Root hair cells are single cells on the outer skin of a root, and each one pushes a long, fine tube out into the soil. These tiny tubes do most of a root’s drinking and feeding, and they also help the root grip the soil around it. The same basic genetic plan builds similar soil-gripping cells in mosses, so plants have relied on this idea since they first moved onto land.1PubMed Central. The evolution of root hairs and rhizoids
Where Root Hair Cells Sit and What They Look Like
The root’s outer layer is made of two kinds of cell: hair cells and hairless cells.2PubMed Central. Cell Fate Determination and the Switch from Diffuse Growth to Planar Polarity in Arabidopsis Root Epidermal Cells Not every skin cell grows a hair, because position decides. In thale cress, a much-studied plant, a skin cell lying over the join between two inner cells becomes a hair cell, while one touching a single inner cell stays smooth.3PubMed Central. Cell Fate Determination and the Switch from Diffuse Growth to Planar Polarity in Arabidopsis Root Epidermal Cells The result is a tidy pattern: hair cells line up in rows, eight of them around this plant’s root, and they rarely sit side by side.4PubMed Central. Cell Fate Determination and the Switch from Diffuse Growth to Planar Polarity in Arabidopsis Root Epidermal Cells Neighbouring cells also signal to one another, so a cell that becomes a hair cell holds its neighbours back.5PubMed Central. Cellular patterns in Arabidopsis root epidermis emerge from gene regulatory network and diffusion dynamical feedback
The cell has a large vacuole (a sap-filled sac), a thin wall, and a nucleus often near the tip end. It has no chloroplasts because it lives in the dark, so it cannot make food and depends on sugar sent down from the leaves. Its thin wall lets water and minerals cross easily, and the long shape trades strength for reach.
You can see root hairs yourself. Sprout cress seeds or a bean on damp kitchen paper in a clear container, and within a few days a white fuzz appears on the young root. The fuzz sits in a band a little behind the root tip, while the very tip looks smooth because hairs have not formed there yet. A cheap hand lens, or a phone camera zoomed in, makes individual hairs easy to spot. Further up the root, older hairs wither away as new ones form nearer the tip, so the hairy zone stays close to the tip as the root lengthens.
Why Root Hairs Make Such a Difference to Surface Area
Root hairs widen the zone of soil a root can draw from and greatly enlarge its absorbing surface.6PubMed. The role of root hairs in water uptake: recent advances and future perspectives Thousands of thin tubes have far more outside surface than a smooth root of the same length. The hairs also press into tiny pores between soil grains, making close contact with the water film around them.7PubMed. The role of root hairs in water uptake: recent advances and future perspectives
The long, thin shape gives a large surface area for absorbing. The thin wall means a short distance for water and minerals to cross. Many mitochondria release energy through respiration, which powers the pumping of minerals into the cell. A large vacuole full of sugary, mineral-rich sap keeps the sap more concentrated than the soil water, which draws water in.
The hairs matter most when supplies are short. In barley, plants with no root hairs did worse under low phosphate combined with severe drought, though hair length itself mattered less.8PubMed Central. What are the implications of variation in root hair length on tolerance to phosphorus deficiency in combination with water stress in barley (Hordeum vulgare)?
How Root Hairs Take In Water
Water enters a root hair cell by osmosis: it flows from where it is more plentiful toward where more dissolved substances are. The cell sap holds more dissolved minerals and sugars than the soil water does, so water flows in. Special protein channels in the cell’s membrane, found in root hairs in rice, let water pass quickly.9PubMed. Characterization of OsPIP2;7, a water channel protein in rice
Once inside, the water makes that cell’s sap a little more dilute than the sap of the next cell inward, so water moves on to it. This repeats cell by cell across the root until the water reaches the central pipes, which carry it up to the leaves. Water constantly evaporating from the leaves keeps pulling more up, so the root hairs keep taking in fresh supplies.
The same logic explains why too much fertiliser looks like drought. If the soil water becomes saltier than the cell sap, water stops flowing into the root hairs. In severe cases the flow reverses and water is pulled out of the root cells. The plant then wilts even in wet soil. The usual fix is to flush the soil with plenty of water to wash the excess salts away.
Waterlogged soil harms root hairs for a different reason. Roots need oxygen to release energy, and flooded soil pushes the air out. Starved cells stop working and can die.
How Root Hairs Collect Minerals
Minerals are often more concentrated inside the root than in the soil, so the cell spends energy pumping them in. This is why root hair cells need oxygen and plenty of mitochondria.
In tomato, root hairs make extra nitrate and ammonium transporters, and some only switch on after the hair meets nitrate.10PubMed Central. Preferential expression of an ammonium transporter and of two putative nitrate transporters in root hairs of tomato Some tomato ammonium transporters are also more active in root hairs than in the rest of the root.11PubMed. Differential regulation of three functional ammonium transporter genes by nitrogen in root hairs and by light in leaves of tomato Pumped-in minerals help keep the sap salty enough for water to keep flowing in by osmosis.
