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Rice cultivar Taichung Native 1 (TN1) is susceptible to Rice tungro spherical virus (RTSV). TW16 is a backcross line developed between TN1 and RTSV-resistant cultivar Utri Merah. RTSV accumulation in TW16 was significantly lower than in TN1, although both TN1 and TW16 remained asymptomatic. We compared the gene expression profiles of TN1 and TW16 infected by RTSV to identify the gene expression patterns accompanying the accumulation and suppression of RTSV. About 11% and 12% of the genes in the entire genome were found differentially expressed by RTSV in TN1 and TW16, respectively. About 30% of the differentially expressed genes (DEGs) were detected commonly in both TN1 and TW16. DEGs related to development and stress response processes were significantly overrepresented in both TN1 and TW16. Evident differences in gene expression between TN1 and TW16 instigated by RTSV included (1) suppression of more genes for development-related transcription factors in TW16; (2) activation of more genes for development-related peptide hormone RALF in TN1; (3) TN1- and TW16-specific regulation of genes for jasmonate synthesis and pathway, and genes for stress-related transcription factors such as WRKY, SNAC, and AP2-EREBP; (4) activation of more genes for glutathione S-transferase in TW16; (5) activation of more heat shock protein genes in TN1; and (6) suppression of more genes for Golden2-like transcription factors involved in plastid development in TN1. The results suggest that a significant number of defense and development-related genes are still regulated in asymptomatic plants even with a very low level of RTSV, and that the TN1- and TW16-specific gene regulations might be associated with regulation of RTSV accumulation in the plants.

Rice tungro disease (RTD) is caused by Rice tungro spherical virus (RTSV) and Rice tungro bacilliform virus (RTBV) transmitted by green leafhoppers. Rice cv. Utri Merah is highly resistant to RTD. To define the RTD resistance of Utri Merah, near-isogenic lines (NIL, BC5 or BC6) developed from Utri Merah and susceptible cv. Taichung Native 1 (TN1) were evaluated for reactions to RTSV and RTBV. TW16 is an NIL (BC5) resistant to RTD. RTBV was able to infect both TN1 and TW16 but the levels of RTBV were usually significantly lower in TW16 than in TN1. Infection of RTSV was confirmed in TN1 by a serological test but not in TW16. However, the global gene-expression pattern in an RTSV-resistant NIL (BC6), TW16-69, inoculated with RTSV indicated that RTSV can also infect the resistant NIL. Infection of RTSV in TW16 was later confirmed by reverse-transcription polymerase chain reaction but the level of RTSV was considerably lower in TW16 than in TN1. Examination for virus accumulation in another NIL (BC6), TW16-1029, indicated that all plants of TW16-1029 were resistant to RTSV, whereas the resistance to RTBV and symptom severity were segregating among the individual plants of TW16-1029. Collectively, these results suggest that RTD resistance of Utri Merah involves suppression of interacting RTSV and RTBV but the suppression trait for RTSV and for RTBV is inherited separately.

Artificial mutations may induce traits that are scarce among natural germplasm sources. This study was conducted to characterize a rice line derived from variety IR64 showing resistance to rice tungro disease (RTD). Approximately 24,000 lines derived from IR64 seeds treated with mutagens were evaluated for reaction to RTD. One of the lines, M4D6 83-1 (MD83), showed enhanced resistance to RTD. MD83 was resistant to rice tungro spherical virus (RTSV) and the virus vector, green leafhopper (GLH; Nephotettix virescens Distant). MD83 was crossed with susceptible varieties IR64 and Taichung Native 1. All F 1 plants were susceptible to RTSV, whereas F 2 and F 3 progenies segregated into resistant and susceptible phenotypes in a 1:3 ratio, indicating that the resistance is controlled by a single recessive locus. Most of the F 3 lines resistant to RTSV were also GLH resistant, suggesting that the resistance to RTSV and GLH was governed by the same locus. It has been difficult to prove whether the genotype MD83 was a product of a mutation event or was represented in the variety IR64 at a very low frequency.

