Water is critical for plant growth and development. As with all crop plants, the growth of sorghum [Sorghum bicolor (L.) Moench] relies on an adequate water supply in the form of rainfall or irrigation well distributed throughout the growing season (Assefa et al., 2010; Eck and Musick, 1979). For example, a medium-to-late sorghum variety maturing between 110 and 130 days would require approximately 450 to 650 mm of water during the growing season, and for this reason, the average yield of dryland sorghum is approximately half that of irrigated sorghum (Assefa et al., 2010). Not surprisingly, the total water supply (available soil water at seedling emergence plus in-season precipitation) is significantly correlated with sorghum grain yield (r ² = 0.834), and for every centimeter increase of available soil water at seedling emergence and in-season precipitation, sorghum grain yield increases by 221 and 164 kg ha⁻¹, respectively (Stone and Schlegel, 2006). This indicates that soil water at seedling emergence is slightly more important for grain yield, probably because the early stages of plant growth (germination, emergence, and seedling establishment) are potentially the most vulnerable to drought stress (Abreha et al., 2022).

Despite yield reduction by drought, sorghum is considered more drought-resistant than many other crop plants (Hadebe et al., 2017) and shows a wide range of morphological, physiological, and biochemical adaptations in response to drought stress (Liu et al., 2024). When exposed to drought, older sorghum leaves are selectively killed, while the younger leaves remain physiologically functional as a result of osmotic adjustment in the younger leaves (Blum, 2005). Sorghum plants can have higher water use efficiency because they can reduce evapotranspiration more efficiently (Tolk and Howell, 2003). This is most likely because drought-tolerant sorghums tend to produce more epicuticular wax on their leaf surface compared to sensitive ones during drought stress (Sanjari et al., 2021). Further support comes from overexpressing a sorghum WINL1, which simultaneously increases total wax/cutin content and drought tolerance in Arabidopsis (Bao et al., 2017). Another reason may be that during drought, drought-tolerant sorghum plants show more leaf rolling than the susceptible lines, reducing the effective evapotranspiration area of the uppermost leaves by approximately 75% (Matthews et al., 1990). In addition to this leaf feature, sorghum plants tend to penetrate deeper into the subsoil (40–135 cm) (Schittenhelm and Schroetter, 2014; Singh and Singh, 1995), and more roots are produced during drought (de Oliveira et al., 2022; Queiroz et al., 2019). A combination of these two factors may account for up to 90% of the total water used by sorghum (Rachidi et al., 1993). Therefore, this root feature (more and deeper roots) has been found to be a major contributor to drought tolerance in sorghum (Wright and Smith, 1983). At the physiological level, drought induces the large central vacuole to form small vesicles when the leaf water potential is at −37 bars; this maintains tonoplast integrity and allows sorghum plants to withstand drought (Giles et al., 1976). It is not surprising that drought elicits extensive genetic (Abreha et al., 2022) and proteomic responses (Li et al., 2020) in sorghum.

Because of its importance, drought tolerance has been extensively mapped in sorghum. By searching drought tolerance-related traits in the Sorghum quantitative trait locus (QTL) Atlas (Mace et al., 2019), 817 loci were identified from 19 studies published before 2019. More recently, Tsehaye et al. (2024) mapped 32 drought-related quantitative trait nucleotides (QTNs) using an association mapping panel of 216 diverse accessions and 17,637 Single Nucleotide Polymorphism (SNP) markers, four of which colocated with previously mapped drought-related QTLs. Faye et al. (2022) mapped 16 pleiotropic associations for drought responses across water stress environments using an association mapping panel of 590 predominantly West African sorghum landraces and 130,709 SNPs. Crop landraces represent local adaptations of domesticated species and contribute novel alleles for adaptation to stressful environments (Dwivedi et al., 2016). Although larger panels (Faye et al., 2022; Tao et al., 2020, 2021) have been used in a sorghum genome-wide association study (GWAS), we found that the sorghum mini core panel is effective in QTL mapping. In a previous study, we mapped and cloned a pleiotropic QTL gene for plant height, days to 50% flowering, biomass, juice yield, and juice sugar content (Upadhyaya et al., 2022) using the sorghum mini core collection of 242 global landraces (Upadhyaya et al., 2009). The mini core panel has since been used to map sorghum panicle architecture (Wang et al., 2021), sorghum plant color (Wang et al., 2024), callus induction and regeneration from mature sorghum seeds (Xu et al., 2025), and additional developmental and reproductive traits (Upadhyaya et al., 2024).

