Identifier

etd-07122007-103137

Degree

Master of Science (MS)

Department

Plant Pathology and Crop Physiology

Document Type

Thesis

Abstract

Atoxigenic Aspergillus flavus was demonstrated by others as a promising biocontrol agent to minimize preharvest aflatoxins in susceptible crops. But the mechanism was unclear. A filter insert-well plate system was used to study the mechanism in lab. There was no inhibition when toxigenic A. flavus isolate 53 and inhibitory atoxigenic isolates were separated by 0.4 µm membrane, approximately 50% inhibition occurred when separated by 12 µm membrane, and complete inhibition occurred when a 74 µm membrane was used. This result suggested that touching or close physical interaction is needed for toxin inhibition and the nutrient competition hypothesis was not supported. Isolate 53 and inhibitory atoxigenic isolate 51 were used to study the timing of intraspecific toxin inhibition. The result showed that inhibition occurred when 0 - 4 day old isolate 51 was added within the first 16-hour growth of isolate 53. However, two-day old isolate 51 inhibited toxin production by two-day old isolate 53 and twenty-four hour old isolate 51 inhibited toxin production by 48-hour old isolate 53. These results suggested that there is a 16-hour “window” for the conidial inhibition ability of atoxigenic isolate but for mycelia, the “window” is expanded to 48 hours. Isolate Af70-GFP was acquired to microscopically examine the touch inhibition interaction. Surprisingly, none of the completely inhibitory atoxigenic isolates from our collection or NRRL 21882 inhibited toxin production by Af70-GFP. Isolate K49 and two Australian isolates were shown to be able to inhibit toxin production by Af70-GFP. The inhibitory abilities of additional atoxigenic isolates were tested with toxigenic isolates 53, Af70s-GFP and NRRL 3357. Different patterns were obtained among those three isolates. These results showed that there was specificity in the touch inhibition interaction. Af70-GFP and isolate K49 were used to continue microscopy work. The growth of Af70s-GFP appeared to be inhibited and vacuoles present in Af70s-GFP were absent when it was paired with K49. Biocontrol once thought to be due to competitive exclusion probably requires close physical growth or touching and displays specificity. Multiple atoxigenic isolates each specific to a subset of the toxigenic isolate population may be needed for an effective biocontrol application.

Date

2007

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Kenneth E. Damann

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