Trends in exploring wheat and barley germplasm for novel disease resistance traits

The emergence of highly virulent fungal pathogens presents a major challenge for the sustained production of wheat (Triticum aestivum) and barley (Hordeum vulgare) crops around the world. For instance, the Ug99 and TTTTF race of the stem rust pathogen Puccinia graminis f. sp. tritici identified in East Africa and Sicily, respectively, has become a serious threat to global food security due to its enhanced ability to infect modern wheat and barley cultivars (Mamo et al. 2015; Singh et al. 2015; Bhattacharya 2017). In addition, the emergence of blast disease in wheat has become a serious concern. New strains of blast recently detected in wheat fields of Brazil and Bangladesh have the potential to cause huge losses (Inoue et al. 2017). While the modern cultivars are highly prone to pathogen attack, historic germplasm collections of wheat and barley and their wild related species offer useful genetic variations that confer resistance against the prevailing fungal pathogen strains (Feuillet et al. 2008; Mamo et al. 2015; Das et al. 2016; Huang et al. 2016; Mondal et al. 2016). Such germplasm collections can be grouped into three categories, known as the primary, secondary and tertiary gene pools. The primary gene pool consists of landraces and progenitor species that have a genome structure resembling the cultivated crop. Hence, chromosomal segments carrying useful genetic variations can be readily introgressed into the targeted crop through direct hybridisation. In wheat, this group consists of the two broadly cultivated species, the hexaploid T. aestivum (AABBDD) and tetraploid T. turgidum (AABB) and their related diploid progenitors such as the diploid A (T. monococcum, T. boeoticum and T. urartu) and D (Aegilops tauschii) genome species.