No abstract

Citrus leprosis is considered the main viral disease for the Brazilian citrus production, particularly for the State of São Paulo, due to the high costs spent for the chemical control of its vector, the tenuipalpid mite Brevipalpus phoenicis. In addition, its global importance has significantly increased in the last years, with the dissemination of the virus to new countries in South and Central America. In Brazil, despite its economical importance and occurrence for more than seven decades, the most significant advances towards understanding the pathosystem interactions have been obtained only in the last ten years. This review focuses on various aspects of the disease, beginning with a historical view, its main characteristics, alternatives for its control, its increasing economical importance in Brazil and abroad, and the new data on the search for understanding the interactions amongst the mite vector, the virus, and the plant host.

Nearly 65,000 citrus EST (Expressed Sequence Tags) have been investigated using the CitEST project database. Microsatellites were investigated in the unigene sequences from Citrus spp. and Poncirus trifoliata. From these sequences, approximately 35% of the non-redundant ESTs contained SSRs. The frequencies of different SSR motifs were similar between Citrus spp and trifoliate orange. In general, mononucleotide repeats appeared to be the most abundant SSRs in the CitEST database, but we also identify di-, tri-, tetra-, penta-and hexanucleotide repeats. The AG/CT and AAG/CTT were the most common dinucleotide and trinucleotide motifs, with frequencies of 54.4% and 25.2%, respectively. Primer sequences flanking SSR motifs were successfully designed and synthesized. After in silico polymorphism analysis, a subset of sixty-eight primers was validated in different Citrus spp. and Poncirus trifoliata. PCR-amplification revealed polymorphism in citrus with all tested primer pairs and showed the potential of these markers for linkage mapping. Our study showed that the CitEST database can be exploited for the development of SSR markers that can amplify Citrus spp. and related genus for comparative mapping and other genetic analyses.

Obtaining new rootstocks that have resistance to biotic and abiotic factors is one of the main goals of breeding programs for citrus. This study evaluated the performance of 42 hybrids of Sunki mandarin (Citrus sunki Hort. ex Tanaka) vs. Poncirus trifoliata cv. Rubidoux, as rootstock for Pera sweet orange. The experiment was conducted in Colômbia, São Paulo, Brazil, in randomized blocks with three replications in a spacing of 6.0 m x 3.5 m. The trees were seven years old and the experiment was conducted without irrigation. We quantified the variables height, diameter and canopy volume, and production of plants, besides the physico-chemical analysis of fruits. After the natural period of drought in the region, the trees were evaluated for resistance to drought, with scale ranging from 1 to 3. Plants were also evaluated for symptoms of citrus sudden death and compatibility canopy/rootstock. Differences were observed in height, diameter and canopy volume of Pera sweet orange grafted on 42 hybrids of Sunki mandarin x Poncirus trifoliata (TSxPT). Differences were observed in the physico-chemical characteristics of fruits of Pera sweet orange grafted on different hybrids TSxPT. Different degrees of drought tolerance were observed and six hybrids were resistant. The hybrids TSxPT 245 and 254 showed incompatibility with variety of Pera sweet orange.

