"Procambium" redirects here. For geologic period, seePrecambrian.
Tunica-corpus model of the apical meristem (growing tip). The epidermal (L1) and subepidermal (L2) layers form the outer layers called thetunica. The corpus (L3) will form the vascular and stem tissues. Cells in the outer layers divide in a sideways fashion relative to each other, which keeps these layers distinct, whereas the lower layer divides in a more random fashion in all directions.[1]
Incell biology, themeristem is a structure composed of specializedtissue found in plants, consisting ofstem cells, known asmeristematic cells, which are undifferentiated cells capable of continuouscellular division. These meristematic cells play a fundamental role inplant growth,regeneration, andacclimatization, as they serve as the source of alldifferentiated plant tissues andorgans. They contribute to the formation of structures such as fruits, leaves, and seeds, as well as supportive tissues like stems and roots.[1]
Meristematic cells aretotipotent, meaning they have the ability to differentiate into anyplant cell type. As they divide, they generate new cells, some of which remain meristematic cells while others differentiate into specialized cells that typically lose the ability to divide or produce new cell types. Due to their active division and undifferentiated nature, meristematic cells form the foundation for the formation of new plant organs and the continuous expansion of the plant body throughout the plant's life cycle.
Meristematic tissues are classified into three main types based on their location and function:apical meristems, found at the tips of roots and shoots;intercalary orbasal meristems, located in the middle regions of stems or leaves, enablingregrowth; andlateral meristems orcambium, responsible forsecondary growth inwoody plants. At the summit of the meristem, a small group of slowly dividing cells, known as the central zone, acts as a reservoir of stem cells, essential for maintaining meristem activity. The growth and proliferation rates of cells vary within the meristem, with higher activity at the periphery compared to the central region.
The termmeristem was first used in 1858 by Swiss botanistCarl Wilhelm von Nägeli (1817–1891) in his bookBeiträge zur Wissenschaftlichen Botanik ("Contributions to Scientific Botany").[2] It is derived from Greek μερίζειν (merizein)'to divide', in recognition of its inherent function.[citation needed]
Apical meristems, also known as the primary meristem, give rise to the primary plant body and are responsible forprimary growth, or an increase in length or height.[3][4] Apical meristems may differentiate into three kinds of primary meristem:
Procambium: lies just inside of the protoderm and develops into primaryxylem and primaryphloem. It also produces thevascular cambium, andcork cambium (part of the secondary meristems but descendants of apical meristematic cells). The cork cambium further differentiates into thephelloderm, or bark, (to the inside) and thephellem, or cork (to the outside). All three of these layers (cork cambium, phellem, and phelloderm) constitute theperiderm. In roots, the procambium can also give rise to thepericycle, which produceslateral roots ineudicots.[5]
After the primary growth, lateral meristems develop as secondary plant growth. This growth adds to the plant in diameter from the established stem but not all plants exhibit secondary growth. There are two types of secondary meristems: the vascular cambium and the cork cambium.
Vascular cambium, which produces secondary xylem and secondary phloem. This is a process that may continue throughout the life of the plant. This is what gives rise towood in plants. Such plants are calledarboraceous. This does not occur in plants that do not go through secondary growth, known asherbaceous plants.[citation needed]
Cork cambium, which gives rise to the periderm, which replaces the epidermis with bark and cork for example.[citation needed]
Apical meristems
Organisation of an apical meristem (growing tip)
Central zone
Peripheral zone
Medullary (i.e. central) meristem
Medullary tissue
Apical meristems are the completely undifferentiated (indeterminate) meristems of a plant. They give rise to primary growth, enabling the elongation of shoots and roots. Apical meristems give rise to three types of primary meristems, which later develop into secondary or lateral meristems, contributing to the plant's lateral expansion.
There are two main types of apical meristems:shoot apical meristem (SAM) androot apical meristem (RAM). The SAM is located at the tips of shoots and produces leaves, stems, and flowers, while the RAM is found at the tips of roots and generates new root tissues. Both types consist of rapidly-dividing cells that remain indeterminate, meaning they continuously produce new cells without a predefined final state, similar tostem cells in animals, which have an analogous behavior and function.
Structurally, apical meristems are organized into distinct zones. The central zone serves as a reservoir of undifferentiated cells, while the peripheral zone generates new organs and tissues. The medullary meristem contributes to vascular development, forming the medullary tissue, which makes up the plant's central structure. The meristem layers also vary depending on the plant type. The outermost layer, called thetunica, determines the leaf edge and margin inmonocots, whereas indicots, the second layer of thecorpus influences leaf characteristics.
