Triticeae Genomics and Genetics, 2025, Vol.16, No.3, 110-119 http://cropscipublisher.com/index.php/tgg 113 4 Key Drought-Responsive Traits and Associated QTLs inTriticeae 4.1 QTLs linked to root architecture and water-use efficiency When it comes to drought resistance, the "ability" of the root system is often the most crucial. Just like the roots can grow deeper and expand outward, this is particularly useful for the wheat tribe in "finding water" during drought. In fact, many years ago, some studies had already locked the QTLS that control such root traits on chromosomes 2B, 4A, 5A and 7B (Peleg et al., 2009). Interestingly, these loci are often crowded together with QTLS of yield or other drought resistance traits, which may indicate that the mechanisms behind their "management" are similar. On the other hand, some QTLS regarding leaf water use efficiency (LWUE) have also been identified, and they are often related to yield. Although some traits are not easy to observe directly, these indirect indicators are actually very valuable for reference when breeding drought-resistant varieties. 4.2 QTLs associated with stay-green, leaf rolling, and canopy temperature When drought strikes, whether the leaves will turn yellow early, whether they will curl, and whether the temperature on the leaf surface will soar-these seemingly small details actually all reveal the "stress response" of crops. Like the "greenness retention" that delays leaf aging, one of its indicators is the chlorophyll content, and the corresponding QTL appears on chromosomes 1A and 6B. As for leaf curling, the currently known related QTLS are concentrated in 3B and 4A (Khaled et al., 2022). And the canopy temperature-which can reflect the transpiration status and water utilization of plants-its significant QTL is mainly also on chromosome 3B. These physiological traits do not exist in isolation; the QTLS corresponding to them are often associated with yield. This also indicates that genotypes that can maintain normal photosynthetic function under drought conditions are worthy of close attention. 4.3 Yield-related QTLs under drought conditions (grain number, biomass, harvest index) Many people regard yield as the ultimate goal, but in fact, in a drought situation, this "outcome" is determined by many factors together, such as the number of grains per panicle, biomass accumulation, and even the harvest index. Interestingly, multiple studies have found that the QTLS of these traits are not isolated. Chromosomal regions like 1B, 1D and 7D can often stably "appear on camera" in different environments. Some QTLS on 7D-b, which are related not only to the 1000-grain weight but also to the heading period and yield, have also been verified in the field experiments under high temperature and drought. There are still many such yield QTLS that are "superimposed" on QTLS with physiological or morphological traits. This overlap suggests that drought adaptation does not rely on a single trait but on a set of closely collaborating genetic mechanisms. 5 Case Study 5.1 Field studies in semi-arid regions identifying consistent QTLs for drought tolerance There are many field trials in semi-arid areas, but the ones that can truly screen out stable QTLS from them still rely on long-term, cross-regional multi-point studies. For instance, in some experiments using double haploid and recombinant inbred line populations, under both irrigation and rain-fed conditions, hundreds of QTLS related to key agronomic traits or physiological indicators have been identified. However, not every QTL can perform consistently in various environments. Those major QTLS on chromosomes 5A, 7A, and 1B can be regarded as "stable" only when they repeat in multiple situations (Tahmasebi et al., 2017). Of course, there are also studies that distinguish specific QTLS under different water pressures through environmental clustering or meta-analysis (Acuna-Galindo et al., 2015; Touzy et al., 2019). These results not only enrich our understanding of the genetic basis of drought tolerance, but also point out several potential areas that may be applicable to multiple ecological regions. 5.2 Validation of QTLs across genetic backgrounds and multi-year trials Ultimately, whether a QTL is trustworthy or not depends on whether it can still hold its own in different varieties and years. Some studies have demonstrated that QTLS at sites such as 5A and 7A perform quite stably both under irrigation conditions and in water-scarce environments (Figure 2) (Gahlaut et al., 2017; Xu et al., 2017). More importantly, they can also be repeatedly detected in different genetic backgrounds, which is no accident. Later, there were studies that conducted Meta-QTL analyses, integrating dozens of results and ultimately summarizing
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