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Plant bio
Legume-rhizobium
| Term | Definition |
|---|---|
| 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:
reub8n
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