This document describes the biology of Triticum aestivum L.(bread wheat), with particular reference to the Australian environment, cultivation and use. Information included relates to the taxonomy and origins of cultivated T. aestivum, general descriptions of its morphology, reproductive biology, development, biochemistry, biotic and abiotic interactions. This document also addresses the potential for gene transfer to occur to closely related species. The purpose of this document is to provide baseline information about the parent organism in risk assessments of genetically modified T. aestivum that may be released into the Australian environment.
In Australia, the majority of wheat grown is T. aestivum and its cultivars. The other wheat species grown in Australia is Triticum turgidum subsp. durum (Desf.) Husn., also known as durum or pasta wheat.The terms ‘wheat’ and ‘bread wheat’ will be used as general terms to refer to T. aestivum throughout this document.
Bread wheat is an annual grass generally grown in Australia as a rotation crop. The varieties grown in Australia are spring wheat varieties although they are grown during the winter growing season and harvested in early summer. Bread wheat is the most widely grown food crop in the world and Australia is one of the four major exporters of wheat in the world.
Worldwide, two species of wheat are commonly grown. The first, T. aestivum, or bread wheat, includes the classes hard ‘red winter’, ‘hard red spring’, ‘soft red winter’, ‘hard white’ and ‘soft white’. The second, T. turgidum subsp. durum, includes the ‘durum’ and ‘red durum’ wheat classes (macaroni or pasta wheats). In Australia, production is limited to these two types. Bread wheat grown in Australia is exclusively white and does not have the red colour typical for most bread wheat grown in the northern hemisphere.
Section 1 Taxonomy
Triticum aestivum L. belongs to the family Poaceae (BEP clade), subfamily Pooideae and tribe Triticeae (Clayton et al. 2015). Synonyms include Triticum vulgare and there are also many synonyms for subspecies and cultivars (Clayton et al. 2015). All names of the members of Poaceae used in this document are currently valid according to The World Checklist of Selected Plant Families (Clayton et al. 2015).
Bread wheat is an allohexaploid (6x) that regularly forms 21 pairs of chromosomes (2n = 42) during meiosis. Chromosomes are organised in the A, B and D genomes (AABBDD). Each genome normally contains seven pairs of chromosomes (Hegde & Waines 2004) and the chromsomes belong to seven homoelogous groups of three (Sears 1954; Hegde & Waines 2004). Chromosomes may be numbered such that the chromosomes of the AB genome are I to XIV and those of the D genome are XV to XXI (Sears 1954), or as 1A, 1B, 1D to 7A, 7B, 7D (Hegde & Waines 2004). Each chromosome in hexaploid wheat has a homologue in each of the other two genomes, however pairing between homoeologous chromosomes of the A, B and D genomes is prevented by a gene, now designated Ph1 (Riley & Chapman 1958; Sears 1976) on chromosome 5B (Riley & Chapman 1958). This gene acts as a dominant gene suppressing pairing of homeologous chromosomes while allowing pairing between homologous chromosomes (from the same genome) (Hegde & Waines 2004). The Ph1 locus has been shown to prevent homoelogous pairing between wheat and several related genomes in hybrids (Riley et al. 1959; Jauhar & Chibbar 1999), but conversely, expression of Ph1 is suppressed in hybrids between bread wheat and some diploid Aegilops species, thus allowing homoeologous pairing of chromosomes (Hegde & Waines 2004). Practical consideration of the role of this gene is needed in both breeding of hybrids and in interpreting phylogenetic relationships (Riley & Chapman 1958; Riley et al. 1959).
This homology in hexaploid wheat and also in tetraploid wheat (AABB) allows a range of chromosomal abnormalities (aneuploidy) to survive, which cannot survive in diploid species such as barley (Hordeum vulgare L.). Sears (1954) described the effects of aneuploidy for each wheat chromosome, including the nullisomics1, monosomics2, telocentrics3 and isochromosomes4. Examination of wheat aneuploids has been important in furthering understanding of the evolution of the genome of modern cultivated wheat.
Currently it is thought that hexaploid wheat is the product of two hybridisation events. In the first, the A genome progenitor combined with the B genome progenitor to form a primitive tetraploid wheat (2n=28, AABB) (Feuillet et al. 2007). Analysis of chloroplast and mitochondrial genomes showed that this hybridisation occurred with the B genome - from the maternal parent (Tsunewaki 1988). The second event involved hybridisation between the tetraploid (AABB) form and the D genome progenitor to form the basic hexaploid configuration, AABBDD (Kimber & Sears 1987; Feuillet et al. 2007), again in the B genome cytoplasm.
While there is still some debate about the origin of the three genomes of T. aestivum (in particular the B genome), there is a degree of consensus for the A and D genomes.
Triticum uratu Thumanjan ex Gandilyan, has been suggested as the progenitor of the A genome in cultivated tetraploid (Feuillet et al. 2007) and hexaploid wheat (Kimber & Sears 1987; May & Appels 1987; Feuillet et al. 2007), as has T. monococcum (Kimber & Sears 1987), a cultivated diploid wheat (Feuillet et al. 2007). Early work (McFadden & Sears 1946a; McFadden & Sears 1946b) identified the D genome progenitor as Aegilops tauschii Coss. Schmal. (formerly Triticum tauschii Coss. or Aegilops triuncialis L.), which is also supported by later authors (Kimber & Sears 1987; Feuillet et al. 2007). A later review summarised much of the earlier work, concluding that T. aestivum originated from a cross between T. turgidum and Ae. tauschii (Matsuoka 2011).
The identity of the B genome donor remains unclear. It was originally proposed that the B genome donor was Aegilops speltoides Tausch (see Sarkar & Stebbins 1956). Feldman (1978) concluded that although Ae. longissima Schweinf. and Muschl in Muschler (as Triticum longissimum (Schweinf. & Muschl.) Bowden) was a candidate for the B genome progenitor, based on genetic compatibility, the lack of geographical contact between this species and tetraploid wheat or wild tetraploid wheat and suggested, as did (Feldman & Kislev 1977), that this was unlikely. The 1977 work suggested that Ae. searsii Feldman and Kislev, formerly believed to be a variant of Ae. longissimi, was the B genome progenitor, as did Nath et al. (1983) (as Triticum searsii). It has more recently been suggested that the original B genome donor of wheat no longer survives in the wild but was probably a member of the Sitopsis section of the Triticeae most closely related to Ae. speltoides (Feuillet et al. 2007). The processes of interspecific hybridisation and the ubiquitous nature of the B genome cytoplasm have been reviewed by Tsunewaki (1991). The origin and taxonomy of cultivated wheat have been reviewed by Feuillet et al. (2007).
The progenitors and selected wild species are listed in Table 1.
Table 1: Chromosome number and genome(s) of selected species of the tribe Triticeae.Chromosome and genome information from Dewey (1984) and Kimber and Sears (1987); taxonomic information from the Kew Royal Botanic Gardens World Checklist of Selected Plant Families (Clayton et al. 2015).