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Abstract:

Distinctive from that of the animal system, the basic plan of the plant body is the continuous formation of a structural unit, composed of a stem with a meristem at the top and lateral organs continuously forming at the meristem. Therefore, mechanisms controlling the formation, maintenance, and development of a meristem will be a key to understanding the body plan of higher plants. Genetic analyses of filamentous flower (fil) mutants have indicated that FIL is required for the maintenance and growth of inflorescence and floral meristems, and of floral organs of Arabidopsis thaliana. FILencodes a protein carrying a zinc finger and a HMG box-like domain, which is known to work as a transcription regulator. As expected, the FIL protein was shown to have a nuclear location. In situ hybridization clearly demonstrated that FIL is expressed only at the abaxial side of primordia of leaves and floral organs. Transgenic plants, ectopically expressing FIL, formed filament-like leaves with randomly arranged cells at the leaf margin. Our results indicate that cells at the abaxial side of the lateral organs are responsible for the normal development of the organs as well as for maintaining the activity of meristems.

Introduction:

Differing from the situation in animal systems, maintenance of the meristem activity throughout life is a key for the patterning of the plant body. The shoot apical meristem and the meristem of the main root are formed in embryonic development and kept active after germination. During vegetative growth, leaves are continuously formed in a strictly controlled fashion from the shoot apical meristem. After the plant shifts to reproductive growth, the meristem converts to an inflorescence meristem, which in turn forms floral meristems. The floral meristem generates floral organs at predetermined positions. Genetic and molecular studies are under way to help understand the genetic regulatory system supporting meristem activity: control of cell division, formation of lateral organ primordia, and maintenance of the meristem structure. Recent studies using Arabidopsis, snapdragon, and maize have started to unveil the genetic basis of these molecular mechanisms (Okada and Shimura 1994).

The leaf meristems are formed in a helical manner at the top of the inflorescence. When the leaf primordia begin to form, the petiole and flattened leaf blade are developed and the leaves show epinasty. Mature leaves show abaxial-adaxial polarity. The adaxial surface bears glossy, dark green epidermal cells, and produces many trichomes. On the contrary, the abaxial surface shows matte, gray-green epidermal cells and does not produce any trichomes. As the plant grows, the adaxial surface of the newly made leaves undergoes a gradual reduction in production of trichomes, and the abaxial surface starts to produce them. An Antirrhinum phantastica (phan) mutant produces filamentous leaves that are radially symmetrical with abaxial characters, and an Arabidopsis phabulosa-1d (phb-1d) dominant mutant shows adaxialized filamentous leaves. Analysis of these mutants suggests that the outgrowth of the leaf blade requires the juxtaposition of the abaxial-adaxial cell fate, and also suggests that the adaxial leaf environment is required for the development of the shoot apical meristem (Waites and Hudson 1995; MacConnell and Barton 1998).

The floral meristems are also formed in a helical manner at the top of the inflorescence. Determination of floral meristem identity and development of the floral meristem are known to be controlled by several genes. FILAMENTOUS FLOWER (FIL), LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER (CAL) genes are major players in this step (Bowman et al. 1993; Kempin et al. 1995; Weigel and Nilsson 1995; Sawa et al. 1999). The floral organ fate is determined by a combination of a set of ABC class genes (Weigel and Meyerowitz 1994).

By using a combined molecular and genetic approach to study the meristem activity that regulates the development of the plant body, we showed that the Arabidopsis FIL gene is responsible for normal floral development (Komaki et al. 1988; Okada and Shimura 1994; Sawa et al. 1999). Here we show the molecular nature of the FIL gene and its spatially controlled expression. We also report the phenotypes of the 35S::FIL plants. By determining the abaxial character, the results indicate that the FIL gene is responsible for the normal development of leaves and floral organs, and for the maintenance of meristem activity.

