Action spectra for polarotropism and phototropism in protonemata of the fern Adiantum capillus-veneris

The action spectrum for polarotropism was determined, using the Okazaki large spectrograph, by brief irradiation with light between 260 nm and 850 nm in single-celled protonemata of the fern Adiantum capillus-veneris L., which had been cultured for 6 days in red light and then in the dark for 15 h. The action spectrum had a peak at around 680 nm. This effect was nullified by subsequent irradiaton with far-red light, and typical red/far-red reversibility was observed, indicating the involvement of phytochrome. Polarized ultraviolet or blue light had no effect on the direction of apical growth. The action spectrum for phototropism was also determined in the red light region by means of brief microbeam irradiation of a flank of the subapical region of the protonema. This spectrum showed a peak at 662 nm which was consistent with the absorption peak of phytochrome, but not with the peak of the action spectrum for polarotropism.     

Blue Light-Induced Phototropic Response and the Intracellular Photoreceptive Site in Adiantum Protonemata

Blue light-induced phototropism in Adiantum protonemata wasinvestigated with microbeam irradiation. Brief irradiation withblue light effectively induced a phototropic response when itwas applied to a half-side of the apical 200d µm regionof a protonema. The phototropic response was partly reversedby the subsequent far-red light irradiation but the full reversalof the response was not observed even when the fluence of far-redlight was increased. In the far-red reversible part of the response,blue/far-red photoreversibility was repeatedly observed. Thus,both phytochrome and a blue light-absorbing pigment (other thanphytochrome) seem to be involved in the blue light-induced phototropicresponse in Adiantum protonemata. Furthermore, detailed studiesof the far-red light effect on the fluence-response curve forblue lightinduced phototropism revealed that the concomitantmediation by the two receptors was limited to the response inthe relatively higher fluence range of blue light and that theblue light-absorbing pigment alone was responsible in the lowerfluence range. In the higher fluence range, the response mediatedby the blue light-absorbing pigment became saturated and thephytochrome response became evident, indicating a differencein the sensitivities of the two receptor pigments to blue light. When various regions of half-sides of protonemata were irradiatedwith a blue microbeam of 10 µm width, irradiation at theapical 5–25 µm region was most effective both forphytochrome- and blue light-absorbing pigment-mediated response,indicating that the site of blue light perception is almostidentical for each response. (Received July 14, 1986; Accepted September 26, 1986)     

Apical Growth of Protonemata in Adiantum capillus-veneris IV. Phytochrome-Mediated Induction in Non-Growing Cells

In non-growing two-celled protonemata of Adiantum capillus-veneris,apical growth was induced most effectively by red light irradiation;half of the samples were induced to grow by 660 nm light ofca. 1.5 J m^–2 and the maximum number by ca. 70 J m^–2.The reciprocity law was valid in this photoinduction. The growthresumption became detectable 6 hr after the light irradiationand reached a plateau within 24 hr irrespective of given fluences.When non-growing samples were irradiated with red light of 4.6W m^–2 for 4 sec or shorter, the effect was fully reversedby a subsequent irradiation with far-red light to the far-redlight control level. But, when the red light was given for 16sec or longer, photoreversibility became partial. An interveningdark period of 2 min between red and far-red light did not significantlyinfluence the photoreversibility so that the escape reactionin the dark may not be attributed to the above-mentioned lossof photoreversibility. By means of a local irradiation with a narrow red light beam(10 µm in width), the apical cell was found to be photosensitivefor the growth induction, but basal cell was not. Photoreceptivesite was not localized in any particular region of the apicalcell, but was rather dispersed in the entire apical cell. (Received January 26, 1981; Accepted March 10, 1981)     

Intracellular Localization and Dichroic Orientation of Phytochrome in Plasma Membrane and/or Ectoplasm of a Centrifuged Protonema of Fern Adiantum capillus-veneris L.

