3.4.38
CHARACTERISATION OF MOLECULAR BASIS OF RESISTANCE TO POWDERY MILDEW IN WHEAT

V DUGGAL, R STRATFORD, G J JELLIS and R SHIELDS

Plant Technology Group, Plant Breeding International, Maris Lane, Trumpington, Cambridge CB2 2LQ, UK

Background and objectives
Expression of various PR (pathogenesis related) proteins is proposed to be associated with disease resistance in various crop species. Suppressers of resistance genes have been reported to exist in hexaploid wheat signifying that the expression of resistance depends upon the genetic background of the plant. In 1981, 'Hobbit sib' (hexaploid wheat) was irradiated with fast neutrons and a series of deletion mutants with increased resistance to rusts and mildew were identified. We wish to test whether the mutations lead to a high constitutive expression of various PR proteins in the resistant mutants.

Material and methods
RNA of these deletion mutants and wild-type 'Hobbit sib' was extracted from the uninfected seedling and adult plant leaf tissues and was hybridised with cDNA probes of various PR proteins. viz. PR1, PR-R homologue, a thaumatin-like protein (TL) a peroxidase, a novel protein PWIRI [1] and two lipid transfer proteins. Northern blots were standardised using a wheat rubisco cDNA probe and the intensity of the different transcript bands was quantified using a phospho-imager. The genotypes were also evaluated for their disease resistance to powdery mildew at both seedling and adult growth stages.

Results and conclusions
Mutant 13-48 was found to be the most resistant at the seedling stage while 13-54 was the most resistant at the adult plant stage. Preliminary results indicate that both of these mutants also had the highest expression levels for a number of the PR proteins. Transcripts of PR1, PR-R homologue, TL, peroxidase and PWIRI were 2-5 times higher in seedlings of 13-48 in comparison to 'Hobbit sib'. In adult plants of 13-54, transcripts of PWIRI and TL were expressed at 2-6 times the level of 'Hobbit sib'. In some mutant seedlings, two transcripts were detected for PR1, PR-R homologue, TL, peroxidase and PWIRI. The higher molecular weight transcript could be the primary RNA transcript of the PR proteins. Further work is in progress to test the consistency of these results and to analyse more PR protein genes. Mutant lines that show enhanced constitutive expression of PR proteins will be analysed further by testing whether PR protein expression co-segregates with resistance using F2 segregating populations. In the long term we will also look to see if any of these mutant lines show more rapid or higher levels of induction of PR protein expression after inoculation with powdery mildew. These results will then be correlated with microscopic studies to see whether the higher PR protein expression levels coincide with cessation of fungal growth.

References
1. Bull et al., 1992. Molecular Plant-Microbe Interactions 5, 516-519.