The genetic and molecular basis of b-proteins in the genus Nicotiana

Abstract

Screening for the pathogenesis-related (b) protein patterns of 11Nicotiana species and 30N. tabacum varieties has revealed both inter- and intraspecific variability and 7 different b-proteins (b₀, b₁, b_1′, b_1″, b₂, b₃ and b₄) have been clearly defined. Their genetic determinants are sexually transmitted independently of theN gene conferring resistance to TMV, and a monogenic inheritance has been demonstrated for one of them (b_1′). Grafting experiments have revealed the existence of a species-aspecific ‘mobile compound’ responsible for the expression of the b-protein genes, the production of which is probably under the control of theN gene. Among the 5 intraspecific and 6 interspecific hybrids studied, one of them, theN. glutinosa x N. debneyi together with its amphidiploid, synthesizes b-protein (b_1″) in a constituve way and possesses a high level of resistance to necrosis-inducing viruses. The amphidiploid is able to transfer these two properties to otherNicotianae not only by crossing but also by grafting; it therefore appears to permanently synthesize the ‘mobile compounds’. Furthermore, the hypersensitive reaction to TMV in these hybrids is only completely broken down at 35 °C, whereas this normally occurs at 30 °C in plants with theN gene.