As these processes happen deep within the parental root, it is critical that overlying tissues undergo cell separation to allow primordium emergence. In Arabidopsis thaliana , three different tissues have to be crossed: the endodermis, the cortex, and the epidermis, each of them composed of one cell layer, resulting in a dramatic impact on primary root structure Fig. Formation of a lateral root strongly impacts the primary root structure.
Before the production of a new LR the epidermal cells are tightly attached A. During the emergence process, the outer tissues undergo cell separation as the primordium pushes its way towards the rhizosphere B, C. Once the LR formation process is over, the primary root still bears the effects of the emergence process D. Note the formation of large gaps between the epidermal cells at sites of emergence arrows. The reasons for such a peculiar developmental pattern remain unclear: protection of the meristematic tissues from external damage, adaptation to a very heterogeneous environment to integrate numerous external and internal signals or simply connection to the primary root vasculature?
The fact is that the mechanisms by which LR emergence is controlled has puzzled scientists for centuries. Several recent reviews have described LR formation as a whole in Arabidopsis Nibau et al. This review focuses on the LR emergence process. The first section will summarize the information gathered by cell biologists over the past decades on lateral root emergence in different plant species.
The second section will focus on more recent work demonstrating the importance of shoot-derived auxin during emergence in Arabidopsis. Finally, the last section will summarize recent findings describing the molecular mechanism of auxin action during LR emergence.
The model plant Arabidopsis thaliana has a very simple root anatomy Dolan et al. The stele consists of a diarch vascular cylinder surrounded by a layer of pericycle cells. The outer tissues are composed of one layer of endodermis, cortex, and epidermis Fig.
Lateral roots originate from a subset of pericycle cells situated in front of the xylem poles Dolan et al. Since the pericycle is located deep within the root, new primordia have to break through the three overlying outer layers of cells Fig. Root sections of emerging lateral root primodia in the two model species Arabidopsis thaliana and Oryza sativa rice.
The Arabidopsis root has a simple structure composed of the stele surrounded by three one-cell layers A. The emerging LR primordium reprogrammes the outer cells to promote cell separation B. Rice has a complex root system made of different root types radicle, embryonic crown root, crown root, large lateral root, and small lateral root.
The rice crown root is composed of the stele that is surrounded by five layers accounting for 5—15 cells in total C. Emergence of a rice LR primordium involves more cell layers and is probably highly regulated D. Cereals have a very different root architecture Hochholdinger et al. In rice, the pericycle is surrounded by the endodermis, the mesodermis, the sclerenchyma layer, the exodermis, and the epidermis Rebouillat et al.
These tissues are composed of up to 20 layers of cells, making the emergence of LRP a challenging biomechanical process Fig.
Whilst the outer tissues represent a barrier that primordia have to pass through, these tissues do not play a passive role during lateral root emergence. The first evidence that outer tissues facilitate emergence is the observation that cell divisions occur in front of the growing primordium.
Endodermal cells adjacent to the LRP can undergo tangential or anticlinal divisions in many dicots Lloret, The new endodermal cells do not form a Casparian strip Peterson and Peterson, probably to facilitate the primordium divisions.
Cortical cell divisions in Allium cepa Casero et al. It is thought that the production of small cells or radial cell walls will facilitate the emergence of the primordium. Moreover, the dividing outer cells can be incorporated within the primordium and participate in LR tissue formation.
Several layers of cortical cells seem to dedifferentiate and to be incorporated into primordia of Cucurbita pepo Charlton, In Cucurbita maxima , the endodermis appears to contribute to the cortex, the epidermis, the cap, and the vascular tissues of the lateral root Mallory et al. The dividing cells can also form temporary structures covering the primordium that are eventually degraded McCully, Indeed, modification of cell wall composition of the outer tissues appears to be necessary to weaken the tight junctions observed between those cells.
This is elegantly demonstrated by the inability of LR to emerge in sulphide-treated rice roots. Sulphide toxicity in soil triggers suberization and thickening of walls within the exodermis and epidermis whereas the mesodermis walls are not modified. LRP growth is not affected in its early stage but when it reaches the two outermost tissues, its progression towards the rhizosphere is blocked Armstrong and Armstrong, Amazingly, the lateral root starts growing upward within the primary root mesodermis Armstrong and Armstrong, This example clearly illustrates the importance of cell wall weakening during emergence.
In summary, diverse strategies have evolved to promote LR emergence in higher plants. The very existence of these regulatory mechanisms shows that an important selective pressure must exist on the ability for a LR to emerge. Considerable progress has recently been made describing the role of auxin during lateral root formation in Arabidopsis Fukaki et al. In the young seedling up to 4 d after germination, auxin is mainly synthesized in the shoot and transported to the root via two pathways of equal importance: phloem-mediated transport and polar transport.
