SREEKUTTAN KS

Nodulation (Nod) factors  are  signaling molecules  produced by bacteria known as rhizobia during the initiation of nodules on the root of legumes.

In response to flavonoids secreted by legume root hairs.

Nod factors are secreted by rhizobia during a tightly regulated signalling pathway which results in the formation of root nodules in leguminous plants, resulting in the rhizobia becoming  bacteroids

1. The leguminous plant releases flavonoids into the area surrounding the plant roots, known as the rhizosphere , when the available nitrogen in the soil is depleted. 

2. Rhizobia in the surrounding soil secrete Nod D, which is a protein that recognizes the flavonoids secreted by the plants.

3. Interaction with the flavonoids activates Nod D, then NodD returns to the rhizobium to induce the transcription of nod ABC genes.

NODULATION PROCESS

4. Nod ABC  proteins modify the Nod factor in response to the flavonoids released by the plant.

5. The Nod factors interact with the plant and cause root hair curling and root hair deformation. This leads to the formation of an infection thread into the root hair.

6. Nod factors cause cortical cell division in the primordium of the plant and form a nodule.

7. The rhizobia move through the plant root hair via the infection thread to the nodule within the plant

8. Rhizobia switch into bacteroid form and are surrounded by a plant-derived symbiosome membrane.

9. Rhizobia begin nitrogen fixation and provide the legume with amino acids, such as glutamine and asparagine, or ureides, such as allantoin and allantoic acid, which are made in the nodule.

10. The legume provides simple sugars produced from plant catabolism to the rhizobia in the nodule.

Nod factors are the important signaling molecules in the symbiotic interaction between rhizobia and leguminous plants.

When they bind to the root hairs of the leguminous plants, they cause root hair deformations, activation of plant genes, initiation of cortical cell division and nodule formation.

 Each of these processes are necessary for the proper formation of root nodules, which enable the symbiotic interaction between rhizobia and leguminous plants.

FUNCTION OF NODE FACTOR

Nod factors can affect other plants in the soil surrounding them.

When Nod factors are present in mixed crop fields, they have the ability to stimulate seed germination, promote plant growth, increase photosynthetic rates, and increase grain yields of legume and non-legume crops.

STRUCTURE OF NODE FACTOR Nod factors are lipochito-oligosaccharides

and have three to five N-acetyl-glucosamines. .

The specific structure of Nod factors is determined by modifications made by Nod genes, which are found in the rhizobium genome.

The Nod genes encode for proteins that modify Nod factors by adding or removing different chemical structures such as sulfates, fatty acids, acetyl groups, and methyl groups to the original lipochito-oligosaccharide structure.

(nod A, nod B and nod C) most extensively studied Nod genes .

The enzymes encoded by the common Nod genes have specific functions.

Host Specificity due to Nod Factors

There are two modes of specificity for rhizobium- legume symbiosis. 

The first is the interaction between the flavonoids, the chemical signals that are derived from the legumes.

The recognition of the flavonoids by the bacteria is dependent on the interactions with NodD. When NodD is activated, it leads to the expression of nod ABC  as well as many other Nod genes, which results in the structural modification of Nod factors

The second mode of specificity is dependent on the structure of the Nod factors themselves.

There are some bacteria that can make several structurally different Nod factors and therefore have different host plants, slight variations in the Nod factor structure leads to host specificity