Short periods of high temperature during meiosis prevent normal meiotic progression and reduce grain number in hexaploid wheat (Triticum aestivum L.)

Heating up meiosis - Chromosome recombination and segregation under high temperatures

Heat stress is one of the major constraints to plant growth and fertility. During the current climate crisis, heat waves have increased dramatically, and even more extreme conditions are predicted for the near future, considerably affecting ecosystems and seriously threatening world food security. Although heat is very well known to affect especially reproductive structures, little is known about how heat interferes with reproduction in comparison to somatic cells and tissues. Recently, the effect of heat on meiosis as a central process in sexual reproduction has been analyzed in molecular and cytological depth. Notably, these studies are not only important for applied research by laying the foundation for breeding heat-resilient crops, but also for fundamental research, revealing general regulatory mechanisms of recombination and chromosome segregation control.

Meiotic instability and irregular chromosome pairing underpin heat-induced infertility in bread wheat carrying the Rht-B1b or Rht-D1b Green Revolution genes

Mutations in the Rht-B1a and Rht-D1a genes of wheat (Triticum aestivum; resulting in Rht-B1b and Rht-D1b alleles) cause gibberellin-insensitive dwarfism and are one of the most important elements of increased yield introduced during the 'Green Revolution'. We measured the effects of a short period of heat imposed during the early reproductive stage on near-isogenic lines carrying Rht-B1b or Rht-D1b alleles, with respect to the wild-type (WT). The temperature shift caused a significant fertility loss within the ears of Rht-B1b and Rht-D1b wheats, greater than that observed for the WT. Defects in chromosome synapsis, reduced homologous recombination and a high frequency of chromosome mis-segregation were associated with reduced fertility. The transcription of TaGA3ox gene involved in the final stage of gibberellic acid (GA) biosynthesis was activated and ultra-performance liquid chromatography-tandem mass spectrometry identified GA1 as the dominant bioactive GA in developing ears, but levels were unaffected by the elevated temperature. Rht-B1b and Rht-D1b mutants were inclined to meiotic errors under optimal temperatures and showed a higher susceptibility to heat than their tall counterparts. Identification and introduction of new dwarfing alleles into modern breeding programmes is invaluable in the development of climate-resilient wheat varieties.

Enhancing climate change resilience in agricultural crops

Climate change threatens global food and nutritional security through negative effects on crop growth and agricultural productivity. Many countries have adopted ambitious climate change mitigation and adaptation targets that will exacerbate the problem, as they require significant changes in current agri-food systems. In this review, we provide a roadmap for improved crop production that encompasses the effective transfer of current knowledge into plant breeding and crop management strategies that will underpin sustainable agriculture intensification and climate resilience. We identify the main problem areas and highlight outstanding questions and potential solutions that can be applied to mitigate the impacts of climate change on crop growth and productivity. Although translation of scientific advances into crop production lags far behind current scientific knowledge and technology, we consider that a holistic approach, combining disciplines in collaborative efforts, can drive better connections between research, policy, and the needs of society.

Histone H3 lysine 27 trimethylation suppresses jasmonate biosynthesis and signaling to affect male fertility under high temperature in cotton

High-temperature (HT) stress causes male sterility in crops, thus decreasing yields. To explore the possible contribution of histone modifications to male fertility under HT conditions, we defined the histone methylation landscape for the marks histone H3 lysine 27 trimethylation (H3K27me3) and histone H3 lysine 4 trimethylation (H3K4me3) by chromatin immunoprecipitation sequencing (ChIP-seq) in two differing upland cotton (Gossypium hirsutum) varieties. We observed a global disruption in H3K4me3 and H3K27me3 modifications, especially H3K27me3, in cotton anthers subjected to HT. HT affected the bivalent H3K4me3-H3K27me3 modification more than either monovalent modification. We determined that removal of H3K27me3 at the promoters of jasmonate-related genes increased their expression, maintaining male fertility under HT in the HT-tolerant variety at the anther dehiscence stage. Modulating jasmonate homeostasis or signaling resulted in an anther indehiscence phenotype under HT. Chemical suppression of H3K27me3 deposition increased jasmonic acid contents and maintained male fertility under HT. In summary, our study provides new insights into the regulation of male fertility by histone modifications under HT and suggests a potential strategy for improving cotton HT tolerance.

Night-time warming in the field reduces nocturnal stomatal conductance and grain yield but does not alter daytime physiological responses

Global nocturnal temperatures are rising more rapidly than daytime temperatures and have a large effect on crop productivity. In particular, stomatal conductance at night (gsn ) is surprisingly poorly understood and has not been investigated despite constituting a significant proportion of overall canopy water loss. Here, we present the results of 3 yr of field data using 12 spring Triticum aestivum genotypes which were grown in NW Mexico and subjected to an artificial increase in night-time temperatures of 2°C. Under nocturnal heating, grain yields decreased (1.9% per 1°C) without significant changes in daytime leaf-level physiological responses. Under warmer nights, there were significant differences in the magnitude and decrease in gsn , values of which were between 9 and 33% of daytime rates while respiration appeared to acclimate to higher temperatures. Decreases in grain yield were genotype-specific; genotypes categorised as heat tolerant demonstrated some of the greatest declines in yield in response to warmer nights. We conclude the essential components of nocturnal heat tolerance in wheat are uncoupled from resilience to daytime temperatures, raising fundamental questions for physiological breeding. Furthermore, this study discusses key physiological traits such as pollen viability, root depth and irrigation type may also play a role in genotype-specific nocturnal heat tolerance.

