Light modulation of the gravitropic set-point angle (GSA).

A study has been made of the means by which light influences the gravitropic set-point angle (GSA) of the nodes of Tradescantia and the hypocotyls of the lazy-2 mutant of tomato. In light-grown Tradescantia there is a light-regulated developmental change in the GSA with the magnitude of this change being dependent on the photon flux density of white light. The photosynthetic inhibitor DCMU abolished the effect of white light. Low fluence rates of red light had no significant effect on the GSA of Tradescantia: It was concluded that there is an interaction between photosynthesis and the GSA in Tradescantia: The light-induced reduction of the GSA of the hypocotyl of lazy-2 tomato has previously been assumed to be solely an action of light acting via phytochrome. However, it can be shown that the GSA of hypocotyls of lazy-2 seedlings grown in white light is sensitive to DCMU and norflurazon treatment, hence the light effects on the GSA of an organ can be mediated via both phytochrome and photosynthesis. The implication of these findings to the study of gravitropism is discussed.

The evolution of secondary metabolism - a unifying model.

Why do microbes make secondary products? That question has been the subject of intense debate for many decades. There are two extreme opinions. Some argue that most secondary metabolites play no role in increasing the fitness of an organism. The opposite view, now widely held, is that every secondary metabolite is made because it possesses (or did possess at some stage in evolution) a biological activity that endows the producer with increased fitness. These opposing views can be reconciled by recognizing that, because of the principles governing molecular interactions, potent biological activity is a rare property for any molecule to possess. Consequently, in order for an organism to evolve the rare potent, biologically active molecule, a great many chemical structures have to be generated, most of which will possess no useful biological activity. Thus, the two sides of the debate about the role and evolution of secondary metabolism can be accommodated within the view that the possession of secondary metabolism can enhance fitness, but that many products of secondary metabolism will not enhance the fitness of the producer. It is proposed that secondary metabolism will have evolved such that traits that optimize the production and retention of chemical diversity at minimum cost will have been selected. Evidence exists for some of these predicted traits. Opportunities now exist to exploit these unique properties of secondary metabolism to enhance secondary product diversity and to devise new strategies for biotransformation and bioremediation.

The use of mutants to probe models of gravitropism.

It has been widely believed for more than 70 years that auxin plays a central role in the induction of differential growth which causes gravitropic curvature. However, this long-standing consensus about a role for auxin in gravitropism has only been achieved by allowing several mutually exclusive models to coexist. Furthermore, because there is no detailed model which is unchallenged by evidence, consensus is now centred on ill-defined models which have a low predictive value, hence are harder to challenge experimentally. An increasing number of mutants with abnormal gravitropic behaviour are becoming available. Such mutants should be very helpful in challenging existing models of gravitropism and in providing new evidence on which to build improved, more precise models. However, to date, most studies of mutants with abnormal gravitropism have been guided, experimentally and conceptually, by the old inadequate and vague models. Consequently, the full potential of modern molecular analysis in aiding our understanding of gravitropism has yet to be realized.

Solving the puzzle of gravitropism--has a lost piece been found?

Attempts to devise models to account for the gravitropic behaviour of plant organs have been limited conceptually by the predominance of studies on the gravitropic behaviour of organs of young seedlings. The dramatic growth responses induced by gravitropic stimulation of young shoots or roots, which rapidly restore the elongating axes to vertical, are experimentally convenient but theoretically limiting because gravitropism needs not simply restore an organ to vertical. Evidence is reviewed that suggests that plant organs must possess a mechanism which will allow them to attain a stable gravitropic position at any angle and that each organ has a characteristic gravitropic set-point angle (GSA). The GSA can be changed developmentally and is also regulated in a reversible manner by environmental parameters such as light. It is speculated that gravity may itself influence the GSA of an organ. The recognition that plant organs can grow at angles other than vertically up or down is not new, but previously it has been accepted that angles other than vertical were the consequence of the vectorial resultant effect of two different, opposing mechanisms. The new GSA concept proposes that a single mechanism might be sufficient to account for all forms of gravitropism in roots and shoots. This unifying concept proposes that the ability to change the angle of an organ with respect to the vertical is part of the basic gravitropic mechanism and that models of gravitropism must be able to account for this important feature.

The gravitropic set-point angle (GSA): the identification of an important developmentally controlled variable governing plant architecture.

