Elucidation of Aegilops tauschii contribution to heat and drought tolerance diversity in bread wheat through genomics and metabolomics

概要

（Format No. 13）

SUMMARY OF DOCTORAL THESIS
Name: Michael Okoi Itam
Title: Elucidation of Aegilops tauschii contribution to heat and drought tolerance diversity in bread

wheat through genomics and metabolomics
（ゲノミクスとメタボロミクスに基づくタルホコムギのパンコムギ耐暑・耐乾性変異
への貢献解明）
Many studies have shown the potential of using A. tauschii for breeding to enhance bread wheat
productivity in drought- and heat-prone areas. However, the diversity in heat and drought resilience
traits in bread wheat has not been fully explored. Also, there is a dearth of knowledge on the
mechanism of combined heat and drought resilience, and a lack of genetic materials for combined
stress resilience breeding. In this research, the heat and drought resilience diversity in bread wheat
lines containing Ae. tauschii introgressions was explored; the genomic regions (including loci,
candidate genes and alleles) regulating the resilience and the underlying physiological and
metabolomic dynamics were highlighted. Also, the practicability of utilizing the diversity of Ae.
tauschii for combined stress resilience breeding was assessed.
In Chapter 1, a wheat diversity panel containing Ae. tauschii introgressions was evaluated
under heat (H) and combined heat-drought (HD) stress in Sudan to identify QTLs associated with
resilience to the combined stress, and to assess the practicability of harnessing Ae. tauschii
diversity for combined stress resilience breeding. Novel alleles and quantitative trait loci (QTLs)
were identified on chromosomes 3D, 5D, and 7A controlling grain yield (GY), kernel number per
spike (KPS), and thousand-kernel weight (TKW), and another on 3D (521–549 Mbp) controlling
GY alone. A strong marker-trait association for GY stability was identified on chromosome 3D
(508.3 Mbp) explaining 20.3% of the variation. Furthermore, leaf traits including canopy
temperature (CT), normalized difference vegetative index (NDVI), and carbon-13 composition
(δ13C) were controlled by five QTLs on 2D (23–96, 511–554, and 606–614 Mbp), 3D (155–171
Mbp), and 5D (407–413 Mbp), some of which were pleiotropic for GY and related traits. Most
MTAs and QTLs were found in the D genome, indicating the potential of using Ae. tauschii
diversity for wheat breeding. Further analysis revealed candidate genes, including GA20ox,
regulating GY stability, and CaaX prenyl protease 2, regulating CT at the flowering stage, under H
and HD stress. As this is the first such study, our results provide genomic landmarks for wheat
breeding to improve adaptation to H and HD conditions under climate change.
In Chapter 2, twenty-four selected wheat lines, were evaluated under a
drought-rewatering-drought cycle for two years in a greenhouse in Tottori, Japan. The objective
was to validate the lines selected in Chapter 1. Drought was imposed by withholding water during
flowering. The results revealed considerable genetic variability in physio-agronomic traits,
reflecting the variation in introgressed segments. High heritability estimates (above 47%) were
recorded for most traits, including days to 50% heading, plant height, and TKW, indicating the
genetic control of these traits which may be useful for cultivar development. The trait-trait
correlations within and between water regimes highlighted a strong association among the genetic
factors controlling these traits. Some lines exhibited superior performance in terms of stress
tolerance index and mean productivity compared with their backcross parent (N61) and elite
cultivars commonly grown in hot and dry areas. Graphical genotyping revealed unique
introgressed segments on chromosomes 4B, 6B, 2D, and 3D in some drought-resilient lines which

may be linked to drought resilience. Therefore, these lines were recommended for further breeding
to develop climate-resilient wheat varieties.
In Chapter 3, I used two classical physiological methods, the fraction of transpirable soil
water threshold (FTSWTh) and drought stress response function, to characterize the water
conservation traits of two selected wheat lines (MSD53 and MSD345) which both contain
introgressed segments from Ae. tauschii but differ in drought resilience. The lines and N61 were
subjected to dry-down conditions. MSD53 had a higher FTSWTh for transpiration decrease than
N61 and MSD345. In terms of drought stress response function, MSD53 had the lowest threshold
suction, suggesting a lower drought resilience capacity compared with MSD345. However,
MSD53 exhibited an effective-water-use trait whereas MSD345 exhibited a water-saving trait
under dry-down conditions. These results are consistent with the reported higher GY of MSD53 in
comparison with MSD345 under drought stress in Sudan, and demonstrate that high FTSWTh
supports effective water use for improved agricultural productivity in drylands. The differences in
water conservation traits between the two MSD lines may be attributed to variation in introgressed
segments, which can be further explored for drought resilience breeding. This study validates the
results in Chapter 1 and 2.
In Chapter 4, my aim was to gain in-depth understanding of drought effect on wheat
metabolism. I exposed wheat N61 plants to progressive drought stress [0 (before drought), 2, 4, 6,
8, and 10 days after withholding water] during the flowering stage and investigated physiological
and metabolomic responses. Key abscisic acid-responsive genes, δ13C and CT played major roles
in wheat response to progressive drought stress. The CT depression was tightly correlated with soil
water potential (SWP). Additionally, SWP at −517 kPa was identified as the critical point for
increasing CT and inducing reactive oxygen species. Metabolome analysis identified four potential
drought-responsive biomarkers, the enhancement of nitrogen recycling through purine and
pyrimidine metabolism, drought-induced senescence based on 1-aminocyclopropane-1-carboxylic
acid and Asn accumulation, and an anti-senescence response through serotonin accumulation
under severe drought stress. These findings provide insights into the molecular, physiological and
metabolite changes involved in drought response which are useful for wheat breeding programs
developing drought-resilient wheat varieties.
In Chapter 5, the physiological and metabolic plasticity of three drought-resilient wheat
lines and N61 were evaluated in response to drought stress at the seedling stage. The results
suggested that the D-genome introgressions from Ae. tauschii to the lines improved their
drought-adaptive traits. Specifically, MNH5 and MSD345 showed higher photosynthesis rates and
triose phosphate utilization than N61 under control conditions, resulting in greater accumulation of
glucose and sucrose in the shoots. However, under drought stress, MNH5 and MSD345 had higher
intrinsic water use efficiency than MSD53 and N61. The total antioxidant capacity and superoxide
dismutase activity increased in the three lines, whereas no significant changes were found in N61
in response to drought stress. Metabolome analysis identified six common drought-induced
metabolites in all of the investigated genotypes. However, four metabolites (adenine, gamma
aminobutyric acid, histidine, and putrescine) each specifically accumulated in one resilient line in
response to drought stress, suggesting that these metabolites are important for drought resilience.
Overall, this work has expanded current knowledge on the role of high diversity breeding
panels for wheat breeding for heat and drought resilience. It has provided in-depth insights on
important genomic regions, and metabolic and physiological dynamics in wheat in response to
heat and drought stress. In the future, high-throughput analyses and validation of these findings
will allow them to serve as effective tools for climate-resilience breeding.

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