Cloning of the FIL locus was done by the map-based chromosome-walking procedure. The FIL locus was mapped to the lower part of chromosome 2 close to RFLP marker m336 (Fig.1) A contiguous map covering a 400-kb region was made adjacent to the marker, and the FIL locus was pinpointed at the left end of a TAC clone, TAC27M5, between the two markers of SBP3 and ML. To confirm the presence of the FIL gene, we used the TAC clone to transform the fil-1 mutant and found wild-type flowers in three independent transgenic plants. Based on the nucleotide sequence of the clone, five ORFs were predicted in the 20-kb region between SBP3 and ML markers. By comparing the nucleotide sequences of the fil-1 mutant, fil-2 mutant, and wild type, we concluded that FIL is an ORF (TIGR accession no. F4L23.30) that encodes a protein of 229 amino acid residues in seven exons, because there was a single-base change in both mutants in one ORF but no sequence aberrations in the other ORFs.

Figure 1. Genomic structure of the FIL region at the lower part of chromosome 2. FIL was isolated by map-based cloning and was mapped between markers SBP3 and ML on clone TAC27M5. Other BAC clones overlapping in the region are shown. Numbers below the molecular markers shown in white boxes indicates the recombination frequency between the marker and the FIL locus (no. of recombinant chromosomes/no. of chromosomes examined).

Cormophytes produce new cells throughout their life and form new organs at regular intervals (some tallophytes do the same). All cells of an embryo display the same activity of division, but as soon as a certain state of differentiation and thus a certain size has been reached, the production of new cells is restricted to special tissues, the meristems.

Typical meristems are found at the tips of all stems and roots. They are called apical meristems. Those of the shoot are usually protected by involucral leaves and the whole complex forms a bud.

The apical meristems are the cause of tip growth that is one of the most striking features of vegetative growth. This contrasts with animal growth, that is an allometric process, i.e. all parts of the body grow in a weay that maintains the body's overall proportions. As soon as an animal has reached its final size, growth ceases. This, however, does not mean that no more cell divisions are taking place. Cell divisions occur all the time, especially in epithelia and in blood stem cells. But this division activity has always the function of replacing old or damaged cells. No such repair occurs in plants. If a leaf or flower or any other part is damaged, it is neither replaced nor repaired. Instead, new organs grow, and this again does not happen in animals.

The cells of a meristem have a potentially unlimited ability to divide. But division is controlled. Many plants, for example, follow the principle of apical dominance, where the activity of the lateral meristems is suppressed while the tip is growing. In this case, information is exchanged between the two tissues in question. If the tip of the main axis is cut off, suppression is abolished and the lateral buds start to sprout.

Many plants like gymnosperms and dicots have extensive lateral meristems (the cambium, the vascular cambium, and the cork cambium) that give rise to growth in girth, also called secondary growth. Their activity may fluctuate in the course of the year and is reflected by annual rings.

Other plants, like monocots, have intercalary meristems. They are actively growing meristems that differ clearly from apical meristems and are located between more or less differentiated tissues usually at the base of each internode. The formation of secondary meristems shows that differentiated cells can reverse their state and go back to a meristematic existence. Their ability to divide is thus not lost, though it is not used unless an adjustment to changed circumstances requires it. The ability of indeterminate growth is widely exploited by the use of cuttings in horticulture. It is also displayed in the formation of roots at the cutting sites of begonia leaves, for example.

But what exactly is a meristem ? The following three examples will help to answer this question.

The marine brown alga Dictyota dichotoma consists of a flat thallus with a thickness of three cell layers. The large cell at its apical tip divides periclinal, while the subapical progenitor cells divide in an anticlinal manner. The progenitor cells themselves display alternating peri- and anticlinal cell division. The thallus enlarges thus both in length and in width. At regular intervalls, the apical cell divides in an anticlinal manner, too, resulting in two apical cells and a subsequent forking of the thallus. Although this example explains the way a division scheme works, it does not explain where the meristematic cells end and where the differentiated ones begin. The classical scheme of a cormophyte given by SACHS says that no clear demarcation exists. Instead, there is a gradient between meristematic and differentiated cells. The meristematic properties are thus not lost during one division, but they decline gradually.