DMC1 stabilizes crossovers at high and low temperatures during wheat meiosis

Effective chromosome synapsis and crossover formation during meiosis are essential for fertility, especially in grain crops such as wheat. These processes function most efficiently in wheat at temperatures between 17-23 °C, although the genetic mechanisms for such temperature dependence are unknown. In a previously identified mutant of the hexaploid wheat reference variety 'Chinese Spring' lacking the long arm of chromosome 5D, exposure to low temperatures during meiosis resulted in asynapsis and crossover failure. In a second mutant (ttmei1), containing a 4 Mb deletion in chromosome 5DL, exposure to 13 °C led to similarly high levels of asynapsis and univalence. Moreover, exposure to 30 °C led to a significant, but less extreme effect on crossovers. Previously, we proposed that, of 41 genes deleted in this 4 Mb region, the major meiotic gene TaDMC1-D1 was the most likely candidate for preservation of synapsis and crossovers at low (and possibly high) temperatures. In the current study, using RNA-guided Cas9, we developed a new Chinese Spring CRISPR mutant, containing a 39 bp deletion in the 5D copy of DMC1, representing the first reported CRISPR-Cas9 targeted mutagenesis in Chinese Spring, and the first CRISPR mutant for DMC1 in wheat. In controlled environment experiments, wild-type Chinese Spring, CRISPR dmc1-D1 and backcrossed ttmei1 mutants were exposed to either high or low temperatures during the temperature-sensitive period from premeiotic interphase to early meiosis I. After 6-7 days at 13 °C, crossovers decreased by over 95% in the dmc1-D1 mutants, when compared with wild-type plants grown under the same conditions. After 24 hours at 30 °C, dmc1-D1 mutants exhibited a reduced number of crossovers and increased univalence, although these differences were less marked than at 13 °C. Similar results were obtained for ttmei1 mutants, although their scores were more variable, possibly reflecting higher levels of background mutation. These experiments confirm our previous hypothesis that DMC1-D1 is responsible for preservation of normal crossover formation at low and, to a certain extent, high temperatures. Given that reductions in crossovers have significant effects on grain yield, these results have important implications for wheat breeding, particularly in the face of climate change.

OsGRF4AA compromises heat tolerance of developing pollen grains in rice

Extreme high temperature at the meiosis stage causes a severe decrease in spikelet fertility and grain yield in rice. The rice variety grain size on chromosome 2 (GS2) contains sequence variations of OsGRF4 (Oryza sativa growth-regulating factor 4; OsGRF4^AA ), escaping the microRNA miR396-mediated degradation of this gene at the mRNA level. Accumulation of OsGRF4 enhances nitrogen usage and metabolism, and increases grain size and grain yield. In this study, we found that pollen viability and seed-setting rate under heat stress (HS) decreased more seriously in GS2 than in its comparator, Zhonghua 11 (ZH11). Transcriptomic analysis revealed that, following HS, genes related to carbohydrate metabolic processes were expressed and regulated differentially in the anthers of GS2 and ZH11. Moreover, the expression of genes involved in chloroplast development and photosynthesis, lipid metabolism, and key transcription factors, including eight male sterile genes, were inhibited by HS to a greater extent in GS2 than in ZH11. Interestingly, pre-mRNAs of OsGRF4, and a group of essential genes involved in development and fertilization, were differentially spliced in the anthers of GS2 and ZH11. Taken together, our results suggest that variation in OsGRF4 affects proper transcriptional and splicing regulation of genes under HS, and that this can be mediated by, and also feed back to, carbohydrate and nitrogen metabolism, resulting in a reduction in the heat tolerance of rice anthers.

A framework for improving wheat spike development and yield based on the master regulatory TOR and SnRK gene systems

The low rates of yield gain in wheat breeding programs create an ominous situation for the world. Amongst the reasons for this low rate are issues manifested in spike development that result in too few spikelets, fertile florets, and therefore grains being produced. Phases in spike development are particularly sensitive to stresses of various kinds and origins, and these are partly responsible for the deficiencies in grain production and slow rates of gain in yield. The diversity of developmental processes, stresses, and the large numbers of genes involved make it particularly difficult to prioritize approaches in breeding programs without an overarching, mechanistic framework. Such a framework, introduced here, is provided around the master regulator target of rapamycin and sucrose non-fermenting-1 (SNF1)-related protein kinase complexes and their control by trehalose-6-phosphate and other molecules. Being master regulators of the balance between growth and growth inhibition under stress, these provide genetic targets for creating breakthroughs in yield enhancement. Examples of potential targets and experimental approaches are described.

Heat stress responses vary during floret development in European spring barley cultivars

The Poaceae, or grasses, include many agriculturally important cereal crops such as rice (Oryza sativa), maize (Zea mays), barley (Hordeum vulgare) and bread wheat (Triticum aestivum). Barley is a widely grown cereal crop used for stock feed, malting and brewing. Abiotic stresses, particularly global warming, are the major causes of crop yield losses by affecting fertility and seed set. However, effects of heat stress on reproductive structures and fertility in barley have not been extensively investigated. In this study we examined three commercial European spring barley varieties under high temperature conditions to investigate the effects on floret development. Using a combination of fertility assays, X-ray micro computed tomography, 3-dimensional modelling, cytology and immunolabelling, we observed that male reproductive organs are severely impacted by increased temperature, while the female reproductive organs are less susceptible. Importantly, the timing of stress relative to reproductive development had a significant impact on fertility in a cultivar-dependent manner, this was most significant at pollen mitosis stage with fertility ranged from 31.6-56.0% depending on cultivar. This work provides insight into how heat stress, when applied during male pollen mother cell meiosis and pollen mitosis, affects barley fertility and seed set, and also describes complementary invasive and non-invasive techniques to investigate floret development. This information will be used to identify and study barley cultivars that are less susceptible to heat stress at specific stages of floral development.

Transcriptomic profiling of wheat stem during meiosis in response to freezing stress

Low temperature injury in spring has seriously destabilized the production and grain quality of common wheat. However, the molecular mechanisms underlying spring frost tolerance remain elusive. In this study, we investigated the response of a frost-tolerant wheat variety Zhongmai8444 to freezing stress at the meiotic stage. Transcriptome profiles over a time course were subsequently generated by high-throughput sequencing. Our results revealed that the prolonged freezing temperature led to the significant reductions in plant height and seed setting rate. Cell wall thickening in the vascular tissue was also observed in the stems. RNA-seq analyses demonstrated the identification of 1010 up-regulated and 230 down-regulated genes shared by all time points of freezing treatment. Enrichment analysis revealed that gene activity related to hormone signal transduction and cell wall biosynthesis was significantly modulated under freezing. In addition, among the identified differentially expressed genes, 111 transcription factors belonging to multiple gene families exhibited dynamic expression pattern. This study provided valuable gene resources beneficial for the breeding of wheat varieties with improved spring frost tolerance.

