Serial Block Face-SEM visualisation of nuclear tubular extensions in a carnivorous plant

Unusual nuclei, with spindle-like tubular projections, in the placental tissue of Utricularia nelumbifolia may be involved in nucleus-to-cell-to-cell communication.
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Bladderworts (Utricularia, Lentibulariaceae) possess an extra-ovular female gametophyte that penetrates the sporophytic tissue (placenta). Some parts of the placenta serve as ‘nutritive tissue’, providing nutrients to the female gametophyte and later to the endosperm. Płachno et al. studied the complexity of nuclei structures of ‘placenta nutritive tissue’ across eleven bladderwort species.

Płachno and colleagues used transmission electron microscopy (TEM) to visualize the complexity of the nuclear structures. They also made 3-D ultrastructural reconstructions, using the serial block face scanning electron microscopy (SBEM) technique. They examined the nuclei of 11 Utricularia species, i.e. U. nelumbifolia, U. reniformis, U. cornigera, U. nephrophylla (sect. Iperua), U. asplundii, U. alpina, U. quelchii (sect. Orchidioides), U. longifolia (sect. Foliosa), U. intermedia, U. minor and U. gibba (sect. Utricularia).

Utricularia nelumbifolia. The localization and general structure of the placental nutritive tissue.
Utricularia nelumbifolia. The localization and general structure of the placental nutritive tissue. (A, B). The general morphology and histology of U. nelumbifolia placenta and nutritive tissue. (A) PL, placenta; VB, vascular bundles; arrows, ovules; encircled, nutritive tissue. Whole mounted preparation stained with DAPI. Fluorescence microscopy. Scale bar = 180 μm. (B) NT, nutritive tissue, the arrow marks the nuclear projection. Inset: a fragment of nutritive tissue at higher magnification; arrows point to nuclear projections. Histocryl semi-thin section stained with DAPI. Fluorescence microscopy. Scale bar = 20 μm. (C, D) Ultrastructure of nutritive tissue cells (NT). CW, cell walls; NU, nucleus; h, chromocentre; arrows, nuclear projections; double arrows, nuclear projection branching; TI, tubule-like inclusions. Material prepared for the SBEM method, transmission electron microscopy (TEM). Scale bars = 2·75 μm in (C) and 1·80 μm in (D). (E) Nutritive cell ultrastructure after fixing for microtubules. Note bifurcating nuclear projection. NU, nucleus. TEM. Scale bar = 1·5 μm. (F) Nutritive cell ultrastructure after fixation in modified Karnovsky fixative. The arrow points to the nuclear projection; NU, nucleus; S, starch grain. TEM. Scale bar = 1·4 μm.

They observed unusual nuclei, with spindle-like tubular projections, in the placental tissue of Utricularia nelumbifolia. The authors call the placental nuclei structures ‘chromatubules’. Due to the apparent association with the plasma membrane and plasmodesmata, they speculate that chromatubules are involved in nucleus-to-cell-to-cell communication.

This paper is part of the Special Issue on Morphology and Adaptation. It is FREE access for a limited period to the end of January 2018. It will then be free access from November 2018.

Reference

Płachno, B. J., Świątek, P., Jobson, R. W., Małota, K., & Brutkowski, W. (2017). Serial block face SEM visualization of unusual plant nuclear tubular extensions in a carnivorous plant (Utricularia, Lentibulariaceae). Annals of Botany, 120(5), 673–680. https://doi.org/10.1093/aob/mcx042


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