Rice tungro disease (RTD) is caused by the interaction between Rice tungro spherical virus (RTSV) and Rice tungro bacilliform virus (RTBV), both of which are transmitted by green leafhoppers (GLH). In order to define the resistance against RTD in rice cv. Matatag 9 which was developed by interspecific hybridization between RTD-susceptible cv. IR64 and Oryza rufipogon, the reactions of Matatag 9 to the viruses and GLH were evaluated in comparison with RTD-susceptible and -resistant rice cultivars. The incidences of infection with RTSV and RTBV in Matatag 9 were significantly lower than those in the susceptible parent cv. IR64; however, no substantial differences in virus accumulation were observed between IR64 and Matatag 9 once infected with the viruses. Symptoms in Matatag 9 infected with RTBV and RTSV were milder than those observed in IR64. A higher level of antixenosis to GLH was observed in Matatag 9 compared with IR64. The levels of antibiosis against GLH in Matatag 9 were comparable with those in another GLH-resistant cultivar, and significantly higher than those in RTD-susceptible cultivars. Collectively, these results suggest that tolerance to tungro viruses and resistance to GLH both contribute to the apparent resistance to RTD in Matatag 9, although possible involvement of other resistance mechanisms cannot be excluded.

The accession of Oryza glaberrima (IRGC Acc. no. 100139) was identified and evaluated as an indicator plant for rice tungro spherical virus (RTSV), a latent virus of rice. This accession was susceptible and sensitive to RTSV infection. When 7‐day‐old seedlings were inoculated with viruliferous leafhopper Nephotettix virescens the infected plants showed stunting, reduced tillering, and pale green leaves 3–4 weeks after inoculation. The visual evaluation of infected plants corresponded well with the serological assay. Accession 100139 could also be used to distinguish plants infected either with rice tungro bacilliform virus (RTBV) or both RTBV and RTSV, based on characteristic symptoms and the life span of infected plants. The indicator plant would be useful in studying the epidemiology of rice tungro viruses without the need for expensive serological assays

The sequence of 5,028 nucleotides, including one open reading frame (ORF), of rice tungro bacilliform virus (RTBV) dsDNA was determined. The predicted translational product comprises 1,675 amino acids and has Mr of 194,134 (p194). The amino acid sequences of three tryptic fragments from the 32 k capsid protein of RTBV (p32) were found in the predicted translational product indicating that the ORF codes for the RTBV capsid protein.

Rice tungro disease (RTD) is a serious constraint to rice production in South and Southeast Asia. RTD is caused by Rice tungro spherical virus (RTSV) and Rice tungro bacilliform virus. Rice cv. Utri Merah is resistant to RTSV. To identify the gene or genes involved in RTSV resistance, the association of genotypic and phenotypic variations for RTSV resistance was examined in backcross populations derived from Utri Merah and rice germplasm with known RTSV resistance. Genetic analysis revealed that resistance to RTSV in Utri Merah was controlled by a single recessive gene (tsv1) mapped within an approximately 200-kb region between 22.05 and 22.25 Mb of chromosome 7. A gene for putative translation initiation factor 4G (eIF4Gtsv1) was found in the tsv1 region. Comparison of eIF4Gtsv1 gene sequences among susceptible and resistant plants suggested the association of RTSV resistance with one of the single nucleotide polymorphism (SNP) sites found in exon 9 of the gene. Examination of the SNP site in the eIF4Gtsv1 gene among various rice plants resistant and susceptible to RTSV corroborated the association of SNP or deletions in codons for Val1060-1061 of the predicted eIF4Gtsv1 with RTSV resistance in rice.

The genome segment 12 (S12) of rice dwarf phytoreovirus (RDV) isolated from the Philippines (RDV-P) and of a variant (RDV-S-6) of RDV severe strain (RDV-S) migrated abnormally slower during polyacrylamide gel electrophoresis than that of the isolate maintained at Hokkaido University (RDV-H). Nucleotide sequence analysis revealed that rearrangement had occurred in these segments, affecting the open reading frame. A polypeptide encoded by S12 (Pns12) of RDV-P had a duplication of 28 amino acids while 1/3 of the carboxyl terminus of Pns12 was deleted in RDV-S-6 by premature termination due to a frameshift. RDV-S is always present in plants infected with the RDV-S-6 variant, suggesting that Pns12 of RDV-S-6 is defective. On the other hand, Pns12 of RDV-P was expressed and appeared to be functional in infected cells in spite of the duplication, as demonstrated by immunoblot analyses using antibody raised against Pns12 expressed inEscherichia coli.