The second example shows how division takes place in some liverworts, in simple cryptogams like horsetail and in many ferns. Contrary to that of many algae, their cormus normally resembles a three-dimensional body with a single tetraedical initial cell at the tip of the axis. It produces progenitor cells by regularly alternating the planes of division. These progenitor cells are given off towards the base. The progenitor cells may again divide in both anti- and periclinal manners.

The tip of phanerogams, my third example, is formed by a whole group of cells. They are not specialized and therefore called vegetation zone (see illustrations of Acacia, Oxypolis, Wheat). The apical meristem is organized into several cell layers. In angiosperms and in some gymnosperms, it has to be distinguished between the outer tunica and the central corpus (A. SCHMIDT, 1924; F. A. L. CLOWES, 1924; A. FÖRSTER, 1943). The meristematic cells divide mostly in an anticlinal manner, while those of the corpus display both planes of division: anti- and periclinal. Some parts of the corpus are dominated by a certain plane of division, thereby determining the first step of differentiation. Usually, the outer layer of the tunica produces the epidermis. The inner tissues of the plant stem from the corpus, the tunica or both.

These schemes are useful guidelines for the understanding of the plant's morphogenesis, since they point out the importance of the information that is contained in the cell's position within the tissue. A cell will only develop into a certain direction, if its position within the tissue is "right". If a cell is transferred to a new position, it will adopt new functions and thus dedifferentiate. At the beginning of the 20th century, G. HABERLAND (Universität Graz, later Berlin) put forward the sentence of the totipotency of plant cells. It states, that each cell of a plant keeps the ability to develop a complete plant. This is true for most, but not all plant cells. And still: growth through division is in fact characteristic for the meristematic state of a cell, but it is by no means restricted to it. A meristem does not only include the initial cells and their immediate progenitors, but also some parts of the shoot.

The situation is similar in root meristems, although their anatomy is different. The shoot apex is protected by the involucral leaves. In roots, this function is exerted by the root cap. Its cells are produced by a specialized root cap meristem and are progressively replaced towards the tip. These cells have to have a high turn-over, since the outmost cell layer is easily damaged by soil particles as a result of the growth movement of the root.

Procambium and Lateral Meristems:

It is normally distinguished between primary and secondary growth of the shoot. The first is the phase of plant development that gives rise to new organs and to the basic plant form, the latter brings about growth in girth and the formation of new vascular tissues (A. de BARY, 1877).

The procambium is the meristematic tissue that produces the primary vascular tissues: xylem and phloem. It develops directly beneath the growing tip next to new leaf primordia. The development of new leaves is thus tightly connected with that of new vascular tissues. The vascular tissues of leaves are also called leaf veins. The cells of the procambium are generally combined in cords. They form an extension of the vascular tissues into the growing tip and do thus provide the connection of the newly developed organ with the conductive systems of the plant. The cells of the procambium are elongated and become even more so in the course of their development. The volume of their vacuoles increases considerably, lending them a lighter and more transparent appearance than their neighboring cells. This is decisive for their differentiation into xylem or phloem cells.

Some of the meristematic cells in plants with secondary growth keep their meristematic state and become cells of the cambium.

What are the causes for the development of the procambium? Two models exist:

1.          They are to be found in the cell-cell interactions that take place between the cells of the bud, that determine the pattern of the leaf positions and the positions of xylem and phloem.

2.          Induction is achieved by fully differentiated tissues. The necessary information is conducted to the bud via already differentiated cells of the procambium.

The first model was confirmed experimentally, since formation and further development of the procambium are undisturbed in isolated bud meristems. The determination of cells takes place before any morphological change can be seen. Differentiation describes the (usually irreversible) changes of structures and functions that result in a certain specialization, while determination means the (usually irreversible) triggering of processes that lead to these changes.

