Project Topics
Get Complete Project Material File(s) Now! »
Evolution of wheat species and associated genetic shifts
Today, wheat is one of the most important staple foods and the second most produced cereal behind maize (USDA 2018). It regroups numerous Triticum species and sub-species, as the best-known T. aestivum aestivum (common wheat, or bread wheat) or T. turgidum durum (durum wheat). The domestication of wheat started about 10 000 years (Salamini et al. 2002; Faris 2014) ago in the Fertile Crescent in the Middle East, during the Neolithic revolution and supported the transition of human societies from hunter-gatherer lifestyle to an agrarian lifestyle (Bonjean 2001; Faris 2014). The ancestor of the modern bread wheat and durum wheat was the wild emmer, T. turgidum dicoccoides, a tetraploid form of wild wheat which came from the hybridation between the wild diploid wheat species T. urartu and a wild diploid relative Aegilops speltoides (Peng et al. 2011; Faris 2014) (Fig 1). Over time, the wild emmer was domesticated by humans, leading to the first subspecies T. turgidum dicoccoidum. Wild and domesticated emmer were distinct, as the last one showed a non-brittle rachis and bigger grains, more convenient to harvest and leading to bigger yield (Salamini et al. 2002; Peng et al. 2011). Several tetraploid domesticated wheat subspecies appeared then, such as T. turgidum durum (durum wheat), T. turgidum polonicum (Polish wheat) or T. turgidum turgidum (Poulard wheat), and are still used today. They are more convenient to harvest than domesticated emmer because of their free-threshing forms (Dubcovsky and Dvorak 2007; Gill et al. 2007). The evolution reached a new step afterwards with the apparition of hexaploid wheat species, T. aestivum, including g T. aestivum aestivum, i.e. the bread wheat (genome AABBDD, 6 x 7= 42 chromosomes, about 17 Gb), which represents the major part of the cultivated wheat today. It is suggested that bread wheat appeared following the hybridization between T. turgidum dicoccoidum (providing genomes AA and BB) and a wild diploid relative, Aegilops tauschii (providing genome DD) (Marcussen et al. 2014).
Morphological and physiological changes related to domestication
Morphological differences can be observed between wild and domesticated wheat species, and this is also the case for other cereals. Indeed, above-ground differences have been well documented, especially those resulting in easier and better harvest: non-brittle rachis, bigger grains and free-threshing form due to soft glumes (Salamini et al. 2002; Gill et al. 2007; Peng et al. 2011; Faris 2014) (Fig 2). Thus, it appeared that in order to raise yield, farmers favored plants presenting big spikelets but compact vegetative parts. This was striking when comparing the domesticated species of another Poaceae, maize, and its wild ancestor the teosinte, which presents clearly more tillered shoots (Gaudin et al. 2014). Wheat ancestors showed also longer shoots than durum or bread wheat (Gurcan et al. 2017; Pour-Aboughadareh et al. 2017). However, very little was investigated about underground traits. Given that the shoot/root ratio remains stable, at least in non-stressful environments (Bastow Wilson 1988; Feller et al. 2015), it is then not surprising that wild relatives of domesticated cereals show more root volume and biomass, like teosinte, which displays a significantly higher root volume than domesticated maize genotypes (Gaudin et al. 2014). Similar results have been shown regarding wheat, as wild wheat relatives show higher root biomass than domesticated wheat species (Waines and Ehdaie 2007; Pour-Aboughadareh et al. 2017; Ahmadi et al. 2018). However, there is a high heterogeneity at an inter- and infra-species level, and it appeared that some ancient species or subspecies present a root biomass lower than bread wheat or durum wheat (Akman 2017). Either way, it can be assumed that wild and domesticated wheat species display different root system morphologies.
In the same way that domestication shaped the genome and the morphology of crop plants such as wheat, it also impacted the physiology of the plants. Thus, Beleggia (2016) showed that transition from wild emmer to domesticated emmer was related to a significant decrease in saturated and unsaturated fatty acids in wheat kernels, and also that the transition from domesticated emmer to durum wheat was related to a significant change in amino acids abundance (notably a drop in alanine, valine, leucine, isoleucine, serine, and threonine concentrations) and sugar abundance (a drop in fructose and glucose concentrations). Moreover, the domestication led to a change in secondary metabolite profiles by the selection of crop genotypes with grains showing lower concentrations in components that could have a detrimental effect on human health or on taste (notably bitter components) (Meyer et al. 2012). It also led to a difference in phenolic compounds, notably flavonoids whose abundance and diversity dropped in grains of domesticated species (Cooper 2015; Şahin et al. 2017). Thus, it is not surprising to observe differences in terms of rhizodeposition contents, and mostly root exudation (i.e. the release of low weight molecular substances mostly derived from photosynthesis products). Ianucci (2017)observed that among wild emmer, domesticated emmer and durum wheat, there were significant differences in exudate composition. For example, the transition between domesticated emmer and durum wheat was marked by a drop in fructose, galactose, mannose and glucose concentrations, but also mannitol and sorbitol. However, the difference is not always in favor of the older genotypes. For instance, durum wheat presented a bigger concentration of sucrose and policosanols (i.e. polymers of primary long-chain alcohols found in plant waxes) than wild or domesticated emmer.
