DOMINANT PLANT VIRUS RESISTANCE GENES CHARACTERIZED AT THE MOLECULAR LEVEL

Most plant disease-resistance (R) genes isolated and characterized to date represent genes whose recognition of their cognate pathogens has been modeled as gene-for-gene interactions (Table). Under this well-known model, complementary pairs of dominant genes are defined by the host-pathogen interaction, one in the host and the other in the pathogen, whose physical interaction, direct or through intermediates, determines the outcome of the encounter. Following pathogen recognition, which occurs via poorly defined mechanisms, the R gene is presumed to activate a signaling cascade that coordinates plant defense responses to block pathogen spread, resulting in an incompatible interaction. Nine dominant plant virus R genes have been isolated and sequenced to date: HRT, RTM1, RTM2, RCY1 from Arabidopsis; and from solanaceous hosts, N,

Table. Naturally occurring plant virus resistance genes for which nucleotide sequences are known

Gene	Plant	Virus	Resistance mechanism	Cloning method	Predicted domains	Year isolated
N	N. tabacum	TMV	Cell-to-cell movement (HR)	Transposon tagging	TIR-NBS-LRR	1994
Rx1	S. tuberosum	PVX	Replication	Positional cloning	CC-NBS-LRR	1999
Rx2	S. tuberosum	PVX	Replication	Positional cloning	CC-NBS-LRR	2000
Sw5	S. esculentum	TSWV	Cell-to-cell movement (HR)	Positional cloning	CC-NBS-LRR	2000
HRT	A. thaliana	TCV	Cell-to-cell movement (HR)	Positional cloning	LZ-NBS-LRR	2000
RTM1	A. thaliana	TEV	Systemic movement	Positional cloning	Jacalin like seq	2000
RTM2	A. thaliana	TEV	Systemic movement	Positional cloning	Jacalin like seq	2000
RCY1	A. thaliana	CMV	Cell-to-cell movement (HR)	Positional cloning	CC-NBS-LRR	2002
Tm22	S. lycopersicum	ToMV	Cell-to-cell movement (HR)	Positional cloning	CC-NBS-LRR	2003
pvr1, pvr12	C. annuum	PVY	Replication	Transposon tagging	eIF4E	2002
pvr11			Cell-to-cell movement (HR)	Candidate approach
mo11	L. sativa	LMV	Replication	Candidate approach	eIF4E	2003
mo12			Tolerance
sbm1	P. sativum	PSbMV	Replication	Candidate approach	eIF4E	2004

CMV, Cucumber mosaic virus; LMV, Lettuce mosaic virus; PSbMV, Pea seed borne mosaic virus; PVY, Potato virus Y; PVX, Potato virus X; TCV, Turnip crinkle virus; ToMV, Tomato mosaic virus; TEV, Tobacco etch virus; TMV, Tobacco mosaic virus; TSWV, Tomato spotted wilt virus.

(Kang B-C et al. 2005. Annu Rev Phytopathol 43: 581-621)

Rx1, Rx2, Sw5, and Tm-22. Except for RTM1 and RTM2 discussed above, all of these cloned virus R genes share structural similarity. HRT, Rx1, Rx2, RCY1, Sw5, and Tm- 22 are Class 2 R genes, proteins that contain a region of leucine-rich repeats(LRRs), a putative nucleotide binding domain (NBS), and an N-terminal putative leucine-zipper (LZ), or other coiled-coil (CC) sequences. The N gene belongs to the Class 3 R gene family, which is similar to Class 2 but with a domain similar to the N terminus of the Toll and Interleukin 1 receptor (TIR) protein instead of the CC domain. Class 2 and Class 3 R proteins lack a transmembrane domain consistent with the intracellular location of viral avirulence factors. These genes define the plant viral pathosystems about which the most is known at the molecular and cellular levels (Figure).

Figure. Structure and location of the six main classes of plant disease resistance proteins. Virus resistance genes are indicated in bold letters. Classes 1–5 are defined based on combinations of a limited number of structural motifs. Class 6 includes R proteins that do not fit into classes 1–5. LRR, leucine-rich repeat; NBS, predicted nucleotide binding site; CC, predicted coiled coil domain; TIR, Toll and interleukin 1 receptor domain

(Kang B-C  et al. 2005. Annu Rev Phytopathol 43: 581-621)

Resistance to TobaccoMosaic Virus in Tobacco Conferred by N

The N gene, introduced into tobacco from Nicotiana glutinosa, is a single dominant gene for HR to TMV that defines a classic model system for plant-virus interaction and for the study of SAR. Below 28◦C, tobacco plants carrying the N allele develop necrotic local lesions within 48 h at the site of TMV inoculation. At higher temperatures, however, HR does not develop, and TMV spreads systemically throughout the plant. If a plant is initially infected at a temperature that allows systemic TMV infection and then subsequently moved to a lower temperature, a lethal systemic necrotic response is observed. The N gene was isolated by insertional mutagenesis using the activator (Ac) transposon system and confirmed by transgenic complementation.

 Deletion- and site-directed mutagenesis indicated that the TIR, NBS, and LRR domains are all required for proper function, although their role in HR is not known. Furthermore, N transcription is up-regulated by TMV infection, producing two transcripts via alternative splicing. Both translation products are necessary at an optimum ratio for resistance to be achieved. Transcriptional activation of several WRKY and MYB transcription factors also results from the N-TMV interaction. Rar1, SGT1, and EDS1, required for signal transduction mediated by most known R genes, are also required for the N gene–mediated resistance in tobacco. It is hypothesized that SGT1 and Rar1 associate with Hsp90 as cochaperones in the assembly or conformational regulation of N protein complex. The multiprotein complex, the COP9 signalosome, which physically interacts with SGT1, is also protein kinases (MAPK), a wound-inducible protein kinase (WIPK), and SA-inducible protein kinase (SIPK) are activated by an upstream MAPK kinase (MAPKK), NtMEK2. Silencing WIPK, SIPK, or NtMEK2 attenuates N gene resistance.

Figure. Domains of the N protein