Department of Botany, Stockholm University, S-10691, Stockholm, Sweden.
Planta. 1992 Oct;188(3):403-13. doi: 10.1007/BF00192808.
The symbiosis between Gunnera and Nostoc was reconstituted using G. chilensis Lam. and G. manicata Linden, respectively, and three different Nostoc strains. Six stages characterised by specific modifications in both the cyanobiont and the host were recognised during the infection process. Mucilage-secreting stem glands developed on the Gunnera stems independent of the presence of cyanobacteria (Stage I). Soon after addition of the Nostoc isolates to the plant apices, an abundant differentiation of motile hormogonia commenced. The cyanobacteria accumulated in the mucilage on the surface of the gland (Stage II), and the hormogonia then proceeded into the stem tissue through intercellular channels (Stage III). At the channel bases, Nostoc was detected between the cell walls of small, densely cytoplasmic Gunnera cells and also in elaborate folds of these (Stage IV). The Gunnera cell walls subsequently dissolved adjacent to the cyanobacteria and Nostoc entered the host cells (Stage V). Once the intracellular association was formed, a high proportion of the vegetative Nostoc cells differentiated into heterocysts (Stage VI). Nostoc changed from being rich in inclusions (particularly cyanophycin) while on the gland surface into a comparatively "non-storing" form during penetration and the early intracellular stages. Bacteria were numerous on the gland surface, fewer in the channels, and were never detected within the Gunnera cells, indicating the existence of specific recognition mechanisms discriminating between conceivable microsymbionts. Mechanisms behind mutual adaptations and interactions between the two symbionts are discussed.
分别使用琴叶榕（Gunnera chilensis Lam.）和曼尼卡榕（G. manicata Linden）以及三种不同的念珠藻菌株，重建了琴叶榕与念珠藻之间的共生关系。在感染过程中，识别出六个阶段，这些阶段的特征是蓝藻共生体和宿主都发生了特定的变化。在没有蓝藻存在的情况下，琴叶榕茎上会发育出分泌黏液的茎腺（第 I 阶段）。将念珠藻分离物添加到植物顶端后不久，大量的游动原丝体开始分化。蓝藻在黏液中积累在腺体的表面（第 II 阶段），然后原丝体通过细胞间通道进入茎组织（第 III 阶段）。在通道底部，在小而密集的细胞质琴叶榕细胞的细胞壁之间以及这些细胞壁的精细折叠中检测到念珠藻（第 IV 阶段）。随后，琴叶榕细胞壁在靠近蓝藻的地方溶解，念珠藻进入宿主细胞（第 V 阶段）。一旦形成细胞内联系，大部分营养性的念珠藻细胞分化为异形胞（第 VI 阶段）。在黏附于腺体表面时，念珠藻富含内含物（特别是藻青素），但在穿透和早期细胞内阶段，念珠藻会变成相对“非储存”的形式。细菌在腺体表面数量众多，在通道中较少，从未在琴叶榕细胞内检测到，这表明存在特异性识别机制，能够区分潜在的微共生体。还讨论了两个共生体之间相互适应和相互作用的机制。
