Ryan Whitford

Widespread deployment of wheat hybrids has been limited by high seed production costs in a commercial environment. This is due to poor outcrossing characteristics of wheat which can, in part, be attributed to poor female organ receptivity to airborne pollen. Here we underpin a detailed characterization of cross-pollination seed set progression in both emasculated wild-type, and a genetically male-sterile wheat line with a comprehensive RNA-Seq and co-expression analysis. We identify three clear phases in the fertilization window: stigma development and onset of receptivity, effective pollination period, and senescence onset and subsequent tissue deterioration. By profiling female-specific marker genes, we determined that physical emasculation of wild-type 'cv. Chris' (Ms1) advances female development 2 to 3 days relative to genic male sterility (ms1d). Gene networks linked to gibberellic acid homeostasis were found associated with styloid and stigma hair elongation, and increased expression of peroxidases associated with peak receptivity (3 days after floret gaping). Further, changes in the expression of exocyst complex components coincided with the onset of stigma hair senescence. Together the applied clustering and differential expression strategy identified biologically meaningful expression signatures, particularly in stigma hairs, that correlate with the effective pollination period and the observed cross-pollination seed set. Overall, the datasets represent a valuable resource to help identify and/or engineer improved cross-pollination seed set, and ultimately leading to greater insights into this complex biological process and a more cost-effective hybrid seed production system.

The commercial realization of hybrid wheat (Triticum aestivum L.) is a major technological challenge to sustainably increase food production for our growing population in a changing climate. Despite recent advances in cytoplasmic- and nuclear-based pollination control systems, the inefficient outcrossing of wheat's autogamous florets remains a barrier to hybrid seed production. There is a pressing need to investigate wheat floral biology and enhance the likelihood of ovaries being fertilized by airborne pollen so breeders can select and utilize male and female parents for resilient, scalable, and cost-effective hybrid seed production. Advances in understanding the wheat genomes and pangenome will aid research into the underlying floral organ development and fertility with the aim to stabilize pollination and fertilization under a changing climate. The purpose of this position paper is to highlight priority areas of research to support hybrid wheat development, including (1) structural aspects of florets that affect stigma presentation, longevity, and receptivity to airborne pollen, (2) pollen release dynamics (e.g., anther extrusion and dehiscence), and (3) the effect of heat, drought, irradiation, and humidity on these reproductive traits. A combined approach of increased understanding built on the genomic resources and advanced trait evaluation will deliver to robust measures for key floral characteristics, such that diverse germplasm can be fully exploited to realize the yield improvements and yield stability offered by hybrids.

Pathogens constantly attack staple crops, leading to substantial yield losses. Plant-pathogen interactions activate endogenous plant-secreted peptides, which act as immunity inducers and are promising breeding targets for enhancing crop resistance to pathogens. However, the identification and mechanisms of immunogenic peptides in staple crops remain largely unexplored. Here, we demonstrated that plant elicitor peptides (TaPeps) in wheat (Triticum aestivum), processed by a metacaspase, are competent to trigger plant immunity and contribute to resistance against Fusarium head blight (FHB). Using exogenous phytocytokine peptide screens, we identified three potential TaPeps acting as elicitors that significantly improve FHB resistance. Mechanistically, these elicitors activate innate immune signals and calcium dynamics in response to the Fusarium pathogen via wheat PEP RECEPTOR 1 (TaPEPR1). Overexpression of endogenous PRECURSOR OF PEPs (TaPROPEPs) further reduces FHB severity. Moreover, we characterized the natural form of TaPeps in planta, revealing that the wheat type-II metacaspase TaMCA-IIa cleaves TaPROPEPs at a conserved arginine residue, promoting TaPep maturation and immune activation. In Tamca-IIa mutants, the efficiency of TaPep maturation was decreased and calcium dynamics were impaired, resulting in FHB susceptibility. Conversely, overexpressing TaMCA-IIa in wheat enhanced the immune response and FHB resistance without causing pleiotropic growth penalties. Our findings highlight TaPeps as potential immune-inducing biologicals for crop protection and uncover the metacaspase-Peps-receptor module in mediating plant disease resistance.

Correct floral development is the result of a sophisticated balance of molecular cues. Floral mutants provide insight into the main genetic determinants that integrate these cues, as well as providing opportunities to assess functional variation across species. In this study, we characterize the barley (Hordeum vulgare) multiovary mutants mov2.g and mov1, and propose causative gene sequences: a C2H2 zinc-finger gene HvSL1 and a B-class gene HvMADS16, respectively. In the absence of HvSL1, florets lack stamens but exhibit functional supernumerary carpels, resulting in multiple grains per floret. Deletion of HvMADS16 in mov1 causes homeotic conversion of lodicules and stamens into bract-like organs and carpels that contain non-functional ovules. Based on developmental, genetic, and molecular data, we propose a model by which stamen specification in barley is defined by HvSL1 acting upstream of HvMADS16. The present work identifies strong conservation of stamen formation pathways with other cereals, but also reveals intriguing species-specific differences. The findings lay the foundation for a better understanding of floral architecture in Triticeae, a key target for crop improvement.

