Rice blast is one of the most widespread and devastating diseases of rice and causes yield losses between 10 and 30% of rice production worldwide in epidemic years (Skamnioti and Gurr 2009). Domestically, major blast epidemics occurred in China has damaged 5.333 million hectares of paddy rice areas, resulting in more than 1.5 billion kilograms of yield loss in the past decade (He et al. 2014). Development of resistant cultivars by using of resistance (R) genes in rice improvement breeding programms are the most economical and environmentally friendly method to combat the disease. However, many resistant cultivars carrying single dominant R genes is generally short-lived due to the dynamic changes in race (pathotype) and composition of the blast pathogen (Dean et al. 2005). Therefore, exploring rice resistance gene resources continuously, identification and utilization of broad-spectrum resistance genes against multiple isolates of M. oryzae have been considered to be one of the best options for crop protection and blast management.
To date, more than 100 blast resistance loci or genes in rice have been identified on all rice chromosomes except chromosome 3 (Ashkani et al. 2016; Sharma et al. 2012), of which 28 major R genes and 5 partial R genes have been cloned and functionally validated (Wu et al. 2021). Among the R genes identified and cloned, many broad-spectrum R genes have been documented and validated, viz., Pi1, Pi2, Pi5, Pi9, Pi33, Pi40, Piz, Piz-t, Pigm, and others (Jeung et al. 2007; Wu et al. 2007), and six of which (Pi2, Pigm, Pi40, Pi9, Piz and Piz-t) were different R gene alleles of the Piz locus located on the short arm near the centromere of rice chromosome 6 (Deng et al. 2006; Qu et al. 2006; Zhou et al. 2006). By BAC sequencing and gene knockout technology, Pi9 was the first cloned gene of the Piz locus, and the lines carrying Pi9 were highly resistant to 43 strains collected from 13 different countries (Qu et al. 2006). Using the same cloning strategy, Zhou et al. (2006) identified Pi2 from the gene cluster composed of 9 tandemly arranged NBS-LRR gene members, and studies showed that that Pi2 was resistant to most of 455 isolates collected from different regions of Philippines and the 792 isolates from 13 major rice regions of China (Chen et al. 1996). Due to the high homology in sequence and structure between Piz-t and Pi2, Piz-t was directly cloned using the in silico cloning method, and comparison revealed that only 8 amino-acid differences within three LRR domains between the Piz-t and Pi2 gene (Qu et al. 2006; Zhou et al. 2006). Especially, the broad-spectrum resistance gene Pigm identified from landrace Gumei 4 (GM4), has been shown to be completely resistant to 50 isolates originating from diverse Chinese and worldwide collections, and was used as an excellent resistance resource in blast resistance breeding for more than 30 years in different varieties cultivated on large surfaces (Deng et al. 2017; Cesari and Kroj. 2017). Deng et al. (2017) revealed that epigenetic regulation of Pi-gmR and Pi-gmS balance the blast resistance and yield in rice, in which Pi-gmR confer broad-spectrum resistance of GM4, and Pi-gmS increase rice production to counteract the yield lost caused by Pi-gmR.
The broad-spectrum resistance of different multiple alleles of Piz locus and the diversity of their sequence and resistance spectrum suggesting that they have great application potential. However, significant differences exist in resistance performance and resistance spectrum of these R genes under different genetic background (Wu et al. 2015), indicated that resistance performance of broad-spectrum resistance genes may require other regulatory factors (Zhou et al. 2019). OsRac1, a small GTPase, associates with and is activated by Pit at the plasmalemma. Once activated, OsRac1 induces ROS production and HR, which contribute to Pit-mediated blast resistance (Kawano et al. 2010). In contrast to OsRac1, the transcription factor OsWRKY45 directly interacts with the CC domain of NLR protein Pb1 at the nucleus to induce quantitative blast resistance (Inoue et al. 2013). Similarly, the homeodomain-containing protein OsBIHD1 physically interacts with Pik-H4 by its CC domain, and is required for Pik-H4-mediated resistance through ethylene-brassinosteroid pathway (Liu et al. 2017). Furthermore, Zhai et al. (2019) discover an RRM class of transcription factor PIBP1s, which directly interacts with the CC domain of PigmR, which could also activate the expression of defense genes OsWAK14 and OsPAL1 directly.
Among various disease symptoms caused by M. oryzae, seedling blast and panicle blast are the most common, but panicle blast directly causes yield loss due to infect the top internodes or panicle of rice and result in barren panicles, chalky kernels, and sterile grain (Titone et al. 2015; Wu et al. 2017). However, the time-consuming and cumbersome nature of inoculating rice panicles with M. oryzae has limited the focus of most studies to seedling blast resistance. Presently, only a few of R genes and QTLs (qPbm11, Pb-bd1, Pi-jnw1, Pb1, Pi64 and Pi68) were confirmed with resistance to panicle blast (Hayashi et al. 2010; Ishihara et al. 2014; Ma et al. 2015; Fang et al. 2019; Wang et al. 2016; Devi et al. 2020). However, the resistance to seedling and panicle blast is often inconsistent, and many varieties with high resistance to leaf blast at the seedling stage show susceptibility to panicle blast at the heading stage (Xiao et al. 2020). Multi-omics analysis also showed that distinct defense-related gene expression is induced by seedling blast and panicle blast, indicated that the genetic mechanisms of seedling blast and panicle blast resistance might differ and are independently controlled by different R genes (Liu et al. 2016; Yan et al. 2020). However, the current research on the molecular mechanism of rice blast resistance is all related to seedling blast (Li et al. 2017). Therefore, it is of great theoretical and practical value to identify the panicle blast resistance genes and analyze the molecular mechanism of panicle blast resistance regulation.
In our previous research, a set of NILs with six resistance alleles of the Piz locus (Piz-t, Pi2, Pigm, Pi40, Pi9 and Piz) were constructed with Yangdao 6 and 07GY31 as the recurrent parent, respectively. We also confirmed that Pigm had important application potential in breeding practice for conferring broad-spectrum resistance to seedling blast and panicle blast in Xian and Geng genetic background (Wu et al. 2016; 2017). However, in the process of introducing Pigm into C134S, an elite photoperiod and thermo-sensitive male sterile (P/TGMS) line widely used in two-line hybrid rice, two advanced backcross inbred sister lines (MSJ13 and MSJ18, BC2F7) were obtained, and resistance identification showed significant differences in the panicle blast resistance between the sister lines, thus we conclude that some of genetic factors might be involved in the panicle blast resistance difference between the sister lines. In this study, QTL analysis were conducted with F2 population deriving from a across between MSJ13 and MSJ18 using Genotyping by Sequencing (GBS) method, and a major QTL qPBR10-1 on chromosome 10 was specifically identified. Additionally, qPBR10-1 was verified among the BC1F2 and BC1F3 population and fine mapped within a 60.6-kb region between ID338 and K1401 markers. and putative candidate genes predicted underlying mapped QTLs that may be involved in genetic regulation of panicle blast resistance traits in Pigm-containing line.