The nature of the resistance in groundnut to rosette disease

Groundnut rosette disease is caused by a complex of three agents, groundnut rosette virus (GRV) and its satellite RNA, and groundnut rosette assistor virus (GRAV); the satellite RNA is mainly responsible for the disease symptoms. Groundnut genotypes possessing resistance to rosette disease were shown to be highly resistant (though not immune) to GRV and therefore to its satellite RNA, but were fully susceptible to GRAV.     

Different variants of the satellite RNA of groundnut rosette virus are responsible for the chlorotic and green forms of groundnut rosette disease*

Groundnut plants with symptoms of rosette disease contain groundnut rosette virus (GRV), but GRV is transmitted by Aphis craccivora only from plants that also contain groundnut rosette assistor virus (GRAV). Two main forms of rosette disease are recognised, ‘chlorotic rosette’ and ‘green rosette’. GRV cultures invariably possess a satellite RNA and this is the major cause of rosette symptoms: satellite-free isolates derived from GRV cultures from Nigerian plants with chlorotic or green rosette, or from Malawian plants with chlorotic rosette, induced no symptoms, or only transient mild mottle or interveinal yellowing, in groundnut. When the satellite RNA species from GRV cultures from Nigerian green or Malawian chlorotic rosette were reintroduced into the three satellite-free isolates in homologous and heterologous combinations, the ability to induce rosette symptoms was restored and the type of rosette induced was that of the cultures from which the satellite RNA was derived. Thus different forms of the satellite are responsible for the different forms of rosette disease. Other forms of the satellite induce only mild chlorosis or mottle symptoms in groundnut. Individual plants may contain more than one form of the satellite, and variations in their relative predominance are suggested to account for the variable symptoms (ranging from overall yellowing to mosaic) seen in some plants graft-inoculated with chlorotic rosette.     

A variant of the satellite RNA of groundnut rosette virus that induces brilliant yellow blotch mosaic symptoms in Nicotiana benthamiana

Some Malawian cultures of groundnut rosette virus (GRV) give rise to variants that, although still causing symptoms of the chlorotic type of rosette in groundnut, induce brilliant yellow blotch mosaic symptoms, instead of the usual veinal chlorosis and mild mottle, in Nicotiana benthamiana. One such isolate (YB) induced the formation in infected plants of a 0.9 kbp dsRNA having extensive sequence homology with molecules of similar size in other naturally occurring isolates of GRV. These dsRNA molecules were shown to be double-stranded forms of single-stranded satellite RNA molecules. Experiments in which the satellite was removed from and restored to isolate YB, or exchanged with those from other GRV isolates, showed that it carries the determinant for yellow blotch mosaic symptoms. Plants inoculated with the 0.9 kbp dsRNA (denatured or undenatured) developed yellow blotch mosaic even when the satellite-free GRV helper was not inoculated until 11 days later. The satellite RNA is therefore a very stable molecule. Prior infection of N. benthamiana with a GRV isolate containing a normal form of the satellite protected against expression of yellow blotch mosaic symptoms when the plants were later inoculated with isolate YB, whereas prior infection with satellite-free isolates did not. This provides a simple method of determining whether a GRV isolate has an associated satellite RNA. The YB satellite seems to be a newly recognised variant additional to those known to cause the chlorotic, green and other forms of groundnut rosette disease.     

GROUNDNUT ROSETTE DISEASE IN TANGANYIKA

Rosette disease of groundnuts in Tanganyika is brought into the crop by infective alatae of Aphis craccivora : spread within the crop is by apterae and alatae. During the dry season the aphids maintain themselves on self-set groundnuts and on two genera of Leguminosae: Vigna and Millettia or Lonchocarpus. No native source of the virus causing rosette disease has been discovered, but self-set groundnuts carry over the virus from one cropping season to the next. Syrphid larvae and other predators are important in controlling the vector. Preliminary spraying trials with 0.5% schradan gave promising results in controlling the aphids on groundnut crops and consequently checking the spread of rosette disease. Selections of the variety Mwitunde showed the lowest incidence of rosette infection and gave the highest yields in trials in 1952.     

