1. Introduction
The root is the bottom part of the plant body axis. It is usually under the ground, although there are some aerial and aquatic roots. The total roots of a plant is known as the root system. The main functions of roots are to anchor the plant to the ground and absorb water and minerals. Roots may perform other functions: storage organ in beets, carrots, or sweet potatoes; hormone synthesis; aeration in aquatic plants; as an organ for plant propagation; and so on. The roots of most plants are associated with some types of fungi to form mycorrhizae, and others, such as leguminous, with some symbiotic bacteria to form nodules. These mutualistic associations greatly improve the absorption of nitrogenous substances by roots. Roots depend on organic molecules synthesized by the aerial photosynthetic organs.
All vascular plants can grow true roots, although they are not formed in some primitive and some epiphyte plants. Some non-vascular plants, like mosses and hepatic plants, show underground structures called ryzhoids that anchor the plant body to the ground and absorb water, but they lack vascular bundles. True roots always have vascular bundles containing xylem and phloem.
Growth rate and extension of roots depend on environmental variables, such as moisture, temperature, season, plant species, and many others. It is estimated that corn roots may grow 5 to 6 cm per day, common grass roots around 10 to 12 cm per day, and tree roots about 3 to 5 cm per day. The depth that roots may reach in the ground is also variable depending on the ground features and plant species, and it can go from a few cm in herbaceous plants to tens of meters in tree roots. The deeper roots usually make plants more resistant to drought.
Root growth and development depend on the organic molecules and hormones coming from the stem. For instance, root growth is reduced during fruit and seed formation. The relation between the size of the stem and the size of the root may vary in different species, and it is influenced by the age of the plant and environmental variables. The ratio root/stem may rank as 0.12 in a tropical forest, 0.5 in corn, and 3 in beet.
Root branches die very often, which depends on the species and season of the year. Thus, plants need to produce new branches continuously, not only to increase the overall size of the root system, but also to maintain it in a steady state. This process demands a lot of energy, mostly from photosynthesis. Roots may use 50–70% of the photosynthesis production of a plant. It is not clear why a plant needs so many roots, but it can be related to the constant search for water and nutrients.
Some tree species have the ability to fuse the roots of different individuals. This behavior has been observed in some tropical species, many pine species, and other angiosperm trees. These connections allow direct communication between a group of individuals through their roots, and therefore they can share water resources, minerals, and other nutrients.
2. Organization
Roots can be embryonic or post-embryonic. Embryonary roots develop during germination and the first stages of plant development. Post-embryonary roots may grow from the stem nodes and from secondary roots, and their role is more relevant in later plant developmental stages.
The root is the first structure that sprouts out of the embryo in the seed. This initial root is known as the radicle. The root system may form from a main root derived from the radicle that branches many times, giving lateral roots. This root disposition is typical of gymnosperms and dicotyledons. In monocot, however, the embryonary root dies or does not produce any new root, and all roots of the adult plant emerge from the stem, so all of them are adventitious and non-branched roots.
The overall organization of the root system is a conserved feature in a group of plants. Gymnosperms and dicot plants show a main or primary root derived from the radicle and many branches or lateral roots (Figure 1). This type of root system is referred to as an axonomorph system or tap root system. They extend very deep into the ground. In axonomorph roots, the main root is conserved throughout the life span of the plant, and it is as old as the plant. However, there is a high mortality rate in the minor lateral branches. It has been estimated that a large part of the smaller lateral branches leave just for a few days, although they constitute the larger proportion of the root system and most of the root tips.
In many monocotyledon species, the primary root is only important during the first stages of development. It is later substituted by many roots emerging from the stem or leaves. All these roots have more or less the same size and length and form a root system known as fibrous roots (Figure 1). They don't get deep in the soil, but they provide good anchorage for the plant body. That is why species with fibrous roots are suitable to prevent soil from being lost by erosion.
The final morphology of the root system is influenced by the soil features, including hard and soft regions, more o fewer cracks, which facilitate or hinder the growth of the roots. A good or poor adaptation of the root system to the ground and its water content depends on the depth the roots are able to reach, the density of radical hairs, and the renewing rate of lateral branches. The main root shows stronger gravitropism, and the angle of the lateral roots with the main root depends on the effects of the auxin hormone. Thus, a lack of auxin leads to lateral roots not showing gravitropism.
