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Plant bio

Legume-rhizobium

TermDefinition
Haber-bosch process - industrial ammonia production - invented 1910 - mainly for fertilisers - expensive metal catalysts - uses high pressure and temperature - meets high demand due to population
Can biological systems help? legume plant + rhizobia bacteria = N2 fixation in nodules
Effects of N starvation on pea plants Nodules form where N is present
Legume - rhizobia synthesis Plant provides carbon and rhizobia provides nitrogen - over 80~ plants form a symbiosis with AM fungi - legumes form this with both
AM fungi provides the plant with water, micronutrients and phosphorus
1. Nodule formations “Molecular dialogue” between host plant & rhizobia bacteria using diffusible signals
Flavonoids host plant secretes these secondary metabolites
Nod factors Lipochitooligosaccharides (LCOs) - Nod factors are essential for nodulation and are important mediators of host-range specificity - mediated by nod genes
2. Host plant signalling Calcium spiking is central to symbiosis signalling - Calcium spiking occurs in the nucleus - One of the earliest measureable responses to Nod factor (within minutes) - Decoded by a unique calcium and calmodulin-dependent protein kinase (CCaMK)
3. Attachment and infection - nod factors induce root hair deformation in legumes - rhizobia form a biofilm and attach to root hairs, then are entrapped in the root hair curl - infection thread formed - hydrolysis of the plant cell wall and invagination of the plasma membrane
4. Symbiotic organ (nodule) formation - bacterial infection must be tightly coordinated with nodule organogenes - plant hormones play a key part in regulating this
Two types of nodules - determinate: round in shape, transient meristem, initiated from outer cortical cells - indeterminate: elongated in shape, tip growing meristem, initiated from inner cortical cells
Bacteroids - released from infection threads as infection droplets - engulfed by cytoplasmic membrane to form peribacteroid units - O2 levels kept low
Biological nitrogen fixation - exclusively a prokaryotic property - via nitrogenase: multi protein component enzyme encoded by the nif genes
Nitrogenase properties - very slow - very energy demanding (16 ATP molecules per N2 fixed) - very oxygen sensitive
Nodule metabolism - Bacteroids are fed carbon in the form of dicarboxylic acids (malate) and amino acids - The plant makes homocitrate, a key component of the FeMoCo cofactor of nitrogenase - N2 fixed and NH4 exploited then assimilated via glutamine synthetase
Rhizobia signalling pathway rhizobia uses nod factors - transcription factors DMI1, DMI2, CASTOR, NENA leads to Ca2+ spiking CCaMK and transcriptional substrate CYCLOPS NSP1&2, ERN1&2, NIN transcription factors lead to nodulation
Created by: rose.coo
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