21. Inheritance of bacterial blight resistance in two accessions of wild rice, Oryza nivara

R. KAUR¹, N. GREWAL¹, A. DAS, Y. VIKAL, J. SINGH, T. S. BHARAJ, J. S. SIDHU and K. SINGH²

Dept. Plant Breeding, Genetics and Biotechnology, Punjab Agricultural University, Ludhiana, 141 004 India

1) Contributed equally for studying inheritance

2)Corresponding author (Email: kuldeep35@yahoo.com)

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv oryzae (Xoo) is one of the most economically serious diseases of lowland irrigated rice and can cause yield losses up to 50% (Gnanamanickam et al. 1999). It is the major disease in Punjab state of India and resistance breeding is the most economical and environmentally safe approach for achieving yield stability (Khush 1993). Extensive cultivation of rice in Punjab began only in 1970s but variability in the pathogen seems very high (Singh et al. in preparation). Currently, more than 20 genes (Xa1 to Xa27) conferring host resistance against various strains of Xoo have been identified (Lin et al. 1996, Zhang et al. 1998, Khush and Angeles 1999, Chen et al. 2002, Yang et al. 2003, Gu et al. 2003) and the majority of these are from cultivated indica rice with Xa4 being most widely used gene in the Indian sub-continent (Vikal et al. 2004). Two genes Xa21 and Xa23 have been identified from two related �eA�f genome wild species, O. longistaminata and O. rufipogon, respectively (Khush et al. 1990, Zhang et al. 1998), while Xa27 is from a distantly related species, O. minuta (Gu et al. 2004). None of the known Xa genes (Xa1 to Xa21) so far tested is effective individually against all the pathotypes of Xoo prevalent in Punjab (our unpublished results). The narrow genetic base of cultivated rice will cause vulnerability to BB because of an increased frequency of newly evolved pathotypes of greater virulence. Pyramiding of known R genes or scouting of new genes with a wider resistance spectrum, are the alternatives for breeding varieties with durable resistance.

O. nivara (Sharma et Shastry), the progenitor of O. sativa subspecies indica (Yamanaka et al. 2003), is widely distributed in South and South East Asia (Vaughan and Morishima 2003). So far no BB resistance gene has been designated from this species. We evaluated 125 accessions of O. nivara against six Xoo pathotypes prevalent in Punjab state of India over a period of three years (Vikal et al. in preparation) and a few accessions showed resistance to all the pathotypes indicating that they may carry some novel gene for broad resistance or several genes that together confer resistance to all six pathotypes. Two accessions, IRGC 81825 (obtained from IRRI, Philippines) and CR 100428 (obtained from CRRI, Cuttak), both originating from India, were resistant to all the six pathotypes and were used for studying the inheritance of BB resistance and its transfer to cultivated rice O. sativa cv PR114. The F₁ of the cross PR114/ O. nivara acc. 81825 showed a resistant reaction whereas F₁ of the cross PR114/ O. nivara acc. 100428 showed a susceptible reaction indicating that the resistance in acc. 81825 is dominant and that of acc. 100428 is recessive. Here we report the inheritance of resistance in these two accessions using F₂ populations.

O. sativa cv PR114 and O. nivara accessions IRGC 81825 and CR 100428 were used for studying inheritance of BB resistance in O. nivara. PR114 is resistant to five pathotypes but susceptible to one pathotype PbXO7 (Singh et al. in preparation) and was used as female parent. The F₂ populations (>900 plants in each cross) were planted in the field with row-to-row and plant-to-plant spacing of 30 x 20 cm. The crop was raised following standard cultural practices. Both the parents and the F₁s were planted along with the F₂ populations.

Pathotype PbXO7 is newly evolved and is the most virulent among six pathotypes. This pathotype was used for inoculating the parents and the F₂ populations. The culture of PbXO7 was revived from stock cultures on PSP medium (200g potato, 10g sucrose, 5g peptone and 20g agar in one litre) for 48 hours at 28^oC. The inoculum was prepared by suspending the bacteria in distilled water and adjusting the optical density to 1.0 (590nm) to give a concentration of approximately 1x10⁹ cells/ml. The plants were inoculated by clipping tips of 5-10 fully opened leaves at the maximum tillering stage (approximately 45 days after transplanting) as per Kauffman et al. (1973). Disease reaction (lesion length in cm) was scored 14 days after inoculation. Plants having lesion lengths of more than 5cm were considered as susceptible and the ones with lesion lengths less than 5cm were considered as resistant. Chi-square (c²) test was used for testing goodness of fit for number of gene(s) segregating in the population.

The pathotype PbXO7 shows virulent reactions on Xa1, Xa2, Xa3, Xa4, xa5, Xa7, xa8, Xa10, Xa11, xa13 and Xa14, whereas Xa21, shows moderate resistance to this pathotype. The two accessions showed resistance against all the six pathotypes for three consecutive years (Fig. 1). The F₁s of both the interspecific crosses showed normal pollen fertility and seed set, hence any distortion due to sterility was not expected in these crosses. This was confirmed by testing the population with a set of microsatellite markers (data not shown). Reaction of F₁s obtained from crosses PR114/O. nivara acc. 81825 and PR114/O. nivara acc. 100428 showed resistant and susceptible reactions, respectively, against pathotype PbXO7, suggesting that resistance in acc. 81825 was dominant and that of acc. 100428 was recessive. Segregation patterns of the F₂ populations are presented in Table 1. The F₂ population of the cross PR114/O. nivara acc. 81825 segregated in 3 resistant: 1 susceptible (��² = 0.052), thereby confirming that BB resistance in the accession 81825 is governed by a single dominant gene. Similarly, the F₂ population of the cross PR114/O. nivara acc. 100428 segregated in 1 resistant: 3 susceptible (��² = 0.005), thereby confirming that BB resistance in the accession 100428 is governed by single recessive gene. Both genes are expected to be novel because of their effectiveness against all the six pathotypes of Xoo prevalent in Punjab. We are now mapping these genes using SSR markers and also transferring these genes into cultivated rice background.

Table 1. Disease reaction of parents, F₁s and F₂ populations of crosses PR114/O. nivara acc. 81825 and PR114/O. nivara acc. 100428 against Xoo pathotype PbXO7

Fig.  1. BB reaction of O. nivara acc. IR81825 to five pathotypes of Xoo (leaves 1-5) and PR114 to pathotype PbXo7 (leaf 6).

Acknowledgements

We gratefully acknowledge IRRI, Philippines and CRRI, Cuttack for supplying the seeds of O. nivara. Financial support for this project by DBT, Govt. of India vide grant No. BT/AB/03/FG-2/2003 is gratefully acknowledged.

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