Photoregulation by the phytochrome family: A physiological study of transgenic plants.

Chapter 1 reviews the evidence, to date, that the individual molecular species of phytochrome have distinct physiological roles and explains why the use of transgenic plants, which express an introduced phytochrome-encoding sequence, is adopted in this thesis as a novel approach to test this hypothesis. Chapter 2 describes the diversity of photomorphogenic responses of etiolated and light-grown wild-type plants. Chapters 3 to 6 detail the photophysiological responses of transgenic tobacco (Nicotiana tabacum) and Arabidopsis thaliana seedlings which, through expression of a transgene, accumulate supra-wild-type levels of either phytochrome A or phytochrome B; comparison is made with an Arabidopsis mutant specifically deficient in phytochrome B. The results indicate that endogenous phytochromes A and B have reciprocal and independent photosensory roles in seedling development, mediating responsiveness to continuous far-red and red, respectively. Persistent expression, in transgenic plants, of phytochrome A is seen to severely impair the shade-avoidance syndrome of light-grown plants, thus phytochrome A is discounted as the photoreceptor for red:far-red ratio perception. A transgene encoding phytochrome A is expressed in horseradish (Armoracia lapathifolia ) in order to supress the normal shade-avoidance responses, with implications for improved crop yield as discussed in Chapter 10. Transgenic overexpression of phytochrome A in Arabidopsis (but not in tobacco) results in enhanced sensitivity of the greening response to red light, relative to wild-type and also phytochrome B-overexpressing seedlings (see Chapter 7). Germination behaviour of transgenic Arabidopsis seed indicates a role for phytochrome B in mediating dark-germination. The action of a heterologous phytochrome A in photoregulation of germination in transgenic tobacco seed is less readily interpreted (see Chapter 8). Expression of a heterologous sequence encoding phytochrome A in the aurea mutant of tomato fails to rescue wild-type phenotype (see Chapter 9). Chapter 11 characterizes a single-cell system for the study of phytochrome activity. Chapter 12 discusses the potential for, and caveats against, the use of transgenic plants in investigations of phytochrome function.