9.1.1 Signal sources: which tissues make hormones? How are hormones synthesised?

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Traditionally, five major hormone classes are described: auxins, cytokinins, gibberellins, abscisic acid and ethylene. Other active compounds have been known for years, and there are new-comers with increasingly strong claims for inclusion. These include brassinosteroids and jasmonic acid in particular, but also polyamines, salicylic acid and oligosaccharides; for details of these, some informative review articles are listed at the end of the chapter.

Most hormones have certain biochemical trends in common: small molecules synthesised from ubiquitous precursors (amino acids, mevalonic acid, nucleotides) sometimes via multi-step pathways, then deactivated by oxidation or conjugation (linking to other small molecules such as glucose and amino acids). Some knowledge of hormone biochemistry will be invaluable in Section 9.3 where we describe genetic and chemical approaches to manipulating plant development through modified hormone biosynthesis and degradation. Further information on hormone biochemistry can be found in many recent reviews and texts (e.g. Davies 1995).



Figure 9.1 Structures and some partial biosynthetic pathways for common members of the five major groups of native plant hormones. (a) The most common natural auxin, indole-3-acetic acid (IAA). (b) Four classes of natural cytokinin: zeatin and its cis isomer, dihydrozeatin, isopentenyladenine. (c) Late stages of gibberellin biosynthesis pathway, showing some key points of genetic and environmental control of amounts of bioactive GA1.  

(i) The GA19 -> GA20 step is under photoperiod control in some long day plants; for example, spinach shows a greater rate of metabolism under long days, and hence production of bioactive GA1, correlating with developmental transition from rosette form to stem elongation.

(ii) GA20 -> GA1 is blocked in many dwarf (short internode) mutants, such as le in pea, d1 in maize and dy in rice. GA20 itself is inactive but becomes active after addition of an hydroxyl (-OH) group to the 3ß position, thus forming GA1. In some other dwarf mutants, the pathway is blocked at steps well before GA19, and shoots of these plants contain almost no detectable gibberellins.

(iii) GA1 -> GA8 is a key reaction regulating amounts of active gibberellins. In this case, addition of an -OH group to the 2ß position inactivates almost every gibberellin, including GA1.

(d) Structure of abscisic acid (ABA)

Auxin in its most common natural form indole-3-acetic acid (IAA; Figure 9.1a), was the first plant hormone to be isolated, and was long thought to be derived exclusively from the amino acid tryptophan. Plants and certain plant pathogenic bacteria synthesise IAA, although the genes, enzymes and reaction intermediates differ between prokaryote and eukaryote. From data on tryptophan-deficient mutants, it now appears that indole may be an alternative starting point for IAA synthesis in some plants (Wright et al. 1991; Normanly et al. 1995). This advance illustrates our incomplete knowledge of even elementary plant hormone biochemistry. Active growing tissues, especially shoot tips and young leaves, synthesise auxins, as do developing fruits and seeds. Roots appear to produce much less auxin, but auxin has vital functions in lateral root development. Plants and bacteria can deactivate auxins by irreversible oxidation involving enzymes such as IAA oxidase, or by covalently linking (conjugating) them to other small molecules: sugars, cyclitols, amino acids. Some conjugates (e.g. IAA-aspartate) act as inactive auxin stores, regenerating active auxin when the link is hydrolysed.


