**1. Introduction**

Rice (*Oryza sativa* L.) is one of the important cereal crops of the world [1]. In Thailand, rice is the major agricultural export, especially Khao Dawk Mali 105 (KDML105) rice. The cooked kernels of

KDML105 rice have a highly prized scent and texture [2]. KDML105 rice is normally grown in the northeast of Thailand, based on rain with limited irrigation [3]. Therefore, it is always a ffected by drought stress, leading to the reduction in growth and yield.

Drought stress a ffects plant morphology, physiology, and molecular mechanisms. Upon drought stress, cell turgor pressure is decreased due to low water potential in cells. This causes a decrease in the relative water content, leaf water potential, stomatal conductance, and transpiration rate [4]. Cell expansion and elongation are inhibited, resulting in the reduction of plant height, leaf area, growth, and yield [5]. Photosynthesis is one of the important physiological mechanisms a ffected by drought stress. The decrease in leaf expansion, leaf area, and stomatal conductance limits CO2 uptake [6]. The photosynthetic pigments (chlorophyll *a*, chlorophyll *b*, and carotenoids) can also be damaged by drought stress, resulting in their degradation and decreased light absorption and maximum photosynthetic rate [7]. During drought stress, phosphoenolpyruvate carboxylase, nicotinamide adenine dinucleotide phosphate-malic enzyme, Rubisco, fructose-1,6-bisphosphatase, and pyruvate orthophosphate dikinase activities are decreased, which can reduce the photosynthetic and electron transport rate [8]. The physiological responses to drought tolerance include osmotic adjustment, osmoprotection, antioxidation, scavenging defense, and photorespiration [9,10].

Kanjoo et al. [11] developed chromosome segmen<sup>t</sup> substitution lines (CSSLs) in the background of variety KDML105. CT9993, a variety with a good rooting system, and IR62266, a variety with high osmotic adjustment ability, were hybridized, and their F1 was used to generate double haploid lines. The double haploid lines were evaluated for yield, yield components, and morpho-physiological characters under drought-stress conditions, defining drought-tolerant quantitative trait loci (DT-QTLs) on chromosomes 1, 3, 4, 8 and 9. The doubled haploid line DH212 carries CT9993 alleles on in all chromosomes, while DH103 has IR62266 alleles on chromosome 8. These lines were selected as donor lines for CSSL development. Repeated crossing to KDML105 resulted in CSSLs with the putative drought-tolerant genes from chromosome 8, donated by DH103, and the CSSLs with the DT-QTL from chromosome, 1, 3, 4 and 9, donated by DH212 [3].

CSSL104 is a drought-tolerant KDML105 CSSL carrying the chromosome 8 introgression from inbred DH103 [11]. Compared to KDML105, CSSL104 had higher relative water content, higher chlorophyll fluorescence (Fv/Fm), and lower leaf drying score under 50% field capacity drought-stress conditions [12]. This implied that the region introgressed from DH103 carried the putative drought-tolerant genes.

To investigate the mechanisms a ffected by the introgression of DH103 genes, the physiological responses to drought stress of CSSL104 were evaluated relative to KDML105 and the DH103 donor line. Then, drought-tolerance genes were predicted based on genomic sequence comparison and co-expression network analysis. Finally, the putative drought-tolerance genes were validated using the corresponding *Arabidopsis* mutants. This study will be beneficial to the future development of drought-tolerant rice.
