PLANT GROWTH AND DEVELOPMENT IN MOLECULAR PERSPECTIVE

• 1 After some false starts in which inactive plant substances were isolated, the isolation and identification of auxin as the growth substance at the meristems and of ethylene as the ripening agent in climacteric fruits represented outstanding achievements.
• 2 In early work, the non-localized origin of auxin at the meristem and its possible transport for coleoptile development were obscured by the superimposition on the results of physiological experiments of the idea of a close parallelism between the plant-growth substances and mammalian hormones. At that time, an absence of chemical instrumentation, suitable for measurement of the tissue levels, compounded the difficulty in interpreting available physiological evidence.
• 3 Member(s) of each of the five groups of naturally occurring plant-growth substances, namely the auxins, cytokinins, gibberellins, ethylene and the growth inhibitors, including abscisic acid, are biologically active at a concentration of 10 μm or less, however, and in this respect they would appear to qualify as candidate phytohormones.
• 4 The sensitivity of plant cells to phytohormones contributes to plant growth and development, and both the variations in sensitivity, for example, of wheat coleoptiles towards growth and the growth of the coleoptiles per se give parallel unimodal relationships with regard to time; the curve representing sensitivity precedes that for growth. A new graphical analysis implies that the growth sensitivity and growth rate functions are mutually interdependent.
• 5 The assumption is made in point 4 that growth substance complexes with receptor protein in growth-sensitive cells, and the concept of receptors would provide explanation for the obvious amplification of effects induced by growth substances.
• 6 Numerous biological situations occur in which the presence of significant amounts of plant hormone controls growth and development. In gravitropism and phototropism, tropistic curvature depends on the difference in physiological concentration of auxin on the two sides of the organ concerned. In infected tobacco plants, the cytokinin to auxin ratios for the tumours determine the kind of development (tumours and shoots, tumours only or tumours plus roots), which takes place.
• 7 Auxin-binding protein has been identified immunologically, and isolated. Work with hormone receptors for gibberellin does not afford unequivocal evidence for more than one primary site of action. Hitherto, no specific receptor protein is known for cytokinins.
• 8 Clear evidence derives, both from structure–activity relationships and from unimodal concentration–response curves, for receptor specificity to auxin action. There is also evidence for a structure–activity relationship in respect of the cytokinin series of compounds.
• 9 From the evidence (points 1–8), there emerges a picture of hormone-induced growth and development of plant cells, which have been made sensitive to hormone through the presence of specific receptor proteins.
• 10 That plant growth and developmental processes involve changes in gene expression would seem to follow from the totipotent nature of meristematic cells, which are capable of specialization in response to phytohormones.
• 11 Auxin regulates de novo synthesis of mRNAs encoding polypeptides essential to the auxin-induced early process of cell elongation. In fact, auxin regulates the concentrations of several authenticated mRNAs and proteins, for example, in elongating soyabean hypocotyl sections.
• 12 Furthermore, two cDNA clones, termed pJCW₁ and pJCW₂ have been isolated with the properties expected of mRNAs involved in the rate-limiting stage of cell elongation. The evidence suggests that the change in relative abundance of the JCW₁ and JCW₂ RNAs is an obligatory auxin-dependent response. Hence, the action of cytokinin in auxin-induced cell elongation would seem to be concerned with the inhibition of rate-limiting proteins, and in fact cytokinin inhibits protein synthesis in excised soyabean hypocotyl.
• 13 Biosystemic experiments on some rapid effects of synthetic auxin growth regulators on mRNA levels in vitro show that there is only partial similarity between those found in pea and soyabean spp. (Leguminosae).
• 14 Two identified sequences, namely TGATAAAAG and GGCAGCATGCA, of two auxin-regulated soyabean genes afford a means for determining whether the auxin-regulation of expression of these genes involves trans-acting regulatory factors.
• 15 The obligatory auxin-induced responses with regard to cell elongation and growth (q.v.) would seem to precede the somewhat mechanical growth properties by which auxin receptive cells secrete H⁺ and lower the pH to yield increased cell-wall plasticity.
• 16 In vertically oriented soyabean seedlings, auxin-regulated RNAs are distributed symmetrically in the elongating region of the hypocotyl, whereas in horizontally oriented seedlings the distribution becomes asymmetrical within a few minutes of horizontal gravitational stimulation. The dynamic expression of auxin-regulated genes is related to the morphogenetic response, initiated by re-distribution of endogenous auxin (point 6).
• 17 In the germination of seeds, the mobilization of food reserves requires hydrolytic enzymes and, in barley grains, gibberellic acid induces de novo biosynthesis of α-amylase and protease. The genetic implications are discussed, and the requirement of dicotyledonous and gymnospermous seeds for the presence of gibberellins is explored.
• 18 In ripening climacteric fruits, ethylene-induced change(s) in gene expression cause de novo biosynthesis of polygalacturonase, which degrades the cell-wall pectin fraction.
• 19 Accordingly, incontestible evidence has been mustered for the proposition that hormone-regulated plant growth and development involves hormone-regulated gene expression.
• 20 As well as the phytohormones, certain environmental factors, such as white light and stress (including anaerobiosis, chilling, heat shock, heavy metal exposure, u.v. light and wounding) have the capacity to regulate gene expression in plants at important stages in growth and development. Discussion at the genetic level focuses on changes produced by:
• 21 The synthesis of phytohormones is significant. For example, as u.v. light-induced regulation of genes produces enzymes for auxin synthesis, it may be responsible in seeds for the endospermal generation of auxins, concerned with the epicotyl/hypocotyl growth in the seedlings.
• 22 Hormones and certain environemental factors (q.v.) initiate some of the numerous stages in plant growth and development, but the regulatory factors are obscure in some other biological situations, such as:
• 23 Utilization of an appropriately re-constituted plant DNA polymerase i in vitro system might enable the type and frequency of misincorporation, produced by plant-growth factors, to be studied. Base-pair substitution changes were produced in strains of crop plant, made resistant to a specific herbicide by genetic engineering (see Hathway, 1989). It is feasible that auxin may behave as a reagent in the chemical sense to effect intramolecular change(s) in some of the sequences concerned, leading to the frame-shift changes observed (see Ainley et al., 1988).

