Introduction

Of the naturally occurring 92 elements of the periodic table, about a quarter are essential to plants. Water and CO2 provide the plant with the elements C, H and O; the remaining necessary elements are obtained by flowering plants as inorganic mineral ions, mostly from the soil solution. Water uptake and ion uptake are to some extent linked, e.g. water uptake mediated by root pressure depends on ion uptake, and the rate of ion uptake tends to increase with increasing rate of transpiration. But the uptake of mineral ions differs greatly from water uptake in that it proceeds against the free energy gradient of the ions and is dependent on metabolic energy. The transport of ions through cellular membranes is mediated by numerous membrane-bound transport proteins which enable the plant to exert considerable control and selectivity over the process. This is vital if the nutritional needs of the plant are to be satisfied. Heterotrophic organisms obtain nearly all their essential elements via plants and the element composition of plants is accordingly of major interest and importance also for human nutrition.

Essential elements

Definition: macronutrients and micronutrients

An element is classed as essential to a plant if the plant cannot complete its life cycle without it and no other element can substitute for it. The effect of the element must also be direct, i.e. it should not act by promoting the uptake of another essential element, or by retarding the absorption of a toxic one.

Introduction

A constant theme underlying the study of plant physiology is that plant growth and development are controlled by the environment. Plants being sessile organisms, it is not surprising that their development is exquisitely sensitive to a wide range of environmental factors and is extremely plastic, i.e. very flexible. There are underlying basic patterns in plant development, but there is considerable regulation by environmental signals of how and when these patterns are expressed.

In addition, there are internal signals within the plant. One of the most important factors influencing the development of a cell is its position within the plant. A plant cell develops depending on its location in relation to neighbouring cells, and this in turn will determine its response to environmental signals. For example, the response to drought of a cell within the leaf will differ in many ways from that of a cell within the root. The key question arises of how a complex set of environmental factors can interact with cells to elicit an appropriate response within a given cell type: what are the internal signals that communicate between cells, and mediate between environmental factors and the plant tissues?

It has been known for decades (if not centuries) that plants contain a range of compounds which have profound effects on many aspects of growth and developmental physiology, and act as a means of communication within the plant. These plant growth hormones, sometimes referred to as plant growth regulators, are still being discovered.

NAMING GENES, PROTEINS AND MUTATIONS

A consistent nomenclature has been adopted for naming genes and proteins. A wild-type gene is written in italic as e.g. CRY1 (for cryptochrome) whilst mutated versions of it are written cry1. The first mutation discovered in the gene would be cry1–1, the second cry1–2, etc. In the case of cryptochrome (and many other proteins, including phytochrome) the receptor consists of a protein (the apoprotein) and a chromophore which together make the functional protein (holoprotein). The apoprotein is written as CRY1 whilst the holoprotein is written as cry1. The same mutants are often isolated by different research groups and are given different names. Once a class of genes has been relatively well studied they are sometimes renamed to avoid confusion.

UNITS OF MEASUREMENT

The system of SI units, Système International d'Unités, was introduced in 1960. In this system, the basic units of mass, length and time are the kilogram (kg), metre (m), and second (s); a number of common units, e.g. litre and hour, are abandoned. Older units, however, still abound even in current scientific literature because of their convenience (and familiarity), and some are retained in this text. Note that there is no full stop after the abbreviations, and no ‘s’ to denote the plural: thus we write 1 m and 10 m.

Introduction

Flowering plants are described as being autotrophic, ‘self-feeding’, capable of synthesizing all their organic material via photosynthesis. But a flowering plant is a complex organism with cells and organs specialized for diverse functions, and only the green photosynthetic cells are truly autotrophic; they must accordingly supply all the non-photosynthetic parts with organic carbon. Over small distances, i.e. between individual cells and within small groups of cells, chemicals can move by diffusion through plasmodesmata, or across plasma membranes by diffusion and by active transport. But organic materials must move for long distances; the growing tips of the roots of a tree are many metres away from the nearest photosynthetic leaves and even in a herbaceous plant diffusion would be too slow for the distances involved. We have already seen (Chapter 3) how water moves in plants over long distances in a specialized transport tissue, the xylem. The subject of this chapter is the long-distance, multidirectional movement or translocation of organic compounds which takes place in the phloem.

Phloem as the channel for organic translocation

Evidence for translocation in the phloem

In flowering plants, the xylem is regularly associated with the phloem, the two together making up the vascular tissues. In young organs the two tissues are in contact; when secondary growth occurs they become separated by the vascular cambium, the meristem which then adds xylem to one side and phloem to the other.