Maize cytosolic invertase INVAN6 ensures faithful meiotic progression under heat stress

Faithful meiotic progression ensures the generation of viable gametes. Studies suggested the male meiosis of plants is sensitive to ambient temperature, but the underlying molecular mechanisms remain elusive. Here, we characterized a maize (Zea mays ssp. mays L.) dominant male sterile mutant Mei025, in which the meiotic process of pollen mother cells (PMCs) was arrested after pachytene. An Asp-to-Asn replacement at position 276 of INVERTASE ALKALINE NEUTRAL 6 (INVAN6), a cytosolic invertase (CIN) that predominantly exists in PMCs and specifically hydrolyses sucrose, was revealed to cause meiotic defects in Mei025. INVAN6 interacts with itself as well as with four other CINs and seven 14-3-3 proteins. Although INVAN6^Mei025 , the variant of INVAN6 found in Mei025, lacks hydrolytic activity entirely, its presence is deleterious to male meiosis, possibly in a dominant negative repression manner through interacting with its partner proteins. Notably, heat stress aggravated meiotic defects in invan6 null mutant. Further transcriptome data suggest INVAN6 has a fundamental role for sugar homeostasis and stress tolerance of male meiocytes. In summary, this work uncovered the function of maize CIN in male meiosis and revealed the role of CIN-mediated sugar metabolism and signalling in meiotic progression under heat stress.

Long-Term Mild Heat Causes Post-Mitotic Pollen Abortion Through a Local Effect on Flowers

Crop reproductive success is significantly challenged by heatwaves, which are increasing in frequency and severity globally. Heat-induced male sterility is mainly due to aborted pollen development, but it is not clear whether this is through direct or systemic effects. Here, long-term mild heat (LTMH) treatment, mimicking a heatwave, was applied locally to tomato flowers or whole plants and followed up by cytological, transcriptomic, and biochemical analyses. By analyzing pollen viability, LTMH was shown to act directly on the flowers and not via effects on other plant tissue. The meiosis to early microspore stage of pollen development was the most sensitive to LTMH and 3 days of exposure around this period was sufficient to significantly reduce pollen viability at the flower anthesis stage. Extensive cytological analysis showed that abnormalities in pollen development could first be observed after pollen mitosis I, while no deviations in tapetum development were observed. Transcriptomic and biochemical analyses suggested that pollen development suffered from tapetal ER stress and that there was a limited role for oxidative stress. Our results provide the first evidence that heat acts directly on flowers to induce pollen sterility, and that the molecular-physiological responses of developing anthers to the LTMH are different from those to severe heat shock.

Assessing the Heat Tolerance of Meiosis in Spanish Landraces of Tetraploid Wheat Triticum turgidum

Heat stress alters the number and distribution of meiotic crossovers in wild and cultivated plant species. Hence, global warming may have a negative impact on meiosis, fertility, and crop productions. Assessment of germplasm collections to identify heat-tolerant genotypes is a priority for future crop improvement. Durum wheat, Triticum turgidum, is an important cultivated cereal worldwide and given the genetic diversity of the durum wheat Spanish landraces core collection, we decided to analyse the heat stress effect on chiasma formation in a sample of 16 landraces of T. turgidum ssp. turgidum and T. turgidum ssp. durum, from localities with variable climate conditions. Plants of each landrace were grown at 18–22 °C and at 30 °C during the premeiotic temperature-sensitive stage. The number of chiasmata was not affected by heat stress in three genotypes, but decreased by 0.3–2 chiasmata in ten genotypes and more than two chiasmata in the remaining three ones. Both thermotolerant and temperature-sensitive genotypes were found in the two subspecies, and in some of the agroecological zones studied, which supports that genotypes conferring a heat tolerant meiotic phenotype are not dependent on subspecies or geographical origin. Implications of heat adaptive genotypes in future research and breeding are discussed.

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

The reproductive compromise under heat stress is a major obstacle to achieve high grain yield and quality in wheat worldwide. Securing reproductive success is the key solution to sustain wheat productivity by understanding the physiological mechanism and molecular basis in conferring heat tolerance and utilizing the candidate gene resources for breeding. In this study, we examined the performance on both carbon supply source (as leaf photosynthetic rate) and carbon sink intake (as grain yields and quality) in wheat under heat stress varying with timing, duration, and intensity, and we further surveyed physiological processes from source to sink and the associated genetic basis in regulating reproductive thermotolerance; in addition, we summarized the quantitative trait loci (QTLs) and genes identified for heat stress tolerance associated with reproductive stages. Discovery of novel genes for thermotolerance is made more efficient via the combination of transcriptomics, proteomics, metabolomics, and phenomics. Gene editing of specific genes for novel varieties governing heat tolerance is also discussed.

Interfered chromosome pairing at high temperature promotes meiotic instability in autotetraploid Arabidopsis

Changes in environmental temperature affect multiple meiotic processes in flowering plants. Polyploid plants derived from whole-genome duplication (WGD) have enhanced genetic plasticity and tolerance to environmental stress but face challenges in organizing and segregating doubled chromosome sets. In this study, we investigated the impact of increased environmental temperature on male meiosis in autotetraploid Arabidopsis (Arabidopsis thaliana). Under low to mildly increased temperatures (5°C-28°C), irregular chromosome segregation universally occurred in synthetic autotetraploid Columbia-0 (Col-0). Similar meiotic lesions occurred in autotetraploid rice (Oryza sativa L.) and allotetraploid canola (Brassica napus cv Westar), but not in evolutionarily derived hexaploid wheat (Triticum aestivum). At extremely high temperatures, chromosome separation and tetrad formation became severely disordered due to univalent formation caused by the suppression of crossing-over. We found a strong correlation between tetravalent formation and successful chromosome pairing, both of which were negatively correlated with temperature elevation, suggesting that increased temperature interferes with crossing-over predominantly by impacting homolog pairing. We also showed that loading irregularities of axis proteins ASY1 and ASY4 co-localize on the chromosomes of the syn1 mutant and the heat-stressed diploid and autotetraploid Col-0, revealing that heat stress affects the lateral region of synaptonemal complex (SC) by impacting the stability of the chromosome axis. Moreover, we showed that chromosome axis and SC in autotetraploid Col-0 are more sensitive to increased temperature than those in diploid Arabidopsis. Taken together, our data provide evidence suggesting that WGD negatively affects the stability and thermal tolerance of meiotic recombination in newly synthetic autotetraploid Arabidopsis.

Heat stress reveals a specialized variant of the pachytene checkpoint in meiosis of Arabidopsis thaliana

Plant growth and fertility strongly depend on environmental conditions such as temperature. Remarkably, temperature also influences meiotic recombination and thus, the current climate change will affect the genetic make-up of plants. To better understand the effects of temperature on meiosis, we followed male meiocytes in Arabidopsis thaliana by live cell imaging under three temperature regimes: at 21°C; at heat shock conditions of 30°C and 34°C; after an acclimatization phase of 1 week at 30°C. This work led to a cytological framework of meiotic progression at elevated temperature. We determined that an increase from 21°C to 30°C speeds up meiosis with specific phases being more amenable to heat than others. An acclimatization phase often moderated this effect. A sudden increase to 34°C promoted a faster progression of early prophase compared to 21°C. However, the phase in which cross-overs mature was prolonged at 34°C. Since mutants involved in the recombination pathway largely did not show the extension of this phase at 34°C, we conclude that the delay is recombination-dependent. Further analysis also revealed the involvement of the ATAXIA TELANGIECTASIA MUTATED kinase in this prolongation, indicating the existence of a pachytene checkpoint in plants, yet in a specialized form.

