A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis

For most plants survival depends upon the capacity of root tips to sense and move towards water and other nutrients in the soil. Because land plants cannot escape environmental stress they use developmental solutions to remodel themselves in order to better adapt to the new conditions. The primary site for perception of underground signals is the root cap (RC). Plant roots have positive hydrotropic response and modify their growth direction in search of water. Using a screening system with a water potential gradient, we isolated a no hydrotropic response (nhr) semi-dominant mutant of Arabidopsis that continued to grow downwardly into the medium with the lowest water potential contrary to the positive hydrotropic and negative gravitropic response seen in wild type-roots. The lack of hydrotropic response of nhr1 roots was confirmed in a system with a gradient in air moisture. The root gravitropic response of nhr1 seedlings was significantly faster in comparison with those of wild type. The frequency of the waving pattern in nhr1 roots was increased compared to those of wild type. nhr1 seedlings had abnormal root cap morphogenesis and reduced root growth sensitivity to abscisic acid (ABA) and the polar auxin transport inhibitor N-(1-naphtyl)phtalamic acid (NPA). These results showed that hydrotropism is amenable to genetic analysis and that an ABA signaling pathway participates in sensing water potential gradients through the root cap.     

Hydrotropism interacts with gravitropism by degrading amyloplasts in seedling roots of Arabidopsis and radish

In response to a moisture gradient, roots exhibit hydrotropism to control the orientation of their growth. To exhibit hydrotropism, however, they must overcome the gravitropism that is dominant on Earth. We found that moisture gradient or water stress caused immediate degradation of the starch anchors, amyloplasts, in root columella cells of Arabidopsis and radish (Raphanus sativus). Namely, development of hydrotropic response was accompanied by a simultaneous reduction in starch content in columella cells. Rapid degradation of amyloplasts in columella cells also occurred in the water-stressed roots with sorbitol or mannitol. Both hydrotropically stimulated and water-stressed roots showed a reduced responsiveness to gravity. Roots of a starchless mutant, pgm1-1, showed an enhanced hydrotropism compared with that of the wild type. These results suggest that the reduced responsiveness to gravity is, at least in part, attributable to the degradation of amyloplasts in columella cells. Thus, the reduction in gravitropism allows the roots to exhibit hydrotropism.     

How amyloplasts,water deficit and root tropisms interact?