     

NON-HORMONAL STIMULATORS AND INHIBITORS OF PLANT GROWTH AND DEVELOPMENT

1. Plants contain growth regulators that are non-hormonal in nature. These regulators change in concentration during ontogeny and when applied exogenously, can either stimulate or depress growth. While the bulk of either the phenolic or terpenoid regulators are localized within the vacuole, they can also be found within other cellular compartments where they may act upon metabolic pathways, modifying either cell multiplication or elongation. 2. Non-hormonal growth regulators may affect the synthesis and/or destruction of phytohormones, mainly indole-3-acetic acid (IAA). These regulators behave non-specifically, modifying the actions of auxins, gibberellins and cytokinins upon growth. 3. A variety of both uncertainties and unresolved contradictions exist that have prevented a thorough elucidation of the mechanisms of actions of both phenolic and terpenoid regulators. These uncertainties and unresolved contradictions include lack of data regarding compartmentalization of many of the inhibitors. This raises the question of whether their intracellular concentrations become elevated sufficiently to affect metabolic pathways in vivo. Exogenously applied regulators of non-hormonal nature usually interfere with growth only at high concentrations. Therefore, the possibility cannot be excluded that under these conditions, reactions occur within the cell that are absent in vivo. 4. The specific properties of natural non-hormonal regulators are similar in certain respects to phytohormones. For example, both of them may be biogenetically bound within metabolic centres: shikimate (phenolics, indoles, alkaloids), bi-benzi (coumarins) or acetate-mevalonate (terpenoids, fluorens, sesquiterpenes, cytokinins). In addition, both non-hormonal regulators and phytohormones exhibit biological activity in growth bioassays. 5. Non-hormonal regulators may possess a number of useful purposes, e.g. test substances such as fusicoccin permit the investigation of the mode of action of phytohormones, specific inhibitors blocking special forms of growth and protectors of phytohormone activity in culture.     

