The speed of intracellular signal transfer for chloroplast movement

The photoreceptors for chloroplast photorelocation movement have been known, but the signal(s) raised by photoreceptors remains unknown. To know the properties of the signal(s) for chloroplast accumulation movement, we examined the speed of signal transferred from light-irradiated area to chloroplasts in gametophytes of Adiantum capillus-veneris. When dark-adapted gametophyte cells were irradiated with a microbeam of various light intensities of red or blue light for 1 min or continuously, the chloroplasts started to move towards the irradiated area. The speed of signal transfer was calculated from the relationship between the timing of start moving and the distance of chloroplasts from the microbeam and was found to be constant at any light conditions. In prothallial cells, the speed was about 1.0 µm min⁻¹ and in protonemal cells about 0.7 µm min⁻¹ towards base and about 2.3 µm min⁻¹ towards the apex. We confirmed the speed of signal transfer in Arabidopsis thaliana mesophyll cells under continuous irradiation of blue light, as was about 0.8 µm min⁻¹. Possible candidates of the signal are discussed depending on the speed of signal transfer.Key words: Adiantum capillus-veneris, Arabidopsis thaliana, blue light, chloroplast movement, microbeam, red light, signalOrganelle movement is essential for plant growth and development and tightly regulated by environmental conditions.¹ It is well known that light regulates chloroplast movement in various plant species. Chloroplast movement can be separated into three categories, (1) photoperception by photoreceptors, (2) signal transduction from photoreceptor to chloroplasts and (3) movement of chloroplasts and has been analyzed from a physiological point of view.² We recently identified the photoreceptors in Arabidopsis thaliana, fern Adiantum capillus-veneris, and moss Physcomitrella patens. In A. thaliana, phototropin 2 (phot2) mediates the avoidance movement,^3,4 whereas both phototropin 1 (phot1) and phot2 mediate the accumulation response.⁵ A chimeric photoreceptor neochrome 1 (neo1)⁶ was identified as a red/far-red and blue light receptor that mediates red as well as blue light-induced chloroplast movement in A. capillusveneris.⁷ Interestingly, neo1 mediated red and blue light-induced nuclear movement and negative phototropic response of A. capillus-veneris rhizoid cells.^8,9 On the mechanism of chloroplast movement, we also found a novel structure of actin filaments that appeared between chloroplast and the plasma membrane at the front side of moving chloroplast.¹⁰ Recent studies using the technique of microbeam irradiation have revealed that chloroplasts do not have a polarity for light-induced accumulation movement and can move freely in any direction both in A. capillus-veneris prothallial cells and in A. thaliana mesophyll cells.¹¹ However, the signal that may be released from photoreceptors and transferred to chloroplasts remains unknown.To understand the properties of the signal for the chloroplast accumulation response, we examined the speed of signal transfer in dark-adapted A. capillus-veneris gametophyte cells and A. thaliana mesophyll cells by partial cell irradiation with a red and/or blue microbeam of various light intensities for 1 min and the following continuous irradiation, respectively.¹²As shown in Figure 1, the relation between the distance of chloroplasts from the microbeam and the timing when each chloroplast started moving toward the microbeam irradiated area (shown as black dots in Fig. 1) was obtained and plotted. The lag time between the onset of microbeam irradiation and the timing of start moving of chloroplasts is the time period needed for a signal to reach each chloroplast. To obtain more accurate data many chloroplasts at various positions were used. The slope of the approximate line indicates the average speed of the signal transfer. Shown with a protonemal cell at the left side of this figure is an instance where the speed of signal transfer from basal-to-apical (acropetal) direction is obtained.Open in a separate windowFigure 1How to calculate the speed of signal transfer in the basal cell of two-celled protonema of Adiantum capillus-veneris. The relationship between the distance of chloroplast position from the edge of the microbeam to the center of each chloroplast as shown in left side of figure and the timing of chloroplast movement initiated shown as the black dots was obtained. Inclination of the approximate lines connecting dots indicates the speeds of the signal transfer.In protonemal cells, which are tip-growing linear cells, the average speed of signal transfer was about 2.3 µm min⁻¹ from basal-to-apical (acropetal) and about 0.7 µm min⁻¹ from apical-to-basal (basipetal) directions. These values were almost constant irrespective of light intensity, wavelength, irradiation period, and the region of the cell irradiated.¹² The difference of speed between basipetal and acropetal directions may be depending on cell polarity. The signal transfer in prothallial cells of A. capillus-veneris and mesophyll cells of A. thaliana was about 1.0 µm min⁻¹ to any direction, probably because they may not have a polarity comparing to protonemal cells or have a weak polarity if any. Thus, the speed of signal transfer must be conserved in most land plants,¹² if not influenced by strong polarity.