Evaluation of progeny of genetically engineered barley plants for resistance to rhizoctonia oryzae and rhizoctonia solani AG-8
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In recent years, farmers in the Pacific Northwest (PNW) have been adopting agricultural practices known as direct-seed, or no-till farming, for both economic and resource conservation benefits. Unfortunately, Rhizoctonia root rot caused by Rhizoctonia solani AG-8 and Rhizoctonia has become a major limiting factor to crop yields in these systems. This situation has developed in part because of an increase in cropping intensity made possible by the additional water captured and saved for crop growth in direct-seed systems, and in part because of apparently higher inoculum potentials for the pathogens when infested host remains are left undisturbed in and on the soil. In addition, this pathogen affects all crops grown in the region making crop rotation ineffective as a means of controlling the disease. As direct-seed systems become more widespread, Rhizoctonia root rot has rapidly become the single most important root disease of cereal crops in the PNW. If direct-seeding is to move forward and succeed in the PNW, cereal genotypes must be identified that have resistance or tolerance to Rhizoctonia root rot. Six barley plans each genetically engineered (transformed) by Ph.D candidate Yongchun Wu in the Plant Pathology Department at Washington State University to express a gene from the soil fungus Trichoderma harzianum, were evaluated to test the hypothesis that the progeny from these plants would be resistant to the root-infecting fungi Rhizoctonia oryzae and/or Rhizoctonia solani AG-8. The purpose of my work was to test progeny of the six transformed barley plants for resistance to Rhizoctonia oryzae and Rhizoctonia solani AG-8 by exposing seedlings to the pathogens in growth chamber trials, and thereby determine whether the ThEn42 gene confers some level of resistance to these pathogens relative to the non-transformed control seedlings. It was hypothesized that there would be a difference in the level of resistance between the seedlings of the transformed plants and the seedlings representing the non-transformed control parent variety. The hypothesis was tested by exposing progeny of all six transformed barley plants and non-transformed control plants to each pathogen as well as no pathogen (healthy control), and then comparing the amount of root disease and size of seedlings for each progeny x pathogen treatment. With the objective of obtaining a complete data set, I tested progeny from the six plants and the progeny of one non-transformed (control) plant by planting seeds in soil with a low level of Rhizoctonia oryzae, a low level of Rhizoctonia solani AG-8, a high level of Rhizoctonia oryzae, a high level of Rhizoctonia solani AG-8, or in pathogen-free soil. The soil was infested with the two respective Rhizoctonia species at the respective "low" and "high" levels by mixing an artificial inoculum source into pasteurized (pathogen-free) soil. Seeds of the six transformed plants as well as one non-transformed (control) plant were planted in small, plastic shaped containers filled with respective infested and pathogen-free soil. The 245 planted containers were completely randomized, placed in trays and allowed to grow in a controlled climate growth chamber at 150 C and 12 hour day/night cycles for 20 days. Data collection commenced during and immediately upon conclusion of the experiment. The emergence date of each plant and the plant height after the 20-day period were recorded as measures of symptoms of Rhizoctonia root rot. These data were compared with the emergence date and plant height of unmodified plants exposed to the same pathogen conditions. I found at least one progeny from each genetic transformation to be resistant to both species causing Rhizoctonia root rot. The plants that have shown resistance have been grown to maturity for seed and may be used as parents for making crosses in barley breeding programs where resistance to Rhizoctonia root rot is needed in order for direct-seed cropping systems to succeed.