Mapping of Rice High-Tillering Dwarf Gene HTD8 and Preliminary Functional Analysis of Its Involvement in Responses to Salt Stress and PEG-Induced Osmotic Stress

Rice (Oryza sativa L.) is an essential staple crop worldwide, and tillering acts as a key target for molecular breeding because of its critical contribution to grain yield. In this study, a high-tillering dwarf mutant designated htd8 was isolated from an ethyl methanesulfonate (EMS)-mutagenized population of indica rice cultivar ‘Shuangkezao (SKZ)’. The mutant exhibits reduced plant height, dramatically increased tiller number, shortened and yellow-chlorotic leaves. Compared with the wild-type (WT), htd8 displays significant decreases in plant height, internode length, panicle branch number and 1000-grain weight, accompanied by a remarkable increase in tiller count. Genetic analysis demonstrated that the htd8 phenotype is governed by a single recessive nuclear gene. By bulked segregant analysis (BSA) sequencing combined with fine mapping, the candidate gene was delimited to a 0.81 Mb genomic interval on rice chromosome 11, where two known tillering-related genes D27 and ZEBRA2 were excluded. Exogenous hormone assays revealed that htd8 had decreased sensitivity to exogenous GA3 but enhanced sensitivity to the strigolactone analog GR24, and both GA3 and GR24 could repress tiller outgrowth in the mutant. Under salt stress and PEG-simulated osmotic stress, htd8 achieved a notably higher survival rate relative to WT. The mutant accumulated more proline (Pro), maintained lower malondialdehyde (MDA) levels, and showed elevated leaf abscisic acid (ABA) content together with the upregulated expression of ABA biosynthetic-related genes. Transcriptome profiling of tiller buds indicated that HTD8 modulates rice tillering and plant architecture by coordinating brassinosteroid, auxin, gibberellin and strigolactone signaling pathways, as well as carbon metabolism redistribution. This work provides theoretical insights into the molecular mechanism underlying high-tillering dwarfism and supplies valuable genetic resources for high-yield and stress-tolerant rice molecular breeding.