Article

In vitro symbiotic seed germination of Oncidium flexuosum (Orchidaceae) by rhizoctonia-like mycorrhizal fungi

Revista Brasileira de Ciência do Solo (Impact Factor: 0.72). 04/2005; 29(2):199-206. DOI: 10.1590/S0100-06832005000200005

ABSTRACT A germinação de sementes e o desenvolvimento de protocórmios de Oncidium flexuosum (Orchidaceae) induzidos simbioticamente são descritos pela primeira vez. As sementes de O. flexuosum foram inoculadas com dez fungos micorrízicos rizoctonióides, previamente isolados de micorrizas de dez espécies de orquídeas neotropicais do Brasil, incluindo O. flexuosum. Foram utilizados um isolado pertencente à espécie Epulorhiza repens, dois pertencentes à Epulorhiza epiphytica, seis de Ceratorhiza spp. e um de Rhizoctonia sp. Sementes inoculadas com o isolado M2 de Ceratorhiza sp., originalmente isolado do sistema radicular de O. flexuosum em habitat natural, promoveu a germinação das sementes em sete dias e em, aproximadamente, 30% das plântulas, houve formação de folhas após 50dias de incubação, apresentando pelotons em algumas células do protocórmio e das radicelas. Os demais isolados promoveram a germinação das sementes; entretanto, não promoveram um desenvolvimento ótimo dos protocórmios. Sementes incubadas na ausência de fungos micorrízicos não germinaram. A especificidade e a alta dependência de O. flexuosum pela associação micorrízica ficaram claras. Aspectos relativos à especificidade, anatomia da interação fungo-planta e a importância da seleção de estirpes fúngicas, previamente ao uso de fungos micorrízicos para o cultivo simbiótico a partir de sementes de O. flexuosum são discutidos.

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    • "It has been assumed that, for epiphytic orchids, dependence upon mycorrhizae is probably not obligate during the first stages of germination, for three reasons: some orchid seeds contain small drops of lipids and protein in the embryo; the rapid assimilation of simple nutrients in in vitro culture media; and after imbibing water, and when exposed to light, the seeds rapidly pass to a photosynthetic state (Arditti 1992). Nevertheless, studies on symbiotic germination in vitro demonstrate dependence upon the mycorrhizae for the differentiation and development of each life stage, and that growth ceases and the plant eventually dies in the absence of a mycorrhizal symbiont (Zettler et al. 1998; Markovina & McGee 2000; Pereira et al. 2005a). "
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    ABSTRACT: Orchids establish symbiosis with Rhizoctonia mycorrhizal fungi, forming the characteristic pelotons within the cells of the root cortex. Under natural conditions, terrestrial and epiphytic orchids have different levels of dependence upon the fungal symbiont, although various authors have mentioned that once orchid plants reach maturity the interaction becomes weaker and intermittent. Recent evidence shows that in some epiphytic orchid species mycorrhization is constant and systematic. In three species of wild orchids from southeast Mexico, we show that mycorrhization is systematically present in roots of different ages, in the wet and dry seasons. We demonstrate that the volume of the root that is colonized depends upon the quantity of rainfall and the diameter of the root, and that rainfall also determines the presence of fresh, undigested pelotons. In very thin roots, mycorrhizal colonization occupies a considerable proportion of the cortex, whereas in thicker roots the proportion of the volume of the root cortex colonized is lower.
    Acta Botanica Brasilica 12/2014; 28(4):512-518. DOI:10.1590/0102-33062014abb3436 · 0.55 Impact Factor
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    • "The control of hyphal invasion in an orchid's tissue is due the production of phytoalexins, which are found in the roots and rhizomes, the production of which is induced by the fungal invasion and colonisation of orchid roots tissues (Gehlert and Kindl 1991; Reinecke and Kindl 1994). Studies about orchid mycorrhizal interactions in Brazil are scarce and, so far, focused on the morphological or molecular identification of mycobiont species (Pereira et al. 2001; Nogueira et al. 2005; Pereira et al. 2003ab; Pereira et al. 2005ab,) and the induction of in vitro germination in seeds (Pereira et al. 2005c). Information about the anatomical aspects of such interactions are restricted, and in most cases, concerned to species from temperate zones. "
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    ABSTRACT: The mycorrhizas are fungi associations considered to be a requirement for the survival of orchids that live naturally in ecosystems, since this group of plants depends on the fungi to germinate, as well as for their establishment. The knowledge of mycorrhizical fungi biodiversity of Brazilian orchids and characteristics of this interaction are of great importance for further reintroduction programs, conservation and handling of these vegetal species. Thus, in this work, we present the anatomical characterisation of mycorrhizas in Neotropical orchids by using a simple staining method. Moreover, unusual hyphae colonisation was also observed.
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    • "The control of hyphal invasion in an orchid's tissue is due the production of phytoalexins, which are found in the roots and rhizomes, the production of which is induced by the fungal invasion and colonisation of orchid roots tissues (Gehlert and Kindl 1991; Reinecke and Kindl 1994). Studies about orchid mycorrhizal interactions in Brazil are scarce and, so far, focused on the morphological or molecular identification of mycobiont species (Pereira et al. 2001; Nogueira et al. 2005; Pereira et al. 2003ab; Pereira et al. 2005ab,) and the induction of in vitro germination in seeds (Pereira et al. 2005c). Information about the anatomical aspects of such interactions are restricted, and in most cases, concerned to species from temperate zones. "
    [Show abstract] [Hide abstract]
    ABSTRACT: The mycorrhizas are fungi associations considered to be a requirement for the survival of orchids that live naturally in ecosystems, since this group of plants depends on the fungi to germinate, as well as for their establishment. The knowledge of mycorrhizical fungi biodiversity of Brazilian orchids and characteristics of this interaction are of great importance for further reintroduction programs, conservation and handling of these vegetal species. Thus, in this work, we present the anatomical characterisation of mycorrhizas in Neotropical orchids by using a simple staining method. Moreover, unusual hyphae colonisation was also observed.
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