The broad bean strain of Tobacco mosaicirus (TMV-B) infects Nicotiana tabacum White Burley systemically whereas the tomato strain of Tomato mosaicirus (ToMV-S1) induces necrotic local lesions and is restricted to inoculated leaves. To examine the possible role of the viral movement protein (MP) in these symptom differences, a chimaeric virus (T/OMP) was produced in which the TMV-B MP gene was replaced by the ToMV-S1 MP gene. T/OMP induced the same symptoms as TMV-B in N. tabacum White Burley. However, in N. tabacum Samsun NN and other plants containing the N resistance gene, T/OMP caused necrotic lesions that were smaller than those produced by TMV-B but similar in size to those of ToMV-S1. We conclude that ToMV MP gene can substitute functionally for the TMV-B MP gene, and that the MP gene influences the size of necrotic local lesions on N-containing hosts.

Summary. Virus isolate Y23V, obtained from squash showing leaf curl symptoms inYunnan, China, was readily differentiated from four studied Chinese begomovirus isolates in reactions with a set of monoclonal antibodies raised against begomoviruses. The complete nucleotide sequence (2714 nts) of the DNA-Alike molecule of Y23V was determined. The DNA-A of Y23V is most closely related to that of tomato yellow leaf curl Thailand virus-[1] (TYLCTHV-[1]) (84% sequence identity). However, the AC1 and AC4 gene of Y23V DNA-A resembled to Pepper leaf curl virus from Bangladesh (PepLCBDV). The DNA-A ofY23V has three distinct regions: the region from 74-2071 nts is 95% identical to TYLCTHV-[1] excluding a 27 nt deletion; the following 386 nts are 91% identical to PepLCBDV and the rest of the DNA-A is not closely related to any reported begomovirus.Y23V, therefore, is considered to have arisen by recombination. The 84%sequence identity ofY23VwithTYLCTHV-[1] allowsY23Vto be considered as a distinct begomovirus species, for which the name squash leaf curl Yunnan virus (SLCYNV) is proposed.

DNAβ is a type of single-stranded (ss) circular satellite DNA found in association with monopartite-genome begomoviruses, such as Tomato yellow leaf curl China virus isolate Y10 (TYLCCNV-Y10). Y10 DNAβ is required for symptom expression in plants but depends on TYLCCNV-Y10 genornic DNA (DNA-A) for replication and encapsidation. When we converted DNAβ into a gene-silencing vector (modified DNAβ (DNAmβ)) by replacing its C1 open-reading frame (ORF) with a multiple cloning site (MCS), it was replicated but no longer induced symptoms in association with TYLCCNV-Y10 DNA-A, so allowing theeffects of gene inserts to be recognized easily. Insertion into DNAmβ of sequences from any of the three host genes (proliferating cell nuclear antigen (PCNA), phytoene desaturase (PDS), and sulfur (Su), or from a transgene (green fluorescent protein (GFP), resulted in silencing of the cognate gene in Nicotiana benthamiana. The silencing persisted for more than a month and was associated with decreased levels of mRNA of the gene targeted. Although DNAmβ probably does not enter meristematic tissue, the PCNAgene could be silenced there. DNAmβ was an effective silencing vector in tested N. glutinosa, N. tabacum Samsun (NN or nn), and Lycopersicon esculentum plants, and was able to silence two genes simultaneously. This satellite DNA vector-based form of virus-induced gene silencing (VIGS) promises to be applicable to other begornovirus/DNAβ systems, which are recently reported to occur in several dicotyledonous crop species, thereby providing a powerful approach to gene discovery and the analysis of gene function in these crops.

Sugarcane mosaic virus (SCMV)was detected in all 62 maize samples collected from eight maize-growing provinces in China showing dwarf mosaic symptoms by immunocapture reverse-transcription polymerase chain reaction (RT-PCR). Maize dwarf mosaic virus (MDMV), Sorghum mosaic virus (SrMV) and Johnsongrass mosaic virus (JGMV), however, were not detected in any of the samples by RT-PCR. Eleven cDNA fragments of approximately 0.8 kilobases covering most of the coat protein (CP) gene of SCMV were sequenced and sequence analysis indicates that these eleven isolates share 98.1 to 100% identity at the amino acid level. Sequence comparison and phylogenetic analysis of the CP genes from the eleven Chinese isolates as well as 21SCMVsubgroup virus isolates indicate that the eleven Chinese virus isolates were closely related to SCMV with 97.0 to 98.1% sequence identity at the amino acid level, while relatively lower sequence identity was found with MDWV, SrMV or JGMV. The results indicate that the Chinese isolates are members of the SCMV species, and thus, SCMV can be considered as the most common and important potyvirus infecting maize in China.

Complete nucleotide sequences of the DNA-A-like molecules of three East African cassava mosaic virus (EACMV) isolates from Kenya (-K, 2801 nt) and Malawi (-MH and-MK, both 2804 nt) were determined. These sequences were compared with that published for a Tanzanian isolate (-T, 2801 nt) and the partial sequence of a third Malawian isolate. Intergenic region sequences of all isolates, and deduced amino acid sequences of their AC1 (Rep) proteins, each formed a tightly related cluster thatwas distinct from the comparable components of other begomoviruses. Other complementary-sense genes (AC2, AC3, AC4) differed between EACMV isolates in a way consistent with the accumulation of point mutations. In contrast, virus-sense genes (CP, AV2) of isolates-MH and-MK differed (substantially for AV2) from those of other EACMV isolates but somewhat resembled those of tomato yellow leaf curl virus-Israel, suggesting they had been acquired by recombination with an unidentified begomovirus.