SHORT AND LONG-DISTANCE SPREAD OF POTATO LEAFROLL LUTEOVIRUS - EFFECTS OF HOST GENES AND TRANSGENES CONFERRING RESISTANCE TO VIRUS ACCUMULATION IN POTATO

Potato leafroll luteovirus (PLRV) movement through phloem of PLRV-resi stant potato clones was examined in experiments in which stem pieces w ere grafted either between infected rootstocks and virus-free suscepti ble scions or between infected scions and virus-free susceptible roots tocks. These test plants permitted either upwards or downwards virus m ovement into the susceptible tissue. Resistant potato clones had eithe r host gene-mediated resistance (H-MR) or transgene-mediated resistanc e (T-NR, conferred by transformation with the PLRV coat protein gene) to PLRV accumulation. The rate of PLRV movement was similar whether st em tissue was taken from H-MR, T-MR or susceptible potato clones. Viru s movement through two graft unions began around 7 days after grafting and was generally complete by about 14 to 16 days. Virus movement occ urred soon after acquiring functional phloem continuity across grafts as demonstrated by tracing with 6(5)-carboxyfluorescein, a phloem-mobi le dye. Most of the delay in virus detection after grafting probably r esulted from the time necessary to develop new phloem strands across g raft unions; subsequent movement of PLRV was rapid suggesting a passiv e process. PLRV infection was largely excluded from external phloem bu ndles in stem tissue of clones with either H-MR or T-MR. This trait wa s less pronounced as tissue aged. The mechanism limiting PLRV invasion of external phloem bundles of the T-MR clones appears to be similar t o that operating in the H-MR clones. Results are discussed in the cont ext of a proposed model of PLRV movement.