The cambium is the prototype of a lateral meristem. It is mono- or multilayered depending on its origin and forms normally a continuous cell layer of tubular shape that is located at the periphery of the shoot or root. It separates xylem from phloem, if present. The cambium develops from the procambium next to the vessels, ensuring a continuity of the meristematic state.

The cambium between the vessels derived from already differentiated, parenchymatous cells. The first type is also called fascicular (within the vascular bundle) cambium, while the latter one is termed interfascicular (between the vascular bundles). The interfascicular cambium is a secondary meristem. A few monocots (Dracaena, Yucca, Aloe and others) have again another type of cambium, the extrafascicular cambium.

The cells of the cambium are often termed initials, since they initiate the formation of specialized progenitor cells after division. Two cell types occur within the cambium:

1.          Fusiform initials. They are the mother cells of all xylem and phloem elements, as well as of all other cells that are oriented parallel to the organ's axis. They are flat, elongated with pointed ends and highly vacuolate. It is their spindle-like shape that caused their name.

2.          Ray initials are nearly isodiametric, small cells that occur often in groups. They develop from fusiform initials or their progenitors. They produce the radially orientated rays in wooden plants (transversal elements).

I. W. BAILEY, who did the basic work on the organization of the cambium in the 20th of this century, gained the following data when comparing a one year old and a sixty years old stem of Pinus strobus (Weymouths Pine).

His data show that both length and number of fusiform initials increases with the age of the stem. The increase in number and the widening (dilatation) of the cambium tube is caused by the widening of the central xylem cylinder, that is produced by the cambium itself.

Secondary vascular elements are given off into opposite directions by periclinal (tangential) division of fusiform cells. The developing xylem elements grow towards the inner cylinder, that of the phloem towards the outer of the cylinder. The progenitors of the initials are organized in radial rows that make it easy to trace back their descent. The increase in girth is compensated by the anticlinal divisions occurring at regular intervals. The activity of the cambium is temperature-dependent: it causes the annual rings. Their thickness, i.e. the activity of the cambium, is determined by extern factors like temperature, day length, soil humidity, temperature and others. Besides its function in the production of vascular tissues, the cambium has an additional capacity for the healing of wounds.

The cork cambium (or phellogen) is a secondary lateral meristem that serves to produce the secondary outer surface, the bark, that replaces the epidermis. It is without exception given off towards the outward direction. Often, though not always, the cork cambium produces cells towards the inner of the stem, that form the phelloderm.

The cork cambium has a rather simple structure, if compared to the cambium. In cross-section, the cells have a right-angled shape, they are flat in radial and tangential sections. Their plasma is highly vacuolated and may contain chloroplasts and tannic acids.

The cork cambium stems from epidermal cells or/and from cells of the underlying parenchyma. It is distinguished between primary and secondary cork cambiumtaking into account the fact that it can be assembled within the stem several times during a plant's life. The cork cambium is monolayered during the first year in some species and becomes multilayered later on. It may be active for several years, sometimes even for the whole life or for just one year. Its activity is, just as that of the cambium, influenced by extern factors.

The stamen output flower has sperm and pollen tube, the column head which goes to pistil gets into after the ovary, developing seed after teaching , having embryo inside seed, no longer is teach egg purely, can make the seed livability enlarge, meet appropriate environment to sproutlace even if.The seed has two main growths to order after sproutlacing, one top in stem, one top in root, distinguish to grow upwards and downwards, the cell which is located caulis department and root growth point calls meristem, is a kind of stem cell(stem cell).Order for the shoot meristem at caulis department's growth of the top.Order for the root meristem in the growth of root top.This orders for the plant two most main growths, knowing together as primary meristem.Also have a growth to order inside the top of the branches and leaves and xylophyta of herbs can make its branches and leaves thick or the stem add thick, this is called secondary meristem, the characteristic like fertilizied egg sort of stem cell can develop various different cells.Plant many cells which have already divided still have stem-cell function during the period of growing, this differs from a great distinguishing feature of the animal for the plant.

