Somatic embryogenesis, micropropagation and plant regeneration of “Early Mature” walnut trees (Juglans regia) that flower in vitro

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Tree Physiology, 24:425–435

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Somatic embryogenesis, micropropagation and plant regeneration of “Early Mature” walnut trees (Juglans regia) that flower in vitro

Christian Breton (1, 2), Daniel Cornu (1), Dominique Chriqui (3), Annie Sauvanet (3), Pierrette Capelli (1), Eric Germain (4) and Christian Jay-Allemand (1, 5)

1. Unité Amélioration Génétique et Physiologie Forestières, Institut National de la Recherche Agronomique, BP 20619, F-45166 Olivet Cedex, France / 2. Corresponding address (christian.breton@orleans.inra.fr) / 3. Laboratoire CEMV, Université Pierre et Marie Curie, Bât. N2, 4 Place Jussieu, F-75252 Paris Cedex 05, France / 4. Unité de Recherche sur les Espèces Fruitières et la Vigne, Institut National de la Recherche Agronomique, BP 81, F-33883 Villenave d’Ornon Cedex, France / 5. Université Montpellier 2, Biochimie et Physiologie Végétale, Place Eugène Bataillon, Case 024, F-34095 Montpellier Cedex 05, France / Received May 5, 2003; accepted August 3, 2003; published online February 2, 2004

Summary

Some walnut trees (Juglans regia L.) originating from central Asia display an early flowering phenotype. These “Early Mature” (EM) trees may produce flowers within months of germination. Secondary flowering waves are also observed within a growing season. Inflorescences may carry male, female and hermaphrodite flowers. Progeny obtained from selected EM trees were cultured in vitro to initiate clonal propagation of these genotypes. Embryogenic lines were established through the culture of immature zygotic embryos. Microshoot lines were obtained from germinated somatic or zygotic embryos. Plants showing EM phenotypes were recovered through direct conversion of somatic embryos or adventitious rooting of microcuttings. During the in vitro propagation phase, flower buds were observed on microshoots after three to six subcultures. Histological analysis showed that most of these flowers were hermaphrodite. In vitro apical buds were used to clone the walnut orthologous cDNAs of the AGAMOUS and APETALA 3 MADS-box genes. Northern blots revealed a preferential expression of both of these homeotic genes in flowers. The results highlight the usefulness of EM lines to study the genetic cues controlling flowering and sexual maturity in woody perennials.

Keywords: flowering, gene expression, homeotic, juvenility, Persian walnut.