Roots and Root Systems Introductory article Nancy M Kerk, Yale University, New Haven, Connecticut, USA Article Contents Ian M Sussex, Yale University, New Haven, Connecticut, USA . Introduction . Parts of the Root System Roots, together with stems and leaves, constitute the major organs of vascular plants. The . The Root Apical Meristem main functions of the root system are to anchor and support the plant, to seek out, absorb, . Anatomy and Histology and transport water and ions from the soil, and to transport and store the products of . Root Branching and the Origin of Lateral Roots photosynthesis from the shoot system. Root Specializations . Root Physiology and Function . Root Genetics Introduction . Root Hairs . Signalling Systems Roots, together with stems and leaves, constitute the major organs of vascular plants. Nonvascular plants such as algae and bryophytes and the most primitive group of vascular plants, the Rhyniophyta, lack a root system, and The Root Apical Meristem other organs perform its functions. The major functions of Each root grows in length through the activity of a the root system are to anchor and support the plant, to seek meristem located at its tip. A meristem may be defined as a out, absorb, and transport water and ions from the soil, population of cells which by their mitotic and cell division and to transport and store the products of photosynthesis activity generate the cells which contribute to the develop- from the shoot system. These functions are facilitated by ing organ and maintain themselves as initial cells. expansion of the root system through continued growth of Meristem cells have frequently been referred to as stem the root tips which are also regions of gravity and moisture cells because of their ability to self-perpetuate. perception. Roots may therefore penetrate to remarkable In the different groups of vascular plants, the cellular depths into the soil. For example, roots of the desert shrub organization of meristems is quite diverse, but their mesquite (Prosopis juliflora) were found at a depth of 53 ultimate functions are similar. In most lower vascular metres below the soil surface. The total extent of the root plants the root meristem is dominated by a single enlarged system may exceed by many times that of the shoot system tetrahedral cell, the apical cell. In gymnosperms and of the same plant. For example, in a plant of winter rye angiosperms there is no single cell or small group of cells (Secale cereale) the surface area of the root system is 130 that differ greatly in size from other cells in the meristem. times that of the shoot system. However, because most Instead, cells are arranged in layers and files that radiate roots are very narrow, the biomass of the root system is out from a common centre. In some plants these cell layers usually less than that of the shoot system. Typical root/ are very conspicuous and converge at the tip of the shoot ratios are in the range 0.2–0.4. meristem. This type of meristem is referred to as closed and is further defined by a sharp boundary between the root tip and root cap. The meristem of Arabidopsis is an extremely Parts of the Root System reduced form of a closed meristem because of the small number of cells that comprise it. The first root of a plant is called the radicle and it originates Within the root apical meristem, the frequency of cell in the embryo. In germinating gymnosperm and dicotyle- divisions is unequal. At the tip there is a group of cells in don seeds the radicle develops as the primary root which which cell division is essentially nonexistent or cell cycle grows directly downward as the tap root and initiates times are very long, in some cases in excess of one week. For lateral roots. This is called a tap root system. Branching this reason, this part of the meristem has been termed the can be elaborated further by the formation of other orders quiescent centre. Proximal to the quiescent centre the rate of lateral roots. In these plants, the root system has its of cell division increases rapidly and this region is referred origin in the embryo independent of the shoot system and is to as the proximal meristem. Distal to the quiescent centre termed allorhyzic. In monocotyledons and the ‘non-seed’ is a layer of cells called the distal or root cap meristem that vascular plants the primary root may be short-lived and the initiates all or much of the root cap. In other plants cell main root system of the plant develops from roots that layering is less conspicuous in the meristem and there is no arise from the shoot either adventitiously or from nodal discrete boundary between the meristem and the root cap. regions. This is called a fibrous root system and is termed This is referred to as an open meristem. homorhyzic because the functional root system originates from shoots. ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net 1 Roots and Root Systems Anatomy and Histology typically elongated tracheids or vessel elements. In the latter the cell end walls are perforated so that the cells form Proximal to the root apical meristem, cells that are derived a continuous tubular system whereas in tracheids the end from it undergo progressive differentiation to form the walls are not perforated and they are connected in tissues and cell types of the mature root. During these staggered files with pit connections that permit water processes, cells are dividing and elongating, and this results movement from cell to cell. The secondary walls of xylem in the root tip being pushed forward through the cells are thick and lignified in patterns that may be annular substratum. Because of the restriction of certain develop- or spiral ringlike thickenings in protoxylem or more mental events to particular regions of the root, zones extensively thickened in scalariform or pitted patterns in recognized as the zone of cell division, the zone of metaxylem. The conducting elements of the phloem elongation, the zone of differentiation, and the zone of transport photosynthates from the shoot system which maturation have been identified. However, there is overlap support root growth and metabolic processes and which of the boundaries of these zones and the zones themselves may be accumulated in storage tissues in the root. The may have different extents at different times during root principal cell types in the phloem are sieve tube elements, development and in different species. which are the conducting cells, and associated companion In the mature root, tissues are arranged in concentric cells. The end walls of the sieve tube elements contain cylinders. The external cylinder is the epidermis. In most perforated sieve plates that permit movement of materials roots this consists of a single cell layer. Some of the from one element to the next. This interconnected series is epidermal cells produce tubular outgrowths which are the called a sieve tube. root hairs. Internal to the epidermis is the cortex which is The root apical meristem actually occupies a subterm- usually several cell layers in thickness, but consists of only inal position at the root tip because in most species of two cell layers in Arabidopsis. The majority of cortical cells vascular plants it is covered by the root cap. The root cap is are parenchymatous and function as storage cells for the formed by anterior derivatives of the apical meristem and it plant. The outermost layer of cortical cells may have probably serves a protective function as the meristem is lignified and suberized radial walls and are then distin- forced through the soil by elongation of the differentiating guished as the exodermis. The innermost cortical cell layer cells behind it. In ferns the root cap is initiated by a division has specialized walls and is identified as the endodermis. at right angles to the long axis of the root that cuts off a This layer is characterized by the presence of Casparian derivative cell from the anterior face of the tetrahedral strips in its radial cell walls. The Casparian strip is a region apical cell. In the higher vascular plants, root cap initial of the wall that is impregnated with suberin and sometimes cells that lie anterior to the quiescent centre divide lignin and its function is to prevent the direct apoplastic transversely to form files of columella cells. In some roots movement of water and ions between the cortex and the the entire root cap appears to be initiated in this way but in vascular cylinder. others lateral parts of the root cap share a common origin Internal to the endodermis is the vascular cylinder. The with cells of the root epidermis. At the surface of the root outermost one or few cell layers of the vascular cylinder is cap mature cells become separated from one another and the pericycle. The pericycle is the principal site of lateral slough off the root tip together with mucigel that lubricates root initiation. Internal to the pericycle are alternating the root penetration of the soil. These cells have been bands of xylem and phloem. In the dicotyledons xylem cells named ‘border cells’ and they perform special functions in typically extend to the centre of the root, but in most relation to root–microbial interactions. monocotyledons the centre of the root is occupied by a pith composed of parenchyma cells. In the monocotyledons there are numerous separate bundles of xylem and phloem that are arranged in a cylinder. In dicotyledons the number Root Branching and the Origin of of alternating xylem and phloem units is much smaller, Lateral Roots typically numbering between two and seven and this number is relatively consistent within each species.