Assessment of Resistance to PVY in Interspecific Hybrids Obtained by Combining Type va Resistance from Nicotiana tabacum with the Resistance from PVY-Immune Species Nicotiana africana

Tobacco veinal necrosis caused by the potato virus Y (PVY) substantially affects yields and crop quality of tobacco. PVY shows high variability in virulence due to numerous mutations and recombinations. The types of resistance available for tobacco include the va type, effective against some PVY isolates but succumbing to others, and the Nicotiana africana-derived type, fully effective in its native genetic milieu but limited to varying degrees of tolerance when transferred to N. tabacum. The objective of this study was to evaluate the pyramiding potential of these two sources of resistance in alloploid hybrids of Nicotiana tabacum x N. africana. To this end, amphidiploids involving N. tabacum cvs. VAM and Wiślica, each being the carrier of a different variant of the va gene, were created and advanced from F1 to two subsequent selfed generations and to BC1 generation with either VAM or Wiślica as the recurrent N. tabacum parent. The hybrid populations thus produced were challenged with two PVY necrotic isolates, IUNG 23 and IUNG 20, mild and severe isolate belonging to the PVYN-Wi and PVYNTN groups, respectively. The mild isolate failed to infect any of the hybrids or parental species. The hybrids varied in their response to IUNG 20. The F1 and F2 populations were composed of asymptomatic hosts to PVY and of those showing mild vein clearing but no necrotic plants. There was a progressive increase in ability to resist PVY in successive amphidiploid generations, with 30 and 10% of fully resistant plants appearing in the F3 amphidiploids involving VAM and Wiślica. Further research is needed to account for the appearance of segregants completely resistant to PVY, since neither N. africana factor is fully expressed in N. tabacum, nor do the va alleles confer resistance against PVYNTN isolates.