Definethese Plants Continue to Develop Vegetatively During and After the Reproductive Phase

Unlike some animal systems in which the germ line is set aside during early embryogenesis, the germ line in plants is established only after the transition from vegetative to reproductive development—that is, flowering. The vegetative and reproductive structures of the shoot are all derived from the shoot meristem formed during embryogenesis. Clonal analysis indicates that no cells are set aside in the shoot meristem of the embryo to be used solely in the creation of reproductive structures (McDaniel and Poethig 1988). In maize, irradiating seeds causes changes in the pigmentation of some cells. These seeds give rise to plants that have visually distinguishable sectors descended from the mutant cells. Such sectors may extend from the vegetative portion of the plant into the reproductive regions (Figure 20.27), indicating that maize embryos do not have distinct reproductive compartments.

Figure 20.27

Clonal analysis can be used to construct a fate map of a shoot apical meristem in maize. Seeds that are heterozygous for certain pigment genes (anthocyanins) are irradiated so that the dominant allele is lost in a few cells (a chance occurrence). All (more...)

Maximal reproductive success depends on the timing of flowering—and on balancing the number of seeds produced with resources allocated to individual seeds. As in animals, different strategies work best for different organisms in different environments. There is a great diversity of flowering patterns among the over 300,000 angiosperm species, yet there appears to be an underlying evolutionary conservation of flowering genes and common patterns of flowering regulation.

A simplistic explanation of the flowering process is that a signal from the leaves moves to the shoot apex and induces flowering. In some species, this flowering signal is a response to environmental conditions. The developmental pathways leading to flowering are regulated at numerous control points in different plant organs (roots, cotyledons, leaves, and shoot apices) in various species, resulting in a diversity of flowering times and reproductive architectures. The nature of the flowering signal, however, remains unknown.

Some plants, especially woody perennials, go through a juvenile phase, during which the plant cannot produce reproductive structures even if all the appropriate environmental signals are present (Lawson and Poethig 1995). The transition from the juvenile to the adult stage may require the acquisition of competence by the leaves or meristem to respond to an internal or external signal (McDaniel et al. 1992; Singer et al. 1992; Huala and Sussex 1993).

Grafting and organ culture experiments, mutant analyses, and molecular analyses give us a framework for describing the reproductive transition in plants (Figure 20.28). Grafting experiments have identified the sources of signals that promote or inhibit flowering and have provided information on the developmental acquisition of meristem competence to respond to these signals (Lang et al. 1977; Singer et al. 1992; McDaniel et al. 1996; Reid et al. 1996). Analyses of mutants and molecular characterization of genes are yielding information on the mechanics of these signal-response mechanisms (Hempel et al. 2000; Levy and Dean 1998).

Figure 20.28

Regulation of the vegetative-to-reproductive transition. Internal and external factors regulate whether a meristem produces vegetative or reproductive structures. Not all of the regulatory mechanisms shown are used in all species, and some species flower (more...)

Leaves produce a graft-transmissible substance that induces flowering. In some species, this signal is produced only under specific photoperiods (day lengths), while other species are day-neutral and will flower under any photoperiod (Zeevaart 1984). Not all leaves may be competent to perceive or pass on photoperiodic signals. The phytochrome pigments transduce these signals from the external environment. The structure of phytochrome is modified by red and far-red light, and these changes can initiate a cascade of events leading to the production of either floral promoter or floral inhibitor (Deng and Quail 1999). Leaves, cotyledons, and roots have been identified as sources of floral inhibitors in some species (McDaniel et al. 1992; Reid et al. 1996). A critical balance between inhibitor and promoter is needed for the reproductive transition.

In some species, meristems change in their competence to respond to flowering signals during development (Singer et al. 1992). Vernalization, a period of chilling, can enhance the competence of shoots and leaves to perceive or produce a flowering signal. The reproductive transition depends on both meristem competence and signal strength (Figure 20.29). Shoot tip culture experiments in several species (including tobacco, sunflower, and peas) have demonstrated that determination for reproductive function can occur before reproductive morphogenesis (reviewed in McDaniel et al. 1992). That is, isolated shoot tips that are determined for reproductive development but are morphologically vegetative will produce the same number of nodes before flowering in situ and in culture (Figure 20.30).

Figure 20.29

The interaction of competence and signal strength. Nicotiana tabacum (Nt) is a day-neutral plant that flowers when the meristem gains competence to respond to internal signals. N. silvestris (Ns) is a long-day plant that flowers when the floral signal(s) (more...)

Figure 20.30

Determination for reproductive development. In this experiment, buds three nodes from the base of pea plants were treated when the plants had three expanded leaves. These buds were determined for reproductive development, and produced the same number (more...)

The "black box" between environmental signals and the production of a flower is vanishing rapidly, especially in the model plant Arabidopsis. The signaling pathways from light via different phytochromes to key flowering genes are being elucidated. Molecular explanations are revealing redundant pathways that ensure that flowering will occur. Light-dependent, gibberellin-dependent, vernalization-dependent, and autonomous pathways that regulate the floral transition have been genetically dissected.

