Summary auto-generated
This study examined potato leafroll luteovirus (PLRV) movement through potato stems in clones with either host gene-mediated resistance (H-MR) or transgene-mediated resistance (T-MR, from coat protein gene transformation). Using grafting experiments that traced virus movement through resistant and susceptible stem sections, researchers measured both upward and downward PLRV spread. PLRV movement began around 7 days after grafting and was complete by 14-16 days, corresponding to the establishment of functional phloem connections (tracked using fluorescent dye). Importantly, virus movement rates through stem tissue were similar regardless of whether tissue came from resistant (H-MR or T-MR) or susceptible potato clones, indicating resistance mechanisms do not slow virus translocation. Instead, resistant clones restricted PLRV infection to internal phloem bundles while susceptible clones showed infection throughout internal and external phloem. This restriction was most pronounced in young tissue but diminished with age. The findings suggest PLRV moves passively through phloem along metabolite gradients, and resistance works by limiting which phloem cells become infected rather than slowing viral movement itself.
Key findings
- PLRV movement through phloem occurred rapidly (7-16 days after grafting) once functional phloem continuity was established, suggesting passive transport along metabolite gradients
- Both host gene-mediated and transgene-mediated resistance showed similar rates of PLRV movement through stem tissue compared to susceptible clones
- Resistant potato clones restricted PLRV infection primarily to internal phloem bundles while susceptible clones showed widespread infection in both internal and external phloem
- Resistance mechanisms appear independent of movement restriction and become less effective in older plant tissue
- The same restriction mechanism operates in both primary and secondary infections and against heavy inoculum pressure from grafted infected tissue
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Abstract
Potato leafroll luteovirus (PLRV) movement through phloem of PLRV- resistant potato clones was examined in experiments in which stem pieces were grafted either between infected rootstocks and virus-free susceptible scions or between infected scions and virus-free susceptible rootstocks. These test plants permitted either upwards or downwards virus movement into the susceptible tissue. Resistant potato clones had either host gene-mediated resistance (H-MR) or transgene- mediated resistance (T-NR, conferred by transformation with the PLRV coat protein gene) to PLRV accumulation. The rate of PLRV movement was similar whether stem tissue was taken from H-MR, T-MR or susceptible potato clones. Virus movement through two graft unions began around 7 days after grafting and was generally complete by about 14 to 16 days. Virus movement occurred soon after acquiring functional phloem continuity across grafts as demonstrated by tracing with 6(5)- carboxyfluorescein, a phloem-mobile dye. Most of the delay in virus detection after grafting probably resulted from the time necessary to develop new phloem strands across graft unions; subsequent movement of PLRV was rapid suggesting a passive process. PLRV infection was largely excluded from external phloem bundles in stem tissue of clones with either H-MR or T-MR. This trait was less pronounced as tissue aged. The mechanism limiting PLRV invasion of external phloem bundles of the T-MR clones appears to be similar to that operating in the H-MR clones. Results are discussed in the context of a proposed model of PLRV movement.