Genetic enhancement of reproductive stage drought tolerance in RPHR-1005R and derivative rice hybrids through marker-assisted backcross breeding in rice (Oryza sativa L.).

BACKGROUND: Drought stress is considered as one of the major production constraints in rice. RPHR-1005R is a restorer line (R-Line) with a popular, medium-slender grain type, and is the male parent of the popular Indian rice hybrid, DRR-H3. However, both the hybrid and its restorer are highly vulnerable to the drought stress, which limits the adoption of the hybrid. Therefore, the selection of the restorer line RPHR-1005R has been made with the objective of enhancing drought tolerance. METHODS AND RESULTS: In this study, we have introgressed a major QTL for grain yield under drought (qDTY 1.1) from Nagina22 through a marker-assisted backcross breeding (MABB) strategy. PCR based SSR markers linked to grain yield under drought (qDTY1.1 - RM431, RM11943), fertility restorer genes (Rf3-DRRM-Rf3-10, Rf4-RM6100) and wide compatibility (S5n allele) were deployed for foreground selection. At BC2F1, a single plant (RPHR6339-4-16-14) with target QTL in heterozygous condition and with the highest recurrent parent genome recovery (85.41%) and phenotypically like RPHR-1005R was identified and selfed to generate BC2F2. Fifty-eight homozygous lines were advanced to BC2F4 and six promising restorer lines and a hybrid combination (APMS6A/RPHR6339-4-16-14-3) were identified. CONCLUSIONS: In summary, the six improved restorer lines could be employed for developing heterotic hybrids possessing reproductive stage drought tolerance. The hybrid combination (APMS6A/RPHR6339-4-16-14-3) was estimated to ensure stable yields in drought-prone irrigated lowlands as well as in directly seeded aerobic and upland areas of India.

Expression profiling of immature florets of IR58025A, a wild-abortive cytoplasmic male sterile line of rice and its cognate, isonuclear maintainer line, IR58025B.

Interaction between gene products encoded by the cytoplasm and nucleus form the core of wild abortive cytoplasmic male sterile (WA-CMS) system of hybrid breeding in rice. Gaining insights into such interactions can be helpful in the development of better three-line rice hybrids and also identify novel male sterility systems. In the present study, the whole transcriptome profiles of immature florets of IR58025A, a WA-CMS line and its isonuclear maintainer line, IR58025B, collected at pre-anthesis stage were compared to delineate the pathways involved in pollen abortion and male sterility. Among the 774 differentially expressed transcripts (DETs), 496 were down regulated and 278 were up regulated in IR58025A compared to IR58025B. The genes associated with oxidative stress response, defense response, etc. were significantly up-regulated, while those associated with respiration, cell wall modifications, pectinesterase activity, etc. were significantly down-regulated in the WA-CMS line. Gene ontology and pathway enrichment analyses revealed the down-regulation of both nuclear and organellar genes involved in key metabolic processes of cell respiration, photosynthesis and other energy yielding metabolites in IR58025A, relative to IR58025B, indicating a general shift toward conservation of energy and other key resources in the florets of WA-CMS line. The data derived from RNA-Seq analysis were validated through qRT-PCR analysis. Based on the results obtained, it can be hypothesized that pollen abortion principally occurs due to up-regulation of pathways leading to oxidative stress leading to energy starvation conditions in consonance with reduced expression of genes associated with the cell wall formation, respiration, and other key metabolic processes.