Introduction
Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a highly damaging disease of rice worldwide. Depending upon the crop stage, degree of susceptibility and environmental conditions, it can lead to yield losses up to 50% (Noh et al., Reference Noh, Lee, Park, Shim, Choi, Kang and Kim2007). During epiphytotic years, the disease appeared at Kresek stage, in some regions of Punjab state of India and yield losses as high as 70% were reported (Raina et al., Reference Raina, Sidhu and Saini1981). Deployment of host plant resistance is recommended as the most effective way of managing the yield losses due to this disease. So far, 39 BB resistance genes (designated in a series from Xa1 to Xa40) have been identified in rice (Brar and Khush, Reference Brar, Khush, Nanda and Sharma2003; Bhasin et al., Reference Bhasin, Bhatia, Raghuvanshi, Lore, Sahi, Kaur, Vikal and Singh2012; Khan et al., Reference Khan, Naeem and Iqbal2014; Dossa et al., Reference Dossa, Sparks, Vera Cruz and Oliva2015; Kim et al., Reference Kim, Suh, Qin, Noh, Reinke and Jena2015; Zhang et al., Reference Zhang, Zhuo, Zhang, Huang, Wang, Xu, Vera Cruz, Li and Zhou2015). However, the resistance have been broken down due to continuously emerging races/pathotypes of Xanthomonas. More than 30 races of the bacterium have been reported worldwide (Noda et al., Reference Noda, Li, Li, Ochiai, Ise and Kaku2001), indicating their highly variable nature. In Punjab State of India alone, there are ten pathotypes and none of the designated BB resistance gene is effective against all the virulence (Lore et al., Reference Lore, Vikal, Hunjan, Goel, Bharaj and Raina2011; Lore et al., Reference Lore, Jyoti and Mangat2013). This emphasizes the need for making constant efforts towards identification of novel sources of BB resistance gene(s). Here, we report screening of 1176 accessions of wild species of rice against two of the most recently evolved Xoo pahotypes viz. PbXo-8 and PbXo-10.
Experimental
The experimental material consisted of 1176 accessions, belonging to AA, CC, EE, FF, BBCC and CCDD genomes from 16 wild species (Table 1) and the African cultivated species O. glaberrima of rice. The seeds were sown in seed beds and 1-month-old seedlings were transplanted in the field with row-to-row distance of 70 cm and plant-to-plant distance of 45 cm. The crop was raised following standard agronomic practices.
Disease reaction of PbXo-8 and PbXo-10 on wild species germplasm of rice
R, resistance; MR, moderate resistance; S, susceptible.
All accessions were inoculated at maximum tillering stage (45-55d) after transplantation by a clip inoculation technique (Kauffman et al., Reference Kauffman, Reddy, Hsieh and Merca1973) with each of the two recently evolved Xoo pathotypes viz. PbXo-8 and PbXo-10 (Lore et al., Reference Lore, Jyoti and Mangat2013). In each accession, 5–7 leaves were clip inoculated on each of the two plants. The PbXo-8 is virulent on Xa1, Xa3, xa8, Xa10, Xa11, xa13, Xa14 and Xa21, whereas pathotype PbXo-10 is virulent on Xa1, Xa3, Xa4, xa5, Xa7, xa8, Xa10, Xa11, Xa38, but shows moderate reaction to Xa21. The disease reaction of near isogenic lines (NILs) of BB resistance genes to the Xoo pathotypes PbXo-8 and PbXo-10 is given in online Supplementary Table S1. The disease reaction was scored on the basis of lesion length (cm) on inoculated leaves for each of the pathotype after 14–16 d of inoculation (Cottyn and Mew, Reference Cottyn, Mew and Goodman2004). The plants with disease score 1–3 (lesion length <5 cm) were scored as resistant, five (lesion length between 5 and 10 cm) as moderately resistant, seven (lesion length 10–15 cm) as moderately susceptible and nine (lesion length >15 cm) as susceptible.
Results
All the 1176 accessions were screened over two consecutive years, 2014 and 2015 against both the pathotypes. Average disease score against Xoo pathotype PbXo-8 and PbXo-10 is presented in Table 1. Details of the disease score of each of the 1176 accessions is presented in online Supplementary Table S2.
BB reaction of germplasm against pathotype PbXo-8
Out of the 1176 accessions screened only 49 accessions showed resistance against the pahtotype PbXo-8. None of the accessions of O. meridionalis, O. glumaepatula, O. officinalis, O. rhizomatis, O. eichingeri, O. brachyantha, O. punctata, O. minuta, O. alta, O. grandiglumis and O. latifolia were resistant against the pathotypes PbXo-8 pathotype (Table 1). Only eight accessions, each of O. nivara (out of 417 accessions studied) and O. longistaminata (out of 103) recorded a disease score of 1–3, indicating presence of resistance gene(s) in these. Out of eight resistant accessions of O. nivara, seven are of Indian origin (Table 2). More than 50% of the accessions of O. glaberrima showed complete to moderate resistance against pathotype PbXo-8. Three of the accessions from Liberia (IRGC102206, IRGC102512 and IRGC102520) and one from Mali (IRGC102445) had immune reaction over 2 years of screening.
