EXAMINATION OF THE LEAF-DROP SYMPTOM OF VIRUS-INFECTED POTATO USING ANTHER CULTURE-DERIVED HAPLOIDS

Necrotic lesions and vein necrosis characteristic of the hypersensitiv e response (HR) controlled by the dominant resistance gene Ny develop in potato cv. Pito after infection with potato virus Y ordinary strain (PVYO) at a low temperature (16/18 degrees C night/day). In contrast, at high temperatures (19/24 degrees C night/day), large coalesced les ions develop in the lower infected leaves, which wither and remain han ging from stems forming the leaf-drop symptom; mosaic symptoms with no necrosis also develop in the top leaves. The genetic basis of the lea f-drop symptom and its dependence on temperature were examined using a novel approach involving 58 haploids (2n = 24) derived from 'Pito' (2 n = 48) through anther culture. These haploids and 'Pito' were graft-i noculated with PVYO at 19/24 to 25 degrees C (night/day). Necrotic sym ptoms were expressed in 28 haploids, of which 18 haploids (phenotype c lass N) developed top necrosis, vein necrosis, or both and necrotic le sions that are characteristic of HR. Ten haploids showed leaf drop sim ilar to 'Pito' (phenotype class LD). Thirty haploids were susceptible and showed only mosaic symptoms (phenotype class S). These data indica ted that necrosis was induced by a single dominant gene, Ny, in the si mplex condition. However, the three distinct phenotypic classes (N, LD , and S) among the haploids grown under the same environmental conditi ons showed that another locus (gene) was involved in modifying the HR triggered by Ny. Data suggested that this locus contains a dominant te mperature-dependent modifier (Tdm) gene that alters the expression of PVY-induced HR at higher temperatures, resulting in leaf drop.