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Ana Fita, Belén Picó, and Fernando Nuez

Roots are critical for plants to withstand environmental abiotic and biotic stresses. Wild taxa are often used as source of variation for improving root systems, as they are adapted to more stressful soil environments than their cultivated relatives. We studied the genetics of traits related to root biomass, root length, and root architecture (considering the primary/secondary and the tertiary root levels) in melon (Cucumis melo L.) in a 2-year assay by examining the root systems of mature plants in 91 F3 families derived from the cross between a wild accession, Pat 81 [C. melo ssp. agrestis (Naud) Pangalo], and a cultivated accession, `Piel de sapo' (C. melo ssp. melo L.). Despite the difficulties of working with adult plants, we found that Pat 81 and `Piel de sapo' differ greatly in their mature root systems, which is in concordance with the results previously obtained with young roots. Pat 81 developed roots with less biomass than `Piel de sapo', but this wild accession had more favorable root length and architectural traits: a higher density of framework roots, more uniformly distributed along the soil profile, longer laterals with a higher density of branches, and a higher number of root orders. This root structure is linked to a deeper rooting ability and to the capacity of exploiting a larger soil volume. The genetic analysis indicated that length and architectural traits are more stable than biomass traits, both between years and between developmental stages. Moderate to low broad- and narrow-sense heritabilites were found for root length and architectural traits, with most of the observed variation due to additive effects. Our results suggest that Pat 81 could be used as donor of valuable genes for increasing root length and improving the root architecture of cultivated melons, producing melons potentially more tolerant to soil stresses. The lack of phenotypic and genetic correlations between length and architectural parameters and root biomass suggest that root structure can be successfully improved without increasing carbon expenditures.

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María Ferriol, Belén Picó, and Fernando Nuez

Cucurbita maxima Duch. is one of the most morphologically variable cultivated species. The Center for Conservation and Breeding of the Agricultural Diversity (COMAV) holds a diverse germplasm collection of the Cucurbita genus, with more than 300 landraces of this species. Morphological and molecular characterization are needed to facilitate farmer and breeder use of this collection. With this aim, the morphological variation of a collection of 120 C. maxima accessions was evaluated. The majority of these accessions originated from Spain, which has acted as a bridge since the 16th century for spreading squash morphotypes between the Americas and Europe. South American landraces (the center of origin of this species) were also included. Eight morphological types were established based on this characterization and previous intraspecific classifications. A subset of these accessions, selected from these classification and passport data, was employed for molecular characterization. Two marker types were used; sequence related amplified polymorphism (SRAP), which preferentially amplifies open reading frames (ORF), and amplified fragment length polymorphism (AFLP). In the main, SRAP marker analysis grouped accessions in accordance to their type of use (agronomic traits) and AFLP marker analysis grouped accessions as to their geographical origin. AFLP marker analysis detected a greater genetic variability among American than among Spanish accessions. This is likely due to a genetic bottleneck that may have occurred during the introduction of squash into Europe. The disparity of the results obtained with the two markers may be related to the different genome coverage which is characteristic of each particular marker type and/or to its efficiency in sampling variation in a population.

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Ana Fita, Belén Picó, Antonio J. Monforte, and Fernando Nuez

The study of the genetic control of natural variation in the root architecture of Cucumis melo L. is complex due to the difficulties of root phenotyping and to the quantitative nature of root traits and their plasticity. A library of near-isogenic lines (NILs), constructed by introgressing the genome of the exotic Korean accession Shongwan Charmi [SC (PI161375)] into the genetic background of the cultivar Piel de Sapo (PS) has recently become available. In this work, we used this population to identify quantitative trait loci (QTLs) controlling variation in root growth and architecture. We studied separately the primary root and the secondary and tertiary root systems during a 15-day period. Heritabilities for the root traits were moderate. Correlation and principal component analysis showed independence among traits measuring root length and root branching level, indicating the possibility of modifying both traits independently. PS and SC clearly differed in plant size. Significant allometric relationships between vine biomass and some root traits were identified. The use of NILs with similar plant size of PS allowed us to avoid the inaccuracies caused by size-dependent variation of root traits. A total of 17 QTLs for root traits in seven linkage groups were identified: three QTLs for primary root length, three QTLs for the diameter of the primary root, three QTLs for secondary root density, three QTLs for the average length of the secondary roots, three QTLs for the percentage of secondary roots bearing tertiary roots, and two QTLs for tertiary root density. In most of these traits, transgressive variation was observed.

