Photocontrol of seed germination of wildtype and long-hypocotyl mutants of Arabidopsis thaliana

This thesis reports research on the photocontrol of seed germination of wildtype and long-hypocotyl mutants of Arabidopsis thaliana. The mutants show reduced photoinhibition of hypocotyl growth in white light in comparison to that of wildtype. In monochromatic light some of the mutants also show no inhibition of hypocotyl growth by red and/or far-red light, while others show no inhibition in blue and UV light. It is proposed that these mutants might either be mutated with respect to red/far-red absorbing phytochrome, another red absorbing photoreceptor or a blue/UV absorbing receptor.Preliminary studies on the breaking of dormancy and subsequent induction of germination by light are reported in chapter 5. It is shown that the imbibition treatment prior to irradiation has a great influence on the level of dormancy of the seed population. Prolonged imbibition results in secondary dormancy. By analysing fluence-response curves, it is shown that induction of secondary dormancy is not the opposite process as that of loss of primary dormancy. During loss of primary dormancy, the reaction partner of Pfr, X, appears to become less limiting, while during the onset of secondary dormancy Pfr appears to become limiting.The seed germination behaviour of the long-hypocotyl mutants and wildtype was investigated. By determining action spectra of the induction and inhibition of induction of seed germination, information was obtained about the receptor pigments involved (chapter 7). It is demonstrated that phytochrome is the sole photoreceptor controlling seed germination. Although seeds can be induced to germinate by blue light, there appears to be no separate blue or UV receptor involved. The relative activity of blue light in the mutants Hy-1 and Hy-2, deficient in spectrophotometrically detectable phytochrome, is comparable to that of wildtype.Although no significant differences in the action spectra of the mutants and wildtype were observed, the fluence-response curves both for the induction and inhibition of induction of germination show differences in form. The mutants Hy-1, Hy-2 and Hy-3 show a shallow fluence-response curve, while the mutants Hy-4 and Hy-5 and wildtype show steeper fluence-response curves.To interpret the differences in the fluence-response curves of the different seed batches, a mathematical model was designed which allows theoretical fluence-response curves to be calculated (chapter 3). In this model it is assumed that there is a normal distribution for the logarithm of the Pfr requirement of individuals within a seed population. The validity of this assumption is supported by experiments in chapter 3, where theoretical and experimental fluence-response curves are shown to coincide. The known formula for the appearance of Pfr upon irradiation was modified to take account of pre-existing Pfr in the seeds. The model takes into account different levels of an overriding factor, affecting germination by a non-phytochrome related process, the total amount of phytochrome, the range of Pfr requirement in the population and differential screening. It is shown that these factors can have a great influence on the form and/or position of the fluence-response curves. Using this model it is suggested that the fluence- response curves of Hy-1, Hy-2 and Hy-3 are shallow because these seed batches have a low level of phytochrome and that the level of dark germination is the result of an overriding factor.To show that the response to Pfr is not only a function of the amount of Pfr in the seeds, but also a function of the duration of Pfr action, the time course of phytochrome action was determined (chapter 6). It is shown that the escape from far-red photocontrol (time course of Pfr action) and the rate of germination is correlated with the sensitivity of the seeds to Pfr. Germination plots versus time can show two phases. The rapid phase of germination is due to those seeds having germination induction satisfied by their endogenous Pfr during imbibition.The model in chapter 3 also enables theoretical action spectra to be calculated both for the induction and inhibition of induction of germination (chapter 4). It is shown that there is no standard action spectrum, the form andlor peak position of a spectrum being determined by the Pfr sensitivity of the seed population, pre-existing Pfr, overriding factor, total phytochrome and differential screening.It is shown that seeds depleted of endogenous Pfr sometimes exhibit biphasic fluence-response (chapter 8). A part of the population is very sensitive to light, while the remaining part shows normal sensitivity. Imbibition conditions determine the proportion of the population responding to very low fluences. The model presented in chapter 3 was modified to fit the biphasic fluence-response curves by assuming that the sensitivity of the seeds to Pfr is determined by Pfr itself, at least at low levels of Pfr.