The NSm proteins of phylogenetically related tospoviruses trigger Sw-5b-mediated resistance dissociated of their cell-to-cell movement function

Abstract

The cell-to-cell movement protein (NS_M) of tomato spotted wilt virus (TSWV) has been recently identified as the effector of the single dominant Sw-5b resistance gene from tomato (Solanum lycopersicum L.). Although most TSWV isolates shows a resistance-inducing (RI) phenotype, regular reports have appeared on the emergence of resistance-breaking (RB) isolates in tomato fields, and suggested a strong association with two point mutations (C118Y and T120N) in the NS_M protein. In this study the Sw-5b gene has been demonstrated to confer not only resistance against TSWV but to members of five additional, phylogenetically-related classified within the so-called "American" evolutionary clade, i.e., Alstroemeria necrotic streak virus (ANSV), chrysanthemum stem necrosis virus (CSNV), groundnut ringspot virus (GRSV), Impatiens necrotic spot virus (INSV) and tomato chlorotic spot virus (TCSV). Remarkably, bean necrotic mosaic virus (BeNMV), a recently discovered tospovirus classified in a distinct American subclade and circulating on the American continent, did not trigger a Sw-5b-mediated hypersensitive (HR) response. Introduction of point mutations C118Y and T120N into the NS_M protein of TSWV, TCSV and CSNV abrogated the ability to trigger Sw-5b-mediated HR in both transgenic-N. benthamiana and tomato isolines harboring the Sw-5b gene whereas it had no effect on BeNMV NS_M. Truncated versions of TSWV NS_M lacking motifs associated with tubule formation, cell-to-cell or systemic viral movement were made and tested for triggering of resistance. HR was still observed with truncated NS_M proteins lacking 50 amino acids (out of 301) from either the amino- or carboxy-terminal end. These data altogether indicate the importance of amino acid residues C118 and T120 in Sw-5b-mediated HR only for the NS_M proteins from one cluster of tospoviruses within the American clade, and that the ability to support viral cell-to-cell movement is not required for effector functionality.

Keywords: Avr; Hypersensitive response; NS(M) protein; Resistance; Sw-5b gene; Tospovirus.