Mucilage Glands and Cyanobacterial Colonization in Gunnera kaalensis (Haloragaceae)

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Abstract
Mucilage-secreting glands develop near the stem apices of the angiosperm Gunnera, Nostoc, a cyanobacterium, is found within channels of the glands and within cavities that form beneath the channels. Eventually, Nostoc penetrates the host cells, and a sheathlike arrangement of Gunnera parenchyma cells results around the invaded area. Mucilage-secreting cells contain numerous dictyosomes, which elaborate vesicles with fibrogranular contents ultrastructurally similar to the extracellular mucilage. The outer walls of the mucilage cells are thickened and contain mucilage aggregates within the wall. The secretion of the mucilage occurs via passage of the aggregates between microfibrils of the cell wall. In addition to the secretory cells, the glands contain cells with dense tannin deposits. Bacterial and fungal cells appear within and between gland cell walls. These organisms are probably involved in aiding the penetration of Gunnera cells by Nostoc.

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