In this study, the objective was to map drought tolerance loci that are pleiotropic for more than one trait or stable across environments. Drought stress was imposed by polyethylene glycol (PEG), which is commonly used to simulate drought in sorghum (Abdel-Ghany et al., 2020; Dugas et al., 2011; Jafar et al., 2004; Pavli et al., 2013; Queiroz et al., 2019), on the sorghum mini core panel. To carry out the mapping, we evaluated seedling shoot/root length, shoot/root fresh/dry weight, germination rate with and without osmotic stress, and drought indices, which were calculated as the ratio of growth under stressed and control conditions in 2020 and 2024 and performed a GWAS on the traits as previously described (Li et al., 2018) using 6,094,317 SNP markers (Upadhyaya et al., 2022; Wang et al., 2021; Xu et al., 2025). We identified 17 QTLs for shoot fresh and dry weight and drought index, along with five QTLs for other traits. A suite of candidate genes landed on by or closest to linked SNPs was also identified.

In this study, we evaluated a sorghum mini core panel for shoot and root growth under simulated drought conditions imposed by 10%/and 20% PEG. The results showed that certain accessions exhibited enhanced root growth—through either increased root number or elongation—under osmotic stress. A greater number of accessions produced more roots rather than longer roots when exposed to 10% PEG. However, at 20% PEG, more accessions exhibited longer roots than increased root number, reflecting the adaptability of some accessions to drought stress. These findings are consistent with those of previous studies (Bibi et al., 2010; de Oliveira et al., 2022; Schittenhelm and Schroetter, 2014; Singh and Singh, 1995). Based on the response of root length and dry weight (RL_STC and RDW_STC), IS 30533 was identified as the most drought-tolerant, while IS 32439 was the most sensitive accession. These accessions may be of particular interest for sorghum breeding programs targeting improved drought tolerance.

Drought stress significantly and negatively impacts sorghum growth (Abreha et al., 2022), especially shoot growth (Jafar et al., 2004), which was most significantly reduced by PEG treatments as measured by shoot fresh and dry weight STC (SDW_STC and SFW_STC) ( Table 1 ). This explains why among the 22 loci identified, 19 were mapped to the STC traits, which reflect the impact on growth, and 17 were mapped to SDW_STC and SFW_STC. Half of the mapped loci are also colocated with 23 previously mapped drought-related QTLs; 15 of these 23 QTLs were mapped to green leaf area, total number of green leaves, or chlorophyll content ( Table 3 ). We also found 19 candidate genes for 12 of the 22 loci. Five of those genes show either preferential or specific expression in the roots ( Supplementary Table S6 ). The relevance of some of these candidate genes to drought tolerance is explained in the following sections.

When exposed to drought stress, the immediate response must be to protect the cell. One candidate gene identified in locus 8–1 encodes Hsp20, which has been found to be induced by drought stress in sorghum (Abdel-Ghany et al., 2020; Zhang et al., 2024). As a small heat shock protein, Hsp20, may form a complex with a variety of non-native proteins to form a first line of defense against protein aggregation during stress (Haslbeck and Vierling, 2015). Drought also induces transporters for various solutes (Dong et al., 2014), and we found one polyol transporter in locus 1-3 and a chloroplastic phosphoenolpyruvate/phosphate translocator (PPT) in locus 2-1. a polyol transporter is an H⁺-dependent plasma membrane carrier that transports mannitol and sorbitol, which protect cells against osmotic stress (Shen et al., 1999) and are induced in grapevines by drought (Conde et al., 2015). PPT imports phosphoenolpyruvate (PEP) to the plastid from the cytosol. A loss-of-function mutant of PPT1 in Arabidopsis results in stunted roots (Staehr et al., 2014), potentially compromising the ability of roots to cope with drought. Regarding transporters, we identified in locus 6 -1 a nucleoporin that is the main transport channel between the cytoplasm and the nucleoplasm, and a maize nucleoporin, ZmNUP58, has been shown to play an important role in the stress response of maize. ZmNUP58 overexpression in maize significantly promotes both chlorophyll content and activities of antioxidant enzymes under drought conditions (Liu et al., 2022). Coincidentally, a chlorophyll fluorescence QTL (QCHLF6.8) (Fiedler et al., 2014) is also colocated in this locus ( Table 3 ), demonstrating the effectiveness of drought QTL mapping using the mini core panel in this study. In sorghum, the stay-green trait contributes to the adaptation to post-flowering drought conditions (Abreha et al., 2022). Since drought reduces sorghum leaf chlorophyll content (Kapanigowda et al., 2013), increased chlorophyll content during drought is a sign of drought tolerance (Kassahun et al., 2010). For this reason, we found at least five QTL clusters from six studies in which stay-green loci overlap with chlorophyll content loci: one each on chromosomes 2 and 10, and three on chromosome 3 ( Supplementary Table S8 ).