Hydrotropism, the differential growth of plant roots directed by a moisture gradient, is a long recognized, but not well-understood plant behavior. Hydrotropism has been characterized in the model plant Arabidopsis. Previously, it was postulated that roots subjected to water stress are capable of undergo water-directed tropic growth independent of the gravity vector because of the loss of the starch granules in root cap columella cells and hence the loss of the early steps in gravitropic signaling. We have recently proposed that starch degradation in these cells during hydrostimulation sustain osmotic stress and root growth for carrying out hydrotropism instead of reducing gravity responsiveness. In addition, we also proposed that abscisic acid (ABA) and water deficit are critical regulators of root gravitropism and hydrotropism, and thus mediate the interacting mechanism between these two tropisms. Our conclusions are based upon experiments performed with the no hydrotropic response (nhr1) mutant of Arabidopsis, which lacks a hydrotropic response and shows a stronger gravitropic response than that of wild type (WT) in a medium with an osmotic gradient.Key words: starch, water deficit, auxin, abscisic acid, gravitropism, hydrotropismRoots of land plants sense and respond to different stimuli, some of which are fixed in direction and intensity (i.e., gravity) while other vary in time, space, direction and intensity (i.e., obstacles and moisture gradients). Directed growth of roots in relation to a gradient in moisture is called hydrotropism and begins in the root cap with the sensing of the moisture gradient. However, since gravity is an omnipresent accompaniment of Earthly life and many living process have evolved with it as a background constant, it is not surprising that root hydrotropism interacts with gravitropism.¹ The hydrotropic response in Arabidopsis, compare with other plants such as pea and cucumber^2,3 is readily observed even in the presence of gravity.^4,5 When Arabidopsis roots are subjected to a water gradient, such that the source of water is placed 180° opposed to the gravity vector, the roots will grow upwards, displaying positive hydrotropism. Therefore, it has been feasible to isolate so far two Arabidopsis mutants affected in their hydrotropic response.^5,6 Analysis of these mutants reveals new insights of the mechanism of hydrotropism. For one hand, the no hydrotropic response (nhr1) mutant lacks a hydrotropic response, and shows a stronger gravitropic response than that of wt and a modified wavy growth response in a medium with an osmotic gradient.^5,7 On the other hand, the mizu-kussei1 (miz1) mutant did not exhibit hydrotropism and showed regular gravitropism.⁶ Hence, the root hydrotropic response is both linked and unlinked from the gravitropic one. Nonetheless, miz1 roots also showed a reduced phototropism and a modified wavy growth response. This indicates that both MIZ1 and NHR1 are not exclusive components of the mechanism for hydrotropism and supports the notion that the root cap has assessment mechanisms that integrate many different environmental influences to produce a final integrated response.⁸ Thus, the physiological phenomena distinctively displayed by roots in order to forage resources from the environment are the result of integrated responses that resulted from many environmental influences sensed in the root cap.In the course of studying how gravity and water availability affected the perception and assessment of each other in root cap cells that generated the final root tropic response, we found that ABA is a critical regulator of the signal transduction mechanism that integrated these two-root tropisms.⁷ For this, we analyzed the long-term hydrotropic response of Arabidopsis roots in an osmotic gradient system. ABA, locally applied to seeds or root tips of nhr1, significantly increased root downward growth in a medium with an osmotic gradient (root length of nhr1 seedlings grown in this medium were on average 12.5 mm and plus 10 µM ABA were 25.1 mm). On the other hand, WT roots germinated and treated locally with ABA in this system were strongly gravitropic, albeit they had almost no starch in amyloplasts of root cap columella cells. Hydrotropically stimulated nhr1 roots, with or without ABA, maintained starch in amyloplastas, as opposed to those of WT. Therefore, the near-absence (WT) or abundant presence (nhr1) of starch granules does not affect the extent of downward gravitropism of roots in an osmotic gradient medium. Starch degradation in the wt might participate in osmoregulation by which root cells maintain turgor and consequently carry out hydrotropism, instead of reducing gravity responsiveness. In fact, it was just recently published that salt-induced rapid degradation of starch in amyloplasts is not likely the main reason for a negative gravitropic response seen under salt stress, because sos mutant roots of Arabidopsis showed negative gravitropic growth without any apparent rapid digestion of starch granules.⁹ Additionally, the stems of overwintering tubers of Potamogeton pectinatus are capable of elongating much faster in the absence than in the presence of oxygen for up to 14 days and its stems has an enhanced capacity for gravitropic movements in completely anoxic conditions.¹⁰ These authors hypothesized that ABA and starch degradation in the starchy tuber sustained stem cell elongation and cell division as well as differential growth required for the gravitropic response in these aquatic plants. These data taken together suggest that in conditions of anoxia, or water stress, ABA and degradation of starch play a critical role in the ability to survive relatively prolonged periods of unfavorable growth conditions. These players are critical when water or minerals are scarce since they regulate the enhancement of root downward growth. However, since roots can trail humidity gradients in soil, they can modulate their branching patterns (architecture) and thus respond to hydrotropism once a water-rich patch is found. Then the response of plants to gravity is principally one of nutrition (shoots to light, roots to mineral and water) and consequently must be regulated according to the long- and short-term environmental variables that occur during the development of the plant.Differential growth that occurs during the gravitropic and phototropic response has been explained according to the Cholodny-Went hypothesis, which states that the lateral transport of auxin across stimulated plant tissues is responsible for the curvature response.¹¹ Analysis of hydrotropism in some Arabidopsis agravitropic auxin transport mutants has demonstrated that these mutations do not influence their hydrotropic response.⁴ Furthermore, current pharmacological studies using inhibitors also indicated that both auxin influx and efflux are not required for hydrotropic response whereas auxin response is necessary for it.¹² These authors suggested a novel mechanism for auxin in root hydrotropism. Here, we analyzed whether asymmetric auxin distribution takes place across hydrotropically-stimulated roots using transgenic plants carrying a responsive auxin promoter (DR5) driving the expression of β-glucuronidase (GUS) or green fluorescent protein (GFP)^13,14 in wt and nhr1 backgrounds. Wt and nhr1 roots hydrotropically stimulated in a system with air moisture gradient⁵ showed no asymmetric expression of the DR5:: GUS or DR5::GFP (Fig. 1A and B). Nonetheless, nhr1 roots showed a substantial decrease in the signal driven by the DR5::GUS and GFP reporters in humidity saturated conditions (Fig. 1A, part b and B, part b), which might indicate that auxin-induced gene expression in the root cap was inhibited. It remains to be determined the significance of this inhibition in the no hydrotropic response phenotype displayed by nhr1 roots. Determination of the DR5::GUS expression in wt and nhr1 roots growing in an osmotic gradient medium for testing long-term hydrotropism revealed that the GUS signal was to some extent diminished in both wt or in nhr1 roots (Fig. 2C and D) compared to those roots growing in normal medium (Fig. 2A and B). An inhibitor of auxin response reduced hydrotropism,¹² and also inhibited auxin-dependent DR5::GUS expression.¹⁵ However, a decrease of DR5::GUS in wt root tips was not an impediment for developing an hydrotropic response. On the other hand, nhr1 roots also showed a decrease of DR5::GUS expression (Fig. 2B and D) and a complete absence of DR5::GFP (data not shown), which did not influence the extent of downward root gravitropism in water deficit conditions. Therefore, it is difficult to assign a role of auxin-induce gene expression in hydrotropism and further studies are required in order to unravel this issue. Furthermore, it needs to be resolved whether these expression studies oppose the idea that gradients in auxin precede differential growth in response to humidity gradients.Open in a separate windowFigure 1DR5:: GUS (A) and DR5::GFP (B) activity in the wild type NHR1 and nhr1 backgrounds. (A) Root tips hydrostimulated in a system with air moisture gradient (C and D) or grown in a saturated water conditions (A and B) stained with 1 mM 5-bromo-4-chloro-3-indolyl-β-d-glucuronic (X-Gluc) acid buffer under the same conditions for 80 min. (B) Root tips hydrostimulated as in (A) (C and D) or grown in a saturated water conditions (A and B) whose green fluorescent signal was visualized by confocal microscopy. Shown are images selected from at least 45 representative root tips. Bar = 29 µm.Open in a separate windowFigure 2Expression of DR5::GUS in wild type NHR1 and nhr1 backgrounds. Roots were hydrotropically stimulated for 8 days in a medium with an osmotic gradient (C and D) or grown in normal medium (A and B) and stained with X-Gluc acid buffer under the same conditions for 80 min. Shown are images selected from at least 50 representative root tips. Bar = 25 µm.Our studies⁷ revealed that ABA is a critical regulator of both root gravitropism and hydrotropism in water deficit conditions, and that the role of auxin under these conditions seems to differ from those observed in several studies thus far published on gravitropism made under well-water conditions. The molecular characterization of NHR1 and from other nhr-like mutants already isolated in our lab will clarify the mechanisms involved in this fascinating tropism.¹⁶     

