Differential accumulation of Potato virus A and expression of pathogenesis-related genes in resistant potato cv. Shepody upon graft inoculation

Potato (Solanum tuberosum L.) cv. Shepody is highly resistant to Potato vir us A (PVA), yielding no visible symptoms after rub inoculation. In 'Shepody ' rootstocks graft-inoculated by PVA-infected scions from a susceptible hos t, we found a resistance consisting of traces of necrosis (necrotic streaks ) in stems and chlorosis in newly emerged leaves. The response was temperat ure dependent, appearing at 15 to 18 degreesC but not at 28 to 31 degreesC. Necrosis was also observed in tubers, appearing first in the bud end and s preading randomly throughout the majority of the tuber. Trace amounts of PV A in plant tissues were detected by a combination of reverse transcription polymerase chain reaction and Southern blot procedures. The virus concentra tion was significantly higher in visibly necrotic areas than in nonnecrotic areas, suggesting that presence of the necrosis may be concentration depen dent. Pathogenesis-related gene expression showed that chitinase A and B, g lucanase B, and PR-10a were associated with chlorotic or necrotic symptoms in leaves, stems, and tubers. Gene expression was markedly more evident in tuber tissues than in leaves and stems. Furthermore, generation of oxidants was also observed within the necrotic areas. Although PVA was detected in necrotic areas of tubers, newly emerged plants from the same tubers were PV A-free.