Natural variation of trichome density on leaf in wild wheat Aegilops tauschii Coss.
Haruhiko Morihiro and Shigeo Takumi
Laboratory of Plant Genetics, Graduate School of Agricultural Science, Kobe University, Nada-ku, Kobe 657-8501, Japan
Corresponding author: Shigeo Takumi
E-mail: takumi@kobe-u.ac.jp
Naturally occurring genetic variation is one of the most important basic resources for plant biology (Koornneef et al. 2004). Aegilops tauschii Coss. (syn. Ae. squarrossa L.), a diploid self-pollinating goatgrass, is the D genome donor of common wheat (Kihara 1944, McFadden and Sears 1944). Ae. tauschii is widely distributed in Eurasia, spreading from northern Syria and Turkey to western China, and shows abundant genetic variation (Dunikov and Goncharov 1993; Dvorak et al. 1998; Matsuoka et al. 2007, 2008, 2009; Mizuno et al. 2010). The species’ primary region of origin is considered to be the Transcaucasus where diploid Aegilops species including Ae. tauschii radiated 2.5-4.5 million years ago (Lubbers et al. 1991; Van Slageren 1994; Dvorak et al. 1998; Huang et al. 2002). Therefore, it was assumed that the eastern accessions had dispersed from the western habitats (Matsuoka et al. 2008; Mizuno et al. 2010).
Plant trichomes are highly specialized epidermal cells implicated in several important functions, including transpiration control, freezing tolerance, and protection against insects, disease, and UV light (Johnson 1975). A lot of Arabidopsis mutants for trichome development on leaves have been isolated and analyzed (Hülskamp et al. 1994).
Here, we reported natural variation of the trichome density on leaves in Ae. tauschii. Trichomes developed on the leaf-blade surface and at the leaf marginal region (Fig. 1). The trichome densities on leaves were varied in the 31 Ae. tauschii accessions (Table 1). For example, short trichomes developed on the adaxial side of leaf blade in most accessions of Ae. tauschii, whereas three accessions such as KU-2074 and KU-2091 showed long trichomes on both adaxial and abaxial sides. Fourteen of the 31 accessions showed trichome-less or -poor phenotype on the leaf sheath (Fig. 2), and these accessions distributed mainly in Afghanistan and Pakistan (Table 1). The trichome-poor accessions were also found in Iran. Trichome-rich phenotype on the leaf sheath was observed mainly in Transcaucasus and northern Iran accessions including 4 accessions of subspecies strangulata. Iran accessions varied in the trichome density on leaf sheath. Therefore, longitudinal cline was found for natural variation of the trichome density on leaves. The trichome-poor accessions spread mainly in eastern habitats such as Afghanistan and Pakistan, implying that the trichome-poor phenotype contributed to eastward dispersal and adaptation to these habitats in Ae. tauschii.
In addition, a novel mutant for trichome development was identified from the natural population of Ae. tauschii. Out of 205 Ae. tauschii accessions (Matsuoka et al. 2009; Takumi et al. 2009), KU-2001 was isolated as a trichome-less mutant. KU-2001, originally collected from Pakistan, showed trichome-less phenotype around the ligule/auricle regions (Fig. 3). In wild type, auricles provided a lot of long trichomes. KU-2001 showed the trichome-poor phenotype on leaf blade and sheath (Fig. 2). KU-2001 is a useful genetic resource to study relationship between trichome, and abiotic and abiotic stress tolerance in wheat. This mutant accession can be supplied from Plant Germ-Plasm Institute, Graduate School of Agriculture, Kyoto University, Japan through the NBRP (National BioResource Project) KOMUGI website (http://shigen.lab.nig.ac.jp/wheat/komugi).
The Ae. tauschii germplasm in the eastern habitats was phenotypically distinct from that in the western habitats (Matsuoka et al. 2008, 2009; Takumi et al. 2009). Together with these previous studies, identification of the spontaneously occurred mutant in this study suggests that the Ae. tauschii populations in the eastern habitats provides unique genetic and morphological variations, and that qualitative mutant varieties might be additionally able to be identified in the eastern habitats such as Pakistan and Afghanistan.
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