UNDERSTANDING THE GENETIC AND PHYSIOLOGICAL MECHANISMS OF DAY AND NIGHT-TIME HEAT STRESS TOLERANCE RELATED TO PHOTOSYNTHESIS AND KEY AGRONOMIC TRAITS IN WHEAT (Triticum aestivum L.)

Kaviraj Singh

doi:10.7273/000006550

Wheat (Triticum aestivum L.) is the second most widely cultivated food crop in the world. Abiotic stress factors particularly high temperature stress causes significant yield reduction in wheat. Every 1°C rise in temperature above the optimum has been shown to result in 5-6% yield loss. Heat stress poses threat to the food productions today which will become even a bigger threat in the future with the changing climate. The increase in global nighttime temperature has been predicted to be 1.4 times higher than that for the daytime temperature, but our understanding of its genetic and physiological basis remains very limited. Thus, the goal of this study was to understand the genetic and physiological underpinnings of the daytime and nighttime heat stress tolerance in wheat. There were two aspects to achieve this goal. First, since the information is so limited, a study was conducted to identify the quantitative trait loci (QTLs) and their corresponding candidate genes controlling high night-time (HNT) stress tolerance. Second, to understand the role of Rubisco activase (Rca) in controlling the photosynthetic potential of wheat under daytime heat stress tolerance in comparison to that under control conditions. With the objective to identify genes controlling the HNT trait, first a doubled-haploid (DH) production method using maize pollination system was optimized and used to develop DH mapping populations (Chapter 4). A DH population developed from a cross between a selection out of cultivar ‘Giza 168’ that is HNT tolerant and PBW 343, a mega variety in SE Asia that turned out to be highly susceptible to HNT. The population along with the parents were evaluated under 30°C night-time conditions while keeping the day temperature to normal growing conditions (22-24°C). The same daytime temperature and 16°C night-time temperature was used as a control. These growing conditions negatively impacted all seven agronomic traits (Days to Heading, Plant Height, Spikelet Number, Tiller Number, Total Spike Weight, Biomass, and Grain Yield). Reduction in the trait values ranged between 0.5 to 35% for the HNT tolerant parent compared to 8 to 75% for the HNT susceptible parent. Trait value reduction under HNT among the DH population for the seven agronomic traits ranged between 8 to 50%. QTL mapping using an average of 1 million sequence reads per DH line enriched for the genic fraction identified 32 QTLs, out of which 19 QTLs were detected under HNT stress treatment and the remaining 13 were for traits under normal growing conditions. The 32 QTLs mapped to 25 chromosomal intervals on 13 of the 21 wheat chromosomes, explaining between 9 to 28% of the cumulative phenotypic variance under HNT stress. The size of QTL intervals ranged between 0.021 to 97.48 Mb, with the number of genes in each interval ranging between 2 and 867. A candidate gene analysis for the smallest QTL intervals revealed eight putative candidate genes for four traits in the six QTL intervals for HNT stress tolerance. With the objective to evaluate variation for the effect of daytime heat stress on photosynthesis, various parameters measuring photosynthetic potential were measured under two heat (flash and acclimation) treatments in comparison to control. Using five distinct controlled condition screening protocols, a systematic screening of ~1,200 wheat lines identified 24 lines with varying heat stress tolerance parameters at different stages of wheat growth. The lines were chosen with care to have complementing heat-tolerance mechanisms. Photosynthetic capacity was measured on the selected 24 lines using CO2 gas exchange, chlorophyll fluorescence, Rubisco carboxylation and was compared to that of Rubisco activase (Rca) expression. On an average, heat stress (40°C) decreased net photosynthesis rate (An) by ~20%. The flash treatment had a pronounced effect on photosynthesis metrices. In five wheat genotypes, An increased with heat treatment (e.g. ~12% in KSG1195). Stomatal conductance, internal CO2 concentration, ratio of intercellular to ambient CO2, transpiration rate, water use efficiency, light adapted PSII maximum efficiency, and observed PSII operating efficiency showed differential response to heat stress with an overall reduction in efficiency under heat stress. Heat stress affects various components of photosynthetic machinery of which Rubisco activation inhibition due to heat sensitive Rca is the most prominent. Detailed comparison of Rca coding genes identified a tandem duplication in the grass lineage. Three isoforms of Rca (TaRca1β, TaRca2α, and TaRca2β) are encoded in wheat and gene expression analysis under control (22°C) and heat (40°C) revealed that these isoforms differ in thermostability. Relative expression of Rca correlated with An and the individual contribution of each isoform was established. Expression of TaRca2β isoform had a strong positive correlation with An under control conditions. Expression of TaRca1β was heat inducible with a higher correlation with An under heat treatment as compared to that under control. Detailed analysis of the promoter region of the two Rca gene copies among various plant species showed insertion of several transposable elements harboring heat responsive elements in the heat inducible copy of the gene. Wheat accessions retaining a high photosynthetic potential and Rca expression at elevated temperatures were identified.

UNDERSTANDING THE GENETIC AND PHYSIOLOGICAL MECHANISMS OF DAY AND NIGHT-TIME HEAT STRESS TOLERANCE RELATED TO PHOTOSYNTHESIS AND KEY AGRONOMIC TRAITS IN WHEAT (Triticum aestivum L.)

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