Structural and functional properties of the coleoptile chloroplast: Photosynthesis and photosensory transduction

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-dependent stomatal opening and phototropic bending. The guard cell chloroplast has been studied in detail but the coleoptile chloroplast is poorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard cell chloroplasts. Coleoptile chloroplasts operated the xanthophyll cycle, and their zeaxanthin content tracked incident rates of solar radiation throughout the day. Zeaxanthin formation was very sensitive to low incident fluence rates, and saturated at around 800–1000 mol m^–2 s^–1. Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 mol m^–2 s^–1. Quenching rates of chlorophyll a fluorescence transients from coleoptile chloroplasts induced by saturating fluence rates of actinic red light increased as a function of zeaxanthin content. This implies that zeaxanthin plays a photoprotective role in the coleoptile chloroplast. Addition of low fluence rates of blue light to saturating red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is analogous to that of the guard cell chloroplast, and implicates these organelles in the sensory transduction of blue light. On a chlorophyll basis, coleoptile chloroplasts had high rates of photosynthetic oxygen evolution and low rates of photosynthetic carbon fixation, as compared with mesophyll chloroplasts. In contrast with the uniform chloroplast distribution in the leaf, coleoptile chloroplasts were predominately found in the outer cell layers of the coleoptile cortex, and had large starch grains and a moderate amount of stacked grana and stroma lamellae. Several key properties of the coleoptile chloroplast were different from those of mesophyll chloroplasts and resembled those of guard cell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.Abbreviations Ant or A antheraxanthin - dv/dt fluorescence quenching rate - Fm maximum yield of fluorescence with all PS II reaction centers closed - Fo yield of instantaneous fluorescence with all PS II reaction centers open - Vio or V violaxanthin - Zea or Z zeaxanthin     

Phytochrome-mediated phototropism in de-etiolated seedlings : occurrence and ecological significance

Phototropic responses to broadband far red (FR) radiation were investigated in fully de-etiolated seedlings of a long-hypocotyl mutant (lh) of cucumber (Cucumis sativus L.), which is deficient in phytochrome-B, and its near isogenic wild type (WT). Continuous unilateral FR light provided against a background of white light induced negative curvatures (i.e. bending away from the FR light source) in hypocotyls of WT seedlings. This response was fluence-rate dependent and was absent in the lh mutant, even at very high fluence rates of FR. The phototropic effect of FR light on WT seedlings was triggered in the hypocotyls and occurred over a range of fluence rates in which FR was very effective in promoting hypocotyl elongation. FR light had no effect on elongation of lh-mutant hypocotyls. Seedlings grown in the field showed negative phototropic responses to the proximity of neighboring plants that absorbed blue (B) and red light and back-reflected FR radiation. The bending response was significantly larger in WT than in lh seedlings. Responses of WT and lh seedlings to lateral B light were very similar; however, elimination of the lateral B light gradients created by the proximity of plant neighbors abolished the negative curvature only in the case of lh seedlings. More than 40% of the total hypocotyl curvature induced in WT seedlings by the presence of neighboring plants was present after equilibrating the fluence rates of B light received by opposite sides of the hypocotyl. These results suggest that: (a) phytochrome functions as a phototropic sensor in de-etiolated plants, and (b) in patchy canopy environments, young seedlings actively project new leaves into light gaps via stem bending responses elicited by the B-absorbing photoreceptor(s) and phytochrome.     

Blue and Green Light-Induced Phototropism in Arabidopsis thaliana and Lactuca sativa L. Seedlings

Exposure time-response curves for blue and green light-induced phototropic bending in hypocotyls of Arabidopsis thaliana (L.) Heynh. and Lactuca sativa L. seedlings are presented. These seedlings show significant phototropic sensitivity up to 540 to 550 nanometers. Since wave-lengths longer than 560 nanometers do not induce phototropic bending, it is suggested that the response to 510 to 550 nanometers light is mediated by the specific blue light photoreceptor of phototropism. We advise care in the use of green `safelights' for studies of phototropism.     

FERONIA is involved in phototropin 1-mediated blue light phototropic growth in Arabidopsis

Plant shoot phototropism is triggered by the formation of a light-driven auxin gradient leading to bending growth. The blue light receptor phototropin 1(phot1) senses light direction, but how this leads to auxin gradient formation and growth regulation remains poorly understood. Previous studies have suggested phot1’s role for regulated apoplastic acidification, but its relation to phototropin and hypocotyl phototropism is unclear. Herein, we show that blue light can cause phot1 to interact with...     

