The Bread wheat (Triticum aestivum L.) constitutes one of the main cereal crops and provides approximately 20% of the global food calorie requirements (Appels et al. 2018). However, like most other crops, wheat is threatened by some pathogenic diseases (Zhang et al. 2022). In particularly, leaf rust or brown rust (Puccinia triticina) is the most widely distributed in wheat pathogen that can cause up to 40% production loss in the susceptible cultivars (Knott 1989; Kolmer 1996; Hou 2023). Several outbreaks of leaf rust disease, with huge yield and production losses in wheat, have been recorded from the past. For example, wheat leaf rust occurrences of 1969 and mid-to-late 1970s across the Southwest and some parts of the Yangtze River Basin in China caused severe wheat yield losses (Dong et al. 2001). Moreover, leaf rust disease outbreak caused significant wheat yield losses across the Sichuan, Gansu, Henan and Anhui regions of China in 2012 (Zhou et al. 2013). Therefore, breeding and deployment of the cultivars resistant to leaf rust has been advanced as the most effective, economical and climate-smart option to sustainably manage (prevent and control) the disease in wheat (Vanzetti et al. 2011; Rosewarne et al. 2012; Dinh et al. 2020). In pursuit of this approach and owing to proliferation of novel virulent leaf rust pathotypes, it is important to discover resistance genes to leaf rust in wheat and its homologous species.
Leaf rust resistance can be broadly categorized into two, i.e., race-specific and race non-specific resistance (Johnson 1988). On one hand, race-specific resistance (or all-stage resistance; ASR) is often controlled by a single gene or a few major genes that evoke the hypersensitive response (HR) to virulent pathotypes (McIntosh et al. 1975). The ASR is overpowered when an existing or new virulent pathotype proliferates in the pathogen population. On the other hand, race non-specific resistance (or adult plant resistance, APR) is often conferred by numerous minor effect genes and offers partial resistance against a broad range of pathotypes. Consequently, APR, or slow rusting resistance, is considered more robust. Thus, in recent years, crop breeders have given much focus to APR or slow rusting resistance.
APR genes confer disease resistance at adult plant stage. Besides, APR is quantitatively inherited and often derived from the additive effects of numerous minor genes (Singh et al. 2005). Characteristically, wheat varieties with slow rusting resistance often exhibit susceptibility responses at earlier (seedling) crop growth but minimal disease severities at later (adult) stage. Additionally, they display extended latent periods, smaller pustule size, low infection frequencies and reduced spore reproduction (Caldwell 1968; Skowrońska et al. 2020). Comparably, APR is often more robust than ASR, and thus, often given much attention by crop breeders.
Up to date, over 200 leaf rust resistance quantitative trait loci (QTLs) have been reported in wheat, of which 80 of these have been permanently catalogued (Pinto et al. 2018; McIntosh et al. 2021). Most of these QTLs harbor qualitative resistance genes that induce hypersensitive response types. About 16 of these leaf rust resistance-related genes have been reported as APR genes, including Lr12, Lr13, Lr22a, Lr22b, Lr34, Lr35, Lr37, Lr46, Lr48, Lr49, Lr67, Lr68, Lr74, Lr75, Lr77, and Lr78 (Herrera-Foessel et al. 2011; Herrera-Foessel et al. 2012; Kolmer et al. 2018; Kolmer et al. 2019; Liu et al. 2021; McIntosh et al. 1966). Meanwhile, despite minor gene resistance being considerably more robust than major gene resistance, minor gene resistance can also be overpowered by slow evolution or rapid proliferations in the pathogen population (McDonald et al. 2002). Plausibly, therefore, it is crucial to identify novel APR genes in wheat cultivars to facilitate breeding and sustainable management of wheat rusts in China.
Currently, molecular markers are routinely employed to map and clone disease resistance genes in wheat. Molecular markers are anchored on cloned gene sequences and are accurate. Among different molecular markers, simple sequence repeat (SSR) marker is the most commonly used because of its utility related to co-dominance, greater levels of polymorphism and chromosome specificity, as well as high repeatability, accuracy and flexibility for manipulation (Röder et al. 1998; Somers et al. 2004). However, the accuracy of associated or linkage markers in predicting the presence of a target gene relies on the closeness of the linkage. Nonetheless, molecular markers circumvent some of the challenges linked to conventional gene postulation, including prediction of individual genes from among multiple interacting genes (Vanzetti et al. 2011).
The major aim of the present study was to map leaf rust APR-related QTLs in a Zhoumai 22/Chinese Spring wheat population comprising 215 F2:3 lines, so as to provide additional valuable molecular markers for leaf rust resistance breeding in wheat. Notably, in our previous research, Zhoumai 22 exhibited resistance to most of the P. triticina pathotypes used in the seedling tests. Thus, we hope that the leaf rust APR-related QTLs, their flanking markers and associated APR genes identified in this study could be useful genomic resources for marker-assisted selection (MAS) and molecular breeding of leaf rust resistant wheat cultivars.