Chassant Reves: Chasing Dreams
A post graduate in Biotechnology. Trying to establish myself as Scientific Writer. Adventurer, Dreamer, Amiable.
Sunday 11 October 2015
COEVOLUTION OF VIRUS RESISTANCE AND VIRAL AVIRULENCE GENES
Avirulence
genes in plant pathogens have been defined by their requirement for disease
resistance in hosts containing corresponding R genes. Plant viruses evolve very
rapidly owing to very short replication cycles, large numbers of genomes within
each cell across many cells per host, and many hosts. For RNA viruses, the
absence of a proofreading function in viral replicases may result in mutation
rates as high as 104 per replication cycle per base. Viral genetic variation
can result from several major genetic processes including mutation,
recombination, and the acquisition of additional genomic sequence. As a
consequence, resistance breaking viral genotypes are known for most host
resistance, especially for genes showing HR. Avirulence determinants are
typically identified by creating chimeric clones derived from viral genotypes
with contrasting virulence and then testing for infectivity. Once an avirulence
domain is identified, site-directed mutagenesis allows identification of
specific point mutations responsible for virulence.
Virtually any part of the viral genome can
define an avirulence determinant. With respect to R genes that confer HR,
avirulence factors include viral RNA polymerase subunits, movement protein, and
CP. Several potyviral avirulence genes have been identified for dominant R
genes that do not show HR. The CI and P3 proteins of Turnip mosaic virus serve
as avirulence determinants for the Brassica napus R genes, TuRBO1 and
TuRBO4/5 (96–98), while SMV HC-Pro and P3 are involved in overcoming Rsv1
in soybean. In contrast to the case for dominant genes where many different
viral components have been identified as avirulence determinants, a pronounced
trend is apparent viral factors that serve as the determinant for pathogenicity
in resistance systems controlled by recessively inherited R genes. Of nine R
gene studies to date, seven identify potyviral VPg as the pathogenicity
determinant for recessive resistance, although the systems in question show
diverse resistance phenotypes: Capsicum pvr1/pvr12 resistance to PVY is
cellular, tobacco va resistance impairs the cell-to-cell movement, and Nicandra
physaloides and Solanum commersonii affect long-distance movement.
The eighth study, also focused on a potyviral system, PsbMV/pea, identified the
P3-6K1 cistron as the pathogenicity determinant. Only one study to date has
focused outside the Potyviridae. In this case, the 3_ untranslated region of
the carmovirus Melon necrotic spotvirus (MNSV) genomic RNA defined the
location of the viral determinant in the by interaction of MNSV with melon. In
the eight cases where the viral elicitor is protein, host recognition of these
viral proteins that serve as pathogenicity determinants is altered by amino
acid substitutions that do not appear to significantly compromise the function of
the protein in pathogenesis. For other microbial pathogens, there often appears
to be a fitness penalty association with mutations from avirulence to virulence.
Although this type of fitness/avirulence tradeoff has not been noted generally
for plant viruses, there are specific examples where this occurs. Isolates of
ToMV capable of overcoming Tm22 gene were found to multiply poorly on
resistant plants. If the Tm22 resistant protein targets a domain of the
viral MP such that this protein is mutated to overcome resistance, these
mutations could result in diminished fitness.
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.
Friday 2 October 2015
Resistance to Cucumber mosaic virus in Arabidopsis Conferred by RCY1
Extensive
examination of 12 Arabidopsis ecotypes identified a CMV-Y-resistant ecotype,
C24. The resistance response of C24 includes suppression of virus multiplication
to a low level, the formation of necrotic lesions at the primary site of virus
infection, and restriction of virus to the inoculated leaves. This resistance response
in C24 is controlled by a single dominant RCY1 (resistance to cucumber mosaic
virus strain Y) gene. The analysis of a series of chimeric viruses constructed from
the avirulent isolate CMV-Y and the virulent isolate CMV-B2 revealed that the
coat protein of CMV-Y serves as the avirulent determinant of resistance in C24.
The RCY1 gene has been mapped in Arabidopsis within the MRC-5 region
on chromosome 5, in which nine other defined resistance genes (RAC3, RPS4,
HRT, TTR1, and five distinct RPP loci) are located. Fine
mapping and sequence comparison of this region from C24 and a CMV-Y susceptible
C24 mutant identified the RCY1 gene encoding 104-kDa CC-NBS-LRR type
protein.