Hypocotyl growth orientation in blue light is determined by phytochrome A inhibition of gravitropism and phototropin promotion of phototropism

How developing seedlings integrate gravitropic and phototropic stimuli to determine their direction of growth is poorly understood. In this study we tested whether blue light influences hypocotyl gravitropism in Arabidopsis. Phototropin1 (phot1) triggers phototropism under low fluence rates of blue light but, at least in the dark, has no effect on gravitropism. By analyzing the growth orientation of phototropism-deficient seedlings in response to gravitropic and phototropic stimulations we show that blue light not only triggers phototropism but also represses hypocotyl gravitropism. At low fluence rates of blue light phot1 mutants were agravitropic. In contrast, phyAphot1 double mutants grew exclusively according to gravity demonstrating that phytochrome A (phyA) is necessary to inhibit gravitropism. Analyses of phot1cry1cry2 triple mutants indicate that cryptochromes play a minor role in this response. Thus the optimal growth orientation of hypocotyls is determined by the action of phyA-suppressing gravitropism and the phototropin-triggering phototropism. It has long been known that phytochromes promote phototropism but the mechanism involved is still unknown. Our data show that by inhibiting gravitropism phyA acts as a positive regulator of phototropism.     

Nastic response of maize (Zea mays L.) coleoptiles during clinostat rotation

Rotation of unstimulated maize (Zea mays L.) seedlings on a horizontal clinostat is accompanied by a strong bending response of the coleoptiles towards the caryopsis, yielding curvatures exceding 100°. The corresponding azimuthal distribution shows two peaks, each of which is displayed by 30° from the symmetry axis connecting the shortest coleoptile and caryopsis cross sections. It is argued that this spatial pattern is not the result of two independent bending preferences, but caused by a one-peaked distribution encountering an obstacle in its central part and thus being split into the two subpeaks. The existence of one preferential direction justifies considering this response to be a nastic movement. Its time course consists of an early negative phase (coleoptiles bend away from the caryopsis) followed 2 h later by a longlasting positive bending towards the caryopsis. In light-interaction experiments, fluence-response curves for different angles between blue light and the direction of the nastic response were measured. These experiments indicate that blue light interacts with the nastic response at two levels: (i) phototonic inhibition, and (ii) addition of nastic and phototropic curvatures. It is concluded that phototropic and phototonic transduction bifurcate before the formation of phototropic transverse polarity. The additivity of nastic and phototropic responses was followed at the population level. At the level of the individual seedling, one observes, in the case of phototropic induction opposing nastic movement, three distinct responses: either strong phototropism, or nastic bending, or an avoidance response which involves strong curvature perpendicular to the stimulation plane. With time the nastic bending becomes increasingly stable against opposing phototropic stimulation. This can be seen from a growing proportion of seedlings exhibiting nastic bending when light is applied at variable intervals after the onset of clinostat rotation. At the transition from instability to stability, this type of experiment produces a high percentage of seedlings displaying the avoidance response. However, no cancelling resulting in zero curvature can be observed. It is concluded that the endogenous polarity underlying the nastic response is different in its very nature from the blue-light-elicited stable transverse polarity described earlier (Nick and Schäfer 1988 b).Abbreviation BL blue light (449 nm)     

PHOT2介导拟南芥下胚轴向光弯曲调节子的筛选与鉴定

向光素(PHOT1和PHOT2)功能冗余调节单侧强蓝光诱导的拟南芥(Arabidopsis thaliana)黄化苗下胚轴向光弯曲表现功能冗余,限制了人们对PHOT2信号转导机制的深入研究。通过化学诱变剂甲基磺酸乙酯(EMS)诱变拟南芥phot1突变体,避开PHOT1基因的干扰,寻找PHOT2下游信号分子。研究结果表明,已成功筛选到1株遗传稳定的下胚轴向蓝光不弯曲突变体。遗传分析结果显示,该突变体可能是PHOT2下游信号分子突变,将其命名为p2sa1(phototropin2 signaling associated1)。用100μmol·m–2·s–1强蓝光单侧照射,phot1p2sa1下胚轴向光弯曲缺失,呈现phot1phot2双突变的表型,然而phot1p2sa1在强蓝光下叶绿体避光正常,明显不同于phot1phot2。实验证实P2SA1可能位于PHOT2的下游,参与调节PHOT2介导的拟南芥下胚轴向光弯曲反应。     

