Time to spare: inefficiencies in wheat hybrid seed production are not due to stigma longevity

Abstract

This article comments on:

Millan-Blanquez M, Simmonds J, Bird N, Manes Y, Uauy C, Boden SA. 2025. Stigma longevity is not a major limiting factor in hybrid wheat seed production. Journal of Experimental Botany 76, 4003–4013. https://doi.org/10.1093/jxb/eraf154

This article comments on:

Millan-Blanquez M, Simmonds J, Bird N, Manes Y, Uauy C, Boden SA. 2025. Stigma longevity is not a major limiting factor in hybrid wheat seed production. Journal of Experimental Botany 76, 4003–4013. https://doi.org/10.1093/jxb/eraf154

The discovery of heterosis and the development of methods for large-scale production of hybrid seeds had a dramatic impact on grain yields in the 20th century. However, hybrid breeding approaches in wheat have been substantially less successful than those in the world’s other two most important grain crops: maize and rice. Because wheat is primarily self-fertilizing, an important barrier in hybrid breeding is the efficient production of hybrid seed. Low seed set has been attributed to the limited developmental window in which stigmas are receptive to pollen, requiring both parents to flower at nearly identical times. Millan-Blanquez et al. (2025) characterize the trajectory of wheat pistil development in the field during hybrid seed production, and show that the timing of stigma degeneration plays a minimal role in limiting seed set. These findings will help shape breeding targets for the development of efficient hybrid wheat production systems of the future.

Changes in climate are predicted to drive yield decreases from 15% to 40% across major wheat-growing regions by the end of the century (Hultgren et al., 2025). It is imperative that all available tools to improve wheat yields are brought to bear to maintain food system stability as these changes occur. Unlike other major grain crops, wheat breeding does not leverage hybrid vigor to increase yields despite estimates that hybrid wheat can achieve 10–20% yield increases relative to inbreds (Gupta et al., 2019). Harnessing the power of hybrid breeding in wheat is an important building block toward meeting increased demand under greater climate uncertainty.

Hybrid seeds are produced by crossing genetically distinct male and female inbred lines to produce better preforming F₁ hybrids. The inefficient production of hybrid seed is a major factor driving the low uptake of hybrid wheat. Wheat is self-fertilizing, which means that outcrossing rates must be artificially increased to produce large numbers of hybrids. Male-sterile female parents were developed in the middle of the 20th century to enforce outcrossing, which led to decades of work to develop hybrid breeding programs in wheat (Gupta et al., 2019). In the 1990s, chemical methods to enforce male sterility were developed and successfully used to further facilitate hybrid production (Gupta et al., 2019). However, seed set on female parents remains low in hybrid production systems, leading breeders to view the yield gains offered by hybrids as not sufficient to justify the increased effort required for seed production (Whitford et al., 2013; Selva et al., 2020). As a result, only 1% of wheat planted worldwide results from hybrid breeding programs (Longin et al., 2012).

Identifying the causes of inefficient hybrid seed production

The low efficiency of hybrid seed production in wheat is often attributed to physical and temporal developmental barriers (Selva et al., 2020) (Fig. 1). The male and female reproductive organs in wheat are partially or even completely enclosed by the palea, lemma, and glumes when pollen is released and the stigma is receptive. Hybridization also requires that the male and female parents flower synchronously so that male pollen can reach female receptive stigma. The time window when efficient pollination can occur is potentially further limited by the short period of wheat pollen viability (∼3 h) and female receptivity (7 d) (Imrie, 1966). To generate efficient wheat hybrid seed production systems, we must tease out which of these traits can improve female and male parents.

Fig. 1.

Developmental barriers to hybridization in wheat hybrid production systems. (A) Physical barriers are related to the exsertion of male or female organs from the wheat floret. Left: a flowering wheat plant; middle: an individual inflorescence (spike); right: an individual floret showing exserted anthers and stigma that are desirable for male and female parents, respectively. (B) Left: diagram of alternating rows of male-fertile (orange) and genetically or chemically induced male-sterile (blue) parents typical of hybrid production systems. Right: density plots showing how elongated stigma receptivity might extend the window of fertilization when parents flower at different times. Diagrams of the stigma transitioning from receptive to degenerated are shown above the plot.

