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 surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.

Function

Roots perform several essential and specialised roles that support plant growth, development and survival. Their primary functions are anchorage, uptake of water and dissolved minerals, and conduction of these resources to the shoot. Beyond these, roots carry out a range of important secondary and adaptive functions — storage of reserves, synthesis of growth regulators, gas exchange in waterlogged environments, facilitation of symbiotic nutrient acquisition, and vegetative propagation.

Primary functions

Anchorage and mechanical support. Roots fix the plant in the substrate and resist mechanical forces ; vigorous root systems are essential for crop stability and prevention of lodging.
Absorption of water and mineral nutrients. Root epidermal cells and root hairs take up soil water and dissolved ions; these are then conducted to the shoot via the xylem. Root architecture determines access to water and nutrients in different soil horizons.

Secondary and specialised functions

Storage of reserve food and water. Many roots accumulate carbohydrates and water that support perennial growth or regrowth after dormancy. Examples include sweet potato and carrot.
Conduction. Roots conduct absorbed water, minerals and some synthesised compounds to the shoot via vascular tissues.
Synthesis of growth regulators. Roots synthesise and modulate plant hormones, influencing shoot development and whole-plant physiology.
Aeration and gas exchange. Specialised structures permit gas exchange where soil oxygen is limited, enabling respiration in mangroves and other hydrophytes.
Symbiotic interactions and enhanced nutrient uptake. Roots form mutualisms that greatly increase nutrient acquisition and stress tolerance. These associations remodel root architecture and function.
Mechanical adaptations and additional support. Adventitious and brace/prop roots provide extra support, anchorage and stability in particular environments.
Vegetative propagation. Adventitious rooting on stems, nodes or cuttings allows many species to reproduce asexually and to regenerate after damage; this is widely exploited in horticulture.

Types of roots (major rooting system)

Plants produce a variety of root systems that differ in origin, structure and function. The two classical, broad categories are taproot and fibrous systems, but several specialised root types — notably adventitious, aerial, prop/stilt, climbing/adhesive, buttress, tuberous and floating roots — are biologically and ecologically important.

Taproot system

A taproot system is dominated by a single, vertically growing primary root from which lateral roots arise. Taproots often function in deep anchorage and in storage of carbohydrates and water. Examples include carrot, dandelion and many true dicots.

Fibrous root system

A fibrous root system consists of numerous, similarly sized roots that form a dense network near the soil surface. In many species this network is composed largely of adventitious roots that arise from the stem base rather than the primary radicle. Fibrous systems are effective at soil binding, rapid uptake of surface nutrients and erosion control. Typical examples are grasses, wheat and rice.

Adventitious roots

Adventitious roots arise from non-root organs and play multiple roles: replacing or supplementing the primary root, providing mechanical support, enabling vegetative propagation, and forming specialized root types. They are especially important in monocots and many cultivated plants, and are a common response to wounding, flooding or other stressors.
Examples and notes: in many monocots the functional root system is adventitious ; maize produces nodal brace/prop roots that stabilise the stem; banyan develops aerial adventitious roots that may become supportive trunks; many cuttings root adventitiously during vegetative propagation.

Anatomy

Root morphology is divided into four zones: the root cap, the apical meristem, the elongation zone, and the hair. The root cap of new roots helps the root penetrate the soil. These root caps are sloughed off as the root goes deeper creating a slimy surface that provides lubrication. The apical meristem behind the root cap produces new root cells that elongate. Then, root hairs form that absorb water and mineral nutrients from the soil. The first root in seed producing plants is the radicle, which expands from the plant embryo after seed germination.
When dissected, the arrangement of the cells in a root is root hair, epidermis, epiblem, cortex, endodermis, pericycle and, lastly, the vascular tissue in the centre of a root to transport the water absorbed by the root to other places of the plant.
Perhaps the most striking characteristic of roots that distinguishes them from other plant organs such as stem-branches and leaves is that roots have an endogenous origin, i.e., they originate and develop from an inner layer of the mother axis, such as pericycle. In contrast, stem-branches and leaves are exogenous, i.e., they start to develop from the cortex, an outer layer.
In response to the concentration of nutrients, roots also synthesize cytokinin, which acts as a signal as to how fast the shoots can grow. Roots often function in storage of food and nutrients. The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizae, and a large range of other organisms including bacteria also closely associate with roots.

Root system architecture (RSA)

Definition

In its simplest form, the term root system architecture refers to the spatial configuration of a plant's root system. This system can be extremely complex and is dependent upon multiple factors such as the species of the plant itself, the composition of the soil and the availability of nutrients. Root architecture plays the important role of providing a secure supply of nutrients and water as well as anchorage and support.
The configuration of root systems serves to structurally support the plant and compete with other plants for uptake of nutrients within the soil. Roots grow in response to specific ecological conditions, which, if changed, can impede a plant's growth. For example, a root system that has developed in dry soil may not be as efficient in flooded soil, yet plants are able to adapt to other changes in the environment, such as seasonal changes.

Terms and components

The main terms used to classify the architecture of a root system are:

Branch magnitude	Number of links
Topology	Pattern of branching
Link length	Distance between branches
Root angle	Radial angle of a lateral root's base around the parent root's circumference, the angle of a lateral root from its parent root, and the angle an entire system spreads.
Link radius	Diameter of root

All components of the root architecture are regulated through a complex interaction between genetic responses and responses due to environmental stimuli. These developmental stimuli are categorized as intrinsic, the genetic and nutritional influences, or extrinsic, the environmental influences, and are interpreted by signal transduction pathways.
Extrinsic factors affecting root architecture include gravity, light exposure, water and oxygen, as well as the availability or lack of nitrogen, phosphorus, sulphur, aluminium and sodium chloride. The main hormones and respective pathways responsible for root architecture development include:

Auxin	Lateral root formation, maintenance of apical dominance and root formation.
Cytokinins	Cytokinins regulate root apical meristem size and promote lateral root elongation.
Ethylene	Promotes crown root formation.
Gibberellins	Together with ethylene, they promote crown primordia growth and elongation. Together with auxin, they promote root elongation. Gibberellins also inhibit lateral root primordia initiation.

Growth

Early root growth is one of the functions of the apical meristem located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells, and undifferentiated root cells. The latter become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues.
Growth from apical meristems is known as primary growth, which encompasses all elongation.
Secondary growth encompasses all growth in diameter, a major component of woody plant tissues and many nonwoody plants. For example, storage roots of sweet potato have secondary growth but are not woody. Secondary growth occurs at the lateral meristems, namely the vascular cambium and cork cambium. The former forms secondary xylem and secondary phloem, while the latter forms the periderm.
In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. The vascular cambium forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" of the stem and root increases. As a result, tissues beyond the secondary phloem including the epidermis and cortex, in many cases tend to be pushed outward and are eventually "sloughed off".
At this point, the cork cambium begins to form the periderm, consisting of protective cork cells. The walls of cork cells contains suberin thickenings, which is an extra cellular complex biopolymer. The suberin thickenings functions by providing a physical barrier, protection against pathogens and by preventing water loss from the surrounding tissues. In addition, it also aids the process of wound healing in plants. It is also postulated that suberin could be a component of the apoplastic barrier which prevents toxic compounds from entering the root and reduces radial oxygen loss from the aerenchyma during waterlogging. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity but are responsible for most water transport through the vascular tissue in stems and roots.
Tree roots usually grow to three times the diameter of the branch spread, only half of which lie underneath the trunk and canopy. The roots from one side of a tree usually supply nutrients to the foliage on the same side. Some families however, such as Sapindaceae, show no correlation between root location and where the root supplies nutrients on the plant.