Roots emerging in the adult plant from the stem, leaves, or roots (differently from the lateral roots of axonomorph root systems) are known as adventitious roots. These roots sprout out after the embryonary period from preformed dormant meristems or from cells near the vascular bundles of the stem and leaves. Some adventitious roots show chloroplasts in their cells. For instance, the roots of ivy are aerial roots, which develop from the stem or leaves. Some plants can propagate through stolons, including strawberries, African violets, or shoots like blackberries. Roots can be generated by these propagating structures. The adventitious roots that are formed following a natural developmental plant are referred to as latent roots, whereas those generated after damage or environmental influence are known as induced roots.
3. Root regions
From the tip to the more mature parts, several regions can be distinguished in roots (Figure 2). Although these regions are observed in most roots, they may show different lengths according to the species and environmental variables.
Apical region. The apical root meristem is found in the apical zone, covered by a protective layer known as the root cap, or calyptra. A group of initial cells (undifferentiated cells) are found in the apical root meristem that give rise to every cell type in the root. Initial cells show a low division rate. Near the initial cells, the procambium, grown meristem, and protodermis are meristems that differentiate into vascular bundles, cortex and epidermis, respectively. The initial cell surrounds the quiescent center, a group of cells that seems to control the behavior of the initial cells. Besides protecting root meristems, the root cap releases mucilaginous substances and dead cells that become lubricants for facilitating the growth and preventing mechanical friction of growing roots.
The root grows through proliferation and elongation of the cells produced by the apical root meristem. Both processes need organic molecules coming from the production and storing centers of the plant body through the vascular system. However, the functional vascular system is several millimeters away from the meristem. Hence, proliferating and elongating cells are also involved in feeding the meristem (see below).
In the central zone of the root cap, there are two columns of cells, collectively known as columnella. There is a gravity sensor in some columnella cells that allows the root to grow to the center of the earth. It is the positive gravitropism of roots. The cells that sense gravity are called statocysts. They are large cells with only a few organelles and cytoskeletal filaments in the center of the cytoplasm. The nucleus is found in the upper half of the cell, and the endoplasmic reticulum and many other organelles are toward the cell periphery. Statocysts contain amyloplasts known as statoliths, which are denser than the cytosol, and therefore they sediment at the bottom part of the cell. Statoliths interact with the plasma membrane, triggering a chemical reaction that releases the hormone auxin, which is transported to the lateral parts of the root. The lateral levels of auxin change according to gravity, and the curvature of the root is modified. This mechanism is absent for some time in the lateral branches, and it appears when they reach some length.
The cell division zone starts after the initial cells. Here, cells divide quite frequently to increase the total number of cells and form all the root tissues. All cells in this zone are going to be differentiated when they stop proliferating.
The elongation zone region of a few millimeters in length where cells increase in size. Roots may grow in length through this cell elongation as well as by the addition of new cells in more apical zones.
In the maturation zone, cells begin to differentiate and get specific cellular features to be functional in each tissue that forms the mature root. Root hairs differentiate from epidermal cells in this region.
The borders between these regions are not clearly defined. For example, cells that become vascular bundles start the differentiation process in the elongation region. In addition, the regions are moving together with the apical root tip during root growth.
Roots may show primary and secondary growth. Primary growth is largely involved in the root length increase, whereas secondary growth leads to a larger diameter. The type of growth and the group of plants (monocot and dicot) are the features we are going to consider when studying the microscopic anatomy. Because roots lack nodes and internodes, the general anatomy is quite similar along the root extension.
4. Functions
Anchoring
Anchoring is an important function performed by roots that is often taken for granted. However, it is essential for plant survival because it keeps plants erect and allows them to withstand adverse environmental conditions like wind, rain, snow, herbivores, supporting the plant body structure, and many others. Indirectly, roots also help to retain the soil.
It is worth noting that stem and root growth have to be coordinated because one depends on the other. For that, there are systemic signals through the plant body. For instance, the root can synthesize small peptides when the nutrients are lacking that travel to the stem and inhibit growth. Other molecules, such as small RNAs, may do the same, but to inform the root about what is happening in the stem.