D6PK AGCVIII Kinases Are Required for Auxin Transport and Phototropic Hypocotyl Bending in Arabidopsis

Phototropic hypocotyl bending in response to blue light excitation is an important adaptive process that helps plants to optimize their exposure to light. In Arabidopsis thaliana, phototropic hypocotyl bending is initiated by the blue light receptors and protein kinases phototropin1 (phot1) and phot2. Phototropic responses also require auxin transport and were shown to be partially compromised in mutants of the PIN-FORMED (PIN) auxin efflux facilitators. We previously described the D6 PROTEIN KINASE (D6PK) subfamily of AGCVIII kinases, which we proposed to directly regulate PIN-mediated auxin transport. Here, we show that phototropic hypocotyl bending is strongly dependent on the activity of D6PKs and the PIN proteins PIN3, PIN4, and PIN7. While early blue light and phot-dependent signaling events are not affected by the loss of D6PKs, we detect a gradual loss of PIN3 phosphorylation in d6pk mutants of increasing complexity that is most severe in the d6pk d6pkl1 d6pkl2 d6pkl3 quadruple mutant. This is accompanied by a reduction of basipetal auxin transport in the hypocotyls of d6pk as well as in pin mutants. Based on our data, we propose that D6PK-dependent PIN regulation promotes auxin transport and that auxin transport in the hypocotyl is a prerequisite for phot1-dependent hypocotyl bending.     

Chloroplast Development in Rye Coleoptiles

In the parenchyma cells of 1-d-old dark-grown rye coleoptiles (Secale cereale) proplastids occurred which sometimes contained starch grains. During coleoptile growth in darkness starch-filled amyloplasts are formed from the preexisting proplastids. No prolamellar bodies were observed in the stroma of the plastids of the etiolated coleoptile. After irradiation of 3-d-old etiolated coleoptiles with continuous white light three different types of plastids occurred. In the epidermal cells proplastids were observed. The parenchyma cells below the stomata of the outer epidermis (above the two vascular bundles) contained mature, spindle-shaped chloroplasts with a well-developed thylakoid system. In the parenchyma cells that surround the vascular bundles amyloplasts with some thylakoid membranes (chloroamyloplasts) occurred. The mesophyll cells of the primary leaves of dark-grown seedlings contained etioplasts with large prolamellar bodies. In the primary leaves of irradiated plants chloroplasts similar to those of the parenchyma cells of the coleoptile were observed. Our results show that the rye coleoptile, which grows underground as a heterotrophic organ, is capable of developing mature chloroplasts upon reaching the light above the soil surface. The significance of this expression of photosynthetic capacity for the carbon economy of the developing seedling is discussed.     

Mutations in the NPH1 locus of Arabidopsis disrupt the perception of phototropic stimuli.

The phototropic response is an important component of seedling establishment in higher plants because it orients the young seedlings for maximal photosynthetic light capture. Despite their obvious importance, little is known about the mechanisms underlying the perception and transduction of the light signals that induce phototropic curvatures. Here, we report the isolation of eight mutants of Arabidopsis that lack or have severely impaired phototropic responses. These nph (for nonphototropic hypocotyl) mutants comprise four genetic loci: nph1, nph2, nph3, and nph4. Physiological and biochemical characterization of the nph1 allele series indicated that the NPH1 locus may encode the apoprotein for a dual-chromophoric or multichromophoric holoprotein photoreceptor capable of absorbing UV-A, blue, and green light and that this photoreceptor regulates all the phototropic responses of Arabidopsis. It appears that the NPH1 protein is most likely a 120-kD plasma membrane-associated phosphoprotein because all of the nph1 mutations negatively affected the abundance of this protein. In addition, the putative NPH1 photoreceptor protein is genetically and biochemically distinct from the HY4 protein, which most likely acts as a photoreceptor for blue light-mediated hypocotyl growth inhibition. Furthermore, the NPH1 and HY4 proteins are not functionally redundant because mutations in either gene alone affect only one physiological response but not the other, thus providing strong support for the hypothesis that more than one blue light photoreceptor is required for the normal growth and development of a seedling.     