Protonemata of the fern Adiantum capillus-veneris grown undercontinuous red light for 6 days were kept in darkness for 15h and subsequently centrifuged 3 times in different directions,so that oil droplets and other cytoplasm were removed from theapical region of the protonemata. Electron micrographs clearlydemonstrated that cell wall, plasma membrane, ectoplasm andmicrotubules remained in the apical and subapical regions afterthe centrifugal treatments. A brief local exposure of the flankof the subapical region of the centrifuged protonemata to amicrobeam of red light effectively induced a phototropic responsetoward the irradiated side, suggesting that phytochrome is locatedin the ectoplasm and/or plasma membrane. When the flank of thecentrifuged protonema was irradiated with linearly polarizedred or far-red light, red light with an electrical vector parallelto the cell surface was more effective than that perpendicularto the cell surface. The direction of the electrical vectorof far-red light for reversion of the preirradiated red lighteffect, however, was opposite. These results suggest that differentdichroic orientations of P_R and P_FR exist in the plasma membraneor ectoplasm. (Received May 26, 1983; Accepted September 1, 1983)     

Phytochrome-Mediated Photoorientation of Chloroplasts in Protonemal Cells of the Fern Adiantum Can be Induced by Brief Irradiation with Red Light

Measuring the ratio of the number of photooriented chloroplaststo the total number of chloroplasts, we found that photoorientationof chloroplasts in protonemata of the fern Adiantum capillus-veneriscould be induced by brief irradiation with polarized red light.After irradiation with red light (R) of 3 or 10 min, orientationalmovement was detected as early as 10 min after the irradiation;it continued during the subsequent dark period for 30–60min, after which chloroplasts gradually dispersed again. WhenR-treated protonemata were irradiated briefly with a second10-min pulse of R, 60 min after the onset of the first irradiation,the orientational response of chloroplasts was again observed.Typical red/far-red photoreversibility was apparent in the response,indicating the involvement of phytochrome. By contrast, irradiationwith polarized blue light for 10 min was ineffective, whileirradiation with blue light (B) at the same fluence for a longerperiod of time clearly induced the photoorientation of chloroplasts.It is likely that longterm irradiation is necessary for theresponse mediated by a blue-light receptor. When protonemata were irradiated with far-red light (FR) immediatelyafter R or after a subsequent dark period of 10 min, the magnitudeof the orientational response was smaller and chloroplasts dispersedmore quickly than those exposed to R alone. When FR was appliedat 50 min, when the response to R had reached the maximum level,chloroplasts again dispersed rapidly to their dark positions.These results indicate that P_FR not only induces the photoorientationmovement of chloroplasts but also fixes the chloroplasts atthe sites to which they have moved as a result of photoorientation. (Received June 2, 1993; Accepted January 11, 1994)     

Phytochrome-mediated polarotropism of Adiantum capillus-veneris L. protonemata as analyzed by microbeam irradiation with polarized light

Summary Perception of polarized light inducing phytochrome-mediated polarotropism in protonemata of the fern Adiantum capillus-veneris L. was analyzed using brief microbeam irradiation with polarized red (R) or far-red light (FR). The polarotropic response inducible by irradiation of the subapical 10–30-m part with polarized R vibrating parallel to the cell axis was nullified by subsequently giving R at the apical 0–2.5-m region. This inhibitory effect of R showed an action dichroism, that is, polarized R vibrating normal to the cell axis was effective but the parallel-vibrating R was not. On the other hand, FR irradiation of the extreme tip after irradiation of the whole cell with depolarized R effectively induced a tropic response. This FR effect also showed action dichroism, with parallel-vibrating polarized FR being more effective than FR vibrating normal to the cell axis. When the apical-dome region and the adjacent subapical 10–20-m region were sequentially irradiated with polarized R vibrating obliquely in different directions, polarotropism took place depending on the vibrating direction of the light given to the apical-dome region. Obliquely vibrating polarized FR given to the apical dome after irradiation of the whole cell with depolarized R also induced polarotropism. Thus, the difference in amount (or percent) of the far-redabsorbing form of phytochrome (Pfr) between the extreme tip and the subapical region appears to be crucial in regulating the direction of apical growth; the difference in Pfr level between the two sides of the protonemal apex may occur mainly at the apical dome. Furthermore, the transition moments of the red-absorbing form of phytochrome (Pr) and Pfr seem to be aligned parallel and normal, respectively, to the cell surface at the periphery of the apical hemisphere.Abbreviations FR far-red light - Pfr far-red-absorbing form of phytochrome - Pr red-absorbing form of phytochrome - R red light     

Changes in microtubule and microfibril arrangement during polarotropism inAdiantum protonemata