Several genetic mapping efforts have identified loci that contribute to male determinants of seed set during hybrid production. Increased anther exsertion and therefore pollen release into the environment has been found to be correlated with hybrid seed set (Boeven et al., 2018). Several studies have found that anther exsertion is heritable and have identified loci that can be targeted by breeding programs to select male parents (Boeven et al., 2016; He et al., 2016; El Hanafi et al., 2021). In general, these studies point to anther exsertion and thus increased pollen dispersal as an important trait for improving male parents.

In contrast, we know less about the role of female receptivity in efficient production of hybrid seed. Loci have been mapped that control stigma length, and presumably exsertion, though this trait appears to be less predictable from genetic data than anther exsertion (El Hanafi et al., 2021; Pallotta et al., 2024). In contrast, the length of time the stigma can receive pollen and its role in hybrid seed production is not well understood. One reason for this lack of knowledge is that the study of carpel development and stigma degeneration involves dissection, microscopic analysis, and manual annotation of images, making it difficult to conduct at a scale relevant to breeding programs. In previous work, Millan-Blanquez et al. (2022) developed a phenotyping approach that allowed them to rapidly generate morphometric data from hundreds of spikes by implementing a machine learning approach to analyze images. This pipeline allowed them to characterize wheat carpel development in the field across multiple cultivars for several years. In this issue, Millan-Blanquez et al. (2025) apply this pipeline to understand differences in carpel development amongst a larger panel of wheat cultivars and to relate the timing of stigma degeneration to hybrid seed set.

Understanding diversity in wheat stigma degeneration

Millan-Blanquez et al. (2025) began by characterizing how female stigma development and degeneration might limit seed set on male-sterile female plants when flowering time is not perfectly synchronized between parents. They hypothesized that selection of female parents with extended carpel developmental times could widen the window of stigma receptivity and increase hybrid seed production. The team explored variation in stigma development across 29 male-sterile wheat parents and found substantial variation in the rate of stigma degeneration and the size of the stigma. They were able to assign accessions to one of six types based on their pattern of stigma development and degeneration across the time course.

Stigma deterioration does not play a major role in hybrid seed set

Millan-Blanquez et al. (2025) then selected six accessions representing this diversity to conduct a multi-year field hybrid seed production experiment using a mix of male parents with variable flowering times. The authors controlled the hybridization window precisely in different plots by removing pollen barriers at progressively later time points, thus limiting the length of the pollination window in some plots. The latest pollination window was restricted to 15 d after the onset of flowing.

Interestingly, the authors found no general effect of limiting the pollination window to later time points. This suggests that the stigma is fully receptive to pollen for a much longer period than previously hypothesized, ∼10–15 d, which is within the window of pollen release in most hybrid breeding programs. One of the six female parents did show a reduction in seed set when the pollination window was limited to later stages of stigma development, suggesting that there may be some genotypes for which the window is narrowed more rapidly, but this is not a general phenomenon. These data indicate that outside of a few unusual accessions, selecting female parents based on the timing of stigma degeneration is not likely to increase seed set in hybrid seed production.

Asynchrony in carpel development buffers the overall effect of stigma degeneration on hybrid seed production

Flowering determines the onset of female receptivity to pollen; however, the exact timing of fertility is determined by development within the wheat inflorescence (Lukac et al., 2012). Stigma receptivity occurs in a wave across the spike, moving from the middle outward toward apical and basal florets. Careful examination of seed set across individual spikes revealed some of the dynamics of seed set that might buffer stigma deterioration across the pollination window. Deterioration of stigmas in early maturing florets slightly decreases seed set in the middle of the spike in later windows. However, the later maturing florets at the top and bottom of the spike are not affected. These observations suggest that stigma deterioration may affect hybrid seed set if males flower much later than females at times not evaluated by this study.

Engineering the wheat hybrid production systems of the future

Millan-Blanquez et al. (2025) find that stigma receptivity of wheat male-sterile plants in hybrid production systems extends 10–15 d after the onset of flowering. Robustness in seed set is partially mediated by asynchrony in floret maturity within spikes. If this result can be generalized across more environments, then it suggests that stigma degeneration is not a promising target to improve hybrid wheat production systems. Instead, traits that increase the exposure of the stigma to external pollen such as stigma size or floret openness are more likely to yield improved crossing efficiencies. Female male-sterile lines exhibiting these characteristics could be important tools for increasing wheat production as global demand increases.

Funding

The author acknowledges funding from the National Science Foundation (IOS-2046256) and from the United States–Israel Binational Agricultural Research and Development Fund (IS-5658–24C).

Data availability

No data was associated with this publication.