Species of the floating, freshwater fern Azolla form a well-characterized symbiotic association with the non-culturable cyanobacterium Nostoc azollae, which fixes nitrogen for the plant. However, several cyanobacterial strains have over the years been isolated and cultured from Azolla from all over the world. The genomes of 10 of these strains were sequenced and compared with each other, with other symbiotic cyanobacterial strains, and with similar strains that were not isolated from a symbiotic association. The 10 strains fell into three distinct groups: six strains were nearly identical to the non-symbiotic strain, Nostoc ( Anabaena ) variabilis ATCC 29413; three were similar to the symbiotic strain, Nostoc punctiforme , and one, Nostoc sp. 2RC, was most similar to non-symbiotic strains of Nostoc linckia. However, Nostoc sp. 2RC was unusual because it has three sets of nitrogenase genes; it has complete gene clusters for two distinct Mo-nitrogenases and an alternative V-nitrogenase. Genes for Mo-nitrogenase, sugar transport, chemotaxis and pili characterized all the symbiotic strains. Several of the strains infected the liverwort Blasia, including N. variabilis ATCC 29413, which did not originate from Azolla but rather from a sewage pond. However, only Nostoc sp. 2RC, which produced highly motile hormogonia, was capable of high-frequency infection of Blasia. Thus, some of these strains, which grow readily in the laboratory, may be useful in establishing novel symbiotic associations with other plants.

中文翻译：

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对来自水蕨 Azolla 的可培养共生蓝藻的比较基因组见解

漂浮的淡水蕨类植物Azolla与不可培养的蓝藻Nostoc zolla形成了一种特征明确的共生关系，后者为植物固氮。然而，多年来，已经从世界各地的Azolla 中分离和培养了几种蓝藻菌株。对其中 10 个菌株的基因组进行了测序，并与其他共生蓝藻菌株以及未从共生关联中分离的类似菌株进行了比较。这 10 个菌株分为三个不同的组：6 个菌株与非共生菌株Nostoc ( Anabaena ) variabilis几乎相同 ATCC 29413；三个与共生菌株Nostoc punctiforme和一个Nostoc sp.相似。2RC，与Nostoc linckia 的非共生菌株最相似。然而，Nostoc sp。2RC 不寻常，因为它有三组固氮酶基因；它具有两种不同的 Mo 型固氮酶和另一种 V 型固氮酶的完整基因簇。Mo-固氮酶、糖转运、趋化性和菌毛的基因表征了所有共生菌株。数种菌株感染了地衣 Blasia，包括N. variabilis ATCC 29413，它不是来自Azolla，而是来自污水池。然而，只有Nostoc sp. 2RC 产生高度活动性的激素，能够高频感染Blasia。因此，这些在实验室中很容易生长的菌株中的一些可能有助于与其他植物建立新的共生关系。