Leaf mass area determines water use efficiency through its influence on carbon gain in rice mutants.

Saving water and enhancing rice productivity are consensually the most important research goals globally. While increasing canopy cover would enhance growth rates by higher photosynthetic carbon gain, an accompanied increase in transpiration would have a negative impact on saving water as well as for sustainability under water-limited conditions. Increased water use efficiency (WUE) by virtue of higher carbon assimilatory capacity can significantly circumvent this trade-off. Here, we report leaf mass area (LMA) has an important canopy architecture trait which when combined with superior carboxylation efficiency (CE) would achieve higher water productivity in rice. A set of 130 ethyl methanesulfonate induced mutants of an upland cultivar Nagina-22 (N22), was screened for leaf morphological traits leading to the identification of mutants differing in LMA. The wild-type, N22, along with a selected low-LMA (380-4-3) and two high-LMA mutants (392-9-1 and 457-1-3), all with comparable total leaf area, were raised under well-watered (100% Field Capacity (FC)) and water-limited (60% FC) conditions. Low Δ13 C and a higher RuBisCO content in high-LMA mutants indicated higher carboxylation efficiency, leading to increased carbon gain. Single parent backcross populations developed by crossing high and the low-LMA mutants with N22, separately, were screened for LMA, Δ13 C and growth traits. Comparison of dry matter accumulation per unit leaf area among the progenies differing in LMA and Δ13 C reiterated the association of LMA with CE. Results illustrated that high-LMA when combined with higher CE (low Δ13 C) lead to increased WUE and growth rates.

Allele-specific analysis of single parent backcross population identifies HOX10 transcription factor as a candidate gene regulating rice root growth.

Understanding the molecular and physiological mechanisms of trait diversity is crucial for crop improvement to achieve drought adaptation. Root traits such as high biomass and/or deep rootedness are undoubtedly important drought adaptive traits. The major aim of this investigation was to functionally characterize a set of ethyl methane sulfonate-induced rice mutants for root traits. We report the identification of a high-root biomass mutant through a novel screening strategy for yield and Δ13 C measurements. The high-root mutant (392-9-1) thus identified, had a 66% higher root biomass compared to wild-type (Nagina-22). Better maintenance of leaf turgor and carbon assimilation rates resulted in lower drought susceptibility index in 392-9-1. Targeted resequencing revealed three non-synonymous single nucleotide variations in 392-9-1 for the genes HOX10, CITRATE SYNTHASE and ZEAXANTHIN EPOXIDASE. Segregation pattern of phenotype and mutant alleles in a single parent backcross F2 population revealed a typical 3:1 segregation for each of the mutant alleles. The number of F2 progeny with root biomass equal to or greater than that of 392-9-1 represented approximately one-third of the population indicating a major role played by HOX10 gene in regulating root growth in rice. Allele-specific Sanger sequencing in contrasting F2 progenies confirmed the co-segregation of HOX10 allele with the root biomass. The non-synonymous mutations in the other two genes did not reveal any specific pattern of co-segregation with root phenotype, indicating a strong role of HOX10, an upstream transcription factor, in regulating root biomass in rice.

Introgression of Root and Water Use Efficiency Traits Enhances Water Productivity: An Evidence for Physiological Breeding in Rice (Oryza sativa L.).

BACKGROUND: Semi-irrigated aerobic cultivation of rice has been suggested as a potential water saving agronomy. However, suitable cultivars are needed in order to sustain yield levels. An introgression of water mining and water use efficiency (WUE) traits is the most appropriate strategy for a comprehensive genetic enhancement to develop such rice cultivars. RESULTS: We report a novel strategy of phenotyping and marker-assisted backcross breeding to introgress water mining (root) and water use efficiency (WUE) traits into a popular high yielding cultivar, IR-64. Trait donor genotypes for root (AC-39020) and WUE (IET-16348) were crossed separately and the resultant F1s were inter-mated to generate double cross F1s (DCF1). Progenies of three generations of backcross followed by selfing were charatcerised for target phenotype and genome integration. A set of 260 trait introgressed lines were identified. Root weight and root length of TILs were 53% and 23.5% higher, while Δ13C was 2.85‰ lower indicating a significant increase in WUE over IR-64. Five best TILs selected from BC3F3 generation showed 52% and 63% increase in yield over IR-64 under 100% and 60% FC, respectively. The trait introgressed lines resembled IR64 with more than 97% of genome recovered with a significant yield advantage under semi-irrigated aerobic conditions The study validated markers identified earlier by association mapping. CONCLUSION: Introgression of root and WUE into IR64, resulted in an excellent yield advantage even when cultivated under semi-irrigated aerobic condition. The study provided a proof-of-concept that maintaining leaf turgor and carbon metabolism results in improved adaptation to water limited conditions and sustains productivity. A marker based multi-parent backcross breeding is an appropriate approach for trait introgression. The trait introgressed lines developed can be effectively used in future crop improvement programs as donor lines for both root and WUE.