Apical meristems are generally found at the tips of roots and stems, but in somearctic plants, they are located in the lower or middle parts of the plant. Thisadaptation is believed to provide advantages in extreme environmental conditions.[citation needed]
Shoot apical meristems are the source of all above-ground organs, such as leaves and flowers. Cells at the shoot apical meristem summit serve as stem cells to the surrounding peripheral region, where they proliferate rapidly and are incorporated into differentiating leaf or flower primordia.[citation needed]
The shoot apical meristem is the site of most of the embryogenesis in flowering plants.[citation needed]Primordia of leaves, sepals, petals, stamens, and ovaries are initiated here at the rate of one every time interval, called aplastochron. It is where the first indications that flower development has been evoked are manifested. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as auxiliary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome.
The shoot apical meristem consists of four distinct cell groups:[citation needed]
Founder cells for organ initiation in surrounding regions
These four distinct zones are maintained by a complex signalling pathway. InArabidopsis thaliana, 3 interactingCLAVATA genes are required to regulate the size of thestem cell reservoir in the shoot apical meristem by controlling the rate ofcell division.[6]CLV1 and CLV2 are predicted to form a receptor complex (of theLRR receptor-like kinase family) to which CLV3 is aligand.[7][8][9] CLV3 shares somehomology with the ESR proteins of maize, with a short 14amino acid region beingconserved between the proteins.[10][11] Proteins that contain these conserved regions have been grouped into the CLE family of proteins.[10][11]
Another important gene in plant meristem maintenance isWUSCHEL (shortened toWUS), which is a target of CLV signaling in addition to positively regulating CLV, thus forming a feedback loop.[14]WUS is expressed in the cells below the stem cells of the meristem and its presence prevents thedifferentiation of the stem cells.[14] CLV1 acts to promote cellular differentiation by repressingWUS activity outside of the central zone containing the stem cells.[6]
The function ofWUS in the shoot apical meristem is linked to thephytohormonecytokinin. Cytokinin activateshistidine kinases which thenphosphorylate histidine phosphotransfer proteins.[15] Subsequently, the phosphate groups are transferred onto two types of Arabidopsis response regulators (ARRs): Type-B ARRS and Type-A ARRs. Type-B ARRs work as transcription factors to activate genes downstream ofcytokinin, including A-ARRs. A-ARRs are similar to B-ARRs in structure; however, A-ARRs do not contain the DNA binding domains that B-ARRs have, and which are required to function as transcription factors.[16] Therefore, A-ARRs do not contribute to the activation of transcription, and by competing for phosphates from phosphotransfer proteins, inhibit B-ARRs function.[17] In the SAM, B-ARRs induce the expression ofWUS which induces stem cell identity.[18]WUS then suppresses A-ARRs.[19] As a result, B-ARRs are no longer inhibited, causing sustained cytokinin signaling in the center of the shoot apical meristem. Altogether with CLAVATA signaling, this system works as anegative feedback loop. Cytokinin signaling is positively reinforced by WUS to prevent the inhibition of cytokinin signaling, while WUS promotes its own inhibitor in the form of CLV3, which ultimately keeps WUS and cytokinin signaling in check.[20]
Unlike the shoot apical meristem, the root apical meristem produces cells in two dimensions. It harbors two pools ofstem cells around an organizing center called the quiescent center (QC) cells and together produces most of the cells in an adult root.[21][22] At its apex, the root meristem is covered by the root cap, which protects and guides its growth trajectory. Cells are continuously sloughed off the outer surface of theroot cap. The QC cells are characterized by their low mitotic activity. Evidence suggests that the QC maintains the surrounding stem cells by preventing their differentiation, via signal(s) that are yet to be discovered. This allows a constant supply of new cells in the meristem required for continuous root growth. Recent findings indicate that QC can also act as a reservoir of stem cells to replenish whatever is lost or damaged.[23] Root apical meristem and tissue patterns become established in the embryo in the case of the primary root, and in the new lateral root primordium in the case of secondary roots.[citation needed]
Inangiosperms, intercalary (sometimes called basal) meristems occur inmonocot (in particular,grass) stems at the base of nodes and leaf blades.Horsetails andWelwitschia also exhibit intercalary growth. Intercalary meristems are capable of cell division, and they allow for rapid growth and regrowth of many monocots. Intercalary meristems at the nodes of bamboo allow for rapid stem elongation, while those at the base of most grass leaf blades allow damaged leaves to rapidly regrow. This leaf regrowth in grasses evolved in response to damage by grazing herbivores and/or wildfires.[citation needed]
