Red light-induced suppression of gravitropism in moss protonemata

Moss protonemata are among the few cell types known that both sense and respond to gravity and light. Apical cells of Ceratodon protonemata grow by oriented tip growth which is negatively gravitropic in the dark or positively phototropic in unilateral red light. Phototropism is phytochrome-mediated. To determine whether any gravitropism persists during irradiation, cultures were turned at various angles with respect to gravity and illuminated so that the light and gravity vectors acted either in the same or in different directions. Red light for 24h (> or = l40nmol m-2 s-1) caused the protonemata to be oriented directly towards the light. Similarly, protonemata grew directly towards the light regardless of light position with respect to gravity indicating that all growth is oriented strictly by phototropism, not gravitropism. At light intensities < or = l00nmol m-2 s-1, no phototropism occurs and the mean protonemal tip angle remains above the horizontal, which is the criterion for negative gravitropism. But those protonemata are not as uniformly upright as they would be in the dark indicating that low intensity red light permits gravitropism but also modulates the response. Protonemata of the aphototropic mutant ptr1 that lacks a functional Pfr chromophore, exhibit gravitropism regardless of red light intensity. This indicates that red light acts via Pfr to modulate gravitropism at low intensities and to suppress gravitropism at intensities < or = 140nmol m-2 s-1.