Polarotropism was induced inAdiantum (fern) protonemata grown under polarized red light by turning the electrical vector 45 or 70 degrees. One hour after the light treatment, tropic responses became apparent in many cells as a slight distortion of the apical dome. Changes in the position of the circumferentially-arranged cortical microtubule band (Mt-band) (Murataet al., 1987) and the arrangement of microfibrils around the subapical part of protonemata were investigated in relation to the polarotropic responses. Twenty minutes after turning the electrical vector, preceding the morphological change of cell shape, the Mt-band began to change its orientation from perpendicular to oblique to the initial growing axis. After 30 min, the Mt-band changed its orientation further under 45 degrees polarized light, but under light rotated 70 degrees, it began to disappear. In phototropic responses induced by local irradiation of a side of the subapical part of a protonema with a non-polarized red microbeam, the Mt-band on the irradiated side disappeared or became faint within 20 min, but neither disappearance nor a change of orientation of Mts occurred on the non-irradiated side. One hour after turning the electrical vector 45 degrees, in half of the cells tested, the innermost layer of microfibrils in the subapical part of the protonema changed its orientation from perpendicular to oblique to the growing axis, corresponding to the changes in the orientation of the Mt-band. After 2 hr, those changes were obvious in all cells examined. The same basic results on the orientation of microfibrils were obtained with protonemata cultured for 2 hr under 70 degrees polarized light. The role of the Mt-band in tropic responses is discussed.     

Intracellular Photoreceptive Site for Blue Light-Induced Cell Division in Protonemata of the Fern Adiantum--Further Analyses by Polarized Light Irradiation and Cell Centrifugation

The intracellular localization of the photoreceptive site forblue light-induced cell division in single-celled protonemataof Adiantum capillus-veneris L. was investigated using polarizedlight irradiation and protonemal cell centrifugation. The responseto irradiation with polarized blue light showed no dependenceon the direction of light polarization. However, centrifugationof the protonemata followed by microbeam irradiation showedthat the site of blue light perception could be displaced togetherwith the nucleus. Centrifugal treatment changed the distributionof intracellular organelles at the time of light exposure andbasipetally displaced the nucleus about 90µm. This treatmenthad no effect on the induction of cell division with blue lightif the protonemata were centrifuged again acropetally afterthe light treatment. Microbeam (30x30 µm²) irradiationwith blue light of the apical 45–75 ßm region,the receptive site of blue light in non-centrifuged cell, didnot induce cell division. However, cell division was inducedby irradiation of the nucleus-containing region, indicatingthat the photoreceptive site was displaced together with thenucleus by the centrifugation. These results suggest that theblue light receptor regulating cell division in Adiantum protonematais not likely to be located on the plasma membrane. (Received February 20, 1986; Accepted May 27, 1986)     

Polarotropism and Photomovement of Chloroplasts in the Protonemata of the Ferns Pteris and Adiantum: Evidence for the Possible Lack of Dichroic Phytochrome in Pteris

The actions of red and blue light in the photomovement of chloroplastsand the polarotropic response were studied in the protonemataof the homosporous ferns Pteris vittata L. and Adiantum capillus-venerisL. In Pteris, polarotropism could be induced with blue lightbut not with red light, while both colors of light were effectivein Adiantum protonemata. The photomovement of chloroplasts inthe two species studied by both polarized light and microbeamirradiation, also revealed similar responses to red and bluelight as the polarotropism; i.e. both colors of light were effectivein Adiantum but only blue light was active in Pteris. The resultsin Adiantum were consistent with previous results, which ledto the conclusion that both phytochrome and a blue light-absorbingpigment are involved in the two responses (Kadota et al. 1982,1984, Hayami et al. 1986, Yatsuhashi et al. 1985). By contrast,phytochrome is not involved in either polarotropism or chloroplastmovement in Pteris. Since the phytochrome system is evidentlyactive in every other photoresponses so far investigated inPteris as well as in Adiantum, the present study suggests thata phytochrome system specific to polarotropism and to photomovementof chloroplasts is absent in Pteris. Discussions are presentedon the possible involvement of two phytochrome populations ina fern gametophyte cell and on the possible lack of dichroicphytochrome in Pteris. (Received October 7, 1988; Accepted March 8, 1989)     

Nuclear behavior during branch formation in a centrifugedAdiantum protonema and the nuclear polarity