Additional sources of resistance to groundnut rosette disease in groundnut germplasm and breeding lines

Groundnut rosette, a virus disease of groundnut (Arachis hypogaea) transmitted by the aphid, Aphis craccivora Koch, reduces yield in susceptible cultivars by 30–100%. Additional sources were sought in germplasm accessions involving 2301 lines from different sources and from 252 advanced breeding lines derived from crosses involving earlier identified sources of resistance to rosette. The lines were evaluated in field screening trials using an infector row technique during 1996 and 1997 growing seasons. Among the germplasm lines, 65 accessions showed high levels of resistance while 134 breeding lines were resistant. All rosette disease resistant lines were susceptible to groundnut rosette assistor virus. This work identified germplasm and breeding lines that will contribute to an integrated management of groundnut rosette disease. These new sources also provide an opportunity to eliminate yield losses due to the rosette disease.     

The Effect of Groundnut rosette assistor virus on the Agronomic Performance of Four Groundnut (Arachis hypogaea L.) Genotypes

The effect of Groundnut rosette assistor virus (GRAV), in the absence of the other two agents (Groundnut rosette virus and its satellite RNA) of the groundnut rosette disease virus complex, was evaluated on the agronomic performance of four groundnut (=peanut) genotypes (CG‐7, ICGV‐SM‐90704, JL‐24 and ICG‐12991) with different botanical characteristics. All genotypes infected with GRAV showed mild yellowing/chlorosis of leaves and the symptoms persisted throughout their growth period. ELISA absorbance values indicated lower amounts of GRAV antigen in ICGV‐SM‐90704 than in the other genotypes. The reduction in leaf area due to GRAV infection varied between 15.5% and 21.7%, whereas the plant height was decreased between 11.3% and 13.4% among the four genotypes. GRAV infection caused 28.4%, 16.9%, 21.7% and 25.5% reduction in the dry weight of haulms in CG‐7, ICGV‐SM‐90704, JL‐24 and ICG‐12991 respectively. Plants infected with GRAV showed greater reduction in seed weight in CG‐7 (52.2%), followed by JL‐24 (46.1%), ICG‐12991 (40.7%) and ICGV‐SM‐90704 (25.7%). These results provide evidence for the first time that GRAV infection, without GRV and sat RNA, affect plant growth and contribute to yield losses in groundnut.     

Sources of resistance to groundnut rosette disease in global groundnut germplasm*

About 6800 groundnut germplasm accessions originating from South America, Africa, and Asia were evaluated for resistance to rosette disease using an infector row technique between the 1990/91 and 1996/97 growing seasons. Of these, 116 germplasm accessions, including 15 short-duration Spanish types, have shown high levels of resistance to rosette disease. A high percentage of these resistant accessions were from West Africa and a few were from Asia and southern Africa. Only one out of 1400 accessions from South America showed resistance to rosette disease. All disease-resistant accessions were susceptible to groundnut rosette assistor virus. This is the first report to identify sources of resistance to rosette disease in groundnut germplasm from Asia and South America. These additional sources of resistance provide an opportunity to broaden the genetic base of resistance to rosette disease. The origins of rosette resistance in groundnut are discussed.     

幽影病毒引起的几种主要植物病害

幽影病毒的基因组不编码外壳蛋白,不形成通常的病毒粒体结构。这类病毒往往和黄症病毒复合侵染引起植物病害,蚜虫传播是病害在田间传播流行的主要方式。对幽影病毒引起的胡萝卜杂色矮缩病、花生丛簇病以及烟草丛顶病等几种主要病害的症状、发生与危害、病原物特性以及病害的控制等进行了综述。     

Deep sequencing reveals a novel closterovirus associated with wild rose leaf rosette disease