Blue Light-Induced Phosphorylation of a Plasma Membrane-Associated Protein in Zea mays L

Blue light induces a variety of photomorphogenic responses in higher plants, among them phototropic curvature, the bending of seedlings toward a unidirectional light source. In dark-grown coleoptiles of maize (Zea mays L.) seedlings, blue light induces rapid phosphorylation of a 114-kD protein at fluence levels that are sufficient to stimulate phototropic curvature. Phosphorylation in response to blue light can be detected in vivo in coleoptile tips preincubated in 32Pi or in vitro in isolated membranes supplemented with [[gamma]-32P]ATP. Phosphorylation reaches a maximum level in vitro within 2 min following an inductive light pulse, but substantial labeling occurs within the first 15 s. Isolated membranes remain activated for several minutes following an in vitro blue light stimulus, even in the absence of exogenous ATP. Phosphoamino acid analysis of the 114-kD protein detected phosphoserine and a trace of phosphothreonine. The kinase involved in phosphorylating the protein in vitro is not dependent on calcium. The 114-kD protein itself has an apparent binding site for ATP, detected by incubating with the nonhydrolyzable analog, 5[prime]-p-fluorosulfonyl-benzoyladenosine. This result suggests that the 114-kD protein, which becomes phosphorylated in response to blue light, may also be capable of kinase activity.     

Morphogenesis of rice and Arabidopsis seedlings in space.

Seedlings of rice (Oryza saliva L.) and Arabidopsis (A. thaliana L.) were cultivated for 68.5 hr in the RICE experiment on board during Space Shuttle STS 95 mission, and changes in their growth and morphology were analyzed. Microgravity in space stimulated elongation growth of both rice coleoptiles and Arabidopsis hypocotyls by making their cell walls extensible. In space, rice coleoptiles showed an inclination toward the caryopsis in the basal region and also a spontaneous curvature in the same direction in the elongating region. These inclinations and curvatures were more prominent in the Koshihikari cultivar compared to a dwarf cultivar, Tan-ginbozu. Rice roots elongated in various directions including into the air on orbit, but two thirds of the roots formed a constant angle with the axis of the caryopsis. In space, Arabidopsis hypocotyls also elongated in a variety of directions and about 10% of the hypocotyls grew into the agar medium. No clear curvatures were observed in the elongating region of Arabidopsis hypocotyls. Such a morphology of both types of seedlings was fundamentally similar to that observed on a 3 D clinostat. Thus, it was confirmed by the RICE experiment that rice and Arabidopsis seedlings perform an automorphogenesis under not only simulated but also true microgravity conditions.     

Uridine and the Control of Phototropism in Oat (Avena sativa L.) Coleoptiles

The short-term growth response of oat (Avena sativa L.) coleoptiles to exogenously applied uridine was studied both in excised apical segments and in the intact seedlings. In both cases growth of coleoptile tissue was inhibited by uridine. The inhibition of coleoptile growth consistently occurred 20–30 min after uridine treatment, which is within the lag period of their phototropic response. Asymmetric application of uridine to coleoptiles in the intact seedlings resulted in their bending toward the direction to which uridine was applied in the absence of light stimulus. These findings suggest that uridine or its metabolites, plays an important role in the phototropism of oat coleoptiles and provide support to the Bruinsma–Hasegawa theory as an alternative to the Cholodny–Went theory for explaining phototropism.     

Differential expression of the auxin primary response gene MASSUGU2/IAA19during tropic responses of Arabidopsis hypocotyls

