Summary The inverse low intensity movement of the Mougeotia chloroplast is a response to polarized red light (R) vibrating parallel (∥) to the long axis of the cell, whereby the chloroplast turns from face to profile position. This response requires a preexisting gradient of phytochrome Pfr produced by preirradiation with polarized red light vibrating perpendicular to the long axis of the cell (⊥). In the present study, the efficiency of the preirradiation is investigated. With increasing energy of R ⊥, the efficiency reaches a maximum and begins to decline as the irradiation time is increased beyond 1 to 3 minutes (fig. 1). This declining efficiency indicates an inhibiting effect of prolonged preirradiation. This inhibiting effect can be simulated by two short preirradiations (R ⊥), separated by a dark interval of 10 min or more (fig. 2, 3). Hence, we are dealing with a time effect rather than with an energy effect, and this time effect is started by the first of the two preirradiations (1st P). To be effective, the inhibitory 1st P needs very little energy, and reciprocity holds within the range of seconds (fig. 5). R ⊥ is more inhibitory than unpolarized R, whereas R ∥ is not inhibitory (fig. 6); instead, R ∥ given after R ⊥ cancels the inhibitory effect of R ⊥ (fig. 7). Even far-red (FR) can substitute for R as an inhibiting 1st P, with ⊥, ∥ and unpolarized FR being about equally effective (fig.8a); here again only little energy is needed and reciprocity holds (fig. 8b). Because of the similar effect of R and FR, a true R-FR antagonism cannot be found, but the last irradiation determines the effect (fig. 9). Nearly all observations point to phytochrome as the photoreceptor pigment of the 1st P. To confirm this, a quantitative comparison has been made between the induction of the normal low intensity movement in Mougeotia (profile to face movement) by R or FR and the inhibitory effect of the 1st P in the inverse low intensity movement. The increasing parts of the dose-response curves coincide fairly well (fig. 11). «Classical» time effects of phytochrome, viz. dark reversion and dark destruction, are unlikely to account for the effect reported here. Hence, a new speculation has been introduced into the Mougeotia system, based upon recent reports on phytochrome binding: Whenever a gradient of Pfr is established, this Pfr is bound, leaving behind a gradient of mainly unbound Pr. Whereas the gradient of bound Pfr should be stable, the gradient of free Pr could be expected to level in a time-dependent process, thus giving rise to completely different starting conditions for the next irradiation.
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Jul 1, 2015
Yin Luo + 2
Open Access