Vernalization genes/class of genes identified in bread wheat to date.
Abstract
In bread wheat (Triticum aestivum L.), flowering time and plant stature are important phenological and agronomic traits for adaptation, yield potential, and yield stability. Timely flowering is critical for production, and the flowering window has to be late enough to avoid early season frosts but early enough to avoid late season stresses such as heat and terminal drought. Flowering time is controlled mainly by vernalization, photoperiod response, and earliness per se genes, which can be exploited to fine‐tune growth and tailor flowering time for the production of desirable wheat cultivars. Tailoring flowering time could help reduce preharvest sprouting problems by escaping high temperatures and late season rainfall, which promote preharvest sprouting, hence yield loss. Concisely summarizing available information on flowering time and identifying research gaps could provide direction for future research. This chapter, therefore, discusses: (i) the progress made in discovering genes involved and the impact of their extensive allelic variation on flowering time, (ii) the potential benefits of tailoring wheat's flowering time to improve yield, and (iii) the benefits of introgressing genes for other complimentary traits, such as semidwarf and preharvest sprouting resistance on advanced lines to achieve higher yield, thus, sustainable food security.
Keywords
- earliness per se
- flowering time
- photoperiod response
- preharvest sprouting
- semidwarf
- vernalization
- yield
1. Introduction
The performance of a wheat cultivar, which is normally measured by its adaptability and yield potential under target environments, is dependent on genetic and environmental factors as well as the interaction between these factors. Timely flowering, that is the switch from the vegetative phase to the reproductive phase, and the duration of the life cycle fine‐tune a cultivar to the targeted environment [1, 2]. Flowering is essential for reproductive success and occurs when conditions are favorable to maximize pollination, seed development, seed dispersal, and subsequent germination [1]. Flowering success is demonstrated by the ability of the plant to efficiently use a range of available resources including water, nutrients, temperature, day length, radiant energy, and relevant endogenous signals to maximize its potential yield and to escape stressful conditions during growth and development [1, 3]. Consequently, there is a need to better understand the genetic control of flowering time in wheat. Understanding the genetic control of the components of the life cycle, although complex, will enable plant breeders to exploit associated genes, thus fine‐tune the growth and development of the crop to fulfill the demands of a specific environment and to increase yield [1, 4]. Discovering genes that control flowering time in wheat have been one of the key research goals for decades [1] and is increasingly gaining importance due to the impact of projected climate change [5]. As a result, many loci influencing flowering time has been successfully mapped and their effects determined [1, 4].
The duration of the life cycle of bread wheat is controlled by numerous genes, including those associated with seed germination, vegetative growth, flowering time, seed maturation, and seed dispersal [6]. These processes form the foundation of the reproductive strategy of flowering plants. The interaction between these genes and the environment defines the ultimate phenotype [7, 8]. Flowering time is an important component of the life cycle with a very wide and complex genetic control. Three groups of genes with major influence on flowering time of wheat include vernalization response genes, photoperiod response genes, and genes controlling the developmental rate (earliness
2. The process of flowering in bread wheat
The process of flowering involves multiple interactions between major genes (vernalization, photoperiod response, and
3. The influence of vernalization genes on the flowering time of bread wheat
Bread wheat is generally classified as spring or winter types according to its response to low temperatures during the vegetative phase [22–24]. Exposure to low temperatures (0–10°C) for several weeks (usually 6–8 weeks) is necessary for the development of tillers and the induction of flowering in winter wheat, whereas tillering and flowering of spring wheat occur regardless of temperature [22, 25]. The flowering models of the temperate cereals indicate that before vernalization,
Substitution line analyses have identified four major series of genes controlling the length of the vernalization period in bread wheat (
Table 1
). According to Yan et al [17], the
Major gene series | Gene(s) comprised | Gene location | Reference |
---|---|---|---|
|
|
Long arms of chromosomes 5A, 5B and 5D, respectively | [31, 34] |
|
Not specified | chromosomes 4B, 4D and 5A | [35] |
|
|
Short arms of chromosomes 7A, 7B and 7D, respectively | [36] |
|
|
Chromosome 5D | [37] |
4. The influence of photoperiod response genes on the flowering time of bread wheat
Photoperiod is the day length and number of long days that a wheat cultivar must reach (a threshold) for floral initiation [38]. The duration of exposure to light can be categorized into three groups namely, short‐day (SD, 11‐14 h), long‐day (LD, 18 h), and day‐neutral (DN) or facultative [39]. The winter wheat and spring wheat varieties can be photoperiod‐sensitive or photoperiod‐insensitive. Photoperiod‐insensitive varieties are early flowering both under SD and LD conditions, in contrast to the photoperiod‐sensitive varieties that require exposure to LD for weeks before they can initiate flowering [38, 40]. Several genes controlling photoperiod response have been successfully identified in wheat (
