Biology:Root hair curling

Nitrogen fixation nodules found on a soybean plant

Root hair curling is the process by which the root hairs of legume plants curl around bacteria to allow an infection into the cortical cells of the roots to form a nitrogen fixing nodule.

Legume plants, just like all plants, need nitrogen in order to grow. Legumes include plants such as alfalfa, beans, peanuts, and lentils. Nitrogen is available in excess in the air, but it that form is not actually usable by plants. The gaseous form (N₂) must be reduced to ammonia (NH₄) in a process known as nitrogen fixation. This nitrogen fixation can occur in many ways, including bacterial symbiosis, adding ammonia-based fertilizer to the soil and ammonia accumulation from decaying matter. The problem is that plant cannot fix nitrogen with their own processes, so they use bacterial symbioses. Different plants associate with different bacteria, but legume plants symbiose with Rhizobium bacteria. These bacteria associate with the roots of the plant, since that is where plants take up nutrients.

The process by which legumes associate with Rhizobium includes many proteins and chemicals. First of all, the legume root itself will release flavonoids in order to attract Rhizobium to itself. Flavonoids will also activate specific genes in the bacteria that initiate nodule formation. These genes are called nod genes, and they excrete nod factors. These nod factors initiate the actual curling of the root. The curling begins with the very tip of the root hair curling around the Rhizobium. Within the root tip, a small tube called the infection thread forms, which provides a pathway for the Rhizobium to travel into the root epidermal cells as the root hair continues to curl.

Partial curling can even be by nod factor alone. An experiment by Esseling et. al. demonstrated this. These experimenters isolated nod factor and applied the chemical at specific points on the root hair. The root hairs curled in the direction of the application, demonstrating the action of a root hair that is attempting to curl around a bacterium. Even application on lateral roots caused curling. This demonstrated that the nod factor, not the bacterium itself, causes the stimulation of the curling.