Stem and the growth on the root order in the process of dividing, in addition to increasing cell number, still have the characteristic that the cell of one part keeps an original stem cell, carry on an abruption continuously, other cells are then carry on dividing, becoming different organization.

In the embryo growth the process, one-cell how does the embryo become a meristem stem cell?

The cell of head at split into 16-cell embryo come to a decision those cells namely will become a stem cell, four cells of red part will express with a kind of gene, this gene is called WUS, regulating the transcription factor of[with] gene performance for the meeting.The cell of yellow part stops an abruption, being called Quiescence center.When the plant embryo develop, then come to a decision a top and bottom caulis the growth position of the root.WUS and Quiescence center will decide the destiny of its surroundings cell after the plant embryo growth complete.

At WUS on of the cell becomes Shoot meristem namely, becoming the stem cell of stem.Becoming root meristem at Quiescence center the cell of the surroundings is the stem cell of root.

The stem-cell number has to maintain certainly, so the WUS will regulate growth of surroundings cell, and open the gene(CLV3) of whole cells, making it become Shoot meristem.And the CLV3 will secrete protein, repressing a WUS performance, this for a take feedback.(negetive feedback control)When stem-cell mount is excessive, will make the CLS3 secrete protein, repressing a WUS function, making the stem-cell amount no longer continue to increase.When stem-cell amount in a little moment, the protein density of CLV3 descends, the WUS starts expressing again, then making a new stem cell.So, after the plant embryo growth complete, caulis root the structure of the stem cell(primary meristem) then and mostly the bottom settle.

When the plant stem grows upwards, will grow leaf.And flower and leafs all develop from the primary meristem of new organization.

Evolving concepts of the organization and function of plant meristems

Caspar:

Wolff first observed the shoot apical meristem of flowering plants in 1759 and recognized that it was the center of organ and cell formation in the shoot. Almost two and a half centuries since Wolff's discovery, the understanding of how meristems are organized and function has increased enormously. Much of this progress resulted from the development of new technologies that allowed previously unanswered questions to be answered, but which often raised new questions that remained unanswered until even newer technologies became available. Thus, progress in understanding meristems has been punctuated by periods of rapid progress followed by periods of relative stasis.

Studies on meristems can be divided into three periods that relate to the available technology. The earliest was observational analysis that had its major impact from the mid 19th century to the mid 20th; this was followed by a period of experimental manipulation of meristems that spanned the 1940s to the 1970s; most recently genetic and molecular analysis that began actively in the 1980s has become the major method of analysis. The following discussion focuses on studies on the shoot apical meristem. Comparable work on the apical meristem of the root is described.

Figure Cellular and molecular characteristics of the shoot apical meristem of Arabidopsis. The boundary of the L1 cell layer is shown within the meristem and into the adjacent primordium. For clarity the boundaries of the layers L2 and L3 are shown within the central zone only. The three cytohistological zones, CZ, PZ, and FZ are indicated. The tunica is composed of the cell layers L1 and L2 and the corpus is composed of underlying L3 cells. A positive signal from WUS maintains the CLV3 expression domain and stem cell identity in L1, L2, and L3 layers at the summit of the meristem. A signal from CLV3 limits the expression domain of WUS to a small region within the corpus. Arabidopsis homologs of the rice gene SHO are proposed to regulate the rate of cell transition from the CZ to the PZ. MGO1 and MGO2 promote the transition of cells from the PZ into the primordium. The lower part of the figure shows the proposed role of cytokinin in stimulating mitotic activity in the SAM by increasing the levels of D-cyclins. Cytokinin is also proposed to increase expression of KNAT1 and KNAT2, which results in increased cytokinin accumulation. AS1 and AS2 are expressed in founder cells of the primordium but negative interactions between them and STM, KNAT1, and KNAT2 maintain the SAM/primordium boundary. (Adapted from Fletcher, 2002.)