When plants begin flowering, the shoot apical meristem is transformed into an inflorescence meristem, which goes on to produce the floral meristem, which produces thesepals,petals,stamens, andcarpels of the flower.[citation needed]
In contrast to vegetative apical meristems and some efflorescence meristems, floral meristems cannot continue togrow indefinitely. Their growth is limited to the flower with a particular size and form. The transition from shoot meristem to floral meristem requires floral meristem identity genes, that both specify the floral organs and cause the termination of the production of stem cells.AGAMOUS (AG) is a floral homeotic gene required for floral meristem termination and necessary for proper development of thestamens andcarpels.[6]AG is necessary to prevent the conversion of floral meristems to inflorescence shoot meristems, but is identity geneLEAFY (LFY) andWUS and is restricted to the centre of the floral meristem or the inner twowhorls.[24] This way floral identity and region specificity is achieved. WUS activates AG by binding to a consensus sequence in the AG's second intron and LFY binds to adjacent recognition sites.[24] Once AG is activated it represses expression of WUS leading to the termination of the meristem.[24]
Through the years, scientists have manipulated floral meristems for economic reasons. An example is the mutant tobacco plant "Maryland Mammoth". In 1936, the department of agriculture of Switzerland performed several scientific tests with this plant. "Maryland Mammoth" is peculiar in that it grows much faster than other tobacco plants.[citation needed]
Apical dominance is where one meristem prevents or inhibits the growth of other meristems. As a result, the plant will have one clearly defined main trunk. For example, in trees, the tip of the main trunk bears the dominant shoot meristem. Therefore, the tip of the trunk grows rapidly and is not shadowed by branches. If the dominant meristem is cut off, one or more branch tips will assume dominance. The branch will start growing faster and the new growth will be vertical. Over the years, the branch may begin to look more and more like an extension of the main trunk. Often several branches will exhibit this behavior after the removal of apical meristem, leading to a bushy growth.[citation needed]
The mechanism of apical dominance is based onauxins, types of plant growth regulators. These are produced in the apical meristem and transported towards the roots in thecambium. If apical dominance is complete, they prevent any branches from forming as long as the apical meristem is active. If the dominance is incomplete, side branches will develop.[citation needed]
Recent investigations into apical dominance and the control of branching have revealed a new plant hormone family termedstrigolactones. These compounds were previously known to be involved in seed germination and communication withmycorrhizal fungi and are now shown to be involved in inhibition of branching.[25]
The SAM contains a population ofstem cells that also produce the lateral meristems while the stem elongates. It turns out that the mechanism of regulation of the stem cell number might be evolutionarily conserved. TheCLAVATA geneCLV2 responsible for maintaining the stem cell population inArabidopsis thaliana is very closely related to themaize geneFASCIATED EAR 2(FEA2) also involved in the same function.[26] Similarly, inrice, theFON1-FON2 system seems to bear a close relationship with the CLV signaling system inArabidopsis thaliana.[27] These studies suggest that the regulation of stem cell number, identity and differentiation might be an evolutionarily conserved mechanism inmonocots, if not inangiosperms. Rice also contains another genetic system distinct fromFON1-FON2, that is involved in regulatingstem cell number.[27] This example underlines theinnovation that goes about in the living world all the time.
Note the long spur of the above flower. Spurs attract pollinators and confer pollinator specificity.(Flower: Linaria dalmatica)Complex leaves ofCardamine hirsuta result from KNOX gene expression
Genetic screens have identified genes belonging to theKNOX family in this function. These genes essentially maintain the stem cells in an undifferentiated state. The KNOX family has undergone quite a bit of evolutionary diversification while keeping the overall mechanism more or less similar. Members of the KNOX family have been found in plants as diverse asArabidopsis thaliana, rice,barley and tomato. KNOX-like genes are also present in somealgae, mosses, ferns andgymnosperms. Misexpression of these genes leads to the formation of interesting morphological features. For example, among members ofAntirrhineae, only the species of the genusAntirrhinum lack a structure calledspur in the floral region. A spur is considered an evolutionaryinnovation because it definespollinator specificity and attraction[citation needed]. Researchers carried outtransposon mutagenesis inAntirrhinum majus, and saw that some insertions led to formation of spurs that were very similar to the other members ofAntirrhineae,[28] indicating that the loss of spur in wildAntirrhinum majus populations could probably be an evolutionary innovation.
