Embryogenic potential of immature zygotic embryos of Passiflora: a new advance for in vitro propagation without plant growth regulators

Abstract

In vitro strategies for Passiflora have been developed owing to its economic and ecological importance. However, plantlet regeneration through somatic embryogenesis has presented some problems, such as the reproducibility of the protocol and formation of abnormal embryos and plantlets. Thus, this study aimed to establish a protocol exploring the embryogenic potential of immature zygotic embryos (IZE) of the wild species Passiflora miniata Vanderpl. and Passiflora speciosa Gardn. Friable calli, which formed on the abaxial surface of the cotyledons, yielded globular, heart-shaped, torpedo and cotyledonary somatic embryos, characterising the embryogenic response as asynchronous. A high percentage of normal regenerants (90 %) was obtained from IZE in media lacking 2,4-dichlorophenoxyacetic acid (2,4-D) in comparison to the value of normal plantlets (60 %) regenerated from mature zygotic embryos inoculated in media with 2,4-D. This result demonstrates that IZE of P. miniata and P. speciosa possess sufficient levels of endogenous phytohormones to trigger a high rate of indirect somatic embryogenesis. All regenerated plantlets had the same genome size and chromosome number as the explant donor plants. Therefore, the indirect embryogenic pathway, employing IZE inoculated into media free of growth regulators, did not cause changes in the karyotype and morphology. Based on these results, IZE should be considered as explant for the establishment of somatic embryogenesis in other species. Besides, a new, reliable and relatively rapid protocol to recover plantlets of P. miniata and P. speciosa yielded several plants, which were acclimatised and used for ornamental purposes and breeding programs, and for reintroduction into biological reserves.