PLANT GROWTH AND DEVELOPMENT IN MOLECULAR PERSPECTIVE

Summary 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).