A gene conferring a hypersensitive reaction to the rice bacterial streak pathogen in maize
--Hulbert, SH; Drake, J; Leach, JE; Senior, L; Stuber, CW

Xanthomonas oryzae pathovars cause two serious diseases of rice. X. oryzae pv. oryzae causes the bacterial blight disease and X. o. pv. oryzicola causes bacterial streak. No single-gene resistances to bacterial streak have been identified in rice. The disease is a significant problem in China, and many of the elite hybrid rice cultivars are particularly susceptible (Xie et al., Chinese J. of Rice Science 4:127-132, 1990).

We tested a number of maize inbred lines to determine their reaction to X.o. pv. oryzicola (strain bls222). Bacteria (109 cfu/ml) suspended in sterile water were infiltrated into seedling leaves, which were then examined one to five days after inoculation. All the maize lines tested were scored as highly resistant to the rice pathogen because none of the lines had any water soaked lesions typical of susceptible rice. Two different reactions were observed. Most of the maize inbreds tested (A188, A619, B14, B73, CM37, K301, Mo20W, N6, Oh545, R168) exhibited a rapid hypersensitive reaction. The intensity of the hypersensitive reaction varied from complete necrosis of the region infiltrated with the bacteria by 48 hours after inoculation, to patches of necrosis throughout the infiltrated regions. Other inbreds, such as H95, Mo17 and Pa405, showed no noticeable necrosis after inoculation. At most, a mild chlorosis was observed at the site of inoculation in these lines.

Inoculation of two small backcross families indicated a dominant gene controlled the necrotic reaction. These families were made previously for the purpose of mapping two rust resistance genes, Rp4 and Rp7. The Rp4 and Rp7 lines both showed the necrotic reactions to X. o. pv. oryzicola. The Rp4 and Rp7 populations were made by crossing the Rp4 or Rp7 lines to H95, and then backcrossing the F1s to the H95 parent. The Rp4 population segregated 27 necrotic to 28 nonnecrotic, and the Rp7 population segregated 22 necrotic to 30 nonnecrotic. Subsequent inoculations of the same seedlings with common rust (Puccinia sorghi) indicated that the gene conferring the necrotic reaction to X. o. pv. oryzicola segregated independently of the Rp4 and Rp7 genes. The locus was designated rxo1, for 'reaction to X. oryzae'.

To determine the map position of the rxo1 gene, recombinant inbred lines from a B73 X Mo17 population (developed at NCSU) were tested with the X. o. pv. oryzicola strain. Of 150 lines tested, 74 showed the strong necrotic reaction of the B73 parent, 75 showed the minimal response of the Mo17 parent, and one family segregated for the reaction. Analysis of rxo1 segregation with the molecular markers already mapped in this population indicated the gene mapped approximately one map unit from umc85. The genomic area around umc85 carries a number of resistance genes (McMullen & Simcox, Mol. Plant-Microbe Interact. 8:811-815, 1995), including genes for resistance to at least two viruses, and one fungal disease (southern corn leaf blight). The rxo1 gene, therefore, appears to map to this same cluster of genes conferring resistance to a diverse variety of pathogens. 

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