Project topics and materials
Get Complete Project Material File(s) Now! »
Transmission and spread
Cassava mosaic geminiviruses (CMGs) are transmitted in a persistent manner by the whitefly, (Bemisia tabaci (Gennadius.) (Homoptera: Aleyrodidae) (Chant, 958; Dubern, 1979, 1994). The first demonstration that cassava mosaic disease (CMD) is transmitted by a whitefly of the genus Bemisia was made in what is now the Democratic Republic of Congo (DRC) by Kufferath and Ghesquiére (1932). Subsequent studies showed that B. tabaci is the sole vector and have refined the understanding of the transmission process through the qualification of the optimum acquisition, latent transmission and retention period (Durben, 1979, 1994). The nature of transmission was proven to be transtadial but not transovarial (Dubern, 1994), with minimum times for acquisition, latent period, and inoculation of 3.5 hrs, 3.5 hrs and 5-10 minutes, respectively. Transmission efficiency has varied from very low (0.15-1.7%) for field-collected insects (Fargette et al., 1985) to moderate (4-13%) for laboratory-reared insects (Durbern, 1994; Maruthi et al., 2002). Recent studies have suggested that there is only limited co-adaptation between virus and vector within Africa, as the frequencies of transmission of different CMGs by B. tabaci populations from geographically distant locations in Africa were not significantly different (Maruthi et al., 2002). The viral coat protein plays a predominant role in virus transmission (Roberts et al., 1984; Briddon et al., 1990). Specificity of whitefly-transmitted geminiviruses (WTGs) probably resides at the haemocoel/salivary gland barrier since a non-transmitting species of whitefly, Trialeurodes vaporariorum Westwood, can acquire ACMV in the haemocoel but not transmit it (Briddon et al., 1990). The rapidity with which CMGs are spread by the whitefly depends on the susceptibility/resistance of the varieties grown, sensitivities of the varieties grown, inoculum or infection pressure, phytosanitation measures and the extent to which CMGs are systemic within the infected plants (Fargette et al., 1994). Whitefly mobility is a key factor in the epidemiology of CMD. The higher incidence of severe CMD in the pandemic in eastern and central Africa is usually associated with high B. tabaci infestation in cassava (Legg, 1999; Ndunguru et al., 2003).
Symptom expression
CMD symptoms described by Storey (1936) included a well-marked mosaic pattern with pale chlorotic areas on leaves, severe stunting and leaf distortion. Less severe symptoms consisted of ill-defined mosaic patterns, green mosaic with slight or absent leaf distortion. Symptoms incited by CMGs vary from mild to severe depending on virus strain, isolate, species, cassava cultivars and environmental factors. In general, cassava plants expressing mild symptoms develop normally with leaves showing mild, light-green mosaic symptoms (Pita et al., 2001a; Fauquet and Fargette, 1990). Plants expressing severe symptoms display extreme shrinking of leaves, along with distortion at the bases of the leaflets and distinct chlorosis as is in the case of the severe strain (EACMV-UG2) or those caused by mixed infection of EACMV-UG and EACMV-UG2 (Pita et al., 2001a). Irrespective of the strains, there is no clear distinction in symptoms produced by ACMV and EACMV. However, mixed infection of the two has resulted in severe symptoms (Fondong et al., 2000; Pita et al., 2001b). The most visible symptom of CMD is the expression of a characteristic leaf mosaic, and young plants are more severely affected than old ones. Symptoms range from barely perceptible mosaic to stunting of plants and extreme reduction of the leaf blades (Fauquet and Fargette, 1990). However, variations in symptom expression and severity within the same cassava variety have been observed in Cameroon by Fondong et al. (1998) and in Uganda by Pita et al. (2001a). Studies conducted by Pascal et al. (1993) demonstrated that expression of BL1 gene (homologue of BC1) was responsible for the disease symptoms suggesting that BL1 may interfere with cell-to-cell movement in the vascular system. Nicotiana benthamiana L. plants inoculated with plasmid carrying BC1, and BV1 and BC1+BV1 genes from African cassava mosaic virus (ACMV) showed symptoms only when both BC1 and BV1 were present (von Arnim et al., 1993) confirming the role of DNA-B in efficient virus spread and symptom induction in N. benthamiana (Stanley, 1983). As a result of the foliar symptoms, the ability of the plant to synthesize food is reduced and there is no root bulking (Thresh et al., 1994).