Introduction

Liquid water is absolutely necessary for life as we know it. Firstly it is the solvent and reaction medium of all living cells, which contain some 75–90% water by weight; secondly it is a reactant in many metabolic processes; and thirdly, as the hydration water of macromolecules, it forms part of the structure of protoplasm, existing as ‘liquid ice’ in a labile but ordered structure. The physicochemical properties of water (H2O) are unique; heavy water (D2O or DHO), containing deuterium, the heavy isotope of hydrogen, differs sufficiently to be toxic. In multicellular organisms, water provides the transport medium. Additionally, for plants, water is one of the raw materials for photosynthesis and produces the turgor pressure of water-filled vacuoles which gives mechanical rigidity to thin-walled tissues, while some movements of plant organs occur as a result of turgor pressure changes. Plant cell expansion is driven by turgor pressure and hence growth rates depend on hydration levels.

On ‘dry’ land, the highly hydrated body of a terrestrial plant in many situations tends to lose water to the environment, especially to the atmosphere, in accordance with gradients of free energy of water. There are few habitats where plants do not suffer some water shortage at least intermittently. The necessity for maintaining an adequate internal water content has been a major factor in the evolution of land plants with respect to structure and numerous aspects of physiology.

Appreciating plants

During a public open day at a university, a child trying to look at a botanical exhibit was dragged away by an impatient parent with the words ‘Come on – we can't spend all day looking at dull green things!’

There is a tendency to consider plants as somewhat dull, passive and inactive. Yet plants face and overcome the same problems as animals: how to obtain nutrients and water, how to survive extreme environmental conditions, how to ensure reproduction and the survival of the next generation. The photosynthetic mode of life has conditioned plants to evolve as sessile organisms; their basic necessities – light, carbon dioxide, water and mineral ions – are ubiquitous and there has therefore been no selection pressure for mobility. An animal may obtain its nutrients and water by skilfully stalking its prey, and learning the path to a pool; this catches human attention as interesting behaviour. A flowering plant obtains nutrients and water by millions of minute root tips constantly growing through the soil, and by pumping ions across root cell plasma membranes with molecular-sized pumps. This is plant behaviour: plant physiology is plant behaviour. It need not be considered dull because it is less spectacular to the eye than what is called animal behaviour. The subsequent hauling up of the absorbed water and minerals to the top of a tree, 100 metres high, might indeed be considered a quite spectacular feat (imagine doing it with a bucket!).

Introduction

All living organisms need a supply of raw materials from which their bodies can be constructed, and a supply of energy. This energy is needed for growth, i.e. for the formation of their bodies, also for the maintenance of their bodies, and for all the various types of work, chemical and mechanical, that are carried out by living systems.

Life as we know it is based on organic compounds of carbon (C). This element accordingly occupies a central place among the raw materials and it is found on earth abundantly in its inorganic forms as carbon dioxide (CO2), carbonate (CO32 −) and bicarbonate (HCO3−). The ultimate energy source for most life forms on earth is the thermonuclear energy of the sun, transmitted to earth as electromagnetic radiation, light. Photosynthesis is the process by which the solar light energy is transformed into the chemical bond energy of organic carbon compounds. Photosynthesis is thus simultaneously a process of energy transduction, and a process by which inorganic carbon is converted to organic form and incorporated into living organisms. Chemically it is a reductive process. Respiration is the process of oxidative breakdown by which the energy stored in the organic products of photosynthesis is tapped for driving metabolism, for growth, for movements, and by which the C is returned to inorganic form again as CO2.

Although it was evident from Darwin's studies of tropisms in plants that informational signals passed from one part of the plant to another, the proof that it was a chemical substance that passed awaited the famous Avena coleoptile experiments of Frits Went (1928). These showed that a molecule (later identified as indole-3-acetic acid [IAA]) was the active agent that was water soluble and would pass across an agar barrier placed between one tissue and another – in his earliest experiments this was between the coleoptile tip (producing IAA) and the IAA-regulated elongating region of the coleoptile below. A tremendous amount of work, both in studying the physiology of this response to IAA, and in identifying the many analogues to IAA, sought the molecular structures required to provide an active molecule. It was from this highly intensive period of plant physiology study that the agricultural revolution of herbicides, defoliants and growth regulators of the 1940s and 1950s was originally generated.

But it was the insect physiologists with their identities of hormone-producing glands and hormone-responding tissues remote from the glands who developed the concepts of target tissues, signalling molecules and receptor sites. Perhaps the most spectacular to record, as an example of the approaches followed later by plant scientists, is the work in the 1930s and 1940s concerning the processes of moulting of larval epidermal skins and of metamorphoses to the adult state (Karlson, 1956).

Cells that we see as permanently committed offer us the opportunity to follow their performance in both excised pieces of plant tissue as well as in planta. With a number of these it has been possible to establish with relative certainty the nature of their target status and the inputs of signals and signalling molecules that they can both perceive and respond to in predictable ways.

Also, it has been possible to follow associations with neighbour cells that influence the pathway to the committed cell state and to deduce certain of the cross-talk and physical communication that leads to a final differentiated condition. Two types of commitment have been considered. The first type is one in which the committed cells remain alive in the body of the plant and their function can therefore be called into operation by the perception of specific signals evoking a one time only response (as is the case with abscission or aleurone cells) or by the differentiation of a response mechanism that can be activated many times without loss of function (as in statocytes and stomata). The second terminally committed cell type to be considered is one that dies in situ amongst its living cell neighbours in the progress of the commitment, but then forms an essential component of the plant's structural architecture and overall function.

