Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...

. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root hair cells improve plant water absorption by increasing root surface area to volume ratio which allows the root hair cell to take in more water. The large

vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...

inside root hair cells makes this intake much more efficient. Root hairs are also important for nutrient uptake as they are main interface between plants and mycorrhizal fungi.

Function

The function all root hairs is to collect water and mineral

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...

s in the soil to be sent throughout the plant. In roots, most water absorption happens through the root hairs. The length of root hairs allows them to penetrate between soil particles and prevents harmful bacterial organisms from entering the plant through the xylem vessels. Increasing the surface area of these hairs makes plants more efficient in absorbing nutrients and interacting with microbes. As root hair cells do not carry out

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

, they do not contain

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...

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 nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known a ...

s in

legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock for ...

plants. The root hairs curl around the bacteria, which allows for the formation of an infection thread into the dividing cortical cells to form the nodule. Having a large surface area, the active uptake of water and minerals through root hairs is highly efficient. Root hair cells also secrete acids (e.g., malic and citric acid), which solubilize minerals by changing their

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

, making the ions easier to absorb.

Formation

Root hair cells vary between 15 and 17 micrometers in diameter, and 80 and 1,500 micrometers in length. Root hairs are found only in the zone of maturation, also called the zone of differentiation. They are not found in the zone of elongation, possibly because older root hairs are sheared off as the root elongates and moves through the soil. Root hairs grow quickly, at least 1 μm/min, making them particularly useful for research on cell expansion. Just prior to and during root hair cell development, there is elevated phosphorylase activity.

Fungal interaction

Root hairs are essential for healthy plant nutrition, especially through their interactions with

symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or para ...

fungi. Symbiotic fungi and root hairs produce mycorrhizal symbioses like arbuscular mycorrhiza, formed by AM fungi, and

ectomycorrhiza An ectomycorrhiza (from Greek ἐκτός ', "outside", μύκης ', "fungus", and ῥίζα ', "root"; pl. ectomycorrhizas or ectomycorrhizae, abbreviated EcM) is a form of symbiotic relationship that occurs between a fungal symbiont, or my ...

, formed by EM fungi. These are very common, occurring in 90% of terrestrial plant species, because of the benefits it brings to both the fungus and plant. Formation of this relationship for EM fungi begins with the colonization of the root hairs. This process begins when the EM fungus adheres to the root hair from the soil. The fungus then secretes diffusible factors, to which root hairs are highly sensitive, allowing the

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

e to penetrate into the epidermal cells and create a Hartig net in the first layers of the root cortex. This highly branched structure serves as an interface between the two organisms as fungal cells adapt to the exchanges that occur between the plant and fungus. 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 hyphae to enter, and there is no Hartig net. Various effects of fungal colonization in root hairs show that this relationship is beneficial to both plant and fungal species, but the main effect is on root hair growth. Fungi actually affect the growth of root hairs if there is water or nutrient deficiency. Since both of these organisms require nutrients and water, their cooperation is essential to their mutual survival. Upon detection of deficiency, the drought stress response of the plant is triggered, causing growth of the root hairs. The mycorrhizae of the fungus then uses its extended system to help the plant find the correct area of nutrition, signaling the direction in which the roots should grow. This makes root growth more efficient, preserving energy for other metabolic processes, which in turn benefits the fungus that feeds off those metabolic products.

Survival

When a new root hair cell grows, it excretes a

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...

that inhibits the growth of root hairs in nearby cells. This ensures equal and efficient distribution of the actual hairs on these cells. Repotting or transplanting a plant can result in root hair cells being pulled off, perhaps to a significant extent, which can cause wilting.

References