RECESSIVE PLANT VIRUS RESISTANCE GENES CHARACTERIZED AT THE MOLECULAR LEVEL

Despite notable progress towards defining the elements that comprise dominant R gene-mediated defense responses in plants, little is known about the nature of plant susceptibility to disease. Owing to the relatively small number of proteins they encode, viruses completely depend on the host factors to complete their life cycle. Typical plant viruses encode 4 to 10 proteins that coordinate the complex biochemistry and intermolecular interactions required for viral infection cycles. Studying recessive virus resistance provides a unique opportunity to reveal host factors required for susceptibility and mechanisms of pathogenesis of the pathogen. Recent findings have confirmed early theoretical predictions that mutations of some host factors will result in recessively inherited resistance to plant viruses. The identification and characterization of host factors in which mutations interrupt viral pathogenesis will provide a new opportunity for understanding viral pathogenesis itself, as well as host responses; this is an approach that has been unavailable to date in the study of dominant resistance. Whether as a consequence of the economic importance of the Potyviridae, the relative prevalence of recessive resistance to this group of viruses, and/or the relative ease with which these viruses can be experimentally manipulated, studies of recessive R genes to date have focused largely on this viral family. Several host genes whose mutations impair the infection cycle of plant viruses, including BCTV,CMV, TEV, TuMV, TMV, TGMV, and TCV, have been identified and characterized in Arabidopsis. The translation initiation factor eIF4Ehas been identified repeatedly as a naturally occurring recessively inherited resistance locus in pepper pvr1, lettuce mo1, and pea sbm1 and has been implicated in barley as a candidate for rym4/5. The eIF4E isoform eIF(iso)4E also has been implicated in Arabidopsis and pepper resistance. The role of eIF4E and eIF(iso)4E in the potyvirus infection cycle is not known. However, the negative effects of mutations in these host factors on the infectivity of various potyviruses in various host plants imply that the effect of these host factors upon potyvirus infection cycle is probably conserved. The common feature linking pvr1/2, sbm1, and mo1 is that the viral avirulence determinants map to a specific region in the VPg, the protein covalently linked to the 5׳end of the viral RNA and perhaps mimicking the m7G cap of eukaryotic mRNAs. In eukaryotic cells, eIF4E binds to the m7G cap as the first step in recruiting mRNA into the translational preinitiation complex. A similar role for eIF4E might be predicted when potyvirus infects plant cells. Although eIF4E has never been shown to bind VPg in infection, VPg or its precursor VPg-Pro interacted with eIF4E or eIF(iso)4E in yeast two-hybrid and in vitro pull-down assays.

In the 1950s, pvr1 and pvr2 in pepper (Capsicum annuum and C. chinense) were initially considered allelic but then two loci were distinguished because of differences in resistance spectra. The allele formerly known as pvr21 is effective only against PVY-0, and pvr22 is effective against both PVY-0 and PVY-1. The allele pvr1 was relatively broad in effect, controlling TEV, PepMoV, and PVY). We now know that only one locus is involved, pvr1, at which at least three resistance alleles and two susceptibility alleles occur.

Point mutations in eIF4E that fall near critical positions for cap-binding function abolish interaction with TEV VPg and determine the range of isolates across three potyviral species that are controlled. Two of these alleles when homozygous block accumulation of the virus in protoplasts. The narrower spectrum allele retards movement of the virus through the plant but has no effect at the protoplast level. In a fourth case, Pepper veinal mottle virus, it appears eIF4E and probably eIF(iso)4E must be mutated to control the virus. In lettuce, mo11 and mo12 control common isolates of LMV. In the homozygous state, mo11 confers resistance, i.e., absence of LMV accumulation; mo12 results in reduced LMV accumulation and lack of symptoms. As observed in pepper, allelic variants of eIF4E, Ls-eIF4E0, Ls-eIF4E1, and Ls-eIF4E2 contained point mutations that result in predicted amino acid substitutions near the cap-binding pocket of the protein. In pea, sbm1 confers resistance to PSbMV pathotypes P1 and P4 as described above, now known to be a consequence of mutations in an eIF4E homolog.

Transient expression of susceptible-eIF4E in a resistant background complemented PSbMV infection by supporting both virus multiplication in primary target cells and cell-to-cell movement. Processes that account for cell-to-cell movement are not well understood, and therefore it is difficult to speculate on a plausible role for eIF4E in virus movement. Nevertheless, in both the pepper and pea systems, variants at an eIF4E locus result in inhibition of movement, as well as extreme resistance. Again, point mutations in the resistant eIF4E allele are located in and around the cap-binding pocket. Similar to pvr1, cap-binding ability of the eIF4E protein is abolished in the resistant eIF4E variant. Recently, rym4/5 for resistance to BaYMV in barley has also been shown to encode eIF4E. In contrast to dominant R genes where resistance to the same or closely related pathogens generally do not occur in syntenic positions, a recessive potyvirus R gene pot1from tomato was mapped to a collinear position with pepper gene pvr2. These results indicate that recessive R genes are highly conserved. Evidence to date indicates that, for the most part, dominant and recessive R genes may not be related mechanistically and evolutionarily.