A representative example of modern varietal selection: the emergence of French modern varieties
The modern breeding of wheat in France has been one of the most efficient breeding programs in the world, and it is very representative of the different steps in the genetic improvement of wheat genotypes that will give higher yield, better grain nutrient content, etc. First, we will review the emergence of old wheat varieties (i.e. the first pure line varieties), selected before 1960 and at the beginning of the massive use of agrochemicals. Then, we will see how the VAT (“Valeur Agronomique et Technologique”) and DHS (“Distinction, Homogénéité et Stabilité”) requirements have drifted the creation of new varieties towards quite homogenous genotypes showing high yield and great bakery quality. Finally, will be discussed how modern agricultural practices and molecular modifications have led to the emergence of modern wheat varieties.
The beginning of the scientific selection: the old varieties
At the end of the 18th century, the first private seed companies appeared, notably Vilmorin-Andrieux in 1774 in France. These private seed companies were the first to develop a more rigorous and scientific method to select wheat genotypes. At that time, public research structures, such as the “Institut national des Sciences et des Arts”, the “Société d’Agriculture de France”, or some years later the “Institut National Agronomique”, focused more on chemistry than on biology and agronomy, and it is Louis de Vilmorin who presented at the end of the 1850s the new method of genealogical selection (Gayon and Zallen 1998; Bonjean 2001). It consisted to sow the wheat progeny separately and not as a mix to obtain pure genotypes, which is the opposite of the mass selection that leads to a high genetic heterogeneity (Gayon and Zallen 1998; Kiszonas and Morris 2018). The individual genotypes became the unit of selection, thus it was the early stages of the fixed varieties as we know them today (Bonnin et al. 2014). It marked the end of the spontaneous hybridizations occurring in the fields, but also of the competition between genotypes in the fields as each individual plant was genetically identical to the others. Vilmorin started then a scientific screening, doing cross-pollination of multiple pure varieties and choosing the ones showing particular agronomic characters of interest, such as disease resistance, heat and cold resistance, or even improved yield (Gayon and Zallen 1998; Bonneuil and Thomas 2012). However, the beginnings were slow and farmers showed very little interest for these new varieties and genetic-based selection methods, favoring their landraces except for the richest farmers. In the early 1930s was created the French official catalogue of varieties for wheat, which collected at that time the information on about 400 varieties (Bonneuil and Thomas 2012; Boulineau and Leclerc 2013; Bonnin et al. 2014). Then, the agronomy during the 1940s was marked by the rise of the Mendelian genetics, and once again, the private seed companies, and notably Vilmorin, were the first ones to select and breed pure varieties, with the purpose to improve their progeny (Roussel et al. 2004; Bonneuil and Thomas 2012; Cavanagh et al. 2013). This scientific selection contributed to the decrease of genetic diversity.
Molecular modifications and agrochemicals to improve yield: the modern varieties
In 1960, appeared in France the semi-dwarf wheat varieties, which are more convenient to harvest. The genes involved in this phenotypic modification are the Rht genes, which came from Japanese wheat varieties (Borojevic and Borojevic 2005; Berry et al. 2015) and allowed the wheat varieties to support increasing concentrations of fertilizers without lodging (Berry et al. 2015). Indeed, between 1960 and 1990, more and more chemicals, notably chemical nitrogen fertilizers, were used (Fig 4). Associated with monoculture, seed densification constrained farmers to use more pesticides in fields, due to increased sensitivity of the crops to fungal diseases (Cao et al. 2015; Parker and Gilbert 2018). Between 1960s and 1980s, only about 20% of selected varieties were commercialized because catalogue registration requirements became more and more difficult to reach (Leclerc 2009; Bonneuil and Thomas 2012). At the end of the 1960s, no more than 80 varieties all in all were still registered in the catalogue. However, the yield of bread wheat was in constant augmentation, and increased from about 2000 kg/Ha in 1950 to about 5000 kg/Ha in 1980 (Brancourt-Hulmel et al. 2003; Brisson et al. 2010), which is an increase similar to the ones observed in other countries like in the USA (Fig 5), while concurrently the use of commercialized seeds for bread wheat increased from 4 to 50 % of the total seeds used.