Table 2
). The
Photoperiod insensitivity is beneficial for crops grown in short‐growing seasons with high summer temperatures in order to avoid heat stress during grain‐filling stages [44, 45, 46]. Earlier flowering conferred by the
5. The interaction of
Vrn
and
Ppd
alleles and its impact on flowering time of bread wheat
Various studies have been conducted to study the interaction between
An epistatic interaction between the
6. The influence of earliness
per se
(
eps
) genes on the flowering time of bread wheat
A third class of genes controlling flowering time of wheat is the earliness
7. The influence of height‐reducing genes on the flowering time of bread wheat
Among the most important growth habit parameters influencing adaptation and yield potential of bread wheat to various environments is plant height. The most common genes for reduced height (
A moderate but significant correlation between flowering time and plant height has been reported in bread wheat [5, 80–82]. Shorter genotypes tend to flower earlier than the taller ones [5]. This effect was proposed to be mainly due to the
8. The potential benefits of tailoring flowering time of wheat in the wheat production industry
Flowering time is a complex trait that is responsible for wide adaptation of wheat (and other cereal crops) to different environments [4, 21, 86]. This trait could be modified or tailored to local climatic conditions to achieve desired characteristics such as improved yield [87, 88]. Similar studies have been conducted successfully whereby high temperatures and drought stress during anthesis and grain filling were avoided through tailoring flowering time of wheat to local climatic conditions [42, 89].
The potential advantage of tailoring flowering time can be used to escape environmental conditions that lead to yield loss, such as high temperatures or conditions that lead to poor wheat quality, such as rain during harvest time. This could contribute to reducing the worldwide physiological phenomenon of preharvest sprouting (PHS). Preharvest sprouting, which is the germination of seed grains in the mother ear before harvest due to humid conditions, is prevalent in wheat‐growing regions experiencing high rainfall during the period of grain maturity and ripening [90]. This results in significant losses in the wheat production industry such as the downgrading of premium milling quality wheat to feed quality [91]. Resistance to PHS is a highly desirable trait sought by plant breeders globally [92, 93]. In addition to breeding for resistance of this trait, tailoring flowering time for the production of early flowering cultivars, which will escape conditions favorable to PHS, could help reduce the problem.
9. The role of diagnostic molecular markers in the detection of allelic variation among the major growth habit genes influencing the flowering time of bread wheat
The fact that current wheat germplasm has not been characterized fully in terms of important agronomic traits limits the use of wheat germplasm to a certain extent. Identifying the alleles of these genes and estimating the effects of their combination on growth, heading date, and ultimately grain yield will enhance the selection of cultivars with wide adaptability to a set of environments [57]. This knowledge can help accelerate the introgression of adaptability and yield‐contributing genes by predicting the best combinations for enhanced yield potential and adaptation [28]. Moreover, the identification of alleles of growth habit genes subsequently leads to the development of a series of molecular markers (allele‐specific DNA markers) for improved identification of these alleles in future [46, 86, 94].
The development of allele‐specific DNA markers has allowed for efficient detection of extensive allelic variation existing among genes controlling flowering time in bread wheat [35, 46, 95]. Through these markers, it has been revealed that the allelic variation at the
For photoperiod response genes, photoperiod insensitivity is induced by indels in the 5’ upstream region of pseudoresponse regulator (PPR) genes, which do not exist in photoperiod‐sensitive varieties [41, 46, 49]. For instance, a 2 kb deletion in the
The
The alleles of
Besides the
10. Concluding remarks and future breeding perspectives
The three classes of genes (
In the view of the current and projected climate change, which will include extreme hot and dry conditions
Selection of favorable alleles could increase the level of variation and/or introduce novel sources of resistance to diseases and unfavorable weather conditions into breeding populations [9, 26, 95, 101]. This allows the transfer of genotypes between regions with different climatic conditions but still maintains their level of agronomic performance [5]. The
Acknowledgments
The authors would like to thank their fellow colleagues: Sandiswa Figlan (Agricultural Research Council, Small Grain Institute), Nondumiso Zanele Sosibo (Agricultural Research Council, Small Grain Institute), Lisemelo Motholo (Agricultural Research Council, Small Grain Institute), Thandeka Nokuthula Sikhakhane (Agricultural Research Council, Small Grain Institute), Learnmore Mwadzingeni (University of KwaZulu‐Natal), and Pamella Ntshakaza (University of KwaZulu‐Natal) for support and contribution to the development of the chapter. This work was funded by the Agricultural Research Council.
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