Overcoming Reproductive Compromise Under Heat Stress in Wheat: Physiological and Genetic Regulation, and Breeding Strategy

The molecular mechanism by which heat stress during the grain filling period inhibits maize grain filling and reduces yield

High temperatures significantly impair plant growth and development by restricting maize grain filling; however, the molecular mechanisms underlying heat stress remain poorly understood. In this study, 350 maize inbred lines were evaluated under field conditions, leading to the identification of heat-tolerant Zheng58 and heat-sensitive Qi319. The two inbred lines were exposed to controlled conditions of 30°C/20°C (optimal) and 42°C/30°C (heat stress) during the grain filling period. Heat stress significantly reduced thousand-kernel weight and seed setting rates, with Qi319 experiencing more pronounced declines. In contrast, Zheng58 showed superior performance, with a grain filling rate 48% higher and seed setting rate 57% greater than Qi319. Transcriptome analysis showed that heat stress disrupted starch biosynthesis and hormonal homeostasis, notably affecting abscisic acid and auxin pathways. Additionally, photosynthetic and transpiration rates in panicle leaves were reduced due to the downregulation of genes related to light-harvesting complexes, photosystem I subunits, and water transport. These findings highlight the critical roles of starch metabolism, hormonal regulation, and photosynthetic efficiency in heat tolerance, offering valuable insights for developing heat-resilient maize varieties to mitigate yield losses under high-temperature conditions.

Physiological and Transcriptome Analyses Provide Insights into the Response of Grain Filling to High Temperature in Male-Sterile Wheat (Triticum aestivum L.) Lines

High-temperature (HT) stress frequently affects the early and middle stages of grain filling in hybrid seed production regions. Photo-thermo-sensitive male-sterile (PTMS) wheat lines, which play a critical role as female parents in hybrid seed production, face challenges under HT conditions. However, the mechanisms governing grain filling in PTMS lines under HT stress remain poorly understood. This study used the BS253 line to investigate the effects of HT on grain filling, primarily focusing on the transition from sucrose unloading to starch synthesis. The findings indicated that HT significantly reduced the grain starch content and weight by 7.65% and 36.35% at maturity, respectively. Further analysis revealed that the expression levels of TaSUT1 and TaSWEETs in grains initially increased after HT stress, paralleling the rise in sucrose content during the same period. The activities of ADP-glucose pyrophosphorylase, UDP-glucose pyrophosphorylase, granule-bound starch synthase, and soluble starch synthase were markedly decreased, indicating that impaired starch synthesis was a key factor limiting grain filling immediately after HT exposure. A total of 41 key regulatory genes involved in sucrose-to-starch metabolism were identified, with HT significantly reducing the expression of genes associated with pathways from sucrose unloading to starch synthesis during the middle and late stages post-HT. Based on the observed ultrastructural changes in the abdominal phloem and sucrose transporter expression levels under HT, we concluded that limited sucrose supply, degradation, and inhibition of starch synthesis collectively constrained grain filling during these stages. Additionally, 11 heat shock proteins and two catalase genes were identified and significantly upregulated during the initial phase post-HT, suggesting their potential role in enhancing sucrose supply at this critical time. More importantly, seven key genes involved in the sucrose-to-starch pathway were identified by weighted gene co-expression network analysis (WGCNA), which provides target genes for their functional research for starch synthase. These findings provide a comprehensive understanding of how HT limits grain filling, identify several genes involved in the sucrose-to-starch pathway, and offer a novel perspective for future research on HT-restricted grain filling across the entire process from sucrose unloading to starch synthesis in developing grains.