Genetic and Physiological Responses to Heat Stress in Brassica napus

Given the current rise in global temperatures, heat stress has become a major abiotic challenge affecting the growth and development of various crops and reducing their productivity. Brassica napus, the second largest source of vegetable oil worldwide, experiences a drastic reduction in seed yield and quality in response to heat. This review outlines the latest research that explores the genetic and physiological impact of heat stress on different developmental stages of B. napus with a special attention to the reproductive stages of floral progression, organogenesis, and post flowering. Several studies have shown that extreme temperature fluctuations during these crucial periods have detrimental effects on the plant and often leading to impaired growth and reduced seed production. The underlying mechanisms of heat stress adaptations and associated key regulatory genes are discussed. Furthermore, an overview and the implications of the polyploidy nature of B. napus and the regulatory role of alternative splicing in forming a priming-induced heat-stress memory are presented. New insights into the dynamics of epigenetic modifications during heat stress are discussed. Interestingly, while such studies are scarce in B. napus, opposite trends in expression of key genetic and epigenetic components have been identified in different species and in cultivars within the same species under various abiotic stresses, suggesting a complex role of these genes and their regulation in heat stress tolerance mechanisms. Additionally, omics-based studies are discussed with emphasis on the transcriptome, proteome and metabolome of B. napus, to gain a systems level understanding of how heat stress alters its yield and quality traits. The combination of omics approaches has revealed crucial interactions and regulatory networks taking part in the complex machinery of heat stress tolerance. We identify key knowledge gaps regarding the impact of heat stress on B. napus during its yield determining reproductive stages, where in-depth analysis of this subject is still needed. A deeper knowledge of heat stress response components and mechanisms in tissue specific models would serve as a stepping-stone to gaining insights into the regulation of thermotolerance that takes place in this important crop species and support future breeding of heat tolerant crops.

The Effects of Brief Heat During Early Booting on Reproductive, Developmental, and Chlorophyll Physiological Performance in Common Wheat (Triticum aestivum L.)

Rising temperatures due to climate change threaten agricultural crop productivity. As a cool-season crop, wheat is heat-sensitive, but often exposed to high temperatures during the cultivation period. In the current study, a bread wheat panel of spring wheat genotypes, including putatively heat-tolerant Australian and CIMMYT genotypes, was exposed to a 5-day mild (34°C/28°C, day/night) or extreme (37°C/27°C) heat stress during the sensitive pollen developmental stage. Worsening effects on anther morphology were observed, as heat stress increased from mild to extreme. Even under mild heat, a significant decrease in pollen viability and number of grains per spike from primary spike was observed compared with the control (21°C/15°C), with Sunstar and two CIMMYT breeding lines performing well. A heat-specific positive correlation between the two traits indicates the important role of pollen fertility for grain setting. Interestingly, both mild and extreme heat induced development of new tillers after the heat stress, providing an alternative sink for accumulated photosynthates and significantly contributing to the final yield. Measurements of flag leaf maximum potential quantum efficiency of photosystem II (Fv/Fm) showed an initial inhibition after the heat treatment, followed by a full recovery within a few days. Despite this, model fitting using chlorophyll soil plant analysis development (SPAD) measurements showed an earlier onset or faster senescence rate under heat stress. The data presented here provide interesting entry points for further research into pollen fertility, tillering dynamics, and leaf senescence under heat. The identified heat-tolerant wheat genotypes can be used to dissect the underlying mechanisms and breed climate-resilient wheat.

The impact of stress combination on reproductive processes in crops

Historically, extended droughts combined with heat waves caused severe reductions in crop yields estimated at billions of dollars annually. Because global warming and climate change are driving an increase in the frequency and intensity of combined water-deficit and heat stress episodes, understanding how these episodes impact yield is critical for our efforts to develop climate change-resilient crops. Recent studies demonstrated that a combination of water-deficit and heat stress exacerbates the impacts of water-deficit or heat stress on reproductive processes of different cereals and legumes, directly impacting grain production. These studies identified several different mechanisms potentially underlying the effects of stress combination on anthers, pollen, and stigma development and function, as well as fertilization. Here we review some of these findings focusing on unbalanced reactive oxygen accumulation, altered sugar concentrations, and conflicting functions of different hormones, as contributing to the reduction in yield during a combination of water-deficit and heat stress. Future studies focused on the effects of water-deficit and heat stress combination on reproduction of different crops are likely to unravel additional mechanisms, as well as reveal novel ways to develop stress combination-resilient crops. These could mitigate some of the potentially devastating impacts of this stress combination on agriculture.

The Formation of Bivalents and the Control of Plant Meiotic Recombination

During the first meiotic division, the segregation of homologous chromosomes depends on the physical association of the recombined homologous DNA molecules. The physical tension due to the sites of crossing-overs (COs) is essential for the meiotic spindle to segregate the connected homologous chromosomes to the opposite poles of the cell. This equilibrated partition of homologous chromosomes allows the first meiotic reductional division. Thus, the segregation of homologous chromosomes is dependent on their recombination. In this review, we will detail the recent advances in the knowledge of the mechanisms of recombination and bivalent formation in plants. In plants, the absence of meiotic checkpoints allows observation of subsequent meiotic events in absence of meiotic recombination or defective meiotic chromosomal axis formation such as univalent formation instead of bivalents. Recent discoveries, mainly made in Arabidopsis, rice, and maize, have highlighted the link between the machinery of double-strand break (DSB) formation and elements of the chromosomal axis. We will also discuss the implications of what we know about the mechanisms regulating the number and spacing of COs (obligate CO, CO homeostasis, and interference) in model and crop plants.