Genomic organization
Cassava mosaic geminiviruses have a genome comprising two circular single-stranded (ss) DNAs (A and B) (Stanley and Gay, 1983; Harrison, 1985). The geminivirus DNAs are encapsidated in twinned (geminate) particles (Bock et al., 1978). The size of DNA-A and DNA-B is about 2.7-2.8 kbp each. DNA-A contains six genes or open reading frames (ORF) distributed between the virion (V) and complementary (C) sense strand of a doublestranded (ds) DNA intermediate (Stanley et al., 1986; Hong and Stanley, 1995). These genes are namely AC1, AC2, AC3 and AC4 on the complementary-sense strand and AV1 and AV2 on the virion-sense (Etessami et al., 1991; Morris et al., 1991; Townsend et el., 1985). DNA-B encodes two genes BC1 on the complementary-sense strand and BV1 on the virion-sense (Etessami et al., 1988) (Fig. 2.1). The two DNA components share only a common region (CR) of approximately 200 bp with high sequence identity of between 90 and 100% (Pita et al., 2001a). The CR contains promoter and sequence elements required for DNA replication and transcription (Zhan et al., 1991; Eagle et al., 1994; Laufs et al., 1995a; Chatterji et al., 1999). The common region is located in the intergenic region on both DNA components (Revington et al., 1989; Lazarowitz et al., 1992) The CR shows one potentially very stable hairpin structure between nucleotides 133 and 165, and contains a GC-rich inverted repeat that could form a stem loop structure, the loop of which is composed almost entirely of A and T residues (Stanley and Gay, 1983). An invariant AT-rich sequence 5-TAATATTAC in the loop is found in all geminivirus genomes (Revington et al., 1989; Lazarowitz et al., 1992).
CHAPTER 1 GENERAL INTRODUCTION
1. 1 References
CHAPTER 2 LITERATURE REVIEW
2.1 CASSAVA (MANIHOT ESCULENTA CRANTZ)
2.2 CASSAVA MOSAIC GEMINIVIRUSES
2.3 References
CHAPTER 3 RESTRICTION AND SEQUENCE ANALYSIS OF PCRAMPLIFIED VIRAL DNAs SUGGESTS THE EXISTANCE OF DIFFERENT CASSAVA MOSAIC GEMINI VIRUSES ASSOCIATED WITH CASSAVA MOSAIC DISEASE IN TANZANIA
3.1 Introduction
3.2 Materials and methods
3.3 Results
3.4 Discussion
3.5 Acknowledgements
3.6 References
CHAPTER 4 CHARACTERIZATION OF A NATURALLY-OCCURING TRUNCATED CASSAVA MOSAIC GEMINIVIRUS ASSOCIATED WITH CASSAVA MOSAIC DISEASE
4.1 Introduction
4.2 Materials and methods
4.3 Results
4.4 Discussion
4.5 Acknowledgements
4.6 References
CHAPTER 5 TWO NOVEL SATELLITE DNA MOLECULES ASSOCIATED WITH BIPARTITE CASSAVA MOSAIC BEGOMOVIRUSES ENHANCE SYMPTOMS AND BREAK RESISTANCE IN CASSAVA GERMPLASM
5.1 Introduction
5.2 Materials and methods
5.3 Results
5.4 Discussion
5.5 Acknowledgements
5.6 References
CHAPTER 6 MOLECULAR BIODIVERSITY OF CASSAVA BEGOMOVIRUSES IN TANZANIA: EVIDENCE FOR THE PRESENCE OF STRAINS OF EAST AFRICAN CASSAVA MOSAIC CAMEROON VIRUS AND RECOMBINATION IN CASSAVA GEMINIVIRUSES
6.1 Introduction
6.2 Materials and methods
6. 3 Results
6.4 Discussion
6.5 Acknowledgements
6.6 References
CHAPTER 7 USE OF PLANT DNA STORED ON FTA CARDS FOR RECOVERY AND MOLECULAR CHARACTERIZATION OF BIPARTITE AND MONOPARTITE GEMINIVIRUSES
7.1 Introduction
7.2 Materials and methods
7.3 ResultS
7.4 Discussion
7.5 Acknowledgements
7.6 References
CHAPTER 8 GENERAL DISCUSSION
8.1 References
SUMMARY
