White-backed planthopper (WBPH) is one of the main pests of rice, which spreads all over the rice area in China and seriously threatens the stable production of rice (Shen et al. 2003). The lack of resistant varieties is one of the main reasons for the epidemic of WBPH. According to the Chinese National Rice Data Center, The database for Chinese rice varieties and their genealogy, among 828 rice varieties approved by the Ministry of Agriculture and Rural Affairs of China(MARA) in 2017–2019, only 2 varieties, one three-line hybrid rice Quanyouhuazhan and one two-line hybrid rice C Liangyou 810, were found with resistance to WBPH and their resistance performance just reached moderate resistance (MR) level (http://www.ricedata.cn/variety/). Therefore, screening the germplasm resources against WBPH for breeding utilization is the key solution to solve the severe problem of WBPH in some areas in China.
At present, the standard seedbox screening test (SSST) method at seedling stage is mainly used to screen rice germplasm resources for resistance to WBPH (Li and Qin 1994; Cheng 2007; Huang FK et al. 2012). However, the operation of SSST is limited by experimental conditions due to the great demand for plants and insects. In addition, it is not suitable to infect excessive pest in identification at the seedling stage. Thus, SSST has the defects of long identification period and low accuracy, and is not ideal for screening stable resistant sources. In order to screen the wild rice for WBPH resistance more efficiently and accurately, the SEM method was used. In this method, a stem represents a plant. The amount of biomass of a stem is much smaller compared with a whole plant, with reduced insect demand and shortened evaluation period. With the limited number of WBPH insects raised for identification, SEM method allows more screening experiments and repeated identification. Combined the verification at the adult stage, it is more efficient and precise in determining stable resistant resources. Therefore, SEM has a good application prospect in screening resistant germplasm resources, constructing mapping population of resistance genes and breeding resistant varieties.
Common wild rice genome contains rich insect-resistant resources, and belongs to the same AA genome as cultivated rice, which has rich genetic diversity and great breeding value (Li et al. 2006; Tang et al. 2008). Therefore, our research group has long been committed to exploring the rice planthopper resistant resources of common wild rice, including more than 30 resources with stable resistance to brown planthopper, and discovered 10 resistance genes, including bph19(t), bph20(t), bph21(t), bph22(t), bph23(t), Bph24(t), Bph27, Bph29, Bph35 and Bph36(Li et al. 2001;Li et al. 2006;Yang et al. 2012;Chen et al. 2009;Wang et al. 2015;Zhang et al. 2020; Li et al 2019).
In this study, 1080 accessions of Guangxi common wild rice from Fusui in the upper reaches of Xijiang river, Yongning in the middle reaches and Pingnan in the lower reaches of Xijiang river were screened for resistance to WBPH by SEM, and 49 resistant resources were identified, accounting for 4.54% of the total. The 49 resistant resources were repeatedly identified with SEM and verified at the adult stage, 15 resources with stable resistance were obtained as the resistance resources. These 15 resistance resources will enrich the genetic basis of resistance to WBPH, and can be used as candidate materials for exploring WBPH resistance genes and breeding WBPH resistant varieties.
According to the analysis of the geographic region and distribution of WBPH resistant accessions, Fusui county had the highest frequency of resistance resources (6.80%), followed by Pingnan county (4.19%), and Yongning district had the lowest resistance frequency (2.78%). The frequency analysis of resistant resources in each area showed that the frequency at Pingnan county was the highest (2.79%), while the frequency difference between Fusui county and Yongning district was very small, which was 1.01% and 1.07% respectively (Table 1). Fusui county with high amount and Pingnan county with high frequency of resistance resources to WBPH will be the key areas of common wild rice materials exploration.
The genetic diversity of wild rice has been widely studied by researchers all over the world. However, the genetic diversity of common wild rice in Guangxi has not been thoroughly elucidated. Xue(2016) studied the enter of genetic diversity and core collection of common wild rice, O, rufipogon Griff., in Guangxi, and found that the main sources of the core collection samples were regional populations from Yongning and Fusui, which were the most important populations of Guangxi common wild rice. But it is not clear how the genetic structure of accessions against WBPH distributed among these populations. In this study, we found that the genetic diversity of resistant accessions from different origins was in the order of Fusui county > Yongning district > Pingnan county. The geographical origin of the resistant resources mutual penetrated with each other in three regions. As shown in Fig. 6C, the entire population was divided into 2 potential genetic branches. The accessions from Pingnan county were all clustered in branch 1, and the accessions of Yongning and Fusui were distributed in both branches respectively. The reason for this phenomenon may be that Fusui and Yongning are located in the upper and middle reaches of Xijiang river Basin, which are adjacent to each other and have relatively higher similarity of diversity, while Pingnan is located in the lower reaches of Xijiang river Basin far away from the former two origins geographically, hence presented larger difference (Fig. 3).
These results suggested that Guangxi common wild rice was an ideal material for collecting germplasm resources with resistance to WBPH and provided a guide for the future exploration of WBPH resistant germplasm resources.