Short-interfering RNAs (siRNAs), the molecular markers of posttranscriptional gene silencing (PTGS), are powerful tools that interfere with gene expression and counter virus infection both in plants and animals. Here, we report the effect of temperature on geminivirus-induced gene silencing by quantifying virus-derived siRNAs and by evaluating their distribution along the virus genome for isolates of five species of cassava geminiviruses in cassava (Manihot esculenta, Crantz) and Nicotiana benthamiana. Cassava geminivirus-induced RNA silencing increased by raising the temperature from 25°C to 30°C, with the appearance of less symptomatic newly developed leaves, irrespective of the nature of the virus. Consequently, nonrecoverytype geminiviruses behaved like recovery-type viruses under high temperature. Next, we evaluated the distribution of virusderived siRNAs on the respective virus genome at three temperatures (25°C, 25°C-30°C, and 30°C). For recovery-type viruses, siRNAs accumulated at moderately higher levels during virus-induced PTGS at higher temperatures, and there was no change in the distribution of the siRNA population along the virus genome. For nonrecovery-type viruses, siRNAs accumulated at strikingly higher levels than those observed for infections with recovery-type viruses at high temperature. As determined for an RNA virus, temperature influences gene silencing for single-stranded DNA geminiviruses. It is possible that other mechanisms besides gene silencing also control geminivirus accumulation at high temperatures. The findings presented here should be taken into consideration when implementing PTGS-based strategies to control plant virus accumulation.Posttranscriptional gene silencing (PTGS) involves sequence-specific suppression of gene expression in diverse eukaryotes. It was first discovered in plants (Napoli et al., 1990). A similar RNA-silencing phenomenon was observed in fungi, termed quelling (Cogoni and Macino, 1997), and in animals, termed RNA interference (Fire et al., 1998). In plants, PTGS serves as a natural antiviral defense response (Waterhouse et al., 2001). As a counter defense, viruses have evolved to encode a protein(s) that suppresses the host PTGS to establish infection in plants ( Vance and Vaucheret, 2001). Occasionally, certain viruses become targets of the induced PTGS, and, consequently, infected plants recover from virus infection (Ratcliff et al., 1997;Chellappan et al., 2004a). There are at least three different pathways in the gene-silencing mechanism: the cytoplasmic siRNA silencing, the endogenous mRNA silencing by microRNAs (miRNAs), and the transcriptional gene silencing by DNA methylation (Baulcombe, 2004). A unifying feature of RNA silencing is the cleavage of long double-stranded RNA (dsRNA) into short-interfering (21-24 nt) RNAs (siRNAs; Hamilton and Baulcombe, 1999) by a ribonuclease III-like enzyme termed DICER (Bernstein et al., 2001). Among the four Dicer-like (DCL) enzymes reported in plants (Schauer et al., 2002), DCL1 is involved in miRNA biogenesi...

A diagnostic survey was conducted in 2002-03 to determine the status of cassava mosaic begomoviruses in Nigeria and to ascertain if the virulent Ugandan variant of East African cassava mosaic virus (EACMV-Ug2) was present. Of the 418 farms visited, 48% had cassava with moderately severe or severe symptoms, whereas 52% had cassava with mild symptoms. These distributions were at random. Of the 1,397 cassava leaf samples examined, 1,106 had symptoms. In polymerase chain reaction tests, 74.1% of the symptom-bearing samples tested positive for African cassava mosaic virus (ACMV) alone, 0.3% for EACMV alone, 24.4% for mixed infections by the two viruses, and 1.2% did not react with any of the primers used. The two viruses also were detected in 32% of the 291 symptomless plants and in the whitefly vector samples. EACMV-Ug2, Indian cassava mosaic virus, and South African cassava mosaic virus were not detected in any of the whitefly or leaf samples. Most farms had ACMV in single infection as well as in mixed infections with EACMV. Most doubly infected plants showed severe symptoms. Two biological variants of ACMV were identified based on symptom expression on cassava in the field. ACMV and EACMV were detected in the leguminous plant Senna occidentalis (L.) Link and the weed Combretum confertum Lams.; these are new natural hosts of the viruses. Although the virulent EACMV-Ug2 was not detected, the occurrence of variants of ACMV and a high proportion of mixed infections by ACMV and EACMV, which could result in recombination events such as the one that produced EACMV-Ug2, demands appropriate measures to safeguard cassava production in Nigeria.

The identification of superior genotypes and mega-environments on the basis of multiple traits is a key objective of multi-environment trials in cassava (Manihot esculenta Crantz). The objective of this study was to examine the genetic variation and genotype 3 environment interaction (GEI) effects for fresh root yield, six other agronomic traits, and severity ratings for cassava mosaic disease (CMD) and cassava green mite (CGM) in 40 genotypes of cassava. Experiments were conducted using a randomized complete-block design with four replications for 2 yr in three representative agro-ecological zones in Nigeria. Site regression (SREG) analysis revealed that GEI was a major source of fresh root yield variation and the different testing sites discriminated among the genotypes. Genotypes TMS 98/0581, TMS 97/4763, TMS 98/ 0002, TMS 99/3073, and M98/0068 were highest yielding at Otobi and Umudike, whereas TMS 98/0510, TMS 97/4779, and TMS 92B/00068 yielded the most at Ishiagu. TMS 98/2226, TMS 92/0325, and M98/0028 had the poorest performance across all locations. Genotypes with the highest yield showed the lowest CMD scores, whereas very tall (well above 2 m) plants had low harvest index on the basis of multiple trait analysis. We identified optimally adapted genotypes for commercial cassava production in different areas in Nigeria.

Cassava mosaic disease (CMD) caused by African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV) is the major constraint to cassava production in Nigeria. Sequences of the DNA-A component of ACMV and EACMCV isolates from leguminous plant species (Senna occidentalis, Leucana leucocephala and Glycine max), castor oil plant (Ricinus communis), a weed host (Combretum confertum) and a wild species of cassava (Manihot glaziovii) were determined. All ACMV isolates from these hosts showed 96-98% nucleotide sequence identity with cassava isolates from West Africa. EACMCV was found only in four hosts (S. occidentalis, L. leucocephala, C. confertum, M. glaziovii), and sequences of these isolates showed 96-99% identity with cassava isolates from West Africa. These results provide definitive evidence for the natural occurrence of ACMV and EACMCV in plant species besides cassava.