A bizarre virus‐like symptom of a leaf rosette formed by dense small leaves on branches of wild roses (Rosa multiflora Thunb.), designated as ‘wild rose leaf rosette disease’ (WRLRD), was observed in China. To investigate the presumed causal virus, a wild rose sample affected by WRLRD was subjected to deep sequencing of small interfering RNAs (siRNAs) for a complete survey of the infecting viruses and viroids. The assembly of siRNAs led to the reconstruction of the complete genomes of three known viruses, namely Apple stem grooving virus (ASGV), Blackberry chlorotic ringspot virus (BCRV) and Prunus necrotic ringspot virus (PNRSV), and of a novel virus provisionally named ‘rose leaf rosette‐associated virus’ (RLRaV). Phylogenetic analysis clearly placed RLRaV alongside members of the genus Closterovirus, family Closteroviridae. Genome organization of RLRaV RNA (17 653 nucleotides) showed 13 open reading frames (ORFs), except ORF1 and the quintuple gene block, most of which showed no significant similarities with known viral proteins, but, instead, had detectable identities to fungal or bacterial proteins. Additional novel molecular features indicated that RLRaV seems to be the most complex virus among the known genus members. To our knowledge, this is the first report of WRLRD and its associated closterovirus, as well as two ilarviruses and one capilovirus, infecting wild roses. Our findings present novel information about the closterovirus and the aetiology of this rose disease which should facilitate its control. More importantly, the novel features of RLRaV help to clarify the molecular and evolutionary features of the closterovirus.     

Detection of groundnut rosette umbravirus infections with radioactive and non-radioactive probes to its satellite RNA

A cloned cDNA copy of the satellite RNA of groundnut rosette virus (GRV), labelled with either ³²P or digoxigenin, was used as a probe to detect the satellite RNA in infected leaves. The test was successfully applied to N. benthamiana and to groundnuts, infected with isolates of GRV from East and West Africa and with isolates which cause different types of symptom in groundnuts, including one which is almost symptomless. Although the probe did not react with extracts from plants infected with GRV isolates from which the satellite RNA had been artificially eliminated, all naturally occurring GRV isolates contain the satellite RNA. The test therefore provides a reliable indicator of infection by GRV.     

Inheritance of resistance to groundnut rosette virus in groundnut (Arachis hypogaea L.)

A method of field screening groundnut seedlings for resistance to groundnut rosette virus (GRV), by means of which over 97% incidence was induced in rows of susceptible test plants, was developed at Chitedze Research Station in Malawi. Two GRV-resistant Virginia cultivars (RG 1 and RMP 40) were crossed with three susceptible cultivars, one from each of the Spanish (JL 24), Valencia (ICGM 48) and Virginia (Mani Pintar) botanical groups. Twelve F₁ reciprocal crosses and their F₂ and backcross generations were produced and the material screened in nurseries in 1985/86 and 1986/87. Seedlings raised from plants which did not become infected in the field were inoculated in the glasshouse in order to eliminate susceptible escapees. The numbers of diseased and healthy individuals in each population were subjected to χ² tests. In the majority of the F₂ populations a good fit was obtained for a ratio of one resistant to 15 susceptible plants, a ratio to be expected if resistance to GRV were determined by a pair of independent complementary recessive genes. This was further supported by data from backcross generations.     

Groundnut rosette and its assistor virus

Chlorotic rosette from Malawi (isolate CR1), passed through Stylosanthes gracilis and S. juncea, was not subsequently transmissible from groundnuts (Arachis hypogaea) by Aphis craccivora or A. gossypii, but with S. mucronata transmissibility was occasionally regained after a period of time. Aphid transmissibility was similarly lost after passage of two isolates (a chlorotic rosette from Rhodesia, CR2, and a green rosette from Nigeria, GR) through soybean (Soja max) and after manual inoculation to groundnuts. Groundnut plants that remained symptomless after exposure to rosette infection by aphids often contained a virus that restored aphid transmissibility when introduced into groundnuts containing the vectorless virus from that isolate. Groundnut rosette disease therefore consists of a symptom-inducing virus that we call groundnut rosette virus (GRV) and a symptomless assistor virus (GRAV) that must be present for aphid transmission. The interactions between the GRV and GRAV of chlorotic and green rosette, and their transmission by different vector races, are described.     

