High irradiance responses involving photoreversible multiple photoreceptors as related to photoperiodic induction of cell division in Euglena

Little is known about the photoreceptors involved in the photoperiodism of unicellular organisms, which we elucidated by deriving their action spectra. The flagellated alga Euglena gracilis exhibits photoperiodism, with a long-day response in cell reproduction. The underlying clock is a circadian rhythm with photoinductive capability, peaking at subjective dusk and occurring at the 26th hour in continuous darkness (DD) when transferred from continuous light (LL); it regulates photoinduction, a high-irradiance response (HIR), of a dark-capability of progressing through cell division. We derived the action spectra by irradiating E. gracilis with monochromatic light for 3 h at around the 26th hour; the action maxima occurred at 380, 450–460, 480, 610, 640, 660, 680, and 740 nm. Except for the maximum at 450–460 nm, which was always a major maximum, the maxima greatly depended on the red (R)/far-red (FR) ratio of the prior LL. The high R/FR ratio resulted in a dominant major peak at 640 nm and minor peaks at 480 and 680 nm, whereas the low ratio resulted in dominant major peaks at 610 and 740 nm and minor peaks at 380 and 660 nm; the critical fluence was minimally about 60 mmol m−2. These HIRs resulted from the accumulation of corresponding low-fluence responses (LFRs) because we found that repetition of a 3-min light/dark cycle, with critical fluences of 1 mmol m−2, lasting for 3 h resulted in the same photoinduction as the continuous 3-h irradiation. Moreover, these LFRs expressed photoreversibility. Thus, photoperiodic photoinduction involves Euglena-phytochrome (640 and 740 nm) and blue photoreceptor (460 nm). Although 380, 480, 610, 660, and 680 nm may also represent Euglena-phytochrome, a definite conclusion awaits further study.