Meiotic recombination is a critical process for plant breeding, as it creates novel allele combinations that can be exploited for crop improvement. In wheat, a complex allohexaploid that has a diploid-like behaviour, meiotic recombination between homoeologous or alien chromosomes is suppressed through the action of several loci. Here, we report positional cloning of Pairing homoeologous 2 (Ph2) and functional validation of the wheat DNA mismatch repair protein MSH7-3D as a key inhibitor of homoeologous recombination, thus solving a half-century-old question. Similar to ph2 mutant phenotype, we show that mutating MSH7-3D induces a substantial increase in homoeologous recombination (up to 5.5 fold) in wheat-wild relative hybrids, which is also associated with a reduction in homologous recombination. These data reveal a role for MSH7-3D in meiotic stabilisation of allopolyploidy and provides an opportunity to improve wheat’s genetic diversity through alien gene introgression, a major bottleneck facing crop improvement.

The development and adoption of hybrid seed technology have led to dramatic increases in agricultural productivity. However, it has been a challenge to develop a commercially viable platform for the production of hybrid wheat (Triticum aestivum) seed due to wheat's strong inbreeding habit. Recently, a novel platform for commercial hybrid seed production was described. This hybridization platform utilizes nuclear male sterility to force outcrossing and has been applied to maize and rice. With the recent molecular identification of the wheat male fertility gene Ms1, it is now possible to extend the use of this novel hybridization platform to wheat. In this report, we used the CRISPR/Cas9 system to generate heritable, targeted mutations in Ms1. The introduction of biallelic frameshift mutations into Ms1 resulted in complete male sterility in wheat cultivars Fielder and Gladius, and several of the selected male-sterile lines were potentially non-transgenic. Our study demonstrates the utility of the CRISPR/Cas9 system for the rapid generation of male sterility in commercial wheat cultivars. This represents an important step towards capturing heterosis to improve wheat yields, through the production and use of hybrid seed on an industrial scale.

The current rate of yield gain in crops is insufficient to meet the predicted demands. Capturing the yield boost from heterosis is one of the few technologies that offers rapid gain. Hybrids are widely used for cereals, maize and rice, but it has been a challenge to develop a viable hybrid system for bread wheat due to the wheat genome complexity, which is both large and hexaploid. Wheat is our most widely grown crop providing 20% of the calories for humans. Here, we describe the identification of Ms1, a gene proposed for use in large-scale, low-cost production of male-sterile (ms) female lines necessary for hybrid wheat seed production. We show that Ms1 completely restores fertility to ms1d, and encodes a glycosylphosphatidylinositol-anchored lipid transfer protein, necessary for pollen exine development. This represents a key step towards developing a robust hybridization platform in wheat.Heterosis can rapidly boost yield in crop species but development of hybrid-breeding systems for bread wheat remains a challenge. Here, Tucker et al. describe the molecular identification of the wheat Ms1 gene and discuss its potential for large-scale hybrid seed production in wheat.

Global food security demands the development and delivery of new technologies to increase and secure cereal production on finite arable land without increasing water and fertilizer use. There are several options for boosting wheat yields, but most offer only small yield increases. Wheat is an inbred plant, and hybrids hold the potential to deliver a major lift in yield and will open a wide range of new breeding opportunities. A series of technological advances are needed as a base for hybrid wheat programmes. These start with major changes in floral development and architecture to separate the sexes and force outcrossing. Male sterility provides the best method to block self-fertilization, and modifying the flower structure will enhance pollen access. The recent explosion in genomic resources and technologies provides new opportunities to overcome these limitations. This review outlines the problems with existing hybrid wheat breeding systems and explores molecular-based technologies that could improve the hybrid production system to reduce hybrid seed production costs, a prerequisite for a commercial hybrid wheat system.

Growth and development are coordinated by an array of intercellular communications. Known plant signaling molecules include phytohormones and hormone peptides. Although both classes can be implicated in the same developmental processes, little is known about the interplay between phytohormone action and peptide signaling within the cellular microenvironment. We show that genes coding for small secretory peptides, designated GOLVEN (GLV), modulate the distribution of the phytohormone auxin. The deregulation of the GLV function impairs the formation of auxin gradients and alters the reorientation of shoots and roots after a gravity stimulus. Specifically, the GLV signal modulates the trafficking dynamics of the auxin efflux carrier PIN-FORMED2 involved in root tropic responses and meristem organization. Our work links the local action of secretory peptides with phytohormone transport.

The Clavata3 (CLV3)/endosperm surrounding region (CLE) signaling peptides are encoded in large plant gene families. CLV3 and the other A-type CLE peptides promote cell differentiation in root and shoot apical meristems, whereas the B-type peptides (CLE41–CLE44) do not. Instead, CLE41 inhibits the differentiation of
Zinnia elegans
tracheary elements. To test whether
CLE
genes might code for antagonistic or synergistic functions, peptides from both types were combined through overexpression within or application onto
Arabidopsis thaliana
seedlings. The CLE41 peptide (CLE41p) promoted proliferation of vascular cells, although delaying differentiation into phloem and xylem cell lineages. Application of CLE41p or overexpression of
CLE41
did not suppress the terminal differentiation of the root and shoot apices triggered by A-type CLE peptides. However, in combination, A-type peptides enhanced all of the phenotypes associated with
CLE41
gain-of-function, leading to massive proliferation of vascular cells. This proliferation relied on auxin signaling because it was enhanced by exogenous application of a synthetic auxin, decreased by an auxin polar transport inhibitor, and abolished by a mutation in the Monopteros auxin response factor. These findings highlight that vascular patterning is a process controlled in time and space by different CLE peptides in conjunction with hormonal signaling.