A new branch was induced on the side wall of fern protonema by cell centrifugation and subsequent polarized red light irradiation after the induction of cell division under white light. Nuclear behavior during the branch formation was analyzed. Immediately after cell division, the two daughter nuclei moved away from the division site in both red and dark conditions. Under continuous irradiation with polarized red light, cell swelling occurred as an early step of branching near the cell dividing wall, even though the nucleus was localized far from the branching site at the beginning of the swelling. After a new branch started to grow, the nucleus returned to the branching site and moved into the new branch from its basipetal end. When a protonema incubated in the dark was centrifuged again acropetally or basipetally just before the irradiation of polarized red light, the rate of apical growth or branch formation was increased, respectively. Moreover, growth of a branched protonema was altered from its former apex or from the branch again by dislocating the nucleus acropetally or basipetally by centrifugation, respectively. These facts suggest that the nucleus has no polarity physiologically, i.e. head and tail, namely either end of the spindle-shaped nucleus can be the nuclear front in a tip-growing protonema.     

Phytochrome-mediated branch formation in protonemata of the mossCeratodon purpureus

We have analyzed light induction of side-branch formation and chloroplast re-arrangement in protonemata of the mossCeratodon purpureus. After 12 hr of dark adaptation, the rate of branch formation was as low as 5%. A red light treatment induced formation of side branches up to 75% of the dark-adapted protonema. The frequency of light induced branch formation differed between cells of different ages, the highest frequency being found in the 5th cell, the most distal cell studied from the apex. We examined the effect of polarized light given parallel to the direction of filament growth. The position of branching within the cell depended on the vibration plane of polarized red light. Branch formation was highest when the electric vector of polarized light vibrates parallel to the cell surface and is fluence rate dependent. The positional effect of polarized red light could be nullified to some extent by simultaneous irradiation with polarized far-red light. An aphototropic mutant,ptr116, shows characteristics of deficiency in biosynthesis of the phytochrome chromophore and exhibits no red-light induced branch formation. Biliverdin, a precursor of the phytochrome chromophore, rescued the red-light induced branching when added to the medium, supporting the conclusion that phytochrome acts as photoreceptor for red light induced branch formation. The light effect on chloroplast re-arrangement was also analyzed in this study. We found that polarized blue light induced chloroplast re-arrangement in wild-type cells, whereas polarized red light was inactive. This result suggests that chloroplast re-arrangement is only controlled by a blue light photoreceptor, not by phytochrome inCeratodon.     

Irradiance-dependent regulation of gravitropism by red light in protonemata of the moss Ceratodon purpureus

Apical cells of protonemata of the moss Ceratodon purpureus (Hedw.) Brid. are negatively gravitropic in the dark and positively phototropic in red light. Various fluence rates of unilateral red light were tested to determine whether both tropisms operate simultaneously. At irradiances ≥140 nmol m⁻² s⁻¹ no gravitropism could be detected and phototropism predominated, despite the presence of amyloplast sedimentation. Gravitropism occurred at irradiances lower than 140 nmol m⁻² s⁻¹ with most cells oriented above the horizontal but not upright. At these low fluence rates, phototropism was indistinct at 1 g but apparent in microgravity, indicating that gravitropism and phototropism compete at 1 g. The frequency of protonemata that were negatively phototropic varied with the fluence rate and the duration of illumination, as well as with the position of the apical cell before illumination. These data show that the fluence rate of red light regulates whether gravitropism is allowed or completely repressed, and that it influences the polarity of phototropism and the extent to which apical cells are aligned in the light path. Received: 19 January 1999 / Accepted: 19 March 1999     

Phytochrome-controlled phototropism of protonemata of the moss Ceratodon purpureus: physiology of the wild type and class 2 ptr–mutants

Phototropism and polarotropism in protonemata of the moss Ceratodon purpureus are controlled by the photoreceptor phytochrome. One class of phototropism mutants is characterised by growing randomly when kept for a prolonged time (5 d or longer) in unilateral red light. It was found that a subclass of these mutants grows faster than the wild type, the rate of cell division and the length of the cells being increased. This difference is found for light-grown and dark-grown filaments. It is therefore suggested that the mutant phenotype neither results from a defect in phytochrome photoconversion nor from a defect in phytochrome-gradient formation. Instead, it is possible that a factor which is involved in both signal transduction of phototropism and regulation of cell size and cell division is deregulated. If dark-grown mutant filaments are phototropically stimulated for 24 h, they show a weak phototropic response. Phototropism and polarotropism fluence-rate effect curves for mutants were flattened and shifted to higher fluence rates compared with those for the wild type. With wild-type filaments, a previously unreported response was observed. At a low fluence rate, half of the filaments grew positively phototropically, while the other half grew negatively phototropically. It seems that under these conditions, a phytochrome gradient with two maxima for the far-red-absorbing form of phytochrome (Pfr) within the cross-section of the cell is displayed by the response of the filaments. At higher fluence rates, all filaments of the wild type grew towards the light. These data and results from microbeam irradiation experiments and from phototropism studies with filaments growing within agar, indicate that light refraction plays an important role in the formation of the Pfr gradient in phototropism of Ceratodon. Received: 10 September 1998 / Accepted: 30 December 1998     