We have examined the expression pattern of an auxin primary response gene, MSG2/IAA19 , during photo- and gravitropic responses of hypocotyls using a transgenic Arabidopsis harboring MSG2/IAA19 promoter::GUS . The upper portion of most etiolated hypocotyls showed uniform β-glucuronidase (GUS) staining with the strongest activity in the pericycle. When hypocotyls were irradiated with unilateral blue light, GUS activity on the concave side of hypocotyls was decreased, resulting in differential GUS staining with a stronger signal on the convex side. The number of differentially stained hypocotyls peaked at 24 h after the onset of the phototropic stimuli, while hypocotyl curvature continued to increase for the entire 36-h experimental period. This result suggests that the MSG2/IAA19 expression precedes the phototropic responses. When seedlings were grown under dim white light, their hypocotyls displayed almost no GUS activity. The light-grown hypocotyls also showed differential GUS staining after phototropic stimuli as result of the increase in GUS activity on the convex side of hypocotyls, especially in the epidermis, the outer cortex and pericycle, although GUS activity was much weaker than that observed in etiolated hypocotyls. Similar but less obvious differential staining was obtained for gravitropic response of hypocotyls. Considering the recent finding that Aux/IAA proteins are immediate targets of the auxin F box receptors, MSG2/IAA19 is likely to act as one of master genes for tropic responses.     

Phytochrome A regulates the intracellular distribution of phototropin 1-green fluorescent protein in Arabidopsis thaliana

It has been known for decades that red light pretreatment has complex effects on subsequent phototropic sensitivity of etiolated seedlings. Here, we demonstrate that brief pulses of red light given 2 h prior to phototropic induction by low fluence rates of blue light prevent the blue light-induced loss of green fluorescent protein-tagged phototropin 1 (PHOT1-GFP) from the plasma membrane of cortical cells of transgenic seedlings of Arabidopsis thaliana expressing PHOT1-GFP in a phot1-5 null mutant background. This red light effect is mediated by phytochrome A and requires approximately 2 h in the dark at room temperature to go to completion. It is fully far red reversible and shows escape from photoreversibility following 30 min of subsequent darkness. Red light-induced inhibition of blue light-inducible changes in the subcellular distribution of PHOT1-GFP is only observed in rapidly elongating regions of the hypocotyl. It is absent in hook tissues and in mature cells below the elongation zone. We hypothesize that red light-induced retention of the PHOT1-GFP on the plasma membrane may account for the red light-induced increase in phototropic sensitivity to low fluence rates of blue light.     

RPT2 is a signal transducer involved in phototropic response and stomatal opening by association with phototropin 1 in Arabidopsis thaliana

Phototropin 1 (phot1) and phot2, which are blue light receptor kinases, function in blue light-induced hypocotyl phototropism, chloroplast relocation, and stomatal opening in Arabidopsis (Arabidopsis thaliana). Previous studies have shown that the proteins RPT2 (for ROOT PHOTOTROPISM2) and NPH3 (for NONPHOTOTROPIC HYPOCOTYL3) transduce signals downstream of phototropins to induce the phototropic response. However, the involvement of RPT2 and NPH3 in stomatal opening and in chloroplast relocation mediated by phot1 and phot2 was unknown. Genetic analysis of the rpt2 mutant and of a series of double mutants indicates that RPT2 is involved in the phot1-induced phototropic response and stomatal opening but not in chloroplast relocation or phot2-induced movements. Biochemical analyses indicate that RPT2 is purified in the crude microsomal fraction, as well as phot1 and NPH3, and that RPT2 makes a complex with phot1 in vivo. On the other hand, NPH3 is not necessary for stomatal opening or chloroplast relocation. Thus, these results suggest that phot1 and phot2 choose different signal transducers to induce three responses: phototropic response of hypocotyl, stomatal opening, and chloroplast relocation.     

A note on blue light-induced potentials in Avena coleoptiles

Transverse electrical potentials were induced by 435.8 nm light, with lateral illumination of coleoptiles of Avena sativa L. cv. Blenda. The potentials were recorded with the aid of the vibrating electrode technique, thus avoiding touching of the plants. The light dose was varied by changing the illumination time, the irradiance always being 3.9.10^-3 W m^-2. The transverse potential varied in time after the start of illumination and the magnitude of it was dose-dependent. Maximum voltages recorded were of the order of 15 mV, the illuminated side of the coleoptile then being negative with respect to the shaded side. Dose response curves were constructed and were very similar to dose response curves published in the literature for phototropic (blue light induced) curvatures.     

