A Microscopical Study of the Development of a Spore-Like Stage in the Life Cycle of the Root-Nodule Endophyte of Alnus glutinosa (L.) Gaertn

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  • A Microscopical Study of the Development of a Spore-Like Stage in the Life Cycle of theRoot-Nodule Endophyte of Alnus glutinosa (L.) GaertnAuthor(s): C. Van Dijk and E. MerkusSource: New Phytologist, Vol. 77, No. 1 (Jul., 1976), pp. 73-91Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2433642 .Accessed: 13/06/2014 02:29

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  • New Phytol. (I 976) 77, 73-9 I .

    A MICROSCOPICAL STUDY OF THE DEVELOPMENT OF A SPORE-LIKE STAGE IN THE LIFE CYCLE OF

    THE ROOT-NODULE ENDOPHYTE OF ALNUS GLUTINOSA (L.) GAERTN.

    BY C. VAN DIJK* AND E. MERKUSt

    Department of Botany, Research Group on Biological Nitrogen Fixation, University of Leiden, Leiden, The Netherlands

    (Received 25 October I974)

    SUMMARY

    A light- and electron-microscopical study of the root-nodule endophyte of Alnus glutinosa (L.) Gaertn. was carried out to investigate the development of a spore-like stage, here called the granule, in the life cycle of the endophyte. Comparison of granule-rich and granule-free root nodules showed that granule formation takes place via local transverse growth of thick endo- phytic hyphae, giving rise to multicellular 'granulated bodies' differing in shape and size. Subse- quently, the cells of these granulated bodies are transformed into granules by cell separation and ultrastructural changes, the most striking of which are cell-wall thickening, reduction of the number of mesosomes, and increased density of the cytoplasm. Granule development takes place both intracellularly and intercellularly. Intracellularly produced granules are eventually liberated by the death of the host cell. Mature granules show a strong resemblance to spores of free-living actinomycetes in their ultrastructure and behaviour.

    It is concluded that, in view of the terminology currently used in the description of members of the Actinomycetales, the term granule should be replaced by spore and thus the term granu- lated body by sporogenous body.

    INTRODUCTION

    Morphological and cytological studies of the root-nodule endophyte of Alnus glutinosa (L.) Gaertn. by Peklo (I9IO), Schaede (I933), Kappel and Wartenberg (I958), Pommer (I956), Becking, de Boer and Houwink (i964) and Gardner (i965) revealed the actino- mycetal character of the endophyte. Intracellular hyphae with a close resemblance to the hyphae of free-living actinomycetes (Becking et al., I964) represent the basic appear- ance of the endophyte. The development of intracellular spherical vesicles is easily deduced from their attachment to and continuity with the tips of hyphal branches. Although the possible function(s) of these complex vesicles has long been uncertain, Akkermans (in press) recently found nitrogen fixation activity in isolated clusters of vesicles. A third morphologically distinct stage in the life cycle of the endophyte, mentioned by Peklo as early as I9IO, has been described as characterized by polyhedral cells about I-2 ,um in diameter, usually lying tightly packed in intercellular spaces, and dead host cells. Most authors have called these particles bacteroids, a term in general use

    * Present address: Institute for Ecological Research, Weevers' Duin Biological Station, Oostvoorne, The Netherlands.

    t Present address: National Institute of Public Health, Bilthoven, The Netherlands.

    73

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  • 74 C. VAN DIJK AND E. MERKUS

    since the descriptions given by Schaede (I933). Becking et al. (I964) introduced the name bacteria-like cells, which was later changed to polyhedral-shaped cells (Becking, I970).

