major brewing companies were unanimous in their estimation that, in light of the above, transgenic malting barley varieties are unlikely to be accepted by the Industry in a foreseeable future. This calls for a renewed interest in using more traditional and publically accepted technologies such as Marker-Assisted Selection (MAS) or the newly developed genome editing technology.
MAS technology implies the use of a set of markers which are closely linked with the target gene(s) for an indirect selection of a specific traits without phenotyping the traits. While a great progress has been achieved in using MAS approach for crop breeding for a range of stresses where the tolerance is conferred by one or two major genes, the progress was more modest when it comes to salinity or drought tolerance. Numerous physiological and morphological traits were used as indirect selection criteria for both salinity and drought tolerance. Leaf wilting, relative water content (RWC) and proline contents are among the most frequently used for drought tolerance (Condon et al. 2002; Richards et al. 2002; Teulat et al. 2003). Physiological and biochemical responses used as selection criteria for salinity tolerance include seed germination under stress conditions, relative water content, wet and dry weight of roots and shoots, chlorophyll content, shoot sodium content, plant survival as well as tissue proline and carbohydrate content (Chen et al. 2005; Shavrukov et al. 2010; Xu et al. 2012).
Proline is a widely distributed osmolyte which protects plants against drought and salinity (Bohnert et al. 1995). It is mainly synthesized from glutamate by two enzymes: pyrroline-5- carboxylate synthetase (P5CS) and pyrroline-5-carboxylate reductase (P5CR) (Szabados and Savoure 2010). Apart from acting as an osmolyte to balance osmotic pressure in cells, proline also plays important roles in regulating cellular reactive oxygen species (ROS) balance (Hong et al. 2000; Yamaguchi and Blumwald 2005), cell signalling and plant development like rapid cell division, floral transition and embryo development (Mytinova et al. 2010). Proline was also shown to be able to affect intracellular ionic homeostasis by controlling ion transport across cellular membranes (Cuin and Shabala 2005, 2007a). Proline level increased dramatically in plants under both drought (Choudhary et al. 2005) and salinity (Yoshiba et al. 1995) conditions, and it was repeatedly suggested that using high proline levels as a biochemical marker may benefit stress breeding programs (reviewed in (Ashraf et al. 2008)). However, higher proline levels were also found in drought-hypersensitive (Hanson et al. 1979;
48
Singh et al. 1972) and salinity-susceptible genotypes (Ashraf and Foolad 2013; Moradi and Ismail 2007), and the causal relationship between proline accumulation and stress tolerance in plants is not that straight forward as initially thought.
In a natural environment, drought and salinity stress are often combined (Katerji et al. 2009). Both drought and salt stress trigger cellular dehydration and cause osmotic stress which then lead to cytosolic and vacuolar volume reduction (Munns 2002; Zhu 2002). Abiotic stress such as cold, drought and salt stress are controlled by many common and conserved regulatory pathways (Rabbani et al. 2003; Yamaguchi-Shinozaki and Shinozaki 2006). Drought tolerance QTL influenced growth under salt stress by reducing salt uptake (Sharma et al. 2011), indicating that some QTL/genes may have pleiotropic effects on multi-stress tolerance.
Both drought and salinity tolerance are quantitatively inherited and controlled by several genetic loci. While many QTLs being reported for drought (Baum et al. 2003; Kalladan et al. 2013; Sayed et al. 2012b; Teulat et al. 2001a) and salinity tolerance (Rivandi et al. 2011; Shavrukov et al. 2010; Xu et al. 2012; Zhou et al. 2012), very few of the linked markers have been successfully used in breeding programs due to the relatively lower heritability of the QTL and other factors affecting the gene expression. The success of using physiological traits as indirect selecting criteria for both drought and salinity tolerance relies on the true correlations between these traits and the tolerance. Most studies used very few varieties to study the relationships between drought/salinity tolerance and different agronomic/physiological traits or simply mapping QTLs for different traits under drought or salinity stress (Kalladan et al. 2013; Teulat et al. 2001a; Teulat et al. 2003; von Korff et al. 2008), which may not necessarily reflect the tolerance genes. This issue was overcome in this work that a doubled haploid (DH) population was used: 1) to investigate the linkage between various agronomic and physiological traits and drought and salinity tolerance, and 2) to identify QTLs controlling tolerance to these two stresses in barley.