Changes in ion fluxes during phototropic bending of etiolated oat coleoptiles

BACKGROUND AND AIMS: This work has been conducted to assist theoretical modelling of the different stages of the blue light (BL)-induced phototropic signalling pathway and ion transport activity across plant membranes. Ion fluxes (Ca(2+), H(+), K(+) and Cl(-)) in etiolated oat coleoptiles have been measured continuously before and during unilateral BL exposure. METHODS: Changes in ion fluxes at the illuminated (light) and shadowed (dark) sides of etiolated oat coleoptiles (Avena sativa) were studied using a non-invasive ion-selective microelectrode technique (MIFE). The bending response was also measured continuously, and correlations between the changes in various ion fluxes and bending response have been investigated. For each ion the difference (Delta) between the magnitudes of flux at the light and dark sides of the coleoptile was calculated. KEY RESULTS: Plants that demonstrated a phototropic bending response also demonstrated Ca(2+) influx into the light side approximately 20 min after the start of BL exposure. This is regarded as part of the perception and transduction stages of the BL-induced signal cascade. The first 10 min of bending were associated with substantial influx of H(+), K(+) and Cl(-) into the light (concave) side of the coleoptiles. CONCLUSIONS: The data suggest that Ca(2+) participates in the signalling stage of the BL-induced phototropism, whereas the phototropic bending response is linked to changes in the transport of H(+), K(+) and Cl(-).     

IAA transport during the phototropic responses of intact Zea and Avena coleoptiles

Summary Transport of indolyl-3-acetic acid (IAA) was studied during the phototropic responses of intact shoots and detached coleoptiles of Zea mays L. and Avena sativa L. The use of a high specific activity [5-³H]IAA and glass micropipettes enabled asymmetric application of the IAA to be made to individual coleoptiles with minimal tissue damage.A unilateral stimulus of 2.59×10^-11 einstein cm^-2 of blue light, probably in the dose range of the first positive phototropic response, caused significant net lateral movement of radioactivity from [5-³H]IAA away from the illuminated side of intact shoots and detached coleoptile apices of both Avena and Zea. The magnitude of the net lateral movement was 15.3% in Zea seedlings and 12.3% in Avena seedlings. Chromatographic analyses indicated that the movement of radioactivity reflected that of IAA. A phototropic stimulus of 1.24×10^-7 einstein cm^-2, which was probably in the second positive dose range, caused significant lateral movement of radioactivity in intact shoots and detached coleoptiles of Zea but not of Avena.In intact Zea seedlings, neither phototropic dosage affected the longitudinal transport of IAA. In intact Avena seedlings, first positive stimulation inhibited longitudinal transport only when the IAA was applied to the illuminated side of the coleoptile, but second positive stimulation inhibited basipetal movement of IAA regardless of the side of application.Exposing the intact seedlings to red light before phototropic stimulation abolished lateral transport after a first positive stimulus in Zea and in Avena.Phototropic stimulation can thus induce a lateral transport of IAA towards the shaded side of the coleoptiles of both Zea and Avena seedlings, and can affect longitudinal movement of IAA in the coleoptile of Avena. However, since phototropic curvature was observed under certain conditions in the absence of either of these effects, the extent to which they are involved in the induction of asymmetric growth in a stimulated coleoptile has yet to be resolved.     

The ARG1-LIKE2 gene of Arabidopsis functions in a gravity signal transduction pathway that is genetically distinct from the PGM pathway

The arl2 mutants of Arabidopsis display altered root and hypocotyl gravitropism, whereas their inflorescence stems are fully gravitropic. Interestingly, mutant roots respond like the wild type to phytohormones and an inhibitor of polar auxin transport. Also, their cap columella cells accumulate starch similarly to wild-type cells, and mutant hypocotyls display strong phototropic responses to lateral light stimulation. The ARL2 gene encodes a DnaJ-like protein similar to ARG1, another protein previously implicated in gravity signal transduction in Arabidopsis seedlings. ARL2 is expressed at low levels in all organs of seedlings and plants. arl2-1 arg1-2 double mutant roots display kinetics of gravitropism similar to those of single mutants. However, double mutants carrying both arl2-1 and pgm-1 (a mutation in the starch-biosynthetic gene PHOSPHOGLUCOMUTASE) at the homozygous state display a more pronounced root gravitropic defect than the single mutants. On the other hand, seedlings with a null mutation in ARL1, a paralog of ARG1 and ARL2, behave similarly to the wild type in gravitropism and other related assays. Taken together, the results suggest that ARG1 and ARL2 function in the same gravity signal transduction pathway in the hypocotyl and root of Arabidopsis seedlings, distinct from the pathway involving PGM.