The angle at which an organ is maintained by gravitropism is characteristic of the organ, its developmental state and the prevailing environmental conditions. We propose that this angle be called the gravitropic set-point angle (GSA), defined as the angle with respect to the gravity vector (with a vertically downward orientation being 0 degrees) at which an organ is maintained as a consequence of gravitropism. Studies or the gravitropic behaviour of organs from trailing plants show that the GSA is subject to developmental regulation. Depending on the developmental age and prevailing environmental conditions, the GSA of an organ can be set at any value between 0 degrees and 180 degrees. The previously reported reversal of the sign of the gravitropic response in such organs, whether this is brought about developmentally or induced by light, represents the change from one common extreme (GSA = 180 degrees, conventionally referred to as negative orthogravitropism) to another (GSA = 0 degrees, or positive orthogravitropism). The concept of a variable gravitropic set-point offers a more unified view of all forms of gravitropic behaviour than has been advanced previously, and places a new constraint on models of gravitropism. Current models of gravitropism appear to be unable to explain either the ability of organs to change their orientation with respect to gravity as they develop, or the re-orientation that can be observed when some organs are exposed to new environmental conditions.

Plant movements caused by differential growth--unity or diversity of mechanisms?

A very wide range of plant organ movements have been described and yet it is not clear how each of them is related to the others. This uncertainty has had two undesirable consequences. Firstly, some workers have accepted the idea of a vague unity in the area and have subsequently been misled by information obtained in one system and applied without adequate justification to a study of a quite different system. Secondly, some researchers have evoked a possible diversity to explain why a particular mechanistic explanation may continue to be valid even when the model fails to explain events of a very similar nature in a slightly different system. We argue that this confusion has resulted from a classification of organ movements which has been based on functional rather than on mechanistic considerations. Mechanistic unity is to be expected on evolutionary grounds. This unity, however, may apply only to certain elements of the stimulus-response chain, at certain levels of organization. It follows from this that in seeking this unity, comparisons should be made between equivalent elements of the stimulus-response chain at the same level of organization in different systems. Only when this is done will theories built around the concept of unity provoke meaningful discussion.

Phenolic biosynthesis, leaf damage, and insect herbivory in birch (Betula pendula).

The effect of both caterpillar herbivory and artificial damage on phenylalanine ammonia lysase (PAL) activity of birch foliage was measured, using an intact cell assay. After artificial damage there was a small increase in PAL activity in damaged leaves but no change in adjacent undamaged ones. Insect grazing produced a larger increase in PAL activity, and the enzyme activity was also increased in adjacent undamaged leaves. Artificial damage increased the phenolic levels of the damaged leaves. Insect grazing caused a larger, longer-lasting increase in phenolic levels and also elevated phenolic levels in undamaged leaves. The possible role of these wound-induced biochemical changes in birch is discussed.

Polysaccharide Synthesis and Turnover in the Cell Walls of Growing and Non-growing Cells of Gravistimulated Sunflower Hypocotyls.

No significant changes were detected in the proportions of the various monosaccharides present in the cell wall polysaccharides of the peripheral cell layers on the upper and lower sides of sunflower hypocotyls which had undergone gravicurvature. The incorporation of radioactive glucose into the major classes of polysaccharide was similar in the walls of growing (convex) and non-growing (concave) cells. Autoradiographic studies showed that most of the newly incorporated material was located in the innermost layers of the cell wall, this being the case at both the upper and lower sides of the curving hypocotyl. During the period of differential growth causing curvature, the turnover of the recently incorporated glucose was not detected. These results may mean that neither the relative rates of synthesis of the main polysaccharides nor the turnover of recently incorporated major polysaccharides were influenced significantly by short term changes in growth rate of the peripheral cell layers.

Solute production and net wall synthesis in the growing and non-growing cells of gravistimulated sunflower hypocotyls.

Solute generation and cell wall synthesis were examined in sunflower hypocotyl peripheral layers, the growth rate of which had been altered by gravistimulation. Measurements of both the concentrations of the major solutes and the osmotic potential showed that although upper cells stopped growing, the solute levels in these cells continued to increase at rates comparable to those in lower cells. This indicated that altered growth rates, generated during gravicurvature, are not based on solute generation but must result from cell wall changes. Gravimetric and precursor incorporation studies showed that net wall synthesis continued in upper cells despite their lack of growth. An ultrastructural study of the epidermal cells on the uppermost (non-elongating) and lowermost (elongating) surfaces of horizontal cucumber hypocotyls showed that the relative amounts of the various membrane fractions were similar in upper and lower cells despite their very different growth rates.