Loss of obligate crossovers, defective cytokinesis and male sterility in barley caused by short-term heat stress

Short-term heat stress during male meiosis causes defects in crossover formation, meiotic progression and cell wall formation in the monocot barley, ultimately leading to pollen abortion. High temperature conditions cause a reduction of fertility due to alterations in meiotic processes and gametogenesis. The male gametophyte development has been shown to be particularly sensitive to heat stress, and even short-term and modest temperature shifts cause alterations in crossover formation. In line with previous reports, we observed that male meiosis in the monocot barley exposed for 24-45 h to heat stress (32-42 °C) partially or completely eliminates obligate crossover formation and causes unbalanced chromosome segregation and meiotic abortion. Depending on the severity of heat stress, the structure and organization of the chromosomes were altered. In addition to alterations in chromosome structure and dynamics, heat treatment abolished or reduced the formation of a callose wall surrounding the meiocytes and interrupted the cell cycle progression leading to cytokinesis defects and microspore cell death.

Male meiotic recombination rate varies with seasonal temperature fluctuations in wild populations of autotetraploid Arabidopsis arenosa

Meiosis, the cell division by which eukaryotes produce haploid gametes, is essential for fertility in sexually reproducing species. This process is sensitive to temperature, and can fail outright at temperature extremes. At less extreme values, temperature affects the genome‐wide rate of homologous recombination, which has important implications for evolution and population genetics. Numerous studies in laboratory conditions have shown that recombination rate plasticity is common, perhaps nearly universal, among eukaryotes. These studies have also shown that variation in the length or timing of stresses can strongly affect results, raising the important question whether these findings translate to more variable field conditions. Moreover, lower or higher recombination rate could cause certain kinds of meiotic aberrations, especially in polyploid species—raising the additional question whether temperature fluctuations in field conditions cause problems. Here, we tested whether (1) recombination rate varies across a season in the wild in two natural populations of autotetraploid Arabidopsis arenosa, (2) whether recombination rate correlates with temperature fluctuations in nature, and (3) whether natural temperature fluctuations might cause meiotic aberrations. We found that plants in two genetically distinct populations showed a similar plastic response with recombination rate increases correlated with both high and low temperatures. In addition, increased recombination rate correlated with increased multivalent formation, especially at lower temperature, hinting that polyploids in particular may suffer meiotic problems in conditions they encounter in nature. Our results show that studies of recombination rate plasticity done in laboratory settings inform our understanding of what happens in nature.

The Heat is On: How Crop Growth, Development and Yield Respond to High Temperature

Plants are exposed to a wide range of temperatures during their life cycle and need to continuously adapt. These adaptations need to deal with temperature changes on a daily and seasonal level and with temperatures affected by climate change. Increasing global temperatures negatively impact crop performance, and several physiological, biochemical, morphological and developmental responses to increased temperature have been described that allow plants to mitigate this. In this review, we assess various growth, development, and yield-related responses of crops to extreme and moderate high temperature, focusing on knowledge gained from both monocot (e.g. wheat, barley, maize, rice) and dicot crops (e.g. soybean and tomato) and incorporating information from model plants (e.g. Arabidopsis and Brachypodium). This revealed common and different responses between dicot and monocot crops, and defined different temperature thresholds depending on the species, growth stage and organ.

Need for speed: manipulating plant growth to accelerate breeding cycles

To develop more productive and resilient crops that are capable of feeding 10 billion people by 2050, we must accelerate the rate of genetic improvement in plant breeding programs. Speed breeding manipulates the growing environment by regulating light and temperature for the purpose of rapid generation advance. Protocols are now available for a range of short-day and long-day species and the approach is highly compatible with other cutting-edge breeding tools such as genomic selection. Here, we highlight how speed breeding hijacks biological processes for applied plant breeding outcomes and provide a case study examining wheat growth and development under speed breeding conditions. The establishment of speed breeding facilities worldwide is expected to provide benefits for capacity building, discovery research, pre-breeding, and plant breeding to accelerate the development of productive and robust crops.

The WtmsDW Locus on Wheat Chromosome 2B Controls Major Natural Variation for Floret Sterility Responses to Heat Stress at Booting Stage

Heat stress at booting stage causes significant losses to floret fertility (grain set) and hence yield in wheat (Triticum aestivum L.); however, there is a lack of well-characterized sources of tolerance to this type of stress. Here, we describe the genetic analysis of booting stage heat tolerance in a cross between the Australian cultivars Drysdale (intolerant) and Waagan (tolerant), leading to the definition of a major-effect tolerance locus on the short arm of chromosome 2B, Wheat thermosensitive male sterile Drysdale/Waagan (WtmsDW). WtmsDW offsets between 44 and 65% of the losses in grain set due to heat, suggesting that it offers significant value for marker-assisted tolerance breeding. In lines lacking the WtmsDW tolerance allele, peaks in sensitivity were defined with reference to auricle distance, for various floret positions along the spike. Other (relatively minor) floret fertility response effects, including at the Rht-D1 dwarfing locus, were considered likely escape artifacts, due to their association with height and flowering time effects that might interfere with correct staging of stems for heat treatment. Heat stress increased grain set at distal floret positions in spikelets located at the top of the spike and increased the size of spikelets at the base of the spike, but these effects were offset by greater reductions in grain set at other floret positions. Potentially orthologous loci on chromosomes 1A and 1B were identified for heat response of flowering time. The potential significance of these findings for tolerance breeding and further tolerance screening is discussed.

The WtmsDW Locus on Wheat Chromosome 2B Controls Major Natural Variation for Floret Sterility Responses to Heat Stress at Booting Stage

Heat stress at booting stage causes significant losses to floret fertility (grain set) and hence yield in wheat (Triticum aestivum L.); however, there is a lack of well-characterized sources of tolerance to this type of stress. Here, we describe the genetic analysis of booting stage heat tolerance in a cross between the Australian cultivars Drysdale (intolerant) and Waagan (tolerant), leading to the definition of a major-effect tolerance locus on the short arm of chromosome 2B, Wheat thermosensitive male sterile Drysdale/Waagan (WtmsDW). WtmsDW offsets between 44 and 65% of the losses in grain set due to heat, suggesting that it offers significant value for marker-assisted tolerance breeding. In lines lacking the WtmsDW tolerance allele, peaks in sensitivity were defined with reference to auricle distance, for various floret positions along the spike. Other (relatively minor) floret fertility response effects, including at the Rht-D1 dwarfing locus, were considered likely escape artifacts, due to their association with height and flowering time effects that might interfere with correct staging of stems for heat treatment. Heat stress increased grain set at distal floret positions in spikelets located at the top of the spike and increased the size of spikelets at the base of the spike, but these effects were offset by greater reductions in grain set at other floret positions. Potentially orthologous loci on chromosomes 1A and 1B were identified for heat response of flowering time. The potential significance of these findings for tolerance breeding and further tolerance screening is discussed.

