CODING CHANGES IN THE 3A CELL-TO-CELL MOVEMENT GENE CAN EXTEND THE HOST-RANGE OF BROME MOSAIC-VIRUS SYSTEMIC INFECTION

The M1 and M2 strains of brome mosaic virus (BMV) both systemically in fect the monocot host barley, but only the M2 strain systemically infe cts the dicot cowpea line TVu-612. We have shown previously that this difference in host range maps primarily to RNAS. To further characteri ze the role of RNA3 in host specificity, a series of RNA3 hybrids were tested, in inoculations with M1 RNA1 and RNA2, for ability to systemi cally infect TVu-612 cowpea. Although all hybrids were amplified well in cowpea protoplasts and all supported systemic infection in barley p lants, only those with the 3a cell-to-cell movement gene of BMV-M2 sup ported systemic infection of cowpea. The sequences of the M1 and M2 3a proteins differ at four positions. Introducing these four coding diff erences individually or in various combinations into M1 RNAS revealed that all four influenced BMV adaptation ID cowpea and that these four differences were sufficient to account for the difference in ability b etween M1 and M2 RNA3s to support systemic infection of this legume. T hese coding changes were also associated with faster spread of infecti on in inoculated cowpea leaves, suggesting that they influence the abi lity to systemically infect TVu-612 cowpea through effects on the rate of cell-to-cell spread. (C) 1995 Academic Press, Inc.