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.

Root traits and cellular level tolerance hold the key in maintaining higher spikelet fertility of rice under water limited conditions.

Reduced spikelet fertility appears to be one of the major factors responsible for the decreased rice grain yield when cultivated under semi irrigated aerobic condition. We demonstrate that genotypes with better root systems coupled with higher cellular level tolerance (CLT) can significantly improve spikelet fertility under semi-irrigated aerobic condition in the field. A set of 20 contrasting rice accessions differing in root traits and CLT with significant molecular diversity were subjected to specific soil moisture regimes during a period between five days before and 10 days after anthesis. Lowest spikelet fertility was observed among the plants grown under water limited (WL) conditions followed by the plants grown aerobically in field conditions (AF). Deep rooted genotypes generally maintained higher spikelet fertility under both WL and AF conditions. Furthermore, genotypes that had high roots biomass as well as high CLT recorded the lowest reduction in spikelet fertility under WL and AF compared with the low root and low CLT genotype. This study emphasised the relevance of combining water acquisition and CLT for improving field level tolerance of rice to water limitation. Such genotypes recorded significantly higher grain yield under stress as well as well watered conditions. The study led to the identification of promising trait donor genotypes which can be exploited in breeding to develop superior trait pyramided cultivars suitable for semi irrigated aerobic cultivation.