1 | INTRODUCTION
The Triticeae tribe (Poaceae), includes economically important annual crops (e.g., wheat, rye, and barley) and crucial perennial forage grasses (e.g., Roegneria , Agropyron , Leymus , andPseudoroegneria ) (Dewey, 1984). The genus Pseudoroegneriaconsists of six diploid (2n = 2x = 14, StSt) and nine autotetraploid species (2n = 4x = 28, StStStSt, Yen & Yang, 2011). The diploidPseudoroegneria species, with St genome, involved in more than 60% of perennial polyploid speciation of Triticeae as maternal donor (Yen & Yang, 2011; Chen et al., 2020). At present, the St-containing species were extensively utilized for wheat breeding material improvement such as wheat stripe rust resistance wheat-Thinopyrum elongatum (StStEEE) translocation line and Fusarium head blight (FHB) resistance wheat-Roegneria ciliaris (StY) disomic addition line (Kong et al., 2018; Yang et al., 2021). Besides, high grass production, excellent disease and saline-alkali resistance characters make the St-containing species to be the ideal germplasm for selecting forage varieties. Thus, the St genome not only account for extensively perennial speciation, but also prominent for forage and crop breeding.
More and more efforts involved the origin of the St genome and genetic relationships among Triticeae species. Molecular data estimated the genus Pseudoroegneria derived is more ancient than theTriticum /Aegilops group (8.0–8.3 Mya) (Zhang et al., 2022). Genomic relationships suggested that the St genome was closely related to the J (=E) genome of (Thinopyrum elongatum ), and diverged from H (Hordeum bogdanii ), D sub-genome, then followed by B and A sub-genome in common wheat (Liu & Wang, 1993; Liu et al., 2007). In recent years, Wang et al (2020) mapped 14 linkage groups (LGs) and identified seven homologous groups of the St genome, and revealed the genome shared-homology between the St and ABD, and H genome of 35% and 24%, respectively (Wang et al., 2020). Comparative cytogenetic karyotype result revealed highly conserved collinearity between St genome and common wheat genome, except a well-known 4A chromosome translocation (Wu et al., 2022). These studies provide partly St genome information, nevertheless, the whole genome homoeology and genetic relationship between the St genome and published Triticeae have not been studied.
Pseudoroegneria plants are predominately cool-season and drought tolerant grasses, which distributed in arid or semi-arid areas. ThePse. libanotica distributes in the lithoid slopes of Lebanon, Iraq, and the north of Iran (Yen & Yang, 2011). Molecular studies indicate that Pse. libanotica is placed at the base of the phylogenetic tree, implying probably a more ancient than species from Eastern Europe (Pse. strigosa ), East Asia (Pse. stipifolia ), and North America (Pse. spicata ) (Yan and Sun, 2011; Gamache et al., 2015). Morphologically, Pse. libanotica is covered by thicken cuticular wax on aerial part, which differs from other diploid Pseudoroegneria species. It is largely reported that the primary function of cuticular waxes is to preventing non-stomata water loss (Larson & Kiemnec, 2003; Fraser et al., 2009). Thus, characterizing the St genome of Pse. libanotica might reveal the regulation of cuticular wax biosynthesis in response to water deficit.
The regulation of cuticle deposition in response to drought stress was found in model species, crops and important economic species such asArabidopsis thaliana , rice, wheat (Triticum aestivum ), maize, soybean (Glycine max ), sesame (Sesamum indicum ), sorghum, oats (Avena sativa ), Medicago sativa , cotton (Gossypium hirsutum ), tree tobacco (Nicotiana glauca ), and pine (Pinus palustris ) (Li et al., 2019; Lewandowska et al., 2020; Lee & Suh, 2022). Multiple gene involved the cuticular wax biosynthesis have been investigated. The plastid Acetyl-CoA carboxylase are responsible for de novo fatty acid biosynthesis (up to C16:0, C16:1 and C18:1) (Roesler et al., 1997; Roudier et al., 2010). Further, the long-chain fatty acids (C16–C18) exported to the cytosol after hydrolysis of ACPs by acyl-ACP thioesterases (Bonaventure et al., 2003). Those 3-ketoacyl-CoA synthase (KCS) isoforms involved in fatty acid elongation (Joubès et al., 2008). Some cytochrome P450s have been reported to be involved in plant cuticle wax synthesis as well (Greer et al, 2007; Zhang et al., 2020). Eceriferum (CER) genes effect different steps of the wax biosynthesis pathway including primary alcohol forming, aldehydes, and alkanes synthesis (Bernard et al., 2012). Researchers investigated distinctive cuticular waxes among different plant species, different tissues and organs, even in different growth and developmental stages (Jetter et al., 2006). Novel genes involved in cuticular wax biosynthesis may be practically used as valuable genetic resource to improve crop drought tolerance in plant breeding (Xue et al., 2017). However, the drought induced wax biosynthesis have not been investigated in crucial wild germplasm in Triticeae.
Given the importance of the St genome in Triticeae and the excellent drought resistance traits of the Pseudoroegneria species, the diploid wild species Pse. libanotica with the more ancient St genome were performed whole-genome sequencing in this study. This study, we will assemble chromosome scale reference St genome by sequencing; identify the genetic relationship especially among Triticeae species using whole genome comparative analysis; elucidate mechanism of fatty acid biosynthesis in Pse. libanotica under drought stress. Those result will provide solid information for the evolution of Triticeae species, and facility better germplasm utilization.