Impact of modern breeding on wheat root morphology
Due to the introduction of dwarfism genes in modern wheat varieties (Fig 7), a decrease of root system size would be expected because of a dynamic balance between roots and shoots (Bastow Wilson 1988; Feller et al. 2015), even if it is worth noting that this balance is not always respected and is influenced by environmental conditions and the plant growth stages (Siddique et al. 1990). Such a negative impact of the dwarfism genes on root morphology has been observed by several authors, who have shown that the Rht1 gene has a significant negative impact on primary and lateral root length and leads to a significant decrease in root biomass (Laperche et al. 2006; Subira et al. 2016). Concurrently, using isogenic wheat lines, it has also been shown that the genes Rht2, Rht8 and Rht12 have a significant impact on root length (Bai et al. 2013). There is also a trend for wheat modern varieties to focus their energy (i.e. their photosynthates) on their reproductive parts, leading to bigger spikelets at the expense of the total biomass of the plant, and thus a better harvest index (Calderini et al. 1995; Guarda et al. 2004; Álvaro et al. 2008). It can be suggested that this energy partitioning is made at the expense of the root system of modern varieties, and it is admitted that between 15 and 50% of daily photosynthates are allocated to the roots for nutrient assimilation and growth, depending on plant species and the environment (Lynch et al. 2011). Thus, in the context of modern breeding, selecting genotypes with abundant root production may have been counterproductive, due to metabolic costs and the potential reduction of energy allocated to reproductive parts and so the yield.
The use of agrochemicals in huge quantity is thought to have driven recent root morphology evolution (Zhang et al. 2013; York et al. 2015) as it has been shown that the concentration of nutrients in soil can lead to a modulation of root morphology (López-Bucio et al. 2003; Lynch 2007). For example, in soil with low phosphorus (P) concentration, i.e. in no or low-input farming system, plants form highly branched root systems by the development of numerous lateral roots and longer root hairs, which allow a better P uptake in the topsoil, were most of the phosphorus sources originating from fertilizers are found (Lynch 2007; Bovill et al. 2013) (Fig 6). Also, it has been shown that in presence of a great P concentration, roots have short hairs (Horst et al. 1993). It has been shown that plants under phosphorus starvation show an accumulation of sugars in their shoot, and it results in a sugar-signalling cascades leading to a relocation of the carbon to the root and a modification of the root system architecture (Hammond and White 2008) (Hammond and White 2008). In a similar way, Shangguan et al. (2004) observed that wheat root growth was negatively correlated to the available nitrogen, notably root length which was significantly reduced at high nitrogen concentration. The improvement of irrigation methods, leading to more available for plants, could also have an impact on the evolution of the root morphology. Indeed, plant genotypes showing deep root growth angle are able to better explore sub-surface soil, and thus to acquire water under drought condition, while it is not a useful trait in non-limiting water status (Ho et al. 2005; Manschadi et al. 2008). It is worth noting, however, that the impact of water deficiency on roots can lead to a reduction or an increase in root biomass depending on wheat genotype (Azarbad et al. 2018).
Table of contents :
Partie 1 Synthèse bibliographique
I – Impact of domestication on genome, morphology and physiology of wheat genotypes
Evolution of wheat species and associated genetic shifts
Morphological and physiological changes related to domestication
Cultivation of wheat after its domestication: the landraces
II – A representative example of modern varietal selection: the emergence of French modern varieties .
The beginning of the scientific selection: the old varieties
The DHS and VAT requirements
Molecular modifications and agrochemicals to improve yield: the modern varieties
III – Consequences of modern wheat breeding on the genome, morphology and physiology of wheat
Drop in wheat genetic diversity
Wheat genes selected by modern breeding
Impact of modern breeding on wheat root morphology
Impact of modern breeding on wheat root physiology
Impact of modern breeding on wheat behavior in field
IV –Abiotic and biotic factors influencing the composition of soil and plant root-associated bacteria
Diversity of soil and plant root-associated bacteria
Impact of soil factors and agricultural practices on the composition of the soil bacterial community
Impact of root physiology and morphology on the composition and activity of the root-associated bacteria
V – The close relationship between the plant-host genotype and its root-associated bacteria.
Influence of plant-host variety on root-associated bacteria
Specificity of interactions between plant genotypes and PGPR
Conclusion
References
Partie 2 Modern breeding and the ability of wheat to interact with the PGPR
Pseudomonas kilonensis F113
Preamble
Abstract
Introduction
Materials and methods
Results
Discussion
Acknowledgments
Conflict of interest
References
Supplementary Information
Partie 3 Symbiotic variations among wheat genotypes and detection of quantitative trait loci for interaction with two contrasted proteobacterial PGPR strains
Preamble
Abstract
Introduction
Material and Methods
Results
Discussion
Acknowledgments
References
Supplementary information
Partie 4 Field analysis of rhizosphere interactions of wheat genotypes stimulating the PGPR Pseudomonas kilonensis F113 and Azospirillum brasilense Sp245 with microbial functional groups relevant
for plant growth
Preamble
Abstract
Introduction
Material and methods
Results
Discussion
Acknowledgements
References
Supplementary information
Partie 5 Discussion générale
Références introduction générale et discussion générale