Sorghum also exhibits physiological and biochemical resistance to drought by scavenging reactive oxygen species (ROS) and changing the activity of its antioxidant enzymes (Liu et al., 2024). For ROS scavenging, a BolA protein identified in locus 10 -1 may play a negative role in ROS scavenging, as a mutation in Arabidopsis BolA causes the plant to produce longer roots and to scavenge ROS, implying an increased capacity to extract deeper soil water (Qin et al., 2015). Another example of a ROS scavenger (Yu et al., 2020) is the three aldo-keto reductases (AKR) in locus 6-2, which are mostly expressed in the roots ( Supplementary Table S6 ). In tomatoes, the majority of AKR genes are induced by drought treatments, and silencing AKR expression reduces drought tolerance due to low proline content and high malondialdehyde content, indicating AKRs’ positive role in regulating drought tolerance in tomatoes (Guan et al., 2023). So is the E3 ubiquitin-protein ligase in locus 1-1. A rice U-box E3 ubiquitin ligase (OsPUB67) was significantly induced by drought, and its overexpression enhances the reactive oxygen species scavenging ability and stomatal closure, which improves drought tolerance (Qin et al., 2020). For antioxidant activities, we found a UDP-glucosyl transferase (UGT) in locus 4-2. Overexpressing two Arabidopsis UGTs, UGT79B2 and UGT79B3, increases drought tolerance thanks to increased anthocyanin accumulation and enhanced antioxidant activity in coping with drought (Li et al., 2017), and similar results have also been reported in rice (Dong et al., 2020).

The last candidate gene to be described is jasmonic acid-amino synthetase1 (JAR1) in locus 5-2. Jasmonic acid (JA) is of central importance in drought stress responses (Wasternack, 2014). JAR1 is involved in conjugating JA to Ile, the bioactive form of JA (Staswick and Tiryaki, 2004). JAR1 plays a major role in JA signaling (Kazan and Manners, 2008), and its expression is upregulated in the early stages of drought and decreased upon persistent drought (Chen et al., 2019). Overexpressing JAR1 reduces water loss during drought, while its mutation lowers JA–Ile content and causes hypersensitivity to drought (Mahmud et al., 2022).

In conclusion, we evaluated a sorghum mini core panel for tolerance to drought simulated by PEG. We confirmed results from previous studies that sorghum plants produced more roots than longer roots at 10% PEG, but at 20% PEG, they produced longer roots than more roots, and PEG reduced shoot growth in all accessions in both years. GWAS identified 22 loci, 19 of which were mapped to the STC traits, and 17 of the 19 were mapped to the STC of shoot weight. Eleven of the 22 loci were colocated with 15 QTLs that had been previously mapped to green leaf area, the total number of green leaves, or chlorophyll content. Of the 19 candidate genes from the 12 loci mapped, five showed either preferential or specific expression in the roots according to GeneAtlas v2. One of the candidate genes from locus 6-1, colocated with a previously mapped chlorophyll fluorescence QTL, was found to increase chlorophyll fluorescence in another study. Sorghum leaf chlorophyll content is closely associated with drought tolerance. IS 30533 was the most tolerant accession, and IS 32439 was the most sensitive accession. The results from this study will facilitate sorghum marker-assisted breeding for drought tolerance.