Comparative transcriptomic analysis of thermally stressed Arabidopsis thaliana meiotic recombination mutants

BACKGROUND

Meiosis is a specialized cell division that underpins sexual reproduction in most eukaryotes. During meiosis, interhomolog meiotic recombination facilitates accurate chromosome segregation and generates genetic diversity by shuffling parental alleles in the gametes. The frequency of meiotic recombination in Arabidopsis has a U-shaped curve in response to environmental temperature, and is dependent on the Type I, crossover (CO) interference-sensitive pathway. The mechanisms that modulate recombination frequency in response to temperature are not yet known.

RESULTS

In this study, we compare the transcriptomes of thermally-stressed meiotic-stage anthers from msh4 and mus81 mutants that mediate the Type I and Type II meiotic recombination pathways, respectively. We show that heat stress reduces the number of expressed genes regardless of genotype. In addition, msh4 mutants have a distinct gene expression pattern compared to mus81 and wild type controls. Interestingly, ASY1, which encodes a HORMA domain protein that is a component of meiotic chromosome axes, is up-regulated in wild type and mus81 but not in msh4. In addition, SDS the meiosis-specific cyclin-like gene, DMC1 the meiosis-specific recombinase, SYN1/REC8 the meiosis-specific cohesion complex component, and SWI1 which functions in meiotic sister chromatid cohesion are up-regulated in all three genotypes. We also characterize 51 novel, previously unannotated transcripts, and show that their promoter regions are associated with A-rich meiotic recombination hotspot motifs.

CONCLUSIONS

Our transcriptomic analysis of msh4 and mus81 mutants enhances our understanding of how the Type I and Type II meiotic CO pathway respond to environmental temperature stress and might provide a strategy to manipulate recombination levels in plants.

The Effect of Photoperiod Genes and Flowering Time on Yield and Yield Stability in Durum Wheat

This study analysed the effect of flowering time as influenced by photoperiod sensitivity genes on yield and yield stability in durum wheat. Twenty-three spring genotypes harbouring different allele combinations at Ppd-A1 and Ppd-B1 were grown in 15 field experiments at three sites at latitudes from 41° to 19° N (Spain, Mexico-North and Mexico-South). Low temperature and solar radiation before flowering and long day length during grain-filling characteristic for the Spanish site resulted in high grain number/m² (GN) and yield (GY), while a moderate GN combined with high solar radiation during grain-filling at Mexico-North led to heavier grains. Allele combination GS100-Ppd-A1a/Ppd-B1a reduced the flowering time up to nine days when compared with Ppd-A1b/Ppd-B1a. Differences in flowering time caused by Ppd-A1/Ppd-B1 allele combinations did not affect yield. Combinations GS105-Ppd-A1a/Ppd-B1b and Ppd-A1b/Ppd-B1b resulted in the highest GN, linked to spikelets/spike, which was higher in GS105-Ppd-A1a/Ppd-B1b due to more grains/spikelet. Flowering time caused by Eps had a minor effect on GN, spikes/m² and grains/spike, but late flowering resulted in reduced grain weight and GY. Allele combinations harbouring alleles conferring a similar photoperiod sensitivity response at Ppd-A1 and Ppd-B1 resulted in greater yield stability than combinations that carry alleles conferring a different response. Allele combination GS100-Ppd-A1a/Ppd-B1a was the most suitable in terms of yield and yield stability of durum wheat cultivated under irrigation within the studied latitudes.

Feeling the heat: developmental and molecular responses of wheat and barley to high ambient temperatures

The increasing demand for global food security in the face of a warming climate is leading researchers to investigate the physiological and molecular responses of cereals to rising ambient temperatures. Wheat and barley are temperate cereals whose yields are adversely affected by high ambient temperatures, with each 1 °C increase above optimum temperatures reducing productivity by 5-6%. Reproductive development is vulnerable to high-temperature stress, which reduces yields by decreasing grain number and/or size and weight. In recent years, analysis of early inflorescence development and genetic pathways that control the vegetative to floral transition have elucidated molecular processes that respond to rising temperatures, including those involved in the vernalization- and photoperiod-dependent control of flowering. In comparison, our understanding of genes that underpin thermal responses during later developmental stages remains poor, thus highlighting a key area for future research. This review outlines the responses of developmental genes to warmer conditions and summarizes our knowledge of the reproductive traits of wheat and barley influenced by high temperatures. We explore ways in which recent advances in wheat and barley research capabilities could help identify genes that underpin responses to rising temperatures, and how improved knowledge of the genetic regulation of reproduction and plant architecture could be used to develop thermally resilient cultivars.

Development of Wild and Cultivated Plants under Global Warming Conditions

Global warming is one of the most detrimental aspects of climate change, affecting plant growth and development across the entire life cycle. This Review explores how different stages of development are influenced by elevated temperature in both wild plants and crops. Starting from seed development and germination, global warming will influence morphological adjustments, termed thermomorphogenesis, and photosynthesis primarily during the vegetative phase, as well as flowering and reproductive development. Where applicable, we distinguish between moderately elevated temperatures that affect all stages of plant development and heat waves that often occur during the reproductive phase when they can have devastating consequences for fruit development. The parallel occurrence of elevated temperature with other abiotic and biotic stressors, particularly the combination of global warming and drought or increased pathogen pressure, will potentiate the challenges for both wild and cultivated plant species. The key components of the molecular networks underlying the physiological processes involved in thermal responses in the model plant Arabidopsis thaliana are highlighted. In crops, temperature-sensitive traits relevant for yield are illustrated for winter wheat (Triticum aestivum L.) and soybean (Glycine max L.), representing cultivated species adapted to temperate vs. warm climate zones, respectively. While the fate of wild plants depends on political agendas, plant breeding approaches informed by mechanistic understanding originating in basic science can enable the generation of climate change-resilient crops.

Heat stress interferes with chromosome segregation and cytokinesis during male meiosis in Arabidopsis thaliana

In higher plants, male meiosis is a key process of microsporogenesis and is crucial for plant fertility. Male meiosis programs are prone to be influenced by altered temperature conditions. Studies have reported that an increased temperature (28°C) within a fertile threshold can affect the frequency of meiotic recombination in Arabidopsis. However, not much has been known how male meiosis responses to an extremely high temperature beyond the fertile threshold. To understand the impact of extremely high temperature on male meiosis in Arabidopsis, we treated flowering Arabidopsis plants with 36-38°C and found that the high-temperature condition significantly reduced pollen shed and plant fertility, and led to formation of pollen grains with varied sizes. The heat stress-induced unbalanced tetrads, polyad and meiotic restitution, suggesting that male meiosis was interfered. Fluorescence in situ hybridization (FISH) assay confirmed that both homologous chromosome separation and sister chromatids cohesion were influenced. Aniline blue staining of tetrad-stage pollen mother cells (PMCs) revealed that meiotic cytokinesis was severely disrupted by the heat stress. Supportively, immunolocalization of ɑ-tubulin showed that the construction of spindle and phragmoplast at both meiosis I and II were interfered. Overall, our findings demonstrate that an extremely high-temperature stress over the fertile threshold affects both chromosome segregation and cytokinesis during male meiosis by disturbing microtubular cytoskeleton in Arabidopsis.