The KNOX family has also been implicated inleaf shape evolution(See below for a more detailed discussion). One study looked at the pattern of KNOX gene expression inA. thaliana, that has simple leaves andCardamine hirsuta, a plant havingcomplex leaves. InA. thaliana, the KNOX genes are completely turned off in leaves, but inC.hirsuta, the expression continued, generating complex leaves.[29] Also, it has been proposed that the mechanism of KNOX gene action is conserved across allvascular plants, because there is a tightcorrelation between KNOX expression and acomplex leaf morphology.[30]
Though each plant grows according to a certain set of rules, each newroot and shoot meristem can go on growing for as long as it is alive. In many plants, meristematic growth is potentiallyindeterminate, making the overall shape of the plant not determinate in advance. This is theprimary growth. Primary growth leads to lengthening of the plant body and organ formation. All plant organs arise ultimately from cell divisions in the apical meristems, followed by cell expansion and differentiation. Primary growth gives rise to the apical part of many plants.[citation needed]
The growth of nitrogen-fixingroot nodules onlegume plants such assoybean andpea is either determinate or indeterminate. Thus, soybean (or bean andLotus japonicus) produce determinate nodules (spherical), with a branched vascular system surrounding the central infected zone. Often,Rhizobium-infected cells have only smallvacuoles. In contrast, nodules on pea, clovers, andMedicago truncatula are indeterminate, to maintain (at least for some time) an active meristem that yields new cells for Rhizobium infection. Thus zones of maturity exist in the nodule. Infected cells usually possess a large vacuole. The plant vascular system is branched and peripheral.[citation needed]
Under appropriate conditions, each shoot meristem can develop into a complete, new plant orclone. Such new plants can be grown from shoot cuttings that contain an apical meristem. Root apical meristems are not readily cloned, however. This cloning is calledasexual reproduction orvegetative reproduction and is widely practiced in horticulture to mass-produce plants of a desirablegenotype. This process known as mericloning, has been shown to reduce or eliminate viruses present in the parent plant in multiple species of plants.[31][32]
Propagating through cuttings is another form of vegetative propagation that initiates root or shoot production from secondary meristematic cambial cells. This explains why basal 'wounding' of shoot-borne cuttings often aids root formation.[33]
Meristems may also be induced in the roots oflegumes such assoybean,Lotus japonicus,pea, andMedicago truncatula after infection with soil bacteria commonly calledRhizobia.[citation needed] Cells of the inner or outer cortex in the so-called "window of nodulation" just behind the developing root tip are induced to divide. The critical signal substance is the lipo-oligosaccharideNod factor, decorated with side groups to allow specificity of interaction. The Nod factor receptor proteins NFR1 and NFR5 were cloned from several legumes includingLotus japonicus,Medicago truncatula and soybean (Glycine max). Regulation of nodule meristems utilizes long-distance regulation known as theautoregulation of nodulation (AON). This process involves a leaf-vascular tissue locatedLRRreceptorkinases (LjHAR1, GmNARK and MtSUNN), CLEpeptide signalling, and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulationphenotype though it is not yet known how this relates to the other AON receptor kinases.[citation needed]
Lateral meristems, the form of secondary plant growth, add growth to the plants in their diameter. This is primarily observed in perennial dicots that survive from year to year. There are two types of lateral meristems: vascular cambium and cork cambium.[citation needed]
In vascular cambium, the primary phloem and xylem are produced by the apical meristem. After this initial development, secondary phloem and xylem are produced by the lateral meristem. The two are connected through a thin layer of parenchymal cells which are differentiated into the fascicular cambium. The fascicular cambium divides to create the new secondary phloem and xylem. Following this the cortical parenchyma between vascular cylinders differentiates interfascicular cambium. This process repeats for indeterminate growth.[34]
Cork cambium creates a protective covering around the outside of a plant. This occurs after the secondary xylem and phloem has expanded already. Cortical parenchymal cells differentiate into cork cambium near the epidermis which lays down new cells called phelloderm and cork cells. These cork cells are impermeable to water and gases because of a substance called suberin that coats them.[35]
^Lohmann, Jan U.; Kieber, Joseph J.; Demar, Monika; Andreas Kehle; Stehling, Sandra; Busch, Wolfgang; To, Jennifer P. C.; Leibfried, Andrea (December 2005). "WUSCHEL controls meristem function by direct regulation of cytokinin-inducible response regulators".Nature.438 (7071):1172–1175.Bibcode:2005Natur.438.1172L.doi:10.1038/nature04270.ISSN1476-4687.PMID16372013.S2CID2401801.
^Hay and Tsiantis; Tsiantis, M (2006). "The genetic basis for differences in leaf form betweenArabidopsis thaliana and its wild relativeCardamine hirsuta".Nat. Genet.38 (8):942–947.doi:10.1038/ng1835.PMID16823378.S2CID5775104.
^Mackenzie, K.A.D; Howard, B.H (1986). "The Anatomical Relationship Between Cambial Regeneration and Root Initiation in Wounded Winter Cuttings of the Apple Rootstock M.26".Annals of Botany.58 (5):649–661.doi:10.1093/oxfordjournals.aob.a087228.
