R. F. Lyndon

Root and shoot meristems: structure and growth

Plants, unlike animals, continue to develop throughout their lives. New structures are formed in a repetitive manner so that the plant has a modular construction. The module of the shoot is a stem internode and node, with a leaf and an axillary bud. The module of the root is a length of root bearing a lateral root. The plant continually increases in size by the addition of extra modules (also called phytomers) (Fig. 2.1). In order to describe the structure of almost the whole plant we only need to know the structure of one module from the shoot and one from the root, the total number of modules, and the branching pattern. On flowering, the new shoot modules are smaller and crowded and the lateral organs differentiate in new ways in the flower, but the basic modular construction persists until the pattern is ended by the formation of the male and female reproductive organs, the gametes, and ultimately, after fertilization, the production of a zygote, which forms a new embryo.

Competence and determination in flowering

When the higher plant changes from vegetative to reproductive growth, a new set of structures, the flower, is formed and represents the expression of genes not previously expressed. The formation of sepals, petals, stamens, and carpels occurs nowhere else in the plant and meiosis occurs only in the stamens and carpels. The transition to flowering, therefore, represents a change in determination of the shoot apical cells and their derivatives. In a vegetative plant already competent to flower (ripe-to-flower), only a change in determination would be required. A lack of competence to flower is most clearly seen in those plants that show a juvenile phase, i.e. a phase in which it is not possible to cause flowering directly (see Ch. 10, section 10.3.5.2). Remembering that competence may be defined as the ability to develop along a particular pathway when given the appropriate signal, plants that flower in response to photoperiod, in which a signal is received from the leaves, may be regarded as competent to flower. In such plants, the leaves are obviously competent to form the floral stimulus, but they may not be in some juvenile plants. Furthermore, it is often far from clear whether this signal from the leaves first has to make the apical meristem competent to flower or whether the meristem is already competent to flower but simply lacks the signal for determination.

Genes and development.

Clearly, genes can regulate development because plant form, such as leaf shape and flower structure, is inherited and differs between species. The necessity of certain genes for development is also shown by mutants such as the series of embryo-lethal mutants in Arabidopsis, which arrest embryo development at various stages. However, the mechanism of action of these and similar genes is not known. We do not know whether they code directly for enzymes that are part of the developmental programme or indirectly for enzymes involved in the synthesis of regulator substances, i.e. growth substances, controlling metabolites, or regulator proteins.

The cellular basis of polarity

To find out how a polar axis originates in the first place, it is necessary to start with a system lacking polarity and to follow what happens as polarity develops. We must start with a single unpolarized cell. This would be possible if we could use an undifferentiated apolar cell in suspension culture, but this would be difficult to recognize and manipulate. The only practical cells to use are the free-living zygotes of some algae, such as the seaweeds Fucus or Pelvetia.

Control of the plane of cell division

The growth and elongation of organs depends on the growth and elongation of the component cells. Growth is accompanied by cell division, except in the final stages of cell expansion during the later stages of growth of leaves, flowers, and fruits and in the proximal parts of the growing zones in stems and roots. In meristems, cell division roughly keeps pace with cell growth so that cells remain approximately the same size. Cell shape also usually remains relatively unchanged because the plane of cell division is usually normal to the predominant direction of growth.

Pattern formation, positional information, and integration of growth

The existence of patterns of cells, tissues, and organs implies coordination of the development of the units making up the patterns. There are basically two types of pattern-forming process. The order may be the result of an assemblage of similar but independent units like tiles, each with its own pattern, pushed together on a board. Alternatively, the pattern may be because of interaction between developing units, which promote or inhibit each others development and so modify the pattern as it forms (differentiation-dependent pattern formation). Are all patterns in plants formed interactively and, if so, what is the nature of the interactions between cells, tissues, and organs, and how do they control plant form?

Competence and determination in differentiation

The development of different types of cells, tissues, and organs from common origins implies that: (1) the cells are competent to differentiate; (2) as development proceeds they lose the capacity to be easily transformed into other cell, tissue, and organ types.

Meristem functioning: formation of branches, leaves, and floral organs

Branching in plants with apical cells is usually by division of the apical cell giving two meristems and so producing dichotomous, or false dichotomous, branching. Moss leaves do not have axillary buds. Axillary meristems are present at the base of the leaves in many ferns but they do not always grow out to form stem or rhizome branches, and sometimes they may form new fronds. In higher plants, the shoot branches by the outgrowth of the axillary buds to form new side shoots. The branched root system is formed by lateral roots, which themselves produce laterals, and so on.