Dmc1 is a candidate for temperature tolerance during wheat meiosis

KEY MESSAGE

The meiotic recombination gene Dmc1 on wheat chromosome 5D has been identified as a candidate for the maintenance of normal chromosome synapsis and crossover at low and possibly high temperatures. We initially assessed the effects of low temperature on meiotic chromosome synapsis and crossover formation in the hexaploid wheat (Triticum aestivum L.) variety 'Chinese Spring'. At low temperatures, asynapsis and chromosome univalence have been observed before in Chinese Spring lines lacking the long arm of chromosome 5D (5DL), which led to the proposal that 5DL carries a gene (Ltp1) that stabilises wheat chromosome pairing at low temperatures. In the current study, Chinese Spring wild type and 5DL interstitial deletion mutant plants were exposed to low temperature in a controlled environment room during a period from premeiotic interphase to early meiosis I. A 5DL deletion mutant was identified whose meiotic chromosomes exhibit extremely high levels of asynapsis and chromosome univalence at metaphase I after 7 days at 13 °C, suggesting that Ltp1 is deleted in this mutant. Immunolocalisation of the meiotic proteins ASY1 and ZYP1 on ltp1 mutants showed that low temperature results in a failure to complete synapsis at pachytene. KASP genotyping revealed that the ltp1 mutant has a 4-Mb deletion in 5DL. Of 41 genes within this deletion region, the strongest candidate for the stabilisation of chromosome pairing at low temperatures is the meiotic recombination gene Dmc1. The ltp1 mutants were subsequently treated at 30 °C for 24 h during meiosis and exhibited a reduced number of crossovers and increased univalence, though to a lesser extent than at 13 °C. We therefore renamed our ltp1 mutant 'ttmei1' (temperature-tolerant meiosis 1) to reflect this additional loss of high temperature tolerance.

Different MicroRNA Families Involved in Regulating High Temperature Stress Response during Cotton (Gossypium hirsutum L.) Anther Development

MicroRNAs (miRNAs) are small molecule RNAs widely involved in responses to plant abiotic stresses. We performed small RNA sequencing of cotton anthers at four developmental stages under normal and high temperature (NT and HT, respectively) conditions to investigate the stress response characteristics of miRNA to HT. A total of 77 miRNAs, including 33 known miRNAs and 44 novel miRNAs, were identified, and 41 and 28 miRNAs were differentially expressed under NT and HT stress conditions, respectively. The sporogenous cell proliferation (SCP), meiotic phase (MP), microspore release period (MRP), and pollen maturity (PM) stages had 10 (including 12 miRNAs), four (including six miRNAs), four (including five miRNAs), and seven (including 11 miRNAs) HT stress-responsive miRNA families, respectively, which were identified after removing the changes in genotype-specific miRNAs under NT condition. Seven miRNA families (miR2949, miR167, and miR160 at the SCP stage; miR156 and miR172 at the MP stage; miR156 at the MRP stage; and miR393 and miR3476 at the PM stage), which had expression abundance of more than 10% of the total expression abundance, served as the main regulators responding to HT stress with positive or negative regulation patterns. These miRNAs orchestrated the expression of the corresponding target genes and led to different responses in the HT-tolerant and the HT-sensitive lines. The results revealed that the HT stress response of miRNAs in cotton anthers were stage-specific and differed with the development of anthers. Our study may enhance the understanding of the response of miRNAs to HT stress in cotton anthers and may clarify the mechanism of plant tolerance to HT stress.

High temperature susceptibility of sexual reproduction in crop plants

Climate change-induced increases in the frequency of extreme weather events, particularly heatwaves, are a serious threat to crop productivity. The productivity of grain crops is dependent on the success of sexual reproduction, which is very sensitive to heat stress. Male gametophyte development has been identified as the most heat-vulnerable stage. This review outlines the susceptibility of the various stages of sexual reproduction in flowering plants from the time of floral transition to double fertilization. We summarize current knowledge concerning the molecular mechanisms underpinning the heat stress-induced aberrations and abnormalities at flowering, male reproductive development, female reproductive development, and fertilization. We highlight the stage-specific bottlenecks in sexual reproduction, which regulate seed set and final yields under high-temperature conditions, together with the outstanding research questions concerning genotypic and species-specific differences in thermotolerance observed in crops. This knowledge is essential for trait selection and genetic modification strategies for the development of heat-tolerant genotypes and high-temperature-resilient crops.

Examining the Effects of Temperature on Recombination in Wheat

Meiotic recombination plays a crucial role in the generation of new varieties. The effectiveness of recombination is limited by the distribution of crossover events, which in wheat and many other crops is skewed toward the distal regions of the chromosomes. Whole-genome sequencing of wheat has revealed that there are numerous important genes in the pericentromeric regions, which are inaccessible to manipulation due to the lack of crossover events. Studies in barley have shown that the distribution of recombination events can be shifted toward the centromeres by increasing temperature during meiosis. Here we present an analysis of the effects of temperature on the distribution and frequency of recombination events in wheat. Our data show that although increased temperature during meiosis does cause an inward shift in recombination distribution for some chromosomes, its overall utility is limited, with many genes remaining highly linked.

Analytical Methodology of Meiosis in Autopolyploid and Allopolyploid Plants

Meiosis is the cellular process responsible for producing gametes with half the genetic content of the parent cells. Integral parts of the process in most diploid organisms include the recognition, pairing, synapsis, and recombination of homologous chromosomes, which are prerequisites for balanced segregation of half-bivalents during meiosis I. In polyploids, the presence of more than two sets of chromosomes adds to the basic meiotic program of their diploid progenitors the possibility of interactions between more than two chromosomes and the formation of multivalents, which has implications on chromosome segregations and fertility. The mode of how chromosomes behave in meiosis in competitive situations has been the aim of many studies in polyploid species, some of which are considered here. But polyploids are also of interest in the study of meiosis because some of them tolerate the loss of chromosome segments or complete chromosomes as well as the addition of chromosomes from related species. Deletions allow to assess the effect of specific chromosome segments on meiotic behavior. Introgression lines are excellent materials to monitor the behavior of a given chromosome in the genetic background of the recipient species. We focus on this approach here as based on studies carried out in bread wheat, which is commonly used as a model species for meiosis studies. In addition to highlighting the relevance of the use of materials derived from polyploids in the study of meiosis, cytogenetics tools such as fluorescence in situ hybridization and the immunolabeling of proteins interacting with DNA are also emphasized.