植物生长调节物质对果实生长发育的调控

本文就赤霉素、细胞分裂素、乙烯利对果实生长发育的调控作用进行了较为系统的论述,并对其在果树上的利用方法、时期、浓度等作了说明。     

THE EFFECT OF CERTAIN BENZAZOLE COMPOUNDS ON PLANT GROWTH AND DEVELOPMENT

Klingensmith , M. J. (Colgate U., Hamilton, N. Y.) The effect of certain benzazole compounds on plant growth and development. Amer. Jour. Bot. 48(1): 40–45. Illus. 1961.—A number of benzazoles, in particular benzimidazole, benzothiazole, and benzotriazole, were examined for their effects on growth of seedlings and established plants. Benzothiazole was the most active in repressing elongation of the primary root of cucumber. Benzimidazole and benzotriazole were about 1/10 as active. Adenine was without effect in reversing the benzazole-induced inhibition of cucumber root elongation and, in fact, supplemented the inhibition caused by benzimidazole and benzothiazole. Application of benzotriazole to the root medium of bean, coleus, tomato, oat and wheat caused a pronounced inhibition of internodal elongation with a stimulation of axillary development. Distinct morphological changes were observed which did not correspond to those produced by other growth regulators. Application of benzimidazole to the root medium of several genera of plants resulted in injury to laminar tissue followed by desiccation, with no concomitant effect on the stem. Application of benzothiazole to the root medium induced development of adventitious roots in bean and tomato. This compound also caused initiation of roots on cultures of tobacco-stem segments, while not suppressing bud development. The benzazoles tested seem to be of a different class of compounds from other growth regulators which have been studied. The responses elicited by treatment with each of the 3 benzazoles are distinctly characteristic and are dependent on the structure of the azole portion of the molecule.     

CALCIUM AND PLANT GROWTH

Calcium as a plant nutrient is characterized by its relatively high content in the plant coupled with a requirement not much higher than that of a micro nutrient element and an exceedingly uneven occurrence in soils. The difficulties in defining its actions are accentuated by a weak biochemical activity. In ecological conditions the secondary consequences of variations in calcium content may be more striking than the direct ones. Electron-microscopical studies have revealed that calcium is required for formation and maintenance of lamellary systems in cell organellae, a fact which might suffice to explain its indispensability for meristematic growth. Calcium is required for cell elongation in both shoots and roots; the common experience that it inhibits shoot elongation is certainly due to calcium additions far above actual requirement. It must be assumed for a rational interpretation of cell elongation that the fundamental mechanism is the same in shoots and roots. The one action which can be ascribed with certainty to calcium is a stabilizing of the cell wall with an increase in rigidity, an effect which, with over-optimal supply, may lead to growth inhibitions. The function is, however, necessary for the normal organization of cell walls. Calcium has, on the contrary, no significant effect on the synthesis of cell wall compounds but appears to act on their proper incorporation into the cell wall. The growth-active calcium may be bound not only to pectins but also to proteins and nucleoproteids in or in close contact with the cell wall. The supposition that calcium interacts directly with auxin in the cell wall has not been verified and does not seem very probable. There are reasons to believe that the points of action of calcium and auxin in the cell wall differ, auxin inducing growth by wall loosening and calcium establishing new wall parts. For submerged organs it may be necessary to consider an indirect effect of calcium on growth by its regulation of cytoplasmic permeability and thus affecting the exudation of growth-active compounds. The ecological problem is to characterize calcifuges (acid soil plants) from calcicoles (base soil or calcareous soil plants). Growth inhibitions on acid soils depend upon poisoning by A1³⁺ and Mn²⁺. Opinions differ as to what extent this can be antagonized by calcium. Lime-induced chlorosis in calcifuges depends upon iron deficiency or iron inactivation in the plant. No acceptable explanation is given, but it might be related to an interaction of calcium carbonate, phosphorus, and iron. A hypothesis that it is linked to formation of organic acids is not tenable in the given form. Plants react to the calcium ions in the concentrations found in soils. Calcifuges have a low calcium-optimum for growth and show growth inhibition at high concentrations. Calcicoles have a high optimum for growth. Calcifuges are resistant to aluminium poisoning. Attempts made to explain the differences in calcium uptake and generally in salt uptake are tentative only, and relevant data are lacking.     

藻类与植物生长物质

高等植物和藻类的发育之间具有相似性.尽管藻类具有多样性,但人们认为它们是比高等植物低级的生物类群,它们的形态比高等植物简单,但多数的发育过程却是相似的1。