RCY1
is allelic to the resistance gene RPP8 against Peronospora parasitica
in the ecotype Lansberg erecta and HRT against TCV in the ecotype
Dijon-17. The RCY1-conferred resistance requires both salicylic acid and
ethylene signaling but not jasmonic acid signaling. Table 4 shows some of the R gene and Avr factor pairs.
Resistance to Turnip crinkle virus in Arabidopsis Conferred by HRT
A
single dominant gene, HRT, was identified for HR resistance to TCV. HRT
is located on chromosome 5 and encodes a CC-NBS-LRR protein with striking similarity
to the RPP8 gene family for resistance to the oomycete Peronospora parasitica.
Despite very high sequence similarity, HRT and RPP8 specifically control
only their cognate pathogens. Analysis of resistance in HRT-expressing transgenic
plants indicated that HRT is necessary but generally insufficient for resistance.
About 90% of the HRT-transformed Col-0 plants developed HR and
Expressed
PR-1 after TCV infection yet remained susceptible to TCV. Full
resistance to TCV required both HRT and a recessive allele rrt.
Later experiments demonstrated that the HRT-/rrt-mediated
response is dependent on EDS1 and independent of RAR1 and SGT1.
In this system, TCV CP is the avirulence determinant recognized by HRT.
A host protein, TIP (TCV interacting protein) that belongs to the NAC family of
transcriptional activators is known to interact with TCV CP. Although the
relevance of this interaction to the mechanism of resistance remains unclear,
this interaction apparently functions to keep TIP out of the nucleus.
Resistance to Tomato mosaic virus in Tomato Conferred by Tm22
Tm22, the second
tobamovirus R gene isolated, is one of the three R genes, Tm1, Tm2, and Tm22, used widely in
tomato breeding to control Tomato mosaic virus (ToMV). The Tm1 gene
from S. hirsutum confers extreme resistance and was mapped to chromosome
2. Tm2 and Tm22, considered to be alleles from S. peruvianum,
are located close to the centromere of chromosome 7. Tm22, considered
the more durable of the two alleles, was isolated by transposon tagging and
encodes an 861 amino acid CC-NBS-LRR protein. The predicted protein from the
susceptible allele tm2 also encodes a CC-NBS-LRR protein that appears comparable
in most respects to the protein encoded by the resistance allele. Analysis of
the nucleotide sequence of resistance-breaking virus isolates indicated that the
MP protein is the avirulence factor in this resistance system. However, different
mutations are required to overcome Tm2 and Tm22.
Resistance to Tomato spotted wilt virus in Tomato Conferred by Sw-5
Economic
considerations have promoted the goal of TSWV-resistant tomato varieties in
plant breeding programs for nearly 70 years. Early genetic studies reported five
genes, Sw-1a, Sw-1b, sw-2, sw-3, and sw-5,
from two species, Solanum pimpinellifolium and Solanum lycopersicum,
all of which were overcome quickly. Sw-5, introgressed from Solanum
peruvianum into tomato, has demonstrated broad and stable resistance. In
resistant genotypes, local necrotic lesions develop on inoculated tissue, and
systemic movement of the virus is restricted. The Sw-5 locus was
isolated by positional cloning and sequenced, revealing that the resistance
allele encodes a CC-NBS-LRR R protein. Sw-5 is remarkably similar to the
tomato Mi gene for nematode resistance with the exception of four heptad
amphipathic leucine zippers at the N terminus. This pronounced similarity
suggests that Sw-5 and Mi may share a common signal transduction pathway.
Sw-5 and its paralogs were mapped to tomato chromosome 9 and chromosome 12
with other fungal, viral, and bacterial R genes. A comparative analysis with
the genus Capsicum, which is considerably diverged from Solanum within
the tribe Solanae, indicated that paralog position was largely conserved
between these genera. In Capsicum, monogenic dominant TSWV resistance
conferred by Tsw showed identical resistance phenotype and
strain-specificity to Sw5, but no cross-hybridization with Sw5 was
detected. When resistance-breaking TSWV strains were analyzed, avirulence
determinants mapped to different subgenomic RNAs.
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