Auxin Uptake and the Rapid Auxin-Induced Growth in Isolated Sections of Helianthus annuus.

When a sunflower (Helianthus annuus L.) hypocotyl section is placed in a solution of 10 micromolar 1-naphthylacetic acid, the majority of the auxin enters via the cut surfaces. However, it is the minority entering radially which causes the typical growth response. Auxin supplied only to the apical cut surface gives a weak, slower response.

The role of the apex in the phototropic curvature of Avena coleoptiles: positive curvature under conditions of continuous illumination.

The differential growth causing second positive phototropic curvature in intact, black-capped and decapitated Avena coleoptiles has been measured. In all cases the curvature is brought about by a cessation in growth of the illuminated side. The fact that shading the apex does not significantly alter the initial steps of differential growth means that the subapical zones can perceive and respond to unilateral illumination. Decapitation significantly reduces coleoptile growth, especially in the most apical zone. However, the fact that differential growth is still evident in the other zones of decapitated coleoptiles within 30 min of unilateral illumination requires one to conclude that the apex cannot be controlling the differential growth in those basal zones.

Stomatal closure in response to xanthoxin and abscisic acid.

The stomata of detached leaves of Commelina communis L., Hordeum vulgare L., Zea mays L., Vicia faba L., Phaseolus vulgaris L. and Xanthium strumarium L. closed when xanthoxin (XAN) was added to the transpiration stream. XAN was approximately half as active as (+)-abscisic acid (ABA) at an equivalent concentration. XAN, like ABA, sensitized stomata of Xanthium strumarium to CO2. In contrast to ABA, XAN was ineffective in closing stomata of isolated epidermal strips of C. communis or V. faba. This may be because XAN added to the transpiration stream is converted to ABA during passage from the xylem to the epidermis.

On the secretion of α-amylase by barley aleurone layers after incubation in gibberellic acid.

Gel filtration and centrifugation studies were used to study the distribution of α-amylase activity in homogenates of barley (Hordeum vulgare L.) aleurone layers. The results obtained were consistent with the hypothesis that α-amylase is secreted via membrane-bound vesicles. The α-amylase activity in an homogenate of barley aleurone layers was derived not only from the enzyme retained in the aleurone cells but also from enzyme previously secreted from the cells but apparently retained by the cell walls. The amount of α-amylase retained by the cell wall was influenced by factors such as the buffer in which the layers were incubated or the presence of Actinomycin D in the incubation medium.

Some effects of applied gibberellic Acid on the synthesis and degradation of lipids in isolated barley aleurone layers.

An analysis of the lipids in isolated barley (Hordeum vulgare L.) aleurone layers after 12 hours incubation in the presence or absence of gibberellic acid showed no quantitative or qualitative changes. Longer incubation periods resulted in some lipid degradation which was greater in the presence of 1 mum gibberellic acid.Glycerolipid synthesis was measured in isolated barley aleurone layers during the first 12 hours of incubation in the presence or absence of gibberellic acid by following the incorporation of (3)H-glycerol. No significant effect of the hormone was found on either the incorporation of glycerol into lipids or on the types of lipid being synthesized.

Enzymatic production of the plant growth inhibitor, xanthoxin.

Incubation of violaxanthin with lipoxygenase and linoleate gave rise to the plant growth inhibitor, xanthoxin; and the yields were reduced to 1/10 and 1/20 by the omission of lipoxygenase and both lipoxygenase and linoleate respecively.

The detection and estimation of the growth inhibitor xanthoxin in plants.

The plant growth inhibitor xanthoxin which can be prepared in vitro by the oxidation of certain xanthophylls has been identified in the ether extracts of the shoots of a wide variety of higher plants. Gas liquid chromatography of an acetylated derivative has been used for its quantitative estimation.Evidence is provided that xanthoxin is a true endogenous inhibitor and that violaxanthin or a related xanthophyll epoxide is its biogenetic precursor. The importance of xanthoxin and its relationship with abscisic acid and other plant growth inhibitors is discussed.