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Secondary endosymbiosis

Secondary endosymbiosis occurs when a host cell - itself a product of primary endosymbiosis - is taken into another cell. This is most often seen when an algal cell is engulfed by another eukaryotic cell, causing a switch in the metabolism of both components. The components in this mutualistic relationship can usually resume independent existence when separated.
Examples:
Cryptomonads result from a protozoan (nonphotosynthetic eukaryotic flagellate) engulfing a red alga.
Chlorarachniophytes similarly result from a flagellated protozoan engulfing a green alga.

Other symbiotic relationships

Cyanobacteria form symbiotic relationships with a variety of plant groups. These are not quite endosymbiotic, i.e. within cells, and more concerned with nitrogen fixation than photosynthesis

The small water fern Azolla has ovoid cavities within the leaves, colonised by the cyanobacterium Anabaena. This forms thick-walled heterocysts which fix atmospheric nitrogen, which presumably explains the high growth rate of this plant. Nitrogen fixation requires anaerobic conditions. Azolla is encouraged to grow in rice paddies, and as it decomposes it provides nutrients which would otherwise be washed out of the soil beneath.

The large rhubarb-like plant Gunnera develops structures at the base of its leaves which function as incubation chambers for the cyanobacterium Nostoc punctiforme which also fixes nitrogen. These glands are produced in conditions of nitrogen deprivation, which are likely to occur in the boggy environments where this plant flourishes.

Cycads are a group of plants (gymnosperms) that also have a symbiotic relationship with nitrogen-fixing cyanobacteria Nostoc spp which inhabit specialized branching structures known as coralloid roots.

Lichens - which used to be classified as a plant group completely separate from others - are a combination of a fungus and either an alga or a cyanobcterium, in a mutualistic symbiotic relationship. The fungus - known as the mycobiont - provides protection to the other organism - the photobiont - which performs photosynthesis.

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