1.2.24
GENETIC DISSECTION OF THE MLA-12 SPECIFIED RACE-SPECIFIC RESISTANCE TO POWDERY MILDEW IN BARLEY

J ORME, A FREIALDENHOVEN and P SCHULZE-LEFERT

Sainsbury Laboratory, John Innes Centre, Norwich, UK

Background and objectives
Previous mutational analysis has identified genetic loci required for Mla-12 specified race-specific resistance against powdery mildew, Erysiphe graminis f. sp.hordei [1, 2]. Mutations at these loci confer susceptibility upon fungal challenge with an isolate carrying avrMla-12. One of these loci, designated Rarl, also plays a key role in a number of resistance reactions specified by other powdery mildew resistance genes. We are seeking to identify further genetic loci required for Mla-12 specified race-specific resistance by a mutational approach.

Materials and methods
Mutagenized seed has been generated from both available mutant rarl alleles (rar1-1 and rar1-2). Screening of M2 families is achieved by inoculating 7-day-old seedlings with a fungal isolate expressing avrMla-12. Seedlings showing an enhanced disease-resistant phenotype compared to the parent mutant rarl susceptible infection phenotype are kept for further analysis. Upon confirmation of the infection phenotype in the M3 generation, these candidates undergo extensive genetic and cytological characterization.

Results and conclusions
One mutant, designated M100:348, showed an enhanced disease-resistance phenotype. Macroscopically, fungal colonies appeared weak, indistinct and with severely limited sporulation compared with the parent rarl phenotype. There was increased chlorosis and necrosis of the infected leaf. The infection phenotype observed was confirmed with two fungal isolates expressing avrMla-12 in the M3. The infection phenotype with a fungal isolate lacking avrMla-12 was identical to that of the parent rarl mutant. Microscopically, there was no change in fungal penetration frequency (assessed by frequency of differentiated haustoria) or plant response (assessed by hypersensitive response, HR, frequency, as indicated by whole-cell autofluorescence in attacked epidermal cells). However, there was a striking increase in trypan blue uptake (indicating a loss of membrane integrity) in mesophyll cells bordering epidermal cells that had undergone an HR by the mutant in comparison to the parent rar1-2. This suggests that the observed increased resistance phenotype is due to a race-specific, post-HR phenomenon.

This mutant has been test-crossed to the parent rarl plant and to the wild-type cultivar Sultan 5 (Mla-12, Rarl). Additional crosses have been made to the Mla-12 backcross lines I10, PIO and S10. Analysis of the F2 segregation of infection phenotypes from these crosses is in progress.

References
1. Torp J, Jorgensen JH, 1986. Canadian Journal of Genetics and Cytology 28, 725-731.
2. Freialdenhoven A, Scherag B, Hollricher K et al., 1994. Plant Cell 6, 983-994.