In previous chapters we outlined the target cell concept, identified the signals and hormones that a cell will encounter and discussed how types of cells can be identified as of particular target status whether during development or on reaching a terminal state of differentiation. Now, over the next chapters, we ask how cells actually recognise signals and question whether the target state dictates, or is dictated by, the mechanisms for signal recognition in vivo. The original description of a hormone, borrowed from the animal world, was a regulatory substance synthesised in one part of the organism and transported to another in which it is recognised and the effect of the hormone becomes manifest. Although the plant has sites of major synthesis of hormone signals and they are all known to be transported, all the evidence tells us that the majority of cells probably contain some level of each hormone and are constantly exposed to the hormones emanating from their neighbours. The plant, after all, is a coenocyte in which all living cells intercommunicate by plasmodesmata and by surface contacts at the cell wall. Of the many signals to which each cell is continuously exposed, why are certain of these perceived and responded to? Or, does a cell respond to all signals that are above a threshold level? If so, how is the threshold level determined and is it fixed or variable?

Animal physiologists deduced the existence of, sought and found receptor proteins on cell surfaces and within the nucleus.

As a first step in developing the target cell concept for plants it is important that the major players in the known informational and signal repertoire are set out briefly at the start of the discussions. There are five major players: auxin, gibberellins, cytokinins, ethylene (and its precursor 1-aminocyclopropane-1-carboxylic acid, ACC), and abscisic acid. The first hormone to be discovered and isolated, auxin, is the best understood, the most important and without doubt the most remarkable. As well, the more recent signal molecules to be discovered are described in greater detail for some of them link more closely to molecules in the animal kingdom.

Auxin

Indole-3-acetic acid (IAA) is the most abundant naturally occurring auxin, with indole-3-butyric acid (IBA) and 4-chloroindole-3-acetic acid (4-Cl-IAA) also occurring naturally (Figure 2.1). IAA was discovered in 1928 by Frits Went (Went, 1928) in the search for the chemical substance that was transported from the apex of the oat coleoptile and caused the cells below to elongate. In higher plants, several pathways of synthesis are possible. IAA is an indole derivative, and both in vivo and in vitro evidence indicates routes of synthesis from the aromatic amino acid, tryptophan, although more recent genetic and biochemical experiments have suggested that tryptophan-independent pathways may also operate to yield the final product (Bartel, 1997).

Auxin biosynthesis

Tryptophan was proposed originally as the precursor of IAA due to structural similarities of the two molecules and when a clearly defined conversion was identified in plant-associated microbes.

Every cell can be considered a target cell, with a status that is subject to change throughout its life until a state of terminal differentiation is reached. On this basis, every cell is slightly different from its neighbour with respect to position and signal response, so that at any one time each cell has a unique target status even though it is a member of an apparently uniform tissue. Although the number of signals that have been identified or described so far are limited, the number of responding target cell types in plants would appear to be unlimited.

The flexibility of an individual cell, or perhaps more correctly, the flexibility of a group of cells to give rise by repeated cell divisions to a whole new plant, is the basis of the concept that plant cells remain totipotent throughout their lives. Horticulturists have used this knowledge in vegetative reproduction following observations that many isolated plant parts will readily regenerate new individuals with all the anatomical and behavioural characters of the parent. Planting a cutting is one thing, where all the coordinating signals and target cells are, as it were, still in operational position. Propagation by pieces of tissue where lines of intertissue communication have been lost is quite another.

The question of how a community of cell types in a callus or suspension culture develops in an organised and temporal fashion into a meristem is essentially unresolved though certain clues give consistency to the concept that specific short-distance signals are operating between them.

In the last two chapters, discussion has been concerned with the evidence for specific hormone receptors and the downstream signalling events in cells that form part of the transduction chain initiated by the binding of a ligand (the hormone) to its respective receptor. Each major plant hormone has been considered and classified as a separate operational entity, but it is clear that while the same hormone can have different effects in different tissues, a similar response in the same tissue can also be brought about by more than one hormone, the interactions involved being highly dependent upon the genetic background of the tissue in question. With the unravelling of intracellular signalling downstream of hormone perception, it is now becoming clear that more than one signal can utilise a particular transduction pathway. In this final chapter, we refer to examples of such apparently duplicated hormonal responses and how this cross-talk in perception and signalling has been revealed through the use of specific phenotypically expressed mutants. The list is not exhaustive but it serves to illustrate the level of flexibility that a cell can sustain, combined with the basic concept of every cell as an individual target cell.

It has become evident that plants are quite versatile in the cross-talk of their molecular communication language, as represented by situations where one hormone can substitute in function for another.