Explain Briefly the symbiotic relationship between a) Lichens, b) Rhizobium Bacteria and leguminous plants..
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Answer:
Rhizobia are nitrogen-fixing bacteria which invade root hairs of leguminous plants and induce, in a specific manner, the formation of root nodules in which they fix nitrogen. The early steps of the symbiosis can be considered as a reciprocal molecular communication between the two partners. Initially, the plant excretes a gene inducer which stimulates the expression of bacterial nodulation genes. These nodulation genes are responsible for the synthesis of extracellular host-specific signals, called nodulation factors. The bacterial nodulation factors were isolated and structurally identified as substituted and N-acylated chitin oligosaccharides. These prokaryotic lipo-oligosaccharide signals play a key role in the symbiosis by controlling the host specificity of the bacteria. They constitute a new class of signalling molecules able to elicit nodule organogenesis in leguminous plants in the absence of bacteria.
Answer:
a) Like all fungi, lichen fungi require carbon as a food source; this is provided by their symbiotic algae and/or cyanobacteria, that are photosynthetic. The lichen symbiosis is thought to be a mutualism, since both the fungi and the photosynthetic partners, called photobionts, benefit.
b) Legumes are able to form a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The result of this symbiosis is to form nodules on the plant root, within which the bacteria can convert atmospheric nitrogen into ammonia that can be used by the plant.
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