By 8 d, phloem transport becomes the dominant pathway Ljung et al. This aerial source of auxin is required to promote LR emergence Bhalerao et al. Indeed, removal of the leaves and cotyledons blocks LR emergence Swarup et al. Roots are also a source of auxin, however LR acquire the ability to synthesize their own auxin only after emergence Ljung et al. It is therefore unlikely that this source of auxin plays a role in emergence.
LR initiation is regulated by auxin originating from the root tip Casimiro et al. Despite these distinct sources, a competition appears to exist between the two processes as shown by statistical analysis and in silico modelling backed up by experimental evidences Lucas et al. This suggests that LR initiation and emergence are distinct but yet interconnected developmental processes competing for the same source of auxin. Auxin modifies cell fate and activates cell division during LR initiation whereas during emergence, auxin is linked with cell separation Boerjan et al.
However, the induction by auxin of a set of genes involved in cell separation brings the question whether the primordium is producing its own enzymes or if they are produced by the outer tissues. Associated with LR primordium growth, these enzymes can facilitate cell separation during emergence.
However, the massive production of cell wall remodelling enzymes at sites of LR emergence does not affect LR primordium integrity.
This might be explained by a difference in cell wall composition between the LR primordium and the parental root. The pectin in the developing LR is largely methylated while that in the overlying cells of the parent root has become demethylated under the action of pectin methyl esterases Laskowski et al.
Together with targeted gene expression, this would restrict cell wall remodelling activity to cells in the outer tissues. The mechanism responsible for the localized pattern of expression of cell wall remodelling genes was recently deciphered in Arabidopsis.
In this region the xylem cells are the first of the vascular tissues to differentiate. Mature Root : the primary tissues of the root begin to form within or just behind the Zone of Cellular Maturation in the root tip.
The root apical meristem gives rise to three primary meristems: protoderm, ground meristem, and procambium. Epidermis : the epidermis is derived from the protoderm and surrounds the young root one cell layer thick. Epidermal cells are not covered by cuticle so that they can absorb water and mineral nutrients. As roots mature the epidermis is replaced by the periderm.
Cortex : interior to the epidermis is the cortex which is derived from the ground meristem. The cortex is divided into three layers: the hypodermis, storage parenchyma cells, and the endodermis. The hypodermis is the suberinized protective layer of cells just below the epidermis. The suberin in these cells aids in water retention.
Storage parenchyma cells are thin-walled and often store starch. The endodermis is the innermost layer of the cortex. Endodermal cells are closely packed and lack intercellular spaces. Their radial and transverse walls are impregnated with lignin an suberin to form the structure called the Casparian Strip. The Casparian Strip forces water and dissolved nutrients to pass through the symplast living portion of the cell , thus allowing the cell membrane to control absorption by the root.
Stele : all tissues inside the endodermis compose the stele. The stele includes the outer most layer, pericycle, and the vascular tissues. The pericycle is a meristematic layer important in production of branch roots. The vascular tissues are made up of the xylem and phloem.
In dicots the xylem is found as a star shape in the center of the root with the phloem located between the arms of the xylem star. New xylem and phloem is added by the vascular cambium located between the xylem and phloem. In monocots the xylem and phloem form in a ring with s the central portion of the root made up of a parenchymatous pith.
Root structures may be modified for specific purposes. For example, some roots are bulbous and store starch. Aerial roots and prop roots are two forms of aboveground roots that provide additional support to anchor the plant.
Tap roots, such as carrots, turnips, and beets, are examples of roots that are modified for food storage Figure. Epiphytic roots enable a plant to grow on another plant. For example, the epiphytic roots of orchids develop a spongy tissue to absorb moisture. The banyan tree Ficus sp. In screwpine Pandanus sp. Roots help to anchor a plant, absorb water and minerals, and serve as storage sites for food. Taproots and fibrous roots are the two main types of root systems. In a taproot system, a main root grows vertically downward with a few lateral roots.
Fibrous root systems arise at the base of the stem, where a cluster of roots forms a dense network that is shallower than a taproot. The growing root tip is protected by a root cap. The root tip has three main zones: a zone of cell division cells are actively dividing , a zone of elongation cells increase in length , and a zone of maturation cells differentiate to form different kinds of cells.
Root vascular tissue conducts water, minerals, and sugars. In some habitats, the roots of certain plants may be modified to form aerial roots or epiphytic roots. Compare a tap root system with a fibrous root system. For each type, name a plant that provides a food in the human diet. Which type of root system is found in monocots?
Which type of root system is found in dicots? A tap root system has a single main root that grows down. A fibrous root system forms a dense network of roots that is closer to the soil surface. An example of a tap root system is a carrot. Grasses such as wheat, rice, and corn are examples of fibrous root systems. Fibrous root systems are found in monocots; tap root systems are found in dicots. Skip to content Plant Form and Physiology. Learning Objectives By the end of this section, you will be able to do the following: Identify the two types of root systems Describe the three zones of the root tip and summarize the role of each zone in root growth Describe the structure of the root List and describe examples of modified roots.
Types of Root Systems Root systems are mainly of two types Figure.
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