Background:
Backcrossing of gamma-irradiated IS21 with MR220 has generated progenies (ML 82-2 and ML 125-2) with improved traits. However, studies on these new mutant lines remain limited. This study aimed to determine and compare the biochemical characteristics and transcriptomic profiles of drought-tolerance-related genes in ML 82-2 and ML 125-2, as well as in the parent lines, IS21 and MR220.

Material and methods:
Seeds were germinated for 14 days under a controlled photoperiod (16 h light and 8 h darkness) at a constant temperature of 25 ± 2°C. Biochemical analyses, including total soluble protein content, specific peroxidase activity (SPA), chlorophyll content, and proline content, were conducted. Transcriptomic profiling was performed using STRING and gene ontology (GO) enrichment analysis.

Results:
ML 82-2 exhibited the highest SPA, which was significantly different from that of MR220 and IS21, as well as significantly different total soluble protein content. However, ML 82-2 did not significantly differ from MR220 in chlorophyll and proline content. Drought stress-responsive genes Os01g0124401, Os08g0473900, and Os08g0518800 were identified in ML 82-2. Conversely, ML 125-2 displayed total soluble protein content and SPA similar to IS21, while chlorophyll and proline content were not significantly different from those of MR220. Drought stress-responsive genes Os10g0471100, Os01g0197100, and Os11g0701400 were identified in ML 125-2.

Conclusions:
ML 82-2 demonstrated improved SPA, whereas ML 125-2 exhibited enhanced total soluble protein content. The identified genes in both mutant lines are associated with drought resistance, with most sharing a similar genomic profile with MR220. These findings contribute to plant physiology studies and stress-responsive gene discovery in rice.