The ancestral angiosperm is believed to have formed a terminal flower directly from the terminal shoot apex (Stebbins 1974). In modern angiosperms, a variety of flowering patterns exist in which the terminal shoot apex is indeterminate, but axillary buds produce flowers. This observation introduces an intermediate step into the reproductive process: the transition of a vegetative meristem to an inflorescence meristem, which initiates axillary meristems that can produce floral organs, but does not directly produce floral parts itself. The inflorescence is the reproductive backbone (stem) that displays the flowers (see Figure 20.20). The inflorescence meristem probably arises through the action of a gene that suppresses terminal flower formation. The CENTRORADIALUS (CEN) gene in snapdragons suppresses terminal flower formation (Bradley et al. 1996). It suppresses expression of FLORICAULA (FLO), which specifies floral meristem identity. Curiously, the expression of FLO is necessary for CEN to be turned on. The Arabidopsis homologue of CEN (TERMINAL FLOWER 1 or TFL1) is expressed during the vegetative phase of development as well, and has the additional function of delaying the commitment to inflorescence development (Bradley et al. 1997). Overexpression of TFL1 in transgenic Arabidopsis extends the time before a terminal flower forms (Ratcliffe et al. 1998). TFL1 must delay the reproductive transition. Garden peas branch one more time than snapdragons do before forming a flower. That is, the axillary meristem does not directly produce a flower, but acts as an inflorescence meristem that initiates floral meristems. Two genes, DET and VEG1, are responsible for this more complex inflorescence, and only when both are nonfunctional is a terminal flower formed (Figure 20.31; Singer et al. 1996).

Figure 20.31

Regulation of inflorescence branching architecture. (A) In the simplest angiosperms, a terminal flower forms directly from the terminal shoot apex. (B) Arabidopsis and snapdragons produce flowers on axillary branches (second-order flowers). (C) In peas, (more...)

The next step in the reproductive process is the specification of floral meristems—those meristems that will actually produce flowers (Weigel 1995). In Arabidopsis, LEAFY (LFY), APETALA 1 (AP1), and CAULIFLOWER (CAL) are floral meristem identity genes (Figure 20.32). LFY is the homologue of FLO in snapdragons, and its upregulation during development is key to the transition to reproductive development (Blázquez et al. 1997). Expression of these genes is necessary for the transition from an inflorescence meristem to a floral meristem. Mutants (lfy) tend to form leafy shoots in the axils where flowers form in wild-type plants; they are unable to make the transition to floral development. If LFY is overexpressed, flowering occurs early. For example, when aspen was transformed with an LFY gene that was expressed throughout the plant, the time to flowering was dramatically shortened from years to months (Weigel and Nilsson 1995). AP1 and CAL are closely related and redundant genes. The cal mutant looks like the wild-type plant, but ap1 cal double mutants produce inflorescences that look like cauliflower heads (Figure 20.33)

Figure 20.32

Floral meristem identity mutants. (A) Wild-type Arabidopsis, (B) leafy mutant, (C) apetala1 mutant, and (D) leafy apetala1 double mutant. (Photographs courtesy of J. Bowman.)

Figure 20.33

Arabidopsis double mutant of ap1 and cal. Since cal alone gives a wild-type phenotype, the double mutant demonstrates the redundancy of these two genes in the flowering pathway. (Photograph courtesy of J. Bowman.)

Floral meristem identity genes initiate a cascade of gene expression that turns on region-specifying (cadastral) genes, which further specify pattern by initiating transcription of floral organ identity genes (Weigel 1995). SUPERMAN (SUP) is an example of a cadastral gene in Arabidopsis that plays a role in specifying boundaries for organ identity gene expression. Three classes (A, B, and C) of organ identity genes are necessary to specify the four whorls of floral organs (Figures 20.34 and 20.35; Coen and Meyerowitz 1991). They are homeotic genes (but not Hox genes) and include AP2, AGAMOUS (AG), AP3, and PISTILLATA (PI) in Arabidopsis. Class A genes (AP2) alone specify sepal development. Class A genes and class B genes (AP3 and PI) together specify petals. Class B and class C (AG) genes are necessary for stamen formation; class C genes alone specify carpel formation. When all of these homeotic genes are not expressed in a developing flower, floral parts become leaflike. The ABC genes code for transcription factors that initiate a cascade of events leading to the actual production of floral parts. In addition to the ABC genes, class D genes are now being investigated that specifically regulate ovule development. The ovule evolved long before the other angiosperm floral parts, and while its development is coordinated with that of the carpel, one would expect more ancient, independent pathways to exist.

Figure 20.34

Wild-type and mutant phenotypes of the Arabidopsis class A (ap2), class B (ap3, pi), and class C (ag) floral organ identity genes. (Model proposed by Coen and Meyerowitz 1991; photographs courtesy of J. Bowman.)

Figure 20.35

ABC model for floral organ specification. Three classes of genes—A, B, and C—regulate organ identity in flowers. The central diagram represents the wild-type flower; surrounding diagrams represent mutants that are missing one or more of (more...)

hermessicert.blogspot.com

Source: https://www.ncbi.nlm.nih.gov/books/NBK10122/