In many ways, cytokinins are opposites of auxins, being synthesised in roots but with most dramatic effects on shoot development. However, shoot tissues can also produce cytokinins, as can developing seeds. A classic example of the latter is coconut milk, the copious liquid endosperm from coconut seed, which is still a popular cytokinin source in plant tissue culture media. Cytokinins were originally named from their ability to promote cell division, but they also function in initiation of new shoot structures, dormancy release and retardation of senescence. Cytokinins are derivatives of adenine, one of the purine bases found in all DNA and RNA. Indeed, cytokinins were originally thought to be products of transfer RNA (tRNA) breakdown. However, based on cytokinin and tRNA composition of pea roots and turnover rates in maize, it was calculated that there were insufficient cytokinin nucleotides in tRNA to account for total cytokinin production (Short and Torrey 1972; Klemen and Klämbt 1974). Instead, a de novo pathway using free adenine nucleotides as substrate appears to predominate. There are also many cytokinin types each with subtle differences in structure. The four main classes of natural cytokinin each have a different five-carbon side-chain attached to the N6 position (Figure 9.1b). The two classes found in tRNA (cis-zeatin and isopentenyladenine) are less biologically active than the major free cytokinin classes, trans-zeatin and dihydrozeatin. Discovery of a cis-trans isomerase that interconverts the two zeatin forms has re-opened the biosynthesis debate, because active trans-zeatin may be made from RNA-derived cis-zeatin (Bassil et al. 1993). The first enzyme in the de novo pathway, isopentenyl transferase (IPT), is well known in bacteria but has yet to be characterised fully from plant tissues. A novel suggestion is that all cytokinins in plants are actually synthesised by bacteria present on and in plant tissues (Holland 1997). If true, this could account for the failure to find plant biosynthetic enzymes, and for the lack of cytokinin biosynthesis mutants. Each cytokinin class exists as base, riboside and nucleotide forms, many of which can readily be metabolically interconverted. This has made it difficult to decide which forms are biologically active in their own right, and which achieve activity only after conversion. As with auxins, inactivation results from conversion to glucosyl or amino acid conjugates (e.g. 9-alanyl zeatin = lupinic acid) or from action of degradative enzymes such as cytokinin oxidase which cleave the side-chain.


Gibberellins were first noticed through symptoms of a disease (known in Japanese as bakanae = foolish seedling) on rice that caused excessive stem elongation. The causative agent, a fungus called Gibberella fujikuroi, contains several different types of gibberellin (abbreviated to GA, after gibberellic acid, the first form discovered), some of which also exist in plants. Plants possess many other unique gibberellins, and collectively there are well over 100 identified compounds. This intimidating complexity can be reduced to a comprehensible level by realising that each species contains only about 25 of these gibberellins and that most gibberellins are biosynthetic intermediates or inactive end-products, and not active in their own right. There are many steps and enzymes involved in building up the 19- and 20-carbon gibberellin molecules from five-carbon mevalonic acid. Several parallel pathways exist differing only in number of hydroxyl (–OH) groups. Hydroxyl groups are the key to gibberellin functions: some positions (3b) are generally essential for activity, whereas others (2b) completely abolish it (Figure 9.1c). Inactivation by conjugation to glucose also occurs. Gibberellin synthesis takes place mainly in developing leaves and stems, in developing seeds and during germination. Gibberellins function in dormancy release and germination, as well as in growth promotion (e.g. stem elongation, fruit tissue expansion).

Abscisic acid

Abscisic acid (ABA) is an unfortunate name because this hormone has little to do with abscission. But once again it tells a story: cotton, one of the plants originally studied, turns out to be an exception in that ABA does promote fruit shedding (Okhuma et al. 1963). ABA is a 15-carbon molecule (Figure 9.1d) and its synthesis occurs from breakdown of carotenoid pigments, especially violaxanthin, a 40-carbon molecule. Previously, mevalonic acid was thought to be the main precursor, with early steps in common with gibberellin biosynthesis. This alternative pathway may operate in tissues such as avocado mesocarp and in tomato seedlings (Milborrow 1983; Willows et al. 1994). ABA is produced in large quantities in water-stressed tissues, especially roots and leaves, but also has a role in seed maturation, dormancy and senescence. ABA concentrations are lowered by oxidative deactivation to phaseic acid or by formation of glucosides.


Ethylene (= ethene; C2H4) is a unique gaseous hormone that diffuses rapidly out of plant tissues. Its immediate precursor is 1-aminocyclopropane-1-carboxylate (ACC) which in turn originates from S-adenosyl methionine, a derivative of another common amino acid:

Methionine                            Enzymes:

       ↓                                     SAM synthase


      ↓                                      ACC synthase


      ↓                                       ACC oxidase


Ethylene is produced in response to cell damage and other stresses such as anoxia. It accumulates rapidly during fruit ripening and senescence, but all living cells produce some ethylene. Oxidation and conjugation can occur, but dissipation into the atmosphere is probably the main ‘means of disposal’.