Phosphorus and iron have their own tricks. The cell pumps out acid through a membrane pump, which helps free and absorb phosphorus.12PubMed. Serendipita indica promotes rice phosphorus uptake by plasma membrane H+-ATPase OsA1-stimulated root hair growth When iron runs short, roots turn on a dedicated iron transporter, and plants lacking it turn yellow and die in soil.13PubMed Central. IRT1, an Arabidopsis Transporter Essential for Iron Uptake from the Soil and for Plant Growth
Soil pH can also cause shortages. Some minerals, iron among them, become locked up in alkaline soil. Yellow leaves with green veins can point to iron trouble, though other shortages look similar, so a soil test helps confirm it before treating with acidic feed or chelated iron.
How a Root Hair Grows
A root hair grows only at its very tip, like toothpaste squeezed out of one end of the tube.14PubMed Central. Root hair growth: it’s a one way street New material is added only at the end, so the base of the hair stays the same width and does not lengthen. Calcium builds up highest at the tip, and its level rises and falls with each growth spurt.15PubMed Central. Root hair growth: it’s a one way street Wall-loosening proteins soften the tip wall so it can stretch,16PubMed Central. Root hair-specific EXPANSIN A7 is required for root hair elongation in Arabidopsis and rice root hairs lacking these proteins stay short.17PubMed. Root hair-specific expansins modulate root hair elongation in rice For the tip to extend, its wall has to be soft enough to stretch while the cell’s inner pressure pushes outward. Behind the tip the wall hardens, which fixes the tube’s shape as the hair lengthens.
Two plant hormones, auxin and ethylene, both make root hairs longer.18PubMed. Auxin and ethylene promote root hair elongation in Arabidopsis They work together for root hair growth even though they clash elsewhere in the plant.19PubMed Central. Auxin, the organizer of the hormonal/environmental signals for root hair growth Ethylene teams up with a master switch protein to turn on a gene that sets hair length.20PubMed Central. Ethylene promotes root hair growth through coordinated EIN3/EIL1 and RHD6/RSL1 activity in Arabidopsis Dozens of genes control different stages, from starting the bulge to lengthening it.21PubMed Central. Genetic Control of Root Hair Development in Arabidopsis thaliana
Root Hairs and the Soil Around Them
When a hairy root was pulled out, it disturbed soil up to four times the root’s own width, while hairless roots barely moved it.22PubMed Central. Root hairs aid soil penetration by anchoring the root surface to pore walls The hairs were gripping soil grains. A hairless root slid out with little hold. This grip anchors the root tip so it can push forward through the soil.23PubMed Central. Root hairs aid soil penetration by anchoring the root surface to pore walls For holding soil against sliding, the thicker roots matter far more than their hairs.24Geoderma. Do root hairs of barley and maize roots reinforce soil under shear stress?
Root hairs also work with other organisms. In peas, beans and clover, nitrogen-fixing bacteria enter through a root hair that curls around them and traps them. The hair then builds an inner tunnel that carries them in, starting 10 to 20 hours after curling.25PubMed Central. Remodeling of the infection chamber before infection thread formation reveals a two-step mechanism for rhizobial entry into the host legume root hair The bacteria end up inside the root, where the plant houses them in bumps called nodules. There they turn nitrogen from the air into plant food.
Helpful root fungi change how many root hairs grow and how long they get, depending on phosphorus levels.26Agriculture. Interactions between Root Hair Development and Arbuscular Mycorrhizal Fungal Colonization in Trifoliate Orange Seedlings in Response to P Levels Root hairs can also help prime the plant’s defences when beneficial soil bacteria are present.27PubMed Central. The Root Hair Specific SYP123 Regulates the Localization of Cell Wall Components and Contributes to Rizhobacterial Priming of Induced Systemic Resistance
For gardeners, this explains transplanting shock. Moving a plant strips many hairs, so it wilts until new ones regrow. Keep the root ball intact and water well after moving.
What Makes Root Hairs Grow Longer or Shorter
Many plants grow longer root hairs when phosphorus is scarce.28Functional Plant Biology. The utility of phenotypic plasticity of root hair length for phosphorus acquisition Longer hairs reach more soil, which matters because phosphorus does not move far in soil, and the hairs can reach beyond the zone the plant has already emptied. How strongly plants do this varies between varieties,29Functional Plant Biology. The utility of phenotypic plasticity of root hair length for phosphorus acquisition which is why breeders are interested. Nitrate and ammonium in the soil also change root hair length and density in thale cress.30PubMed. Regulation of length and density of Arabidopsis root hairs by ammonium and nitrate If crops had better root hairs, they might manage with less fertiliser and water.31PubMed. Significance of root hairs in developing stress-resilient plants for sustainable crop production
Root hairs are not the same as fine roots. A fine root is made of many cells and has its own tip, while a root hair is one cell’s extension. Fine roots last longer; hairs are short-lived.
Gardeners can still look after the hairs they have. Loose, crumbly soil lets hairs spread between grains, while hard, packed soil gives them little room. Avoid compacting beds by walking on them, since squashed soil has fewer air gaps and pores. Water evenly, because swinging between drowning and drying damages the fine hairs. Feed modestly, since heavy feeding can make the soil salty enough to stop water entering.