Studies on the transmission of groundnut rosette virus by Aphis craccivora Koch

Four strains of groundnut rosette virus were transmitted by a race of Aphis craccivora (Koch) from groundnut in Nigeria. Two of these strains, both from East Africa, were transmitted only by A. craccivora from Kenya. A fifth isolate, from Nigeria, was not transmissible by either race. The two races of aphids have been shown elsewhere to be distinct biotypes. Most A. craccivora needed longer than 24 h feeding on infected groundnuts to acquire virus, and many needed 2–3 days of feeding on healthy plants to cause infection, even after several days on infected plants. The delays partly reflect the slow uptake of virus and possibly a period needed for virus multiplication in aphid tissue but some is lost through resistance of the test plants to infection. In consecutive feeding experiments Natal Common variety could be infected soon after aphids had left the source of virus, but a more resistant Nigerian variety sometimes needed several more days. The frequency of inoculation by aphids, or the concentration of virus in the inocula or both, increased with time, but the times at which aphids were able to infect plants was also dependent on variety.     

Resistance in groundnut (Arachis hypogaea L.) to Aphis craccivora (Koch).

The behaviour, development and reproductive capacity of Aphis craccivora, vector of a number of groundnut viruses, are compared on a range of susceptible and resistant genotypes. Field trials demonstrated no significant difference between genotypes in the rate of arrival of alates, but population development was slower, and subsequent population decline faster, on the genotype EC 36892 (ICG 5240). Behavioural studies in the screenhouse, likewise showed no inhibition to alighting onto EC 36892 though choice tests demonstrated a significant redistribution of the population in favour of the susceptible genotype TMV 2 (ICG 221) over the following 10 h. In clip cage experiments, development was faster and nymphal numbers were higher on the genotype TMV 2 compared to EC 36892.     

Comparison of the coat protein of groundnut rosette assistor virus with those of other luteoviruses

The coat protein gene of groundnut rosette assistor virus (GRAV) was cloned and sequenced. The deduced amino acid sequences of the coat protein and of another protein encoded in a different, overlapping, reading frame resemble those of other luteoviruses. Four monoclonal antibodies against GRAV, prepared using denatured coat protein as immunogen, also reacted with some other luteoviruses in ELISA. Nevertheless, they will be useful as reagents for the identification of GRAV infections in groundnut.     

A Simple Procedure to Detect a dsRNA Associated with Groundnut Rosette

A low-molecular weight double-stranded (ds) RNA [900 base pairs (bp)] associated with groundnut rosette disease can be used as a diagnostic tool. A simple procedure has been developed that is rapid, reliable, and requires only standard electrophoresis equipment and ultraviolet light for detection of nucleic acid bands. Using this procedure, the dsRNA was detected only in groundnut plants with green rosette of chlorotic rosette symptoms. It was not found in uninoculated groundnut plants, in symptomless groundnut plants with groundnut rosette assistor virus, or in groundnut plants infected with several other known groundnut viruses. In studies with northern blots of extracts from rosette-diseased and healthy plants, 5′-endlabeled dsRNA only hybridized to a 900 bp dsRNA from diseased plants. The 900 bp dsRNA was not infectious and its origin remains obscure.     

A plant virus movement protein forms ringlike complexes with the major nucleolar protein, fibrillarin, in vitro

