Infection of maize by the Fny strain of cucumber mosaic virus (CMV) and resistance against infection by the M strain of CMV were mapped to the coat protein gene on RNA 3 of CMV, using biologically active cDNA clones of Fny-CMV RNAs 1, 2, and 3 and RNAs 2 and 3 of M-CMV, as well as chimeras constructed between cDNA clones of M-CMV and Fny-CMV RNA 3. Changes in the coat protein gene of M-CMV RNA 3 at both positions 129 (Leu to Pro) and 162 (Thr to Ala) were required to overcome the resistance against M-CMV in maize. Resistance to M-CMV in maize was correlated with an inability to detect virus accumulation in the inoculated leaves. Since the coat protein of CMV is involved in virus movement, but not virus replication, the data suggest that the resistance in maize to M-CMV is due to the inability of the M-CMV coat protein to promote the cell-to-cell movement of CMV in maize.
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