Chloroplast proliferation based on their distribution and arrangement inAdiantum protonemata growing under red light

Chloroplast proliferation was investigated inAdiantum protonemata growing under continuous red light. Cell division is absent when cells are grown under red light. The chloroplast number increases as the cell length increases, therefore the chloroplasts divide in the absence of cell division. Chloroplasts in the basal part of the filamentous protonemal cell migrate gradually toward the cell apex, but there is no large net migration from the tip to the base or vice versa, indicating that chloroplast division takes place in the apical part of the protonemata. Chloroplast number in the apical 100 μm was maintained at about 200 during cell growth at least over eight days. The chloroplasts were either dumbbell- or ellipsoid-shaped. Dumbbell-shaped chloroplasts are abundant everywhere in a protonema, ranging from 30 to 50% of the total chloroplasts. The dumbbell-shaped chloroplasts attached to or very close to the plasma membrane seem to be the ones that are dividing but the dumbbell-shaped ones in the other regions do not divide. These data support the hypothesis that a signal from the plasma membrane induces the dumbbell-shaped chloroplasts to divide.     

Positive and negative tropic curvature induced by microbeam irradiation of protonemal tip cells of the moss Ceratodon purpureus

The photoreceptor phytochrome mediates tropic responses in protonemata of the moss Ceratodon purpureus. Under standard conditions the tip cells grow towards unilateral red light, or perpendicular to the electrical vector of polarized light. In this study the response of tip cells to partial irradiation of the apical region was analysed using a microbeam apparatus. The fluence response curve gave an unexpected pattern: whereas a 15-min microbeam with light intensities around 3 micro mol m (-2) s (-1) induced a growth curvature towards the irradiated side, higher light intensities around 100 micro mol m (-2) s (-1) caused a negative response, the cells grew away from the irradiated side. This avoidance response is explained by two effects: the light intensity is high enough to induce photoconversion into the active Pfr form of phytochrome, not only on the irradiated but also on the non-irradiated side by stray light. At the same time, the strong light on the irradiated side acts antagonistically to Pfr. As a result of this inhibition, the growth direction is moved to the light-avoiding side. Such a Pfr-independent mechanism might be important for the phototropic response to distinguish between the light-directed and light-avoiding side under unilateral light.     

Bimodal Polarotropism of Vaucheria to Polarized Blue Light: Parallel Polarotropism at High Fluence Rate Corresponds to Negative Phototropism

Vaucheria (Xanthophyceae) exhibited cruciform polarotropism when they were grown under polarized white or blue light for several days. The coexistence of two groups of branches growing perpendicular and parallel to the electric vector (E-vector) resulted in cruciform polarotropic orientation. Such polarotropic bending was, however, not detected within 24 hr. As the fluence rate of polarized white or blue light increased, parallel orientation to the E-vector became dominant. Polarized red light produced exclusively perpendicular polarotropism. This shift in pattern was much obvious in V. terrestris sensu G?tz than V. sessllis and V. dichotoma. Since the photoperception is restricted to the tip of the apical dome and since this region receives maximum photons when the E-vector is at a right angle to the cell axis, Vaucheria becomes oriented normal to the E-vector as far as the fluence rate is optimum. The direction of growth is expected to change into parallel to the E-vector when the fluence rate is supraoptimum. The perpendicular (normal) and parallel polarotropism of Vaucheria, thus, correspond to positive and negative phototropism, respectively. Orientation of photoreceptor molecules is suggested to be predominantly parallel to the surface of the apical dome. Received 14 June 1999/ Accepted in revised form 19 November 1999     

Blue- and red-light action in photoorientation of chloroplasts in Adiantum protonemata

