9.3.1 Manipulating growth and development with applied plant growth regulators

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Plant growth regulators (PGRs) are a diverse group of chemicals classified by their ability to modify plant development and/or biochemical processes. They include not only the native plant hormones already discussed and their synthetic analogues, but also many other compounds that influence hormone phy-siology, especially inhibitors of hormone biosynthesis and compounds that block hormone action, perhaps by interfering with receptor binding.

How specific can we be?

From the preceding sections, it is clear that hormones are multifunctional, and responses depend on dose, site of ap-plication and developmental stage. In theory at least, we have the potential to influence almost any developmental process, and over the past 60 years probably just about every PGR on the shelf has been tried out. Controlling plant height, inducing flowering, increasing fruit numbers, generating seedless fruit, inducing seed germination — all worthy aims often with commercial success as reward for the ‘successful’.

But all is not so simple. For every 100 attempts, probably only one becomes regular practice in agriculture or biotechnology. Why? The multifunctionality, the variability of response between genotypes, between tissues of different age, modification of response by environmental factors — all these can thwart the best planned strategy even with the ‘right’ dose, timing and placement on the plant. In pharmacological terms, the side effects are often stronger and more undesirable than the targeted response. Here we select a few examples which have found commercial application.

Stem elongation and gibberellins

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Figure 9.13 The effects of paclobutrazol, an inhibitor of gibberellin biosynthesis, on shoot growth and flowering of poinsettia, a popular ornamental pot plant. The treated plant on the right has shorter internodes, darker green leaves and slightly enhanced flowering, all characteristics of gibberellin depletion.

(Photograph copyright © ICI Australia, reproduced with permission)

Gibberellins are well known for effects on dormancy, germination, flowering and fruit development, but one of their most studied roles is in modifying stem elongation. This is a ‘rate’ process rather than an ‘all-or-nothing’ on/off switch, so we predict graded changes in cell growth rates as gibberellin concentrations are modified upwards or downwards. The tools are gibberellin mutants (deficient either in the capacity to pro-duce active gibberellins or the capacity to respond to them; see Section 9.3.2), several gibberellins available in commercial quantities, and several compounds that more or less specifically inhibit gibberellin biosynthesis (Figure 9.13). Thus we have possibilities of examining the effects of genetically or chemically altering gibberellin levels. The almost universal result is that plants with low gibberellin concentrations end up shorter (more dwarfed) than those with higher levels. There are limits to the range over which this applies, that is, there is a ceiling growth rate beyond which no response is elicited by extra gibberellin, but in the best-studied examples, such as pea, the classic log dose–linear response relationship seems to hold quite well (Figure 9.14).

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Figure 9.14 Genetic and hormonal control of development are illustrated by the relationship between active gibberellin content and internode length in pea genotypes with different alleles at the Le locus, a gene which encodes an enzyme for synthesis of bioactive GA1. The wild-type Le has a normal enzyme and a tall stature, le is dwarfed with almost no enzyme activity. Note the linear relationship between internode length and log of gibberellin content, showing a very wide range of concentrations over which the plant can detect gibberellins. This graph is similar to the classical 'dose-response' plots used in early hormone research to test biological activity of exogenously applied compounds.

(Based on Ross et al. 1989)

Parthenocarpic fruit: auxin and gibberellins

One highly desirable characteristic in most fruit crops (though not in nut crops!) is seedlessness. This occurs spontaneously in banana because of its triploid genotype, and in certain kinds of citrus because of early embryo abortion. In some mandarin types, an auxin spray before or just after bloom induces fruit to set without seeds. In certain grape varieties such as sultana (known elsewhere as Thompson Seedless) the seed starts to develop but then aborts and added gibberellin is required to promote normal fruit development (see frontispiece to Chapter 11). In both cases, the applied hormone is thought to be substituting for what would normally be generated by the growing seed (Figure 9.6). This gives an insight into how fruit and seed development are intimately coordinated. Auxins can cause similar seedless fruit in other crops such as citrus, but later applications can lead to abscission, probably via induction of ethylene synthesis, and are useful for fruit thinning on trees that otherwise tend to bear excessive numbers of undersized fruits.

Tissue culture: auxin and cytokinins — the essential hormones

When a plant breeder or horticulturalist finds a new, potentially valuable plant, the first priority is usually to multiply it. Often the plant may be infertile or progeny may be genetically inferior. It then becomes necessary to use vegetative propagation. Remember that many plant cells have a remarkable property called totipotency (Section 10.2), so almost any fragment of a plant (or explant) can be used to regenerate new genetically identical plants, called clones. The most dramatic advances in plant propagation, resulting in the ability to generate millions from one, are due to tissue culture or micropropagation. Not all plants spontaneously enter into useful forms of regeneration when cultured: indeed most need some form of chemical persuasion. The most powerful control comes from use of two hormones, auxin and cytokinin. Auxin tends to promote cell expansion and, together with cytokinins, induces cell division. On top of these fundamentals of tissue growth, auxin causes cells to become organised, sometimes simply into vascular tissue but more importantly to form roots. Cytokinins on the other hand induce shoot formation. The ratio of concentrations of auxin to cytokinin, as discussed in Section 9.2.2, appears to determine the type of development that ensues.

Legislation, safety and moral issues

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Table 9.3

In addition to the physiological issues, increasing awareness of environmental pollution and potential dangers of exposure to hazardous chemicals have led to critical examination of use of all types of chemicals, especially on food crops. Initial concerns were over pesticides, especially organochlorines, but then spread to include PGRs. A few of these have made news headlines.

The selective herbicide 2,4,5-T (2,4,5-trichlorophenoxy-acetic acid) is a synthetic auxin that kills dicotyledonous (broadleaf) plants but has relatively little effect on grasses (incidentally an excellent example of plants differing in sensitivity to the same dose of hormone), and therefore found widespread use for removing dicotyledonous weeds from lawns and cereal crops. Its inclusion in the chemical warfare substance Agent Orange is notorious, but in fact it was an impurity (a highly carcinogenic dioxin compound) in the commercial preparation which led to worldwide withdrawal of the chemical. 2,4,5-T itself is not particularly toxic (Table 9.3). Preparations of a related compound, 2,4-D (2,4-dichloro-phenoxyacetic acid) contain no dioxins and are still used as herbicides and in plant tissue culture.

Alar, also known as daminozide, is a plant growth retardant which was used widely on apples to modify fruit shape and skin colour. Some evidence in the 1980s suggested chronic toxicity symptoms were attributable to this chemical, and it was rapidly withdrawn from use. Sales of apples, even untreated ones, plummeted at the time, a dramatic example showing how a small amount of scientific data can have massive economic consequences. Subsequent investigations concluded that there were no substantiated toxic effects but, harmful or not, the lasting impression in the general public has meant Alar has not been widely reintroduced (Caswell et al. 1991; O’Rourke 1990). This treatment was never essential for fruit production because it was used mainly for cosmetic changes concerning size and appearance rather than to improve yield or nutritional quality. In general, legal restrictions, safety concerns and public perceptions are leading researchers and agrochemical companies to seek alternative means to achieve the same results, some of which are described next.

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