Auxin a natural plant growth regulator

Auxin, or indol-3-yl-acetic acid (IAA), is an endogenous plant growth regulator that plays a crucial role in the division, differentiation and elongation of plant cells. At the organ and whole plant level it has a profound influence on many aspects of plant physiology, including seedling morphology, geotropism, phototropism, apical dominance, leaf senescence and abscission, flowering, and fruit setting and ripening. It is synthesised via tryptophan-dependent and tryptophan-independent pathways in meristematic, actively growing tissues and is found throughout the plant body in concentrations ranging from 1 to 100|gIAAkg-1 fresh weight. Young seedlings and tissues that are rapidly growing and elongating contain relatively higher concentrations of auxin than mature tissues and it is believed that younger tissues are the most sensitive to this growth regulator.

During the last 60 years many studies have investigated the effects of exogenous auxins on plant growth, with the general conclusions that (a) auxins can inhibit as well as stimulate plant growth in a concentration-dependent manner; and (b) different tissues show differential sensitivity to applied auxin. Growth inhibition caused by supra-optimal auxin concentrations is largely attributable to auxin--nduced ethylene evolution. Thus, once a critical level of auxin is reached which is tissue-specific, ethylene is produced, and this causes relative inhibition of growth (Figures 7.1 and 7.2).

Table 7.2 Some examples of weed seedlings controlled by auxin-type herbicides (modified from Martin, 1987 and various proceedings from the British Crop Protection Conferences).

Weed

MCPA (1945)

Mecoprop (1957)

Dichlorprop (1961)

Clopyralid (1975)

Dicamba + mecoprop + MCPA

Benazolin + clopyralid

Quinmerac (1985)

Sinapis arvensis (charlock)

S

S

S

S

S

Capsella bursa-pastoris (shepherd's purse)

S

S

S

S

S

Chenopodium album (fat hen)

S

S

S

S

S

Galium aparine (cleavers)

R

S

S

S

S

S

Stellaria media (chickweed)

R

S

S

S

S

Polygonum lapathifolium (pale persicaria)

R

R

S

S

S

Polygonum persicaria (redshank)

R

R

S

S

S

Bilderdykia convolvulus (black bindweed)

R

R

S

S

S

Tripleurospermum maritimum (scentless mayweed)

R

R

R

S

S

S

Cirsium arvense (creeping thistle)

R

R

S

S

R

S

Veronica hederifolia (ivy-leaved speedwell)

R

R

R

R

R

S

Lamium purpureum (red deadnettle)

R

R

R

R

S

S

R, Resistant; S, susceptible.

R, Resistant; S, susceptible.

Figure 7.1 Ethylene evolution by scentless mayweed following application of 10"3 M clopyralid or indol-3-yl-acetic acid (IAA) (after Thompson and Cobb, 1987).

Figure 7.2 Effect of exogenous auxin (IAA) on growth (solid curves) and ethylene production (dashed curves) by roots and stems (from Goodwin and Mercer, 1983).

Figure 7.2 Effect of exogenous auxin (IAA) on growth (solid curves) and ethylene production (dashed curves) by roots and stems (from Goodwin and Mercer, 1983).

Klaus Grossmann and colleagues at BASF have proposed a link between hydrogen peroxide production and tissue damage in Galium aparine when treated by auxin-type herbicides (Grossmann et al2001t . They envisage that as a consequence of auxin t herbicide treatment, ethylene synthesis is stimulated, accompanied by an increase in the

oxida

oxida oxidation products (inactive)

Biosynthesis v

IAA POOL

Physiological action conjugates (potentially active)

Figure 7.3 The control of auxin concentration in vivo (from Goodwin and Mercer, 1983).

biosynthesis of the hormone abscisic acid (ABA). Ethylene induces senescence, while the ABA induces stomatal closure and hence the cessation of carbon assimilation by photosynthesis. Since the treated plant is still exposed to light, these workers consider that H2O2 accumulates resulting in oxidative damage that also contributes to weed phytotoxicity.

Auxin concentration -n vivo is tightly controlled by the relative rates of biosynthesis and degradation, with a further layer of complexity evident when conjugation is taken into account (Figure 7.3). Auxin synthesis is complex and the pool size governed by oxidation and/or conjugation. It has been known since 1947 that plant tissues are capable of the oxidative degradation of IAA by a so-called IAA oxidase and that this enzyme activity is rapid and widespread in plant tissues. However, the characterisation of this activity is awaited and it remains to be convincingly demonstrated that it can be separated from plant peroxidases. Certainly in elongating tissues, low oxidase activity is thought to ensure a relatively high auxin concentration (approximately 10_6M), and in roots lower auxin concentrations (approximately 10- 10M) result from measurably higher oxidase activity. Many in vitro studies with peroxidases, especially those isolated from horseradish, have suggested that the oxidation of auxin is under the control of many naturally occurring substances, including phenols and other growth regulators, but supporting data in vivo is lacking.

Auxin conjugation to glucose, amino acids and myo-inositol may serve as storage forms or auxin-reservoirs, which may be hydrolysed to free auxin when necessary, especially following seed germination. This has recently assumed major physiological significance and importance with the finding that concentrations of conjugated auxins can be much higher -n vivo than that of free IAA. The principal amino acid conjugate in vegetative tissues appears to be IAA-aspartate, which is formed by L-aspartate-N-acylase, an enzyme induced by all natural and synthetic auxins. Glucose esters are also common auxin conjugates and they are formed from pre-existing glucosyltransferases. Thus, although much rigorous work remains to be done, auxin synthesis, degradation and conjugation appear to interact, with the result that natural auxin concentrations appear to be tightly controlled in vivo . More detailed reviews of auxin metabolism may be found in Normanly (1997) and Woodward and Bartel (2005).

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