Huanglongbing (HLB) is presently a major threat to the citrus industry. Because of this disease, millions of trees are currently dying worldwide. The putative causal agent is a motile bacteria belonging to Candidatus Liberibacter spp., which is transmitted by psyllids. The bacteria is responsible for the synthesis of callose at the phloem sieve plate, leading to the obstruction of the pores that provide connections between adjacent sieve elements, thus limiting the symplastic transport of the sugars and starches synthesized in leaves to the other plant organs. The Persian triploid lime (Citrus latifolia) is one of the most HLB-tolerant citrus varieties, but the determinants associated with the tolerance are still unknown. HLB-infected diploid Mexican lime (Citrus aurantiifolia) and Persian lime were investigated. The leaf petiole was analyzed using scanning electron microscopy (SEM) to observe callose deposition at the phloem sieve plate. Leaf starch contents and detoxification enzyme activities were investigated. In the field, Persian lime leaves present more limited symptoms due to HLB than the Mexican lime leaves do. Photosynthesis, stomatal conductance, and transpiration decreased compared with control plants, but values remained greater in the Persian than in the Mexican lime. Analysis of the petiole sieve plate in control petiole samples showed that pores were approximately 1.8-fold larger in the Persian than in the Mexican lime. SEM analyses of petiole samples of symptomatic leaves showed the important deposition of callose into pores of Mexican and Persian limes, whereas biochemical analyses revealed better detoxification in Persian limes than in Mexican limes. Moreover, SEM analyses of infected petiole samples of asymptomatic leaves showed much larger callose depositions into the Mexican lime pores than in the Persian lime pores, whereas biochemical traits revealed much better behavior in Persian limes than in Mexican limes. Our results reveal that polyploids present specific behaviors associated with important physiological and biochemical determinants that may explain the better tolerance of the Persian lime against HLB compared with the Mexican lime.

--Introduction. The severe impact of papaya bacterial canker in the West Indies justified launching a program to transfer the genetic resistance/tolerance identified in local germplasm to good-quality cultivars. As a first step, a germplasm collection from the Caribbean was gathered and its morphological and genetic diversity assessed. Materials and methods. The total sample included 50 accessions from the Lesser Antilles, Cuba, Venezuela, Costa Rica and Hawaii. Morphological data from a subsample of 29 accessions were submitted to analysis of variance, principal component analysis and Neighbor-Joining cluster analysis. Nine isozyme systems were tested, and the corresponding data were processed for cluster analysis and genetic parameters, including the fixation index, diversity indices and heterozygosity. Results. Eighteen of the 47 Caribbean and Venezuelan accessions exhibited hermaphroditism. Venezuelan accessions displayed the widest morphological diversity, while those from Barbados were distinguished by fruit quality. Sexual type affected leaf and flower traits. Cluster analysis shows little morphological differentiation between origins, with the relative grouping of the accessions from Guadeloupe or Barbados. Only four isozyme systems were polymorphic, producing 34 distinct zymotypes, with a relatively high heterozygosity and a particular diversity in Venezuela, Guadeloupe and Barbados. Geographic structuration appears limited in the cluster analysis among individuals; however, it is clearer when considering the six best-represented populations, with one cluster representing Venezuela, Trinidad and Barbados, and one for Guadeloupe, Martinique and Grenada. Discussion. The low level of fixation is attributed to dioecy and to the low level of selection. The relative isolation of island populations has favored geographic differentiation, albeit limited. The differentiation between the good-quality germplasm from Barbados and the tolerant populations from Martinique and Guadeloupe encourages the development of a marker-assisted introgression scheme.Caribbean / Venezuela / Carica papaya / germplasm / isoenzymes Organisation de la diversité morphologique et génétique des papayers des Caraïbes et du Vénézuéla.Resumé --Introduction. Pour lutter contre le chancre bactérien qui empêche le développement du papayer aux Antilles, un programme visant à transférer les facteurs de résistance génétique de génotypes locaux vers des cultivars commerciaux a été mis sur pied. Dans une première étape, une collection de germplasm caribéen a été réunie pour en étudier la diversité morphologique et génétique. Matériels et méthodes. L'échantillon comprend 50 accessions des Petites Antilles, Cuba, Venezuela, Costa Rica et Hawaï. Les données morphologiques d'un sous-échantillon de 29 accessions ont été soumises à l' analyse de variance, l'analyse en composantes principales et la classification par la méthode du Neighbor Joining. Neuf systèmes enzymatiques ont été testés et les données correspondantes ont été traitées par anal...

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Rootstocks are crucial for the sustainability of the citrus industry worldwide. Diploid intergeneric Citrus × Poncirus hybrids have contributed considerably to citrus rootstock improvement and their tetraploid (doubled-diploid) forms are important resources for the creation of a new generation of tetraploid rootstocks. To optimize the efficiency of tetraploid rootstock breeding strategies, more knowledge is required on inheritance in the allotetraploid genitors. A set of 159 new SNP markers that fully distinguish Poncirus trifoliata (L.) Raf. from Citrus species was developed from polymorphisms mined in GBS data and used to establish a genetic map of tetraploid citrumelo (C. × paradisi Macfad. × P. trifoliata) and to analyze the meiotic behavior of tetraploid citrumelo and citrandarin (C. reticulata Blanco × P. trifoliata). The tetraploid citrumelo genetic map was highly syntenic and collinear with the clementine reference genome. The apparent intergeneric recombination rate was strongly limited by high preferential chromosome pairing, resulting in intermediate inheritance with disomic tendency. Such inheritance, also observed in tetraploid citrandarin, results in the transmission by the diploid gametes of a high rate of intergeneric heterozygosity. It is therefore expected that a large part of the genetic value selected in the original diploid intergeneric rootstock is transmitted to the tetraploid sexual progenies.