    Arcularius (I928) suggested that 'bacteroids' might represent a quite different organ- ism, but Schaede (I933) concluded that they were mainly produced by fragmentation of endophytic hyphae. In some cases, however, he observed the disintegration of spherical vesicles, leading to the formation of bacteroids. Electron-microscopical studies of the endophyte (Becking et al., I964; Gardner, I965) did not contribute to a satisfactory reconstruction of the development of the bacteroids. Becking et al. (I964) suggested that they are only formed by the fragmentation of tightly interlaced hyphae in dead cells, whereas Gardner (I965) came to the conclusion that they are only formed by the frag- mentation of spherical vesicles with complete septa. Kappel and Wartenberg (I958) con- cluded from light-microscopical observations that bacteroids originate from hyphae via an intermediate multicellular stage they called the Spindel and which they supposed to be identical to the Schlauchen and Blasen described by Schaede (I933).

    Despite the divergent views with respect to the development of bacteroids or bacteria- like cells, it is generally believed that they represent some kind of a resting stage, capable of survival and nodulation. But although Kappel and Wartenberg (I958) were able to obtain nodulation of alder plants with crude isolates of them, strict proof of the infective capacity of bacteroids has not yet been given.

    To avoid any premature implicit conclusion concerning morphological and functional relationships with other organisms, the names bacteroid and bacteria-like cell will be replaced here by the more neutral term granule, since we prefer to wait until sufficient knowledge allows a definite use of the term spore. This paper presents the results of a light- and electron microscopical study of the development of bacteroids (further to be called granules) performed to contribute to the understanding of a probably important part of the life cycle of the endophyte of A. glutinosa root-nodules. Unless otherwise stated the root-nodule endophyte of A. glutinosa will be referred to in this paper as the endophyte. The name Frankia alni, introduced by Becking (I970), will not be used, because the present study was restricted to the endophyte of only one Alnus species and there is no certainty that the results apply in all details to the life cycles of the root-nodule endophytes of other Alnus species.

    MATERIAL AND METHODS

    Collection of root-nodules Root-nodules of Alnus glutinosa were collected in the vicinity of Leiden from several

    alder shrubs bearing granule-rich or granule-free root-nodules. Samples of both granule types were also obtained from alder plants cultivated on a liquid medium. For this purpose, plants were raised from seed collected from native trees. The seeds were washed, surface sterilized with a o.i1% bromine solution, and left to germinate at z2zC in the dark in a glass vessel provided with wet gravel. After io days, a modified Hoagland solution (Quispel, I954) reduced to half strength was added to the gravel and the seedlings were exposed to 220C, 70-80% relative humidity, and I7 h illumination by 30,000 Lux Philips TL 65/33 alternating with 7 h darkness. After 3 weeks, plants with two exposed leaves were transferred to jars filled with Crone's solution containing KNO3 (Bond, I95i), Fe-EDTA, and trace elements, according to Allen and Arnon (1955). One week before inoculation, the nutrient solution was replaced by Crone's solution without

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  • Endophytes of Alnus 75

    nitrogen. Root-nodule formation was initiated by bringing 500 mg (fresh-weight) crushed lobes of granule-rich or granule-free field nodules into 400-ml jars containing twelve alder plants on nitrogen-free Crone's solution. Nutrient solution was refreshed weekly and when necessary plants were transferred to larger jars. Cultivated nodules were collected over a period of 4 weeks to 6 months after inoculation of the plants.

    Preparation of nodule sections for light microscopy Peripheral lobes of cultured and field-sampled root nodules were fixed in FAPA

    (50 ml 40?0 formaldehyde, 250 ml 96% ethanol, I5 ml propionic acid, and I5 ml glacial acetic acid) and embedded in diglycol stearate via an ethanol-xylol series, after which 6-pum sections were cut on a rotary microtome and stuck to object slides with Haupt adhesive (Johansen, I940). Staining was carried out with Toluidine Blue (Feder and O'Brien, i968) or Schiff's reagent. Epon sections 2 /um thick, prepared as described for electron microscopy, were fixed to glass slides on a hot plate and stained with Toluidine Blue I % in a I % borax solution for light-microscopical observation. Fresh sections cut about 20 ,um thick with a hand microtome were transferred to a drop of water on a microscope slide,

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