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Andrés Cáceres, Gorka Perpiña, María Ferriol, Belén Picó, and Carmina Gisbert

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Ana Fita, Belén Picó, Rita C.S. Dias, and Fernando Nuez

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Ana Fita, Cristina Esteras, Belén Picó, and Fernando Nuez

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Juan J. Ruiz, Santiago García-Martínez, Belén Picó, Muquiang Gao, and Carlos F. Quiros

We studied the genetic variability of some traditional tomato (Lycopersicon esculentum L. Mill.) cultivars of Spain, and established their relationships using both simple sequence repeats (SSR) and sequence related amplified polymorphism (SRAP) markers. These included cultivars from different locations of three main types, Muchamiel, De la pera, and Moruno. Additionally we tested two other local cultivars, `Valenciano' and `Flor de Baladre', plus a small sample of commercial cultivars and a few wild species. Both types of markers resolved the cultivars from different groups, but SSR failed to distinguish some of those classified under the same group. All the De la pera cultivars clustered together by genetic similarity with the SRAP markers. The other traditional cultivars, which are grown in a wider geographic range, formed a more diffuse group, which included the commercial cultivar Roma. The Mexican cultivar Zapotec, a breeding line, and the virus-resistant commercial hybrid `Anastasia' were the most distant of all the cultivars. The latter hybrid had higher similarity to the wild species due to introgressed segments from them carrying the resistance genes. Similar results were observed for SSR markers but with a lower level of resolution. This information would be useful to facilitate tomato germplasm conservation and management efforts.

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Gorka Perpiñá, Jaime Cebolla-Cornejo, Cristina Esteras, Antonio J. Monforte, and Belén Picó

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Rita de C.S. Dias, Belén Picó, Javier Herraiz, Amparo Espinós, and Fernando Nuez

Vine decline is limiting muskmelon production in many growing areas. Monosporascus cannonballus Pollack and Uecker and Acremonium cucurbitacearum Alfaro-García, W. Gams, and J. García-Jiménez are the main causal agents of this disease in Spain. The wild accession Pat81 (Cucumis melo subsp. agrestis Jeffrey) has shown to be highly resistant in naturally infested fields and after artificial inoculations. In three greenhouse experiments conducted over two seasons, the root structure of Pat81 was examined and compared to the highly susceptible commercial cultivar Amarillo Canario (AC). Pat81 produced a more vigorous, branched, and longer root system, conferring to this accession a higher capacity for the uptake of water and nutrients, even after inoculation using naturally infested soil. To determine the plasticity of the root systems, the effect of five different soil substrates on root growth was assayed. The root morphology was highly influenced by the soil substrate. Differences between genotypes appeared at 10 weeks after transplanting using sand as soil substrate. An organic substrate made up of well-decomposed peat and sand minimized the genotype × substrate interactions, and facilitated root analysis. This substrate allowed bringing the sampling date forward to flowering (at 7 weeks after transplanting). The maximum root length, the number and size of lateral roots (diameter 0.5-1 mm) and branching order, consistently differed between the two genotypes in most of the assayed substrates. These easily measurable root traits can be used as selection criteria in healthy soils to breed a larger root system more tolerant to stress. In addition, in inoculated soils the greater root absorbent area and the reduced lesion intensity of Pat81 could have applications to increase vine decline resistance of cultivated melons. By using segregant populations derived from the cross AC × Pat81, we are trying to modify the root structure of muskmelon in order to offer a genetic alternative to the expensive strategy of grafting muskmelon varieties onto rootstocks resistant to soil stresses.

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Juan J. Ruiz, Belen Pico, Genyi Li, Vincent D'Antonio, Bryce Falk, and Carlos F. Quiros

Resistance to Celery mosaic virus (CeMV) in celery [Apium graveolens L. var. dulce (Mill.) Pers.] is recessive and determined by the single gene, cmv. We report discovery of two polymerase chain reaction-based dominant markers tightly linked to cmv in segregating F2 and BC1 populations. Marker me1em2 is associated to the dominant (susceptibility allele) and the second marker, me8em2, to the recessive (resistance allele). Simultaneous screening for both markers in segregating populations allows for identification of both homozygous and heterozygous genotypes for disease resistance. This marker system can be used for early seedling selection, which will simplify and speed development of celery cultivars resistant to CeMV.