Fibrillarin, one of the major proteins of the nucleolus, has methyltransferase activity directing 2′-O-ribose methylation of rRNA and snRNAs and is required for rRNA processing. The ability of the plant umbravirus, groundnut rosette virus, to move long distances through the phloem, the specialized plant vascular system, has been shown to strictly depend on the interaction of one of its proteins, the ORF3 protein (protein encoded by open reading frame 3), with fibrillarin. This interaction is essential for several stages in the groundnut rosette virus life cycle such as nucleolar import of the ORF3 protein via Cajal bodies, relocalization of some fibrillarin from the nucleolus to cytoplasm, and assembly of cytoplasmic umbraviral ribonucleoprotein particles that are themselves required for the long-distance spread of the virus and systemic infection. Here, using atomic force microscopy, we determine the architecture of these complexes as single-layered ringlike structures with a diameter of 18-22 nm and a height of 2.0 ± 0.4 nm, which consist of several (n = 6-8) distinct protein granules. We also estimate the molar ratio of fibrillarin to ORF3 protein in the complexes as approximately 1:1. Based on these data, we propose a model of the structural organization of fibrillarin-ORF3 protein complexes and discuss potential mechanistic and functional implications that may also apply to other viruses.     

Agricultural diversification: the potential for underutilised crops in Africa's changing climates.

Two of the greatest challenges currently facing humanity are the potential consequences of climate change and the actual consequences of reduced agricultural diversity. This paper considers the consequences of both climate change and reduced agricultural diversity on global food security and nutrition. The inextricable link between climate change and crop diversity is examined, particularly in the context of crop production in Africa where most agricultural diversity exists and where climate change will have most impact. The Green Revolution, often seen as a model for increasing global agricultural productivity, is reconsidered in terms of its failure to make a significant impact in hostile tropical environments such as those of much of Africa. An alternative or, at least, a complementary strategy, is advocated where we might better harness the huge repository of indigenous plant species cultivated and conserved by local communities for many generations across variable climates. An example is given of multidisciplinary research on bambara groundnut (Vigna subterranea), an ancient grain legume grown, cooked, processed and traded mainly by subsistence women farmers in sub-Saharan Africa. The experience gained on bambara groundnut is considered as a basis for similar efforts on many other potentially useful underutilised food crops in the climates of the future.     

Development of microsatellite markers for bambara groundnut (Vigna subterranea L. Verdc.) — an underutilized African legume crop species

Bambara groundnut is an indigenous African legume crop plant. It is largely grown by subsistence farmers, but can also be used as a cash crop to supplement family income. The fact that it is highly drought tolerant means that it has considerably potential to provide part of food security in regions of the world where water availability is a serious issue. As one part of the international effort to understand and improve this species, we report here the first 10 microsatellite markers derived from bambara groundnut.     

Resistance to groundnut rosette disease in wild Arachis species

One hundred and sixteen accessions representing 28 species in the genus Arachis were evaluated for resistance to groundnut rosette disease using an infector row technique during the 1996/97, 1997/98, 1998/99 and 1999/2000 growing seasons at Chitedze, Malawi. Of these, a total of 25 accessions belonging to Arachis diogoi (1 accession), A. hoehnei (2), A. kretschmeri (2), A. cardenasii (2), A. villosa (1), A. pintoi (5), A. kuhlmannii (2), A. appressipila (3), A. stenosperma (5), A. decora (1), and A. triseminata (1) showed resistance to the groundnut rosette disease. No visible disease symptoms were observed in several accessions belonging to A. appressipila, A. cardenasii, A. hoehnei, A. kretschmeri, A. villosa, A. pintoi, A. kuhlmannii, and A. stenosperma. Some accessions in A. appressipila, A. diogoi, A. stenosperma, A. decora, A. triseminata, A. kretschmeri, A. kuhlmannii, and A. pintoi were resistant to all three components of rosette, Groundnut rosette ass is tor virus (GRAV), Groundnut rosette virus (GRV) and its satellite RNA (sat. RNA). Two accessions in A. stenosperma and one accession in A. kuhlmannii showed the presence of all three components of the rosette disease. Several wild Arachis accessions were resistant to GRAV. All the accessions of A. batizocoi (4), A. benensis (2), A. duranensis (46), A. dardani (1), A. ipaensis (1), A. magna (1), A. monticola (3), A. oteroi (1), A. pusilla (4), and A. valida (2) were susceptible to rosette disease. In all these accessions, infected plants were chlorotic and severely stunted. The value of exploitation of the resistance in wild Arachis species in rosette resistance breeding programmes is discussed.