Brassinosteroid deficiency caused by the mutation of the HvDWARF gene influences the reactions of barley to powdery mildew

This study investigated the impact of mutation in the HvDWARF gene, encoding C6-oxidase involved in brassinosteroid (BR) biosynthesis, on disease resistance of barley against the fungal pathogen powdery mildew (Blumeria graminis f. sp. hordei). The mutation results in significant BR deficiency in leaf tissue. Since increase in the BR content by their exogenous application was reported to improve plant resistance to viral, bacterial and fungal infection, we wanted to find out, if lowered content of BR may affect plant resistance to this pathogen. The 527DK mutant carrying the aforementioned mutation and its respective wild type Delisa were inoculated by powdery mildew fungus (Bgh, race A6). However, no symptoms of disease were visually noted neither on 527DK nor on Delisa. Microscopic observations showed that after germination of the conidia the germ tubes started to penetrate, developed appressorium but could not develop further in the two genotypes. The cell wall thickening (papillae) around the penetration was only visible. However, inoculated plants showed changes in some of parameters describing photosynthetic efficiency or water relations. Similarly, alterations in hormonal homeostasis not only between wild type and mutant, but between control and infected plants were observed. Inoculation with the pathogen induced especially accumulation of compounds from auxin group (indole-3-carboxylic acid, oxindole-3-acetic acid, 4-chloroindole-3-acetic acid and 5-chloroindole-3-acetic acid). Interestingly, the stronger effect was observed in plants with mutation in the HvDWARF gene. Taking into account that the HvDWARF gene mutation finally did not affect resistance to powdery mildew (no symptoms appearance), we suggest that the conclusion of our study may be of importance for breeding of barley.