1.5 Major Resistance Gene Exploitation and their Effectiveness

Resistance genes to Pst can be broadly categorized into two main classes, namely, major and minor resistance genes. The major gene-based resistance can be called all-stage resistance. Minor resistance genes also called partial, non-race specific, slow-rusting, durable or adult plant resistances (Rosewarne et al., 2013).

Till now, 80 Yr genes (denoted from Yr1 to Yr80) have been formally designated and approximately 100 Yr genes temporarily designated (McIntosh et al., 2017; Chen and Kang, 2017). Among these, Yr11, Yr12, Yr13, Yr14, Yr16, Yr18, Yr29, Yr30, Yr34, Yr46, Yr48, Yr49, Yr58, Yr60, Yr68, Yr71, and Yr75 confer APR, and Yr36, Yr39, Yr52, Yr59, Yr62 confer higher-temperature adult-plant (HTAP) and Yr54, Yr77, Yr78, Yr79, Yr80 are quantitative trait loci (QTL) conferring resistance (Han et al., 2018). The others confer all-stage resistance.

Both gene postulation and molecular markers are used to characterize Yr gene in China. Currently, some molecular markers for important genes such as Yr1, Yr2, Yr5, Yr9, Yr10, Yr15, Yr17, Yr18, Yr24, and Yr26 have been developed and used to test these Yr genes (Table 1.1). The others genes were postulated by a series of Pst strains and known Yr gene cultivars.

Table 1.1 Yr genes linked markers, their primer sequences and references

Table 1.1 Yr genes linked markers, their primer sequences and references (continuous)-1

Table 1.1 Yr genes linked markers, their primer sequences and references (continuous)-2

Table 1.2 Resistance genes identified in various wheat varieties/lines using molecular markers and/or gene postulation techniques

Research indicated that just fewer genes such as Yr1, Yr2, Yr3, Yr9, have been widely used in Chinese wheat varieties. In recent years, cultivars carrying Yr10, Yr26 have been released more than before. By now these genes have lost their resistance already. The other genes have only a low frequency in Chinese commercial wheat cultivars. Table 1.2 described the probable Yrgenes frequency in Chinese commercial wheat cultivars, wheat landraces and breeding lines. Virulence monitoring and field trap nursery showed that only Yr5, Yr15 and Yr18 expressed good resistance in China at present. Only a small number of Chinese commercial cultivars carry these genes, and needs to be incorporated into the future Chinese lines.

Yr18 (Lr34/Yr18/Sr57/Pm38/Ltn1) is a very important slow-rusting resistance gene which conferred pleiotropic APR, to yellow rust, leaf rust, stem rust, powdery mildew, and conferring the phenotypic marker of leaf tip necrosis (Rosewarne et al., 2013), and has been widely used in many countries (Ali et al., 2017, 2018). Molecular characterization of 231 Chinese wheat commercial cultivars and 422 landraces indicated that only 6.1% commercial cultivars and 85.1% landraces carried Yr18. The Southwest Winter Wheat Region has a higher frequency than that of the whole China (Yang et al., 2008). Thus, developing slow-rusting cultivars from landraces will be a promising aspect in future.