Question

Consider the transmission of symbionts to new hosts. For horizontally transmitted symbionts, successful establishment of the symbiosis depends on communication between the host and the symbiont. Choose 1 symbiosis and describe, in detail, the reciprocal communications involved in establishing the symbiosis.

Answer 1

Examples of horizontal transmission include the rhizobia in legume root nodules,

Benefit; The plant gains the ability to grow in nitrogen poor soils, and the bacteria gain a protected niche where they multiply and get sufficient nutrients so mutual benefit from symbiosis was observed.

Reciprocal communications involved in establishing the symbiosis is explained below.

Formation of root nodule:

1. Rhizobia are free living in the soil until they are able to sense flavonoids, derivatives of 2-phenyl-1.4-benzopyrone, which are secreted by the roots of their host plant triggering the accumulation of a large population of cells and eventuallly attachment to root hairs. The flavonoid is at the highest concentration at the root and interacts with the product of bacterial nodD gene. The nodABCFELMN gene products are involved in the synthesis of a group of signal molecules (Nod factors) that induce nodule morphogenesis The nodD gene produces the protein, nodD, which is the sensor that recognizes chemicals excreted by host plant roots.

2. Rhizobia colonize the soil in the vicinity of the root hair in response to the flavonoids. This process is autoregulated where favonoids stimulate Nod factor production, which stimulates flavonoid secretion. These flavonoids then promote the DNA binding activity of NodD which belongs to the LysR family of transcriptional regulators and triggers the secretion of nod factors after the bacteria have entered the root hair.

3. Response to Nod factors is extremely fast and the disruption of cell wall happens very quickly. Disruption of crystallization of cell walls take place, thereby allowing entrance by the rhizobia. At the same time Rhizobia multiply in the rhizosphere. The root hair is then stimulated and curls to the side where the bacteria are attached which stimulates cell division in the root cortex.

A second mechanism, used especially by rhizobia which infect aquatic hosts, is called crack entry. In this case, no root hair deformation is observed. Instead the bacteria penetrate between cells, through cracks produced by lateral root emergence

4. A "shepherd's crook" is formed and entraps the rhizobia which then erode the host cell wall and enter near the root hair tip. An infection thread is formed as rhizobia digest the root hair cell wall. Free-living Rhizobium bacteria are converted to bacteroids as the infection elongates by tip growth down root hair and toward epidermal cells.

5. Infection thread branches and heads toward the cortex and a visibly evident nodule develops on the root as the plant produces cytokinin and cells divide. Nodules can contain one or more rhizobial strains and can be either determinant (lack a persistent meristem and are spherical) or indeterminate (located at the distal end of cylindrically shaped lobes) . Many infections are aborted due to a breakdown in communication between rhizobia and the host plant leaving nodule number strictly regulated by the plant.

6. Once inside the nodule, rhizobia are released from the infection thread in a droplet of polysaccharide. A plant-derived peribacteroid membrane, which regulates the flow of compounds between the plant and bacteroid, quickly develops around this droplet via endocytosis. This process keeps the microbes "outside" the plant where the rhizobia are intracellular but extracytoplasmic. The loss of the ammonium assimilatory capacity by bacteroids is important for maintaining the symbiotic relationship with legumes.

7. Inside the nodule, these bacteroids fix atmospheric nitrogen into ammonium, using the enzyme nitrogenase. The plant also provides the bacteroid oxygen for cellular respiration, tightly bound by leghaemoglobins, plant proteins similar to human hemoglobins. This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity

Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant by bacteriods. In return, the plant supplies the bacteria with carbohydrates in the form of organic acids