Temperature-dependent jumonji demethylase modulates flowering time by targeting H3K36me2/3 in Brassica rapa

Global warming has a severe impact on the flowering time and yield of crops. Histone modifications have been well-documented for their roles in enabling plant plasticity in ambient temperature. However, the factor modulating histone modifications and their involvement in habitat adaptation have remained elusive. In this study, through genome-wide pattern analysis and quantitative-trait-locus (QTL) mapping, we reveal that BrJMJ18 is a candidate gene for a QTL regulating thermotolerance in thermotolerant B. rapa subsp. chinensis var. parachinensis (or Caixin, abbreviated to Par). BrJMJ18 encodes an H3K36me2/3 Jumonji demethylase that remodels H3K36 methylation across the genome. We demonstrate that the BrJMJ18 allele from Par (BrJMJ18Par) influences flowering time and plant growth in a temperature-dependent manner via characterizing overexpression and CRISPR/Cas9 mutant plants. We further show that overexpression of BrJMJ18Par can modulate the expression of BrFLC3, one of the five BrFLC orthologs. Furthermore, ChIP-seq and transcriptome data reveal that BrJMJ18Par can regulate chlorophyll biosynthesis under high temperatures. We also demonstrate that three amino acid mutations may account for function differences in BrJMJ18 between subspecies. Based on these findings, we propose a working model in which an H3K36me2/3 demethylase, while not affecting agronomic traits under normal conditions, can enhance resilience under heat stress in Brassica rapa.

Insights into Salinity Tolerance in Wheat

Salt stress has a detrimental impact on food crop production, with its severity escalating due to both natural and man-made factors. As one of the most important food crops, wheat is susceptible to salt stress, resulting in abnormal plant growth and reduced yields; therefore, damage from salt stress should be of great concern. Additionally, the utilization of land in coastal areas warrants increased attention, given diminishing supplies of fresh water and arable land, and the escalating demand for wheat. A comprehensive understanding of the physiological and molecular changes in wheat under salt stress can offer insights into mitigating the adverse effects of salt stress on wheat. In this review, we summarized the genes and molecular mechanisms involved in ion transport, signal transduction, and enzyme and hormone regulation, in response to salt stress based on the physiological processes in wheat. Then, we surveyed the latest progress in improving the salt tolerance of wheat through breeding, exogenous applications, and microbial pathways. Breeding efficiency can be improved through a combination of gene editing and multiple omics techniques, which is the fundamental strategy for dealing with salt stress. Possible challenges and prospects in this process were also discussed.

Improving wheat seedling quality through deep ploughing and soil compaction at sowing in lime concretion black soil

The straw incorporation in lime concretion black soil compromises the emergence and quality of winter wheat seedlings in Huaibei Plain, China, lowering the potential of wheat productivity. To overcome the disadvantage, a two-year field experiment was conducted in 2017-18 and 2018-19 to investigate the effects of different tillage modes on seedling emergence and subsequent seedling growth, and final grain yield (GY) in winter wheat. The modes are rotary tillage with compaction after sowing (RCT), rotary tillage after deep ploughing (PT) and rotary tillage after deep ploughing with compaction after sowing (PCT), with the traditional rotary tillage (RT) method as the control. Compared to RT, greater soil moisture content (SMC) at the seedling stage was observed in deep ploughing or compaction treatment, and the highest SMC was achieved in PCT; the time of reaching the maximum number of seedlings was 1 d sooner in RCT or PT, and 3 d in PCT; the seedling number in RCT, PT and PCT was significantly increased by 32.6%, 34.5% and 61.5% respectively. The population size, shoot and root growth of winter wheat in ploughing mode was significantly enhanced than that of rotary treatment at the over-wintering stage; compared to no compaction after sowing, plant growth in compaction treatments was significantly promoted with greater plant population size and height of seedlings. At harvest, GY in RCT, PT and PCT was significantly improved by 5.87%, 10.8% and 16.4%, respectively, compared to RT and the highest GY was achieved in PCT by up to 8, 350.1 kg ha-1 due to the increased spike number. In conclusion, the seedling quality in the straw incorporation practice was improved through rotary after deep ploughing and compaction after sowing for lime concretion black soil in Huaibei Plain, China or a similar soil type.

Silicon Induces Heat and Salinity Tolerance in Wheat by Increasing Antioxidant Activities, Photosynthetic Activity, Nutrient Homeostasis, and Osmo-Protectant Synthesis

Modern agriculture is facing the challenges of salinity and heat stresses, which pose a serious threat to crop productivity and global food security. Thus, it is necessary to develop the appropriate measures to minimize the impacts of these serious stresses on field crops. Silicon (Si) is the second most abundant element on earth and has been recognized as an important substance to mitigate the adverse effects of abiotic stresses. Thus, the present study determined the role of Si in mitigating adverse impacts of salinity stress (SS) and heat stress (HS) on wheat crop. This study examined response of different wheat genotypes, namely Akbar-2019, Subhani-2021, and Faisalabad-2008, under different treatments: control, SS (8 dSm^-1), HS, SS + HS, control + Si, SS + Si, HS+ Si, and SS + HS+ Si. This study's findings reveal that HS and SS caused a significant decrease in the growth and yield of wheat by increasing electrolyte leakage (EL), malondialdehyde (MDA), and hydrogen peroxide (H₂O₂) production; sodium (Na⁺) and chloride (Cl^-) accumulation; and decreasing relative water content (RWC), chlorophyll and carotenoid content, total soluble proteins (TSP), and free amino acids (FAA), as well as nutrient uptake (potassium, K; calcium, Ca; and magnesium, Mg). However, Si application offsets the negative effects of both salinity and HS and improved the growth and yield of wheat by increasing chlorophyll and carotenoid contents, RWC, antioxidant activity, TSP, FAA accumulation, and nutrient uptake (Ca, K, and Mg); decreasing EL, electrolyte leakage, MDA, and H₂O₂; and restricting the uptake of Na⁺ and Cl^-. Thus, the application of Si could be an important approach to improve wheat growth and yield under normal and combined saline and HS conditions by improving plant physiological functioning, antioxidant activities, nutrient homeostasis, and osmolyte accumulation.

Effects of Spraying KH2PO4 on Flag Leaf Physiological Characteristics and Grain Yield and Quality under Heat Stress during the Filling Period in Winter Wheat

As one of the most important wheat-producing areas in China, wheat is prone to heat stress during the grain filling period in the Huang-Huai-Hai Plain (3HP), which lowers yields and degrades the grain quality of wheat. To assess the effects of spraying potassium dihydrogen phosphate (KH₂PO₄) on the physiological traits in flag leaves and grain yield (GY) and quality under heat stress during the filling period, we conducted a two-year field experiment in the winter wheat growing seasons of 2020-2022. In this study, spraying water combined with heat stress (HT), 0.3% KH₂PO₄ (KDP), and 0.3% KH₂PO₄ combined with heat stress (PHT) were designed, and spraying water alone was used as a control (CK). The dates for the spraying were the third and eleventh day after anthesis, and a plastic film shed was used to impose heat stress on the wheat plants during the grain filling period. The results showed that spraying KH₂PO₄ significantly improved the chlorophyll content and net photosynthesis rate (Pn) in flag leaves compared with the non-sprayed treatments. Compared with CK, the Pn in HT decreased by 8.97% after heat stress, while Pn in PHT decreased by 7.44% compared to that of KDP. The activities of superoxide dismutase, catalase, and peroxidase in flag leaves were significantly reduced when the wheat was subjected to heat stress, while malonaldehyde content increased, and the enzyme activities were significantly enhanced when KH₂PO₄ was sprayed. Heat stress significantly decreased the contribution rate of dry matter accumulation (DM) after anthesis of wheat to grain (CRAA), whereas spraying KH₂PO₄ significantly increased the CRAA and harvest index. At maturity, the DM in CK was significantly higher than that in HT, KDP was significantly higher than PHT, and KDP had the highest DM. Compared with CK, the GY in KDP significantly increased by 9.85% over the two years, while the GY in HT decreased by 11.44% compared with that of CK, and the GY in PHT decreased by 6.31% compared to that of KDP. Spraying KH₂PO₄ after anthesis primarily helped GY by maintaining a high thousand grain weight to lessen the negative effects of heat stress on wheat. Moreover, heat stress significantly reduced protein concentration, wet gluten content, dough development time, and hardness index in grains of mature, while spraying KH₂PO₄ maintained a sufficient grain quality under the conditions of achieving higher yields. Overall, spraying KH₂PO₄ after anthesis could enhance the heat stress resistance of wheat and maintain the photosynthetic capacity of flag leaves, ensuring the dry matter production and reducing the negative effects on grain yield and quality in the 3HP.