Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/1999
Publication Date: N/A
Interpretive Summary: Fusarium head blight (scab) has emerged as an extremely destructive disease of wheat and barley, especially in the upper Midwest where losses have been catastrophic for several years. The disease is caused by a fungus that attacks heads of plants, damaging the grain as it develops. Efforts to control the disease in the field and to develop resistant varieties are limited by a lack of understanding of infection pathways by the fungus in the head. Further, we don't know if resistant reactions can protect the head. The investigation reported here is the first modern attempt to follow fungus infection processes microscopically and also a first look at when defense genes in heads are activated in response to fungus attack. The results show that the fungus develops beneath the plant cuticle (the waxy covering of plant tissues) and can enter through stomates (breathing pores on the plant surface). Tissues underlying stomates are colonized within 48-76 hr after heads are inoculated. Defense genes were shown to b turned on as early as 6 to 12 hours after inoculation, giving peak activity at 36-48 hr. Two defense genes (PR4 and PR5) were activated more in a partially resistant than a susceptible variety, implicating the two genes in resistance. The results provide plant scientists a base of information essential for further research on resistance and control of Fusarium head blight.
Technical Abstract: Fusarium Head Blight (FHB) of wheat is a crippling disease that causes severe economic losses in many of the wheat-growing regions of the world. Temporal patterns of fungus development and transcript accumulation of defense response genes were studied in Fusarium graminearum-inoculated wheat spikes within the first 48-76 hours after inoculation (hai). Microscopy of inoculated glumes revealed that the fungus penetrated throug stomata, exhibited subcuticular growth along stomatal rows, colonized glume parenchyma cells and sporulated within 48-76 hai. No major differences in the timing of these events were found between the Sumai 3 (resistant) and Wheaton (susceptible) genotypes. In complementary experiments, RNA was extracted from spikes at several time intervals up to 48 hai and temporal expression patterns were determined for defense response genes encoding peroxidase, PR1, PR2 (B-1,3-glucanase), PR3 (chitinase), PR4 and PR5 (thaumatin-like protein). In both genotypes, transcripts for the six defense response genes accumulated as early as 6-12 hai during F. graminearum infection and peaked at 36-48 hai. Greater and earlier PR4 and PR5 transcript accumulation was observed in Sumai 3 as compared to Wheaton. Implications of both biological and molecular information of the early events of F. graminearum infection in wheat in developing strategies for FHB resistance are discussed.