Root hair

Drawing of root tip, showing young root hairs

Root hair, or absorbent hairs, are tubular outgrowths of an epidermal cell of a root, a hair-forming cell on the epidermis of a plant root. These structures are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, also called the zone of differentiation^[1] of the root. Just prior to and during root hair cell development, there is elevated phosphorylase activity.^[2] Plants absorb water through the roots from the soil by bulk flow^{[citation needed]}. Root hair cells are adapted to this process by increasing root surface area for the purpose of taking in more water. The large vacuole inside root hair cells makes this intake much more efficient^{[citation needed]}.

Function[]

Most water absorption happens in the root hairs. The root hairs are long which allows them to penetrate between soil particles, and prevent harmful bacterial organisms from entering the plant through the xylem vessels^{[citation needed]}. Increasing the surface area of these hairs allows plants to be more efficient in absorbing nutrients and establish relationships with microbes.^[3] Cross-section of root hair cell: a roughly rectangular shape with a long, thin tail extending to the right and a nucleus at the top left. This happens because the water in the soil has a higher water potential than the cytoplasm of the root hairs. The function of root hairs is to collect water and mineral nutrients that are present in the soil and take this solution up through the roots to the rest of the plant. As root hair cells do not carry out photosynthesis, they do not contain chloroplasts.

Formation[]

Root hair cells are outgrowths at a tip of the plant's roots. Root hair cells vary between 15 and 17 micrometers in diameter, and 80 to 1,500 micrometers in length.^[4] They are found only in the zone of maturation, and not the zone of elongation, possibly because any root hairs that arise are sheared off as the root elongates and moves through the soil.^{[citation needed]} Root hairs grow quickly, at least 1μm/min, making them particularly useful for research on cell expansion.^[5]

Fungi Interaction[]

The function of root hairs are essential for healthy plant nutrition, but there are factors in the environment that can bring a positive change to them. Enhancements to root hair structure that could be obtained for the plant highly depends on their interactions with the surrounding fungi. Symbiotic fungi are known to produce mycorrhizal symbioses like arbuscular mycorrhiza, formed by AM fungi, and ectomycorrhiza, formed by EM fungi, with the root hairs they interact with.^[6] These types of mycorrhizae seems to be the most common relationship,^[7] for which can be classified as occurring in 90% of terrestrial plant species,^[8] because of the benefits it brings to both the fungus and plant.

Formation of this symbiotic relationship for EM fungi first begins with the colonization of the root hairs. This process begins when the EM fungus adheres to the root hair from the soil.^[9] The fungus then secretes diffusible factors that the root hairs are highly sensitive to which allow the hyphae to penetrate into the epidermal cells and in the first layers of the root cortex ^[9] to create a Hairtig net. This highly branched structure serves as an interface between the two cooperating organisms as fungal cells adapt to the exchanges that will soon occur between the plant and newly attached fungus.^[10] This process is similar to how AM fungi colonize root hairs, but instead of diffusible factors, they secrete hydrolases to relax the cell wall which allows the entry of the mycorrhizal hyphae and there is no Hairtig net present.^[9]

There are plenty of effects of fungal colonization in root hairs that prove that this relationship is beneficial to both plant and fungal species, but the main effect fungal species have shown to do in root hair focuses on their growth. Studies have shown that fungi actually organizes and controls the growth of root hairs depending on water or nutrient deficiency.^[9] Since both of these organisms require nutrients and water, their cooperation with each other is essential if they both want to survive in their environment. Upon detection of deficiency, the drought stress response of the plant is triggered causing growth of the root hairs.^[8] The mycorrhizae of the fungus then uses its extended system to help the plant find the correct area of nutrition signaling the direction of where the roots should grow.^[9] The control of the direction of growth of the root hair can be seen as an important role in this symbiotic relationship as it saves the plant time and energy trying to figure out where it should grow. That energy can instead be used for other metabolic processes which in turn helps the fungus that feeds off those metabolic products.

Importance[]

Root hairs form an important surface as they are needed to absorb most of the water and nutrients needed for the plant. They are also directly involved in the formation of root nodules in legume plants. The root hairs curl around the bacteria which allows for the formation of an infection thread through into the dividing cortical cells to form the nodule.^[11]

Having a large surface area, the active uptake of water and minerals through root hairs is highly efficient. Root hair cells also secrete acid (H⁺ from malic acid) which exchanges and helps solubilize the minerals into ionic form, making the ions easier to absorb.^[12]

Survival[]

This way, the root hair coverage stays the same. When a new root hair cell grows, it excretes a hormone so that the other cells in close proximity to it are unable to grow one of these hairs. This ensures equal and efficient distribution of the actual hairs on these cells.^{[citation needed]}

The act of re-potting or transplanting a plant can result in root hair cells being pulled off, perhaps to a significant extent, and such plants may therefore wilt for some time as a result.

See also[]

• Mycorrhizae
• Trichome

References[]

Further reading[]