Water absortion
Water absorption is another essential function of roots, although it is not clear what part of the root is more important for this function. Not much water enters at the apical radical meristem level, mostly because there is no functional vascular system. Root hairs are supposed to be a major entry pathway for water. However, it is also plausible that other regions that are more mature and even suberized, may have an important role in water absorption because non-suberized regions are scarce and small, so it is difficult to explain how the total water needed can enter through them. In some species, a large part of the water absorption is done by the fungi hyphae (mycorrhizae) associated with roots. Anyway, the water passively streams from the soil to the vascular system of the root through cell-cell communication mediated by plasmodesmata. However, water is not totally free to cross cell membranes, so cells can modulate the water flux through water channels known as aquaporins. Nutrients from the soil also enter the plant through the roots. Cells have membrane transporters that let nutrients come into the cell and then are transported from cell to cell through plasmodesmata.
As mentioned above, fungi and roots associate to get water, but also nutrients like nitrogen and phosphorous. Fungi may form endo or ectomycorrhizae. Endomycorrhizae invade the root tissues and branches within the cells, whereas ectomycorrhizae remain outside the cytoplasm. The hyphae extend through the soil and greatly increase the area to search for water and minerals. The plant provides organic molecules to the fungi.
Roots are responsible for finding water by growing in length through the ground. This searching makes roots an important soil transformer, not only because they can break rocks and retain soil, that otherwise is dragged away by rain and wind. On the other hand, the roots are influenced by soil features, such as water content, porosity, and nutrients. Soil is usually classified according to the proportion of sand, clay, and lime. The size of the particles is important because it determines the porosity of the soil. During growth, a root can cross ground regions with different features that are adapted to each of them. Roots can also affect the features of the closest soil. For instance, the root cap releases mucilage (polysaccharides) as a type of exudate, which recruits microorganisms that change the peri-root environment's properties. Curiously, this exudate, known as "rhyzosheath" is larger in dry soils and is absent from more mature regions of root. It can favor the retention of water and minerals around the root.
Nitrogen absortion
Nitrogen is a crucial element for plants and animals, which is uptake by the roots and incorporated in molecules by a process known as nitrogen fixation. Most nitrogen is in gas form, which cannot be directly used to form proteins. Many plants engage in a mutualist symbiotic relationship with bacteria (Rhyzhobium), which can fix the gas nitrogen. This association can be readily observed because a structure known as nodules is formed in the roots. Nodules consist of a core containing the bacteria, surrounded by root vascular bundles, and all wrapped by the endondermis.
Other functions
Some roots are specialized in less typical functions. For example, plants growing in wet or puddled soils usually have aerial roots that facilitate the exchange of gases, as in manglers or floating aquatic plants. In parasitic plants, roots are adapted to get nutrients from other plants. The carrot and beet roots are storage organs. In soil regions with low phosphorous content, some dicot plants grow groups of small roots that release organic acids and phosphatases that help to absorb phosphorous. There are contractile roots in monocots and dicots. They appear in plants that forms bulbs or corms, such as the saffron or dandelion. The root contraction brings the stem closer to the ground surface, so the bulb is in a good position. They are called geophyte plants. The root contraction is a consequence of the volume changes in the perenchyma cortical cells.
4. Evolution
All vascular plants have true roots showing positive gravitropism and a root cap protecting the apical tip. The first land plant did not grow true roots, but rhizoids. Fossils indicate that true roots have been independently discovered several times during evolution by seed plants, lycophytes, and ferns. The ancestor of vascular plants lacks true roots, as do those that gave rise to lycophytes and seed plants; therefore, true roots in these groups were independently developed. Roots have not changed much in their general morphology during evolution, probably because the selection pressure of the ground is not as variable as the aerial environment.
Lycophytes have roots with ancestral features like branching by bifurcation and lack endodermis. Their root meristems may be uni- or multi-cellular. Euphyllophytes do not branch by bifurcation. The roots of ferns generate secondary roots from the endodermis and seed plants from the pericycle. Roost shows a proto-stelle, or an organization that may derive from a proto-stelle. This is a feature of primitive plant stems, so it is suggested that the root emerged as a stem adaptation to the land.
Rizhoids are unicellular or multicellular root-like structures that emerge from the gametophyte of bryophytes, lycophytes and monilophytes. True roots emerge from the sporophyte of vascular plants.
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Bibliography ↷
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Rost TL. 2011. The organization of roots of dycotiledonous plants and the positions of control points. Annals of botany. 107: 1213-1222. doi: 10.1093/aob/mcq229.
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