The complete nucleotide sequence of rice tungro spherical virus (RTSV) strain Vt6, originally from Mindanao, the Philippines, with higher virulence to resistant rice cultivars, was determined and compared with the published sequence for the Philippine-type strain A (RTSV-A-Shen). It was reported that RTSV-A was not able to infect a rice resistant cultivar TKM 6 (10). RTSV-Vt6 and RTSV-A-Shen share 90% and 95% homology at nucleotide and amino-acid levels, respectively. The N-terminal leader sequence of RTSV-Vt6 contained a 39-amino acids-region (positions 65 to 103) which was totally different from that of RTSV-A-Shen; the difference resulted from frame shifting by nucleotide insertions and deletions. To confirm the amino-acid sequence differences of the leader polypeptide, the same region was cloned and sequenced using a newly obtained variant of RTSV-type 6, which had been collected in the field of IRRI, and seven field isolates from Mindanao, the Philippines. Since all the sequences of the target region are identical to that of the Vt6 leader polypeptide, the sequence difference in the leader region seems not to correlate with the virulence of Vt6.

Rice tungro bacilliform virus (RTBV) is one of the two viruses that cause tungro disease. Four RTBV strains maintained in the greenhouse for 4 years, G1, G2, Ic, and L, were differentiated by restriction fragment length polymorphism (RFLP) analysis of the native viral DNA. Although strains G1 and Ic had identical restriction patterns when cleaved with Pst1, BamHI, EcoRI, and EcoRV, they can be differentiated from strains G2 and L by EcoRI and EcoRV digestion. These same endonucleases also differentiate strain G2 from strain L. When total DNA extracts from infected plants were used instead of viral DNA, and digested with EcoRV, identical restriction patterns for each strain (G2 and L) were obtained from roots, leaves, and leaf sheaths of infected plants. The restriction patterns were consistent from plant to plant, in different varieties, and at different times after inoculation. This technique can be used to differentiate RTBV strains and determine the variability of a large number of field samples.

Tungro is a composite disease of rice induced by dual infection with rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). Four strains of RTBV, designated L, G1, G2, and Ic, were isolated on the basis of their characteristic symptoms on rice cultivars FK135 and Taichung Native 1 (TN1). On FK135, strain Ic caused severe stunting, reduced tillering, narrow leaves, and distinct interveinal chlorosis (striping and/or mottling) on the leaf blades, while G1 and G2 caused only mild stunting and foliage was a normal green. On TN1, G1 and Ic caused mild stunting but no discoloration of the foliage; G2 caused severe stunting and yellow to orange discoloration similar to that caused by the type strain L. Thus, G1 and Ic can be readily differentiated by the reaction of cultivar FK135, while G1 and G2 can be differentiated by that of TN1. Cross-inoculation experiments showed that strains G1 and Ic were cross protective. In other studies, a virulent strain of RTSV (Vt6) was isolated from tungro-infected plants collected from Mindanao, Philippines. The cultivar TKM6, like many other cultivars resistant to type strain A, was susceptible to strain Vt6. TKM6 plants infected with both RTBV and strain Vt6 showed severe stunting and discoloration

The polymerase chain reaction (PCR) was 103-104 times more sensitive for detecting rice tungro bacilliform virus (RTBV) DNA extracted from infected rice plants than was enzyme-linked immunosorbent assay (ELISA). The greater sensitivity of PCR enabled the detection of the virus in individual RTBV-exposed leafhoppers, Nephotettix virescens, a semipersistent vector of RTBV. Detection of RTBV in the vector by ELISA has been impossible. To evaluate resistance to RTBV, seedlings of rice cultivars Utri Merah, Utri Rajapan, or Balimau Putih, were inoculated with RTBV, and infection of the cultivars with the virus was indexed by PCR and ELISA [...]

Nucleic acids and proteins of rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV) were analyzed by gel electrophoresis. RTSV contains single-stranded RNA of 3.7×10 6 Da and two major proteins of 24 and 23 kDa. RTBV contains circular double-stranded DNA of 8.3 kbp and a single major protein of 32 kDa. The RTBV-DNA is 2.1-2.3 μm in length and has two discontinuities, one in each of the two strands, having different polarities. Endonuclease cleavage sites in the RTBV-DNA molecule are mapped (...)