9.4 Concluding remarks

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Figure 9.15 Although ripening of climacteric fruit such as tomato is normally associated with autocataclytic ethylene synthesis (see Chapter 11), the physiological response to this increased ethylene level depends on prescence of ethylene receptors. In this experiment, the expression of the tETR gene, which codes for a presumed receptor, was quantified by the relative amount of tETR mRNA present. In mature green fruit, the gene is scarcely expressed, but maximal levels are present in the first visible stages of ripening colour change known as 'breaker' (B). Fruit are fully red by seven days after breaker (B + 7), by which time receptor gene expression has declined again. The developmental regulation of receptor levels may be a key safeguard preventing premature onset of ripening until the fruit and seeds are at the right stage of development. Deliberate manipulation of hormone receptor expression by genetic engineering may become a powerful tool for controlling tissue-specific and developmental-time-based hormone responses.

(Based on Payton et al. 1996; reproduced with permission of Kluwer Academic Publishers)

For many years, plant hormone research focused on measure-ment of hormone levels. Based on responses to applied growth regulators, a widespread notion has been that plants regulate many developmental processes by actively modifying endo-g-enous hormone concentrations. However, despite extremely sensitive and accurate assay techniques, there remain scant examples where normal plants (i.e. not mutant, not inhibitor treated, not genetically engineered) show large changes in endogenous hormone concentration at the site of action. Changes are usually much smaller than predicted, with some exceptions such as 50-fold increases in xylem ABA delivered to leaves in response to water status or 20-fold cytokinin level increases during dormancy release (Tardieu and Davies 1992; Turnbull and Hanke 1985). Much discussion through the 1980s permanently altered ideas on how hormones work in plants, in particular shifting the focus to control of hormone perception and signal transduction, not just to the control of signal levels. The notion of control by changing tissue sensitivity to hormones is not new, but was vigorously proposed by Trewavas (1981) and others. However, in the absence of well-defined receptors, this theory was hard to test other than by traditional dose–response biological assays. Sensitivity is normally equated with presence of a suitable receptor system, but insensitivity (i.e. lack of response) can be the result of failure of any one of the many events between receptor and final physiological action, or sometimes the side effect of disruption of quite unrelated processes. The upshot has been expanded research on signal transduction (Trewavas and Malhó 1997), and a more balanced approach to the possible means of regulating hormone signalling and action.

As discussed above, attempting to modify plant devel-opment through applying PGRs has been popular for many years, often referred to as the ‘spray and pray’ or ‘spray and weigh’ approach. Since the late 1980s, by inserting genes for enzymes of hormone biosynthesis into plants or modifying their expression, alternatives to PGR treatments have been generated. In this way, a plant modifies its own hormone concentrations, avoiding the need for external applications and potentially reducing amounts of chemicals used in agricultural and food industries. However, the responses are very similar in both cases: often in addition to the desired response, we find one or more side effects which frequently limit the usefulness of either technique. The problem lies not only in the multiple functions of plant hormones but also in their mobility within the plant; for example, it is quite hard to prevent root-synthesised cytokinin moving to the shoot in the xylem sap. We conclude this chapter with an idea: if instead of genetically altering hormone concentrations, the receptor or downstream events are modified, we may be then able to generate much more precise tissue-specific control. Receptors, being proteins, will be effectively immobile. Indeed, we already know that some hormone receptor genes are regulated naturally during development. In tomato fruit, the tETR gene which codes for an ethylene-binding protein is hardly expressed at all until the fruit starts to ripen (Figure 9.15). This means that the fruit remains very insensitive to ethylene until the stage of development when the seeds are mature. Regulation of a hormone response in this case confers an adaptive advantage by reducing the likelihood of premature triggering of ripening and seed dispersal before maturity. Much current research is seeking ontogenetically regulated and especially tissue-specific promoters to link to a wide range of genes including those responsible for regulation of hormone concentrations, and this could now be extended to include genes for hormone perception and action.