New Delhi, India
Indian Agricultural Research Institute (IARI)
Mandya, Karnataka, India
Univ. Agric. Sci. Regional Research Station
Udaipur, Rajasthan, India
Rajasthan College of Agriculture
Towards molecular marker mapping of genes conferring resistance to sorghum downy mildew (Peronosclerospora sorghi) in maize --Nair, SK, Setty, TA, Rathore, RS, Kumar, R, Singh, NN, Prasanna, BM One of the major factors limiting the productivity of maize in the tropical Asian region is the increased incidence of insect pests and diseases. Among the various maize diseases in south and southeast Asia, downy mildews (DM) are considered to be major diseases. Sorghum downy mildew (SDM) in maize, caused by Peronosclerospora sorghi, is one of the most important among the DM diseases prevalent in India. Another severe form of DM disease found in the state of Rajasthan in India is Rajasthan downy mildew (RDM), caused by Peronosclerospora heteropogoni, which does not infect sorghum, but is capable of infecting maize and a wild grass, Heteropogon contortus. There is no published information so far about resistance/susceptibility of Indian maize inbreds in the public domain to these two important DM diseases. A research program was initiated under the Asian Maize Biotechnology Network (AMBIONET) with the following objectives: (i) to analyze the responses of all the important maize inbred lines released under the All India Coordinated Maize Improvement Project (AICMIP) to SDM and RDM infection; (ii) to study the genetic basis of resistance to SDM and RDM diseases; and (iii) to map the genes conferring resistance to sorghum downy mildew in maize using microsatellite or Simple Sequence Repeat (SSR) markers, and carry out marker-assisted selection for SDM resistance. The present investigation is a component in this effort.

Responses of Indian maize inbred lines to SDM and RDM infection: A set of 47 Indian maize inbred lines were evaluated in a Randomized Block Design with two replications under artificial infection at the two DM 'hot spots' in India, Mandya in Karnataka for sorghum downy mildew and Udaipur in Rajasthan for Rajasthan downy mildew, during the monsoon (Kharif) seasons of 1999 and 2000. The 47 Indian maize genotypes included 37 elite inbreds and 10 inbred lines developed at the Agricultural Research Station (of the University of Agricultural Sciences, Bangalore), Nagenahalli, Karnataka, designated as 'NAI' lines. The NAI lines were derived from the DM resistant maize germplasm obtained from Thailand and the Philippines in the 1970s. Disease scoring was carried out on the basis of percent DM incidence in each genotype: 0 to 10% - Resistant; >10 to 30% - Moderately resistant; >30 to 50% - Moderately susceptible; and >50% - Susceptible. The following are the salient findings from the experiments carried out in Mandya and Udaipur during 1999 and 2000:

® Only one inbred line, NAI116, showed a strong resistance response to both sorghum downy mildew and Rajasthan downy mildew among the Indian maize lines. Almost all the elite Indian maize inbreds, including those used in hybrid seed production in the public sector, are highly vulnerable to SDM infection.

® Genotypes found to be resistant to RDM infection at Udaipur showed a differential response to SDM infection at Mandya. Therefore, there appear to be distinct differences in host-pathogen interaction at Mandya and Udaipur; the DM pathogen at Udaipur probably has lower virulence in comparison with the one in Mandya. This conclusion is also reinforced by the observation that all the inbreds found to be highly susceptible to SDM at Mandya showed varying responses to RDM infection at Udaipur.

This study emphasizes the urgent need to transfer DM resistance to the elite maize lines which are parents of important single cross, double cross and three way cross hybrids in India. A systematic analysis to understand the nature of inheritance of resistance to the disease is highly important for this effort. As a step towards this direction, various crosses were carried out in 6 x 6 diallel and 9 x 5 line x tester mating designs involving resistant, moderately resistant, moderately susceptible, and susceptible inbred lines identified through the present study. These cross combinations were evaluated in randomized complete block design during Monsoon season of 2000 under artificial inoculation at both Mandya and Udaipur. Statistical analysis of the results obtained from both locations is currently in progress and is expected to provide better understanding of the modes of inheritance of resistance to SDM and RDM infection in India.

Analysis of SSR polymorphism among SDM resistant and susceptible lines: A set of seven elite, SDM susceptible inbred lines (CM115, CM116, CM117, CM119, CM123, CM133 and CM139) along with the resistant line, NAI116, were screened using 108 SSR markers at the AMBIONET-India Lab at the Maize Genetics Unit, Indian Agricultural Research Institute, New Delhi. Super Fine Resolution agarose gel electrophoresis was used to detect SSR polymorphism, following the protocol suggested by Senior and Heun (1998). Two genotypes, CM139 and CM117 showed higher polymorphism with NAI116 than other inbred lines. Since CM117 revealed a high frequency of 'double bands', CM139, an elite inbred line which is also being used in the Indian hybrid maize programme as the female parent of a popular single-cross hybrid 'Parkash', has been selected as one of the parents for generating the mapping population. A total of 44 polymorphic SSR markers have been identified so far between NAI116 and CM139 (Table 1). Fifty-two SSR markers, found to be monomorphic during this study shall be reconfirmed using PAGE + silver staining protocol standardized under the AMBIONET programme. The team is presently analyzing an additional set of 150 SSR markers for detection of additional polymorphic SSR loci. A backcross mapping population has been generated using CM139 as the female parent and NAI116 as male parent. Genotyping of the mapping population using polymorphic SSR markers and phenotyping under SDM artificial infection at Mandya (during monsoon season, 2001) shall be carried out for molecular marker mapping and consequently, marker-assisted selection for SDM resistance in maize.

Table 1. Preliminary information on SSR loci found to be polymorphic between SDM resistant (NAI116) and susceptible (CM139) Indian maize inbreds.
Chromosome Polymorphic SSR loci
1 phi097, bnlg615, bnlg400, bnlg504, umc1331
2 dupssr21, magE.05, bnlg371, bnlg198
3 phi099, 1452, phi073, phi053, bnlg420, bnlg1182
4 bnlg1729, phi079, bnlg490, dupssr34
5 phi113, bnlg105, bnlg653, bnlg278
6 phi078, phi102, phi070
7 phi057, bnlg339, bnlg572, bnlg469, phi116
8 phi115, phi014, bnlg1065, bnlg240, bnlg1056, phi015
9 bnlg1272, phi033, phi022, bnlg127, bnlg619
10 phi050, bnlg210


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