The Nostoc–Gunnera symbiosis

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Abstract
Gunnera L. develops a complex and intimate symbiosis with N2-fixing cyanobacteria of the genus Nostoc, which renders the plant independent of combined nitrogen. The Nostoc-Gunnera symbiosis exhibits unique features compared to other cyanobacterial-plant symbioses: it is for example the only one that involves a flowering plant (angiosperm), the cyanobacterium infects specialized gland organs located on the stems of the host and once it has passed into the interior of the gland the cyanobacterium also enters the Gunnera cells where it starts to differentiate the highest frequency of heterocysts (the N2-fixing cells) recorded in any cyanobacterial population. Gunnera has attracted scientific attention also for the following reasons: the genus has a peculiar geographic distribution of its subgenera and species in the Southern Hemisphere. It differs morphologically and anatomically from related plants and also shows an anomalous polystelic vascular system (polystely). This review gives an updated account of present knowledge concerning the Nostoc-Gunnera symbiosis. Emphasis will be on the development of the symbiotic tissue (the gland), the structure and function of the prokaryotic N2-fixing cyanobacterium, the infection process and on the relationship between the pro- and eukaryotic partners prior to and following the establishment of symbiosis. CONTENTS Summary 379 I. Introduction 379 II. The Gunner a plant 380 III. The microsymbiont(s) 383 IV. The symbiosis 384 V. The gland 385 VI. The infection process 388 VII. Specificity 391 VIII. Impacts on the cyanobiont 392 IX. N2 fixation and release 393 X. Photosynthesis 396 XI. Concluding remarks 397 Acknowledgements 398 References 398