The Nature of the Resistance of Agrotricums to Wheat Streak Mosaic Virus

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Abstract
The agrotricums, C.I. 15092, C.I. 15321 and C.I. 15322, [derived lines from Agropyron spp.] possess a hypersensitive type of resistance to wheat streak mosaic virus (WSMV). Following inoculation with WSMV, chlorotic or necrotic local lesions develop on inoculated leaves and increase in size until the leaves die. All 3 agrotricums are resistant to WSMV in the field, but under greenhouse or growth chamber conditions only C.I. 15092 remains resistant. In C.I. 15092, WSMV replicates in the inoculated leaf and is translocated in both directions within the leaf. Virus can be transferred from the lesions and the titer increases until 5 days postinoculation, then declines as necrosis ensues. Virus was purified from inoculated leaves but was not detected in roots or noninoculated leaves of C.I. 15092 by infectivity assay. Virus particles and pinwheel inclusion bodies were found only in inoculated leaves. Resistance was not affected by light intensity or plant age at inoculation, but was broken at 35.degree. C. The percentage of plants that became systemically infected was influenced by the length of heat treatment and the period that elapsed between inoculation and heat treatment. Resistance also was broken by daily treatment of inoculated plants with tannic acid, acridine orange, cycloheximide, or actinomycin D. Agrotricum C.I. 15092 is a systemic host of foxtail, brome and barley stripe mosaic viruses. Necrotic local lesions did not develop following infection by those 3 viruses.