9.2.4 Modified gene expression

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With the advent of more sophisticated methods of detecting changes in gene expression, it has become clear that many responses to plant hormones operate through up- and down-regulation of specific genes. In some cases, hormone signals elicit gene expression within a few minutes, so speed of response is no longer a diagnostic tool for deciding whether a hormone is acting directly on physiological processes. Analysis of DNA sequences in the promoters of hormone-regulated genes has shown common short sequences, typically four to twelve nucleotides long, called ‘response elements’, which are unique to each hormone class and are essential for hormone action (Table 9.2). Provided other essential factors are present, these response elements allow a single hormone potentially to regulate expression of whole suites of genes each carrying the same response element.


Table 9.2

(a)  Gibberellin- and ABA-induced gene expression in germinating cereals

The ‘opposing forces’ of gibberellin and ABA in seed dormancy were described in Chapter 8, but these hormones also appear to operate in regulating metabolism in the seed during the germination phase. This is best known in cereal seeds. Gibberellins, produced in the embryo and scutellum, move through the endosperm to the aleurone layer, where they stimulate expression of a suite of genes coding mostly for enzymes that hydrolyse stored starch, protein and lipids. These resources are contained in the endosperm into which the enzymes are secreted. The released products move to the embryo and provide it with energy and the building blocks required for the rapid growth of the developing seedling. The best-known genes are those for alpha-amylase which degrades starch to sugars. If, however, ABA is applied before or with the gibberellin, then the alpha-amylase gene expression is much reduced. We now know from DNA sequence analysis of the gene promoter in rice, wheat and barley that there are specific response elements upstream from the transcription start (see Section 10.3) which are necessary for gibberellin or ABA to be effective (Table 9.2). If either the gibberellin-response or ABA-response element is excised or deleted from the gene, then the response to that hormone is lost. The time scale involved in switching on gene expression is around 1 h and measurable increase in enzyme activity takes somewhat longer (Higgins et al. 1976; Figure 9.5).

(b)  Auxin-induced growth genes

Some of the most rapid changes in gene expression yet found in plants are those that occur in response to auxin application to growing tissues. The work of Guilfoyle on the SAUR (short auxin upregulated) and GH3 genes of soybean has shown enhanced mRNA levels within 2–5 min (Guilfoyle et al. 1992). We do not yet know exactly what the functions of the gene products are but they are more abundant in faster growing cells, for example on the lower side of a horizontally placed hypocotyl as it responds to light or gravity (Figure 9.10b). In this case, no additional auxin was supplied, and the distribution of gene expression may reflect localisation of endogenous auxin. Two interpretation are possible: (1) this is proof that modified endogenous auxin levels are involved in tropisms; (2) this simply demonstrates that auxin-induced and tropism-induced growth involve expression of the same growth-associated genes.