An action spectrum for the low-fluencerate response of chloroplast movement in protonemata of the fern Adiantum capillus-veneris L. was determined using polarized light vibrating perpendicularly to the protonema axis. The spectrum had several peaks in the blue region around 450 nm and one in the red region at 680 nm, the blue peaks being higher than the red one. The red-light action was suppressed by nonpolarized far-red light given simultaneously or alternately, whereas the bluelight action was not. Chloroplast movement was also induced by a local irradiation with a narrow beam of monochromatic light. A beam of blue light at low energy fluence rates (7.3·10^-3-1.0 W m^-2) caused movement of the chloroplasts to the beam area (positive response), while one at high fluence rates (10 W m^-2 and higher) caused movement to outside of the beam area (negative response). A red beam caused a positive response at fluence rates up to 100 W m^-2, but a negative response at very high fluence rates (230 and 470 W m^-2). When a far-red beam was combined with total background irradiation with red light at fluence rates causing a low-fluence-rate response in whole cells, chloroplasts moved out of the beam area. When blue light was used as background irradiation, however, a narrow far-red beam had no effect on chloroplast distribution. These results indicate that the light-oriented movement of Adiantum chloroplasts is caused by red and blue light, mediated by phytochrome and another, unidentified photoreceptor(s), respectively. This movement depends on a local gradient of the far-red-absorbing form of phytochrome or of a photoexcited blue-light photoreceptor, and it includes positive and negative responses for both red and blue light.Abbreviations BL blue light - FR far-red light - Pfr far-red-absorbing form of phytochrome - Pr red-absorbing form of phytochrome - R red light - UV ultraviolet     

Light-induced synchrony of cell division in the protonema of the fern,Pteris vittata

Summary In protonemata of Pteris vittata grown for 6 days under red light, which brings about a marked depression of mitotic activity, the first division of the cells was synchronously induced by irradiation with blue light, and subsequent cell divisions were also promoted. The peak of the mitotic index reached a maximum of about 70% at 11.5 hrs, and 90% of all protonemata divided between the 11th and 13th hour after exposure to blue light. When the protonemata were continuously irradiated with blue light, synchronism of the next cell division in the apical cells decreased to a mitotic index of about 30%, and further divisions occurred randomly.The synchronization of cell division was found to be a combined effect of red and blue light. Red light maintained the cells in the early G₁ phase of the cell cycle; blue light caused the cells to progress synchronously through the cell cycle, with an average duration of 12 hr. By using ³H-thymidine, the average duration of the G₁, S, G₂ and M phases was determined to be about 3.5, 5, 2.5 and 1 hr, respectively.Synchronous cell division could be induced in older protonemata grown for 6 to 12 days in red light and even in protonemata having two cells. It could be repeated in the same protonema by reexposure to red light for 24 hrs or more before another irradiation with blue light.     

Heart-Shaped Prothallia of the Fern Adiantum capillus-veneris L. Develop in the Polarization Plane of White Light

When red light-precultured filamentous protonemata of Adiantumcapillus-veneris were cultured under linearly polarized whitelight, heart-shaped prothallia developed in the plane parallelto the vibration plane of electrical vector of polarized light,and were directed toward the light source. When the polarizationplane was rotated during the culture, the prothallial wingstwisted correspondingly to develop in the new plane. Continualobservation of the early steps of prothallial development witha time-lapse video system revealed that the apical cell of protonemaafter the first transverse cell division became flattened inthe vibration plane of electrical vector of polarized light,and that the first longitudinal cell division, that is, thefirst step in the transition from one-dimensional to two-dimensionalgrowth, as well as the subsequent cell divisions, occurred perpendicularlyto the electrical vector. (Received February 20, 1986; Accepted May 7, 1986)     

Expansion of Vaucheria Cell Apex Caused by Blue or Red Light

Vaucheria geminata shows typical tip-growth. Symmetrical irradiationof the cell apex with strong blue or red light caused expansionat the very apex. The apical expansion required not only a largedosage, but also a long duration (>5 min), of irradiation.The expansion took place ca. 6 min after the onset of lighttreatment. The action spectrum for the response revealed thatblue light was the most effective: red light was two ordersof magnitude less effective. DCMU completely cancelled the expansioneffect of red light, yet the drug totally failed to inhibitthe effect of blue light. The expansion is thus controlled byboth the blue light-absorbing pigment system and photosynthesis.The expansion is a three-dimensional growth promotion event,i.e., it is accompanied by simultaneous longitudinal growth.Blue light caused another longitudinal growth promotion, a positivelight-growth response. The light-growth response is the firstfound in such a tip-growing cell. (Received January 17, 1981; Accepted February 27, 1981)