The randomly amplified polymorphic DNA (RAPD) technique was used to distinguish nucellar and zygotic seedlings resulting from crosses between the ‘Montenegrina’ (Citrus deliciosa Tenore) and‘ King’ (C. nobilis Loureiro) tangerines. The aim of the present study was to develop tangerine varieties with a reduced number of seeds and organoleptic characteristics similar to the ‘Montenegrina’ tangerine. Embryos were isolated from seeds, cultivated in vitro, and acclimated in a greenhouse. Four random primers were used to identify 54 plants of sexual origin from a total of 202 individuals. The degree of polymorphism of each primer was reflected in the number of zygotic plants obtained per primer. Cluster analysis of parents and progeny separated the individuals into distinct groups with a maximum genetic dissimilarity of 20%.
A técnica RAPD (random amplified polymorphic DNA) foi utilizada para distinguir plântulas nucelares e zigóticas resultantes do cruzamento entre as tangerineiras ‘Montenegrina’ (Citrus deliciosa Tenore) e ‘King’ (C. nobilis Loureiro). Este cruzamento foi realizado objetivando a obtenção de variedades de tangerineiras com características organolépticas de fruto semelhantes à tangerina ‘Montenegrina’ e menor número de sementes. Embriões foram isolados das sementes e cultivados in vitro e aclimatizados em casa de vegetação. Utilizando-se de 4 primers de seqüência randômica foram identificadas 54 plantas de origem sexual de um total de 202 indivíduos. O grau de polimorfismo de cada primer refletiu no número de plantas zigóticas obtidas por primer, sendo o total de zigóticos identificados pela soma das informações geradas pelos 4 primers. Análise de agrupamento com os parentais e a progênie separou os indivíduos em grupos distintos com uma dissimilaridade genética máxima de 20%

A muda cítrica é o insumo mais importante na for

Although the citriculture is one of the most important economic activities in Brazil, it is based on a small number of varieties. This fact has contributed for the vulnerability of the culture regarding the phytosanitary problems. A higher number of varieties/genotypes with potential for commercial growing, either for the industry or fresh market, has been one of the main objectives of citrus breeding programs. The genetic breeding of citrus has improved, in the last decades, due to the possibility of an association between biotechnological tools and classical methods of breeding. The use of molecular markers for early selection of zygotic seedlings from controlled crosses resulted in the possibility of selection of a high number of new combination and, as a consequence, the establishment of a great number of hybrids in field experiments. The faster new tools are incorporated in the program, the faster is possibility to reach new genotypes that can be tested as a new variety. Good traits should be kept or incorporate, whereas bad traits have to be excluded or minimized in the new genotype. Scion and rootstock can not be considered separately, and graft compatibility, fruit quality and productivity are essential traits to be evaluated in the last stages of the program. The mapping of QTLs has favored breeding programs of several perennial species and in citrus it was possible to map several characteristics with qualitative and quantitative inheritance. The existence of linkage maps and QTLs already mapped, the development of EST and BAC library and the sequencing of the Citrus complete genome altogether make very demanding and urgent the exploration of such data to launch a wider genetic study of citrus. The rising of information on genome of several organisms has opened new approaches looking for integration between breeding, genetic and genome. Genome assisted selection (GAS) involves more than gene or complete genome sequencing and is becoming an import support in breeding programs of annual and perennial species. An huge information amount can be derivate from genome analysis. The use and benefit of such informations will depend on the genetic basis of the breeding program.

The genetic inheritance of resistance to leprosis, the most important viral disease of citrus in Brazil, was characterized through the phenotypic assessment of 143 hybrids resulting from crosses between tangor 'Murcott' (Citrus sinensis x C. reticulata) and sweet orange 'Pêra' (C. sinensis), considered to be resistant and susceptible to the disease, respectively. All plants were grafted onto Rangpur lime (C. limonia) and inoculated with Citrus leprosis virus, cytoplasmic type through the infestation with viruliferous mites, Brevipalpus phoenicis. The experiments were arranged in a completely randomized block design with 10 replicates. Incidence and severity of the disease in leaves and stems as well as plant growth parameters (plant height and stem diameter) were recorded for 3 years after the infestation with the viruliferous mites. The average values of all variables were analyzed using principal component analysis, discriminant factorial analysis, estimation of the clonal repeatability coefficients, and frequency of the distributions of the average values for each measured variable. The principal component analysis resulted in the identification of at least two groups with resistance and susceptibility to leprosis, respectively. About 99% of all hybrids were correctly classified according to the discriminant factorial analysis. The broad-sense heritability coefficients for characteristics associated with incidence and severity of leprosis ranged from 0.88 to 0.96. The data suggest that the inheritance of resistance to leprosis may be controlled by only a few genes.