Detailed description of the accessions resistant to Xoo pathotypes PbXo-8 and PbXo-10
a IRGC and CR denote accessions from International Rice Genetic Resources Centre, IRRI Philippines and Central Rice Research Institute (CRRI), Cuttack, India, respectively.
BB reaction of germplasm against pathotype PbXo-10
Among the A genome species, O. longistaminata had considerable number of accessions resistant to PbXo-10 followed by O. rufipogon and O. nivara (Table 2). Out of 103 accessions of O. longistaminata studied, 16 accessions had disease score between (1 and 3), whereas 51 accessions showed partial resistance. In O. rufipogon, out of 382 accessions screened, eight showed resistant reaction and 70 accessions showed partial resistance and 289 accessions were susceptible. None of the accessions of other AA genome species, O. glaberrima, O. barthii, O. meridionalis and O. glumaepatula had shown any resistance. Among other genomes, O. officinalis (CC) and O. punctata (BBCC) had two and one resistant accessions, respectively. Out of 17 species screened against PbXo-10, 12 species did not show any resistance to Xoo pathotype PbXo-10.
Discussion
Only two accessions of O. longistaminata (IRGC92624 and IRGC92644) had resistant reaction against both the pathotypes, whereas remaining wild species accessions had differential reaction. The newly evolved Xoo pathotype PbXo-10 was found to be more virulent than PbXo-8, as majority of wild species accessions did not have complete resistance. This indicates highly variable pathogenicity of Xoo and hence emphasizes the need for new BB resistance genes. The wild species germplasm of rice is known to harbour useful genes for BB resistance. These include Xa21 from O. longistaminata (AA) (Ronald et al., Reference Ronald, Albano, Tabien, Abenes, Wu, McCouch and Tanksley1992), Xa23 from O. rufipogon (AA) (Zhang et al., Reference Zhang, Lin, Zhao, Wang, Yang, Zhou, Li, Chen and Zhu1998), Xa27 from O. minuta (BBCC) (Gu et al., Reference Gu, Tian, Yang, Wu, Sreekala, Wang and Yin2004), Xa29 from O. officinalis (CC) (Tan et al., Reference Tan, Ren, Weng, Shi, Zhu and He2004), Xa32 from O. australiensis (EE) (Zheng et al., Reference Zheng, Wang, Yu, Liang and Zhao2009), Xa33 from O. nivara (Natarajkumar et al., Reference Natarajkumar, Sujatha, Laha, Srinivasa Rao, Mishra, Viraktamath, Hari, Reddy, Balachandran, Ram, Sheshu Madhav, Shobha Rani, Neeraja, Ashok Reddy, Shaik and Sundaram2012) and Xa38 from O. nivara (AA) (Cheema et al., Reference Cheema, Grewal, Vikal, Sharma, Lore, Das, Bhatia, Mahajan, Gupta, Bharaj and Singh2008), but only Xa21 is being exploited. In this study, we identified accessions from O. glaberrima, O. barthii, O. punctata, O. officinalis and O. australensis as source of resistance against newly evolved pathotpes. Presence of resistance in these wild species to the prevalent Xoo pathotypes (up to PbXo-7) was also reported by our group at Punjab Agricultural University (PAU) (Vikal et al., Reference Vikal, Das, Patra, Goel, Sidhu and Singh2007; Bhatia et al., Reference Bhatia, Lore, Vikal, Gupta and Singh2010). Only nine of the reported accessions of O. glaberrima had also shown resistance to PbXo-8, whereas all previously identified accessions were susceptible to the new variants of Xoo. The accessions with variable resistance reaction to different pathotypes could be utilized in combinations in resistance breeding programmes. Two of the BB resistance genes from O. glaberrima in the background of O. sativa cv. Pusa 44 (Bhasin et al., Reference Bhasin, Bhatia, Raghuvanshi, Lore, Sahi, Kaur, Vikal and Singh2012; Singh et al., Reference Singh, Kaur, Lore, Sahi, Bhatia, Mahajan, Gupta, Kumari, Singh, Komal, Mangat and Brar2013) and Xa38 gene from O. nivara in the background of O. sativa cv. PR114 are already available at PAU. The detailed study on inheritance and mapping of putative regions of BB resistance genes from O. rufipogon is under process. The identified resistant accessions were from India, Senegal, Guinea Cambodia, Thailand and many different countries and might have different genes for BB resistance and could be utilized singly or in combination for improving rice yield under stress.
Supplementary material
To view supplementary material for this article, please visit http://dx.doi.org/10.1017/S1479262116000149
Acknowledgements
The Authors are grateful to the Indian Council of Agricultural Research (ICAR), New Delhi for financial support under ICAR Niche Area of Excellence vide project number F. No. 10(9)2011-EPD. Authors also express their gratitude to Dean Post graduate studies of PAU, who is coordinating this project. We are also grateful to IRRI, Philippines and CRRI (Now NRRI), Cuttack India for sharing wild species germplasm.
