Gravity-dependent reactions of the moss Pohlia nutans protonemata.

In darkness, protonemata of Pohlia nutans (Hedw.) grew negatively gravitropically (upwards). However, not all filaments became gravitropic immediately after transfer to darkness. Some of them (~20%) for several days grew in different directions with respect to gravity. The apical cells of those protonemata predominantly contained multiple chloroplasts. The intensity of chlorophyll fluorescence rapidly decreased in the apical cells of such protonemata while starch content increased in comparison with upright growing protonemata. Light, especially in the red and blue part of the spectrum, inhibited protonemal gravitropism. Red light induced stronger inhibitory effects than blue light. Red light of 1.0 to 1.5 micromoles m-2 s-1 intensity induced bud differentiation in apical cells on almost all side branches of main protonemal filaments. Bright fluorescence of F-actin bundles in the tip of apical protonematal cells and a delicately fluorescing network enclosing plastids basal to the tip in a sedimentation zone were visualized. Bright fluorescence of actin as local patches and fine prominent axially oriented bundles was observed in cells of gametophore buds.

Gravitropism and phototropism in protonemata of the moss Pohlia nutans (Hedw.) Lindb.

The gravitropism of protonemata of Pohlia nutans is described and compared with that of other mosses. In darkness, protonemata showed negative gravitropism. Under uniform illumination they grew radially over the substrate surface, whereas unilateral illumination induced positive phototropic growth. Gravitropism was coupled with starch synthesis and amyloplast formation. Protonematal gravitropic growth is more variable than the strict negative gravitropism of Ceratodon chloronema.

Gravi- and photostimuli in moss protonema growth movements.

Moss protonemal growth direction is controlled by at least three factors, photo-, gravi- and autotropism. It is possible to experimentally separate these factors and to control selectively their morphological appearance. In darkness protonema grow negatively gravitropically, and unilateral illumination initiated positive phototropism. Red light suppressed auto- and gravitropism, blue light suppressed only gravitropism. Green light allowed both gravi- and autotropism. The effect of light on gravitropism might involve changes in starch synthesis.

Behavior of amyloplasts in photo- and gravitropism of the moss protonema.

The protonema of mosses Ceratodon purpureus and Pottia intermedia is negatively gravitropic in darkness and grows on the substrate surface under illumination. However, the putative mechanisms of these growth responses are not well understood so far. For gravitropism, sedimentation of amyloplasts has been widely assumed to be the first step of the signal transduction chain. This model was supported by numerous observations where amyloplasts' number or size in a protonema apical cell correlated with its gravisensitivity. Unlike multicellular graviperceptive organs, a protonema apical cell is the same site for both of gravity and light perception and realization of growth movements. In addition, red light is known to change the cell responses to gravity. Therefore, we analysed the influence of red light on the events associated with graviperception and growth movements of protonema apical cells, namely: plastid behavior, size, and number, starch content, chlorophyll fluorescence intensity and alpha-amylase activity, under gravistimulation of dark-grown protonema.