A possible involvement of autophagy in amyloplast degradation in columella cells during hydrotropic response of Arabidopsis roots

Seedling roots display not only gravitropism but also hydrotropism, and the two tropisms interfere with one another. In Arabidopsis (Arabidopsis thaliana) roots, amyloplasts in columella cells are rapidly degraded during the hydrotropic response. Degradation of amyloplasts involved in gravisensing enhances the hydrotropic response by reducing the gravitropic response. However, the mechanism by which amyloplasts are degraded in hydrotropically responding roots remains unknown. In this study, the mechanistic aspects of the degradation of amyloplasts in columella cells during hydrotropic response were investigated by analyzing organellar morphology, cell polarity and changes in gene expression. The results showed that hydrotropic stimulation or systemic water stress caused dramatic changes in organellar form and positioning in columella cells. Specifically, the columella cells of hydrotropically responding or water-stressed roots lost polarity in the distribution of the endoplasmic reticulum (ER), and showed accelerated vacuolization and nuclear movement. Analysis of ER-localized GFP showed that ER redistributed around the developed vacuoles. Cells often showed decomposing amyloplasts in autophagosome-like structures. Both hydrotropic stimulation and water stress upregulated the expression of AtATG18a, which is required for autophagosome formation. Furthermore, analysis with GFP-AtATG8a revealed that both hydrotropic stimulation and water stress induced the formation of autophagosomes in the columella cells. In addition, expression of plastid marker, pt-GFP, in the columella cells dramatically decreased in response to both hydrotropic stimulation and water stress, but its decrease was much less in the autophagy mutant atg5. These results suggest that hydrotropic stimulation confers water stress in the roots, which triggers an autophagic response responsible for the degradation of amyloplasts in columella cells of Arabidopsis roots.     

An altered hydrotropic response (ahr1) mutant of Arabidopsis recovers root hydrotropism with cytokinin

Roots are highly plastic and can acclimate to heterogeneous and stressful conditions. However, there is little knowledge of the effect of moisture gradients on the mechanisms controlling root growth orientation and branching, and how this mechanism may help plants to avoid drought responses. The aim of this study was to isolate mutants of Arabidopsis thaliana with altered hydrotropic responses. Here, altered hydrotropic response 1 (ahr1