Breeding crops to feed 10 billion

Crop improvements can help us to meet the challenge of feeding a population of 10 billion, but can we breed better varieties fast enough? Technologies such as genotyping, marker-assisted selection, high-throughput phenotyping, genome editing, genomic selection and de novo domestication could be galvanized by using speed breeding to enable plant breeders to keep pace with a changing environment and ever-increasing human population.

Leagues of their own: sexually dimorphic features of meiotic prophase I

Meiosis is a conserved cell division process that is used by sexually reproducing organisms to generate haploid gametes. Males and females produce different end products of meiosis: eggs (females) and sperm (males). In addition, these unique end products demonstrate sex-specific differences that occur throughout meiosis to produce the final genetic material that is packaged into distinct gametes with unique extracellular morphologies and nuclear sizes. These sexually dimorphic features of meiosis include the meiotic chromosome architecture, in which both the lengths of the chromosomes and the requirement for specific meiotic axis proteins being different between the sexes. Moreover, these changes likely cause sex-specific changes in the recombination landscape with the sex that has the longer chromosomes usually obtaining more crossovers. Additionally, epigenetic regulation of meiosis may contribute to sexually dimorphic recombination landscapes. Here we explore the sexually dimorphic features of both the chromosome axis and crossing over for each stage of meiotic prophase I in Mus musculus, Caenorhabditis elegans, and Arabidopsis thaliana. Furthermore, we consider how sex-specific changes in the meiotic chromosome axes and the epigenetic landscape may function together to regulate crossing over in each sex, indicating that the mechanisms controlling crossing over may be different in oogenesis and spermatogenesis.

Temperature-induced changes in the wheat phosphoproteome reveal temperature-regulated interconversion of phosphoforms

Wheat (Triticum ssp.) is one of the most important human food sources. However, this crop is very sensitive to temperature changes. Specifically, processes during wheat leaf, flower, and seed development and photosynthesis, which all contribute to the yield of this crop, are affected by high temperature. While this has to some extent been investigated on physiological, developmental, and molecular levels, very little is known about early signalling events associated with an increase in temperature. Phosphorylation-mediated signalling mechanisms, which are quick and dynamic, are associated with plant growth and development, also under abiotic stress conditions. Therefore, we probed the impact of a short-term and mild increase in temperature on the wheat leaf and spikelet phosphoproteome. In total, 3822 (containing 5178 phosphosites) and 5581 phosphopeptides (containing 7023 phosphosites) were identified in leaf and spikelet samples, respectively. Following statistical analysis, the resulting data set provides the scientific community with a first large-scale plant phosphoproteome under the control of higher ambient temperature. This community resource on the high temperature-mediated wheat phosphoproteome will be valuable for future studies. Our analyses also revealed a core set of common proteins between leaf and spikelet, suggesting some level of conserved regulatory mechanisms. Furthermore, we observed temperature-regulated interconversion of phosphoforms, which probably impacts protein activity.

Effect of Ppd-A1 and Ppd-B1 Allelic Variants on Grain Number and Thousand Kernel Weight of Durum Wheat and Their Impact on Final Grain Yield

The main yield components in durum wheat are grain number per unit area (GN) and thousand kernel weight (TKW), both of which are affected by environmental conditions. The most critical developmental stage for their determination is flowering time, which partly depends on photoperiod sensitivity genes at Ppd-1 loci. Fifteen field experiments, involving 23 spring durum wheat genotypes containing all known allelic variants at the PHOTOPERIOD RESPONSE LOCUS (Ppd-A1 and Ppd-B1) were carried out at three sites at latitudes ranging from 41° to 27° N (Spain, Mexico-north, and Mexico-south, the latter in spring planting). Allele GS100 at Ppd-A1, which causes photoperiod insensitivity and results in early-flowering genotypes, tended to increase TKW and yield, albeit not substantially. Allele Ppd-B1a, also causing photoperiod insensitivity, did not affect flowering time or grain yield. Genotypes carrying the Ppd-B1b allele conferring photoperiod sensitivity had consistently higher GN, which did not translate into higher yield due to under-compensation in TKW. This increased GN was due to a greater number of grains spike-1 as a result of a higher number of spikelets spike-1. Daylength from double ridge to terminal spikelet stage was strongly and positively associated with the number of spikelets spike-1 in Spain. This association was not found in the Mexico sites, thereby indicating that Ppd-B1b had an intrinsic effect on spikelets spike-1 independently of environmental cues. Our results suggest that, in environments where yield is limited by the incapacity to produce a high GN, selecting for Ppd-B1b may be advisable.

Non-destructive, high-content analysis of wheat grain traits using X-ray micro computed tomography

BACKGROUND

Wheat is one of the most widely grown crop in temperate climates for food and animal feed. In order to meet the demands of the predicted population increase in an ever-changing climate, wheat production needs to dramatically increase. Spike and grain traits are critical determinants of final yield and grain uniformity a commercially desired trait, but their analysis is laborious and often requires destructive harvest. One of the current challenges is to develop an accurate, non-destructive method for spike and grain trait analysis capable of handling large populations.

RESULTS

In this study we describe the development of a robust method for the accurate extraction and measurement of spike and grain morphometric parameters from images acquired by X-ray micro-computed tomography (μCT). The image analysis pipeline developed automatically identifies plant material of interest in μCT images, performs image analysis, and extracts morphometric data. As a proof of principle, this integrated methodology was used to analyse the spikes from a population of wheat plants subjected to high temperatures under two different water regimes. Temperature has a negative effect on spike height and grain number with the middle of the spike being the most affected region. The data also confirmed that increased grain volume was correlated with the decrease in grain number under mild stress.

CONCLUSIONS

Being able to quickly measure plant phenotypes in a non-destructive manner is crucial to advance our understanding of gene function and the effects of the environment. We report on the development of an image analysis pipeline capable of accurately and reliably extracting spike and grain traits from crops without the loss of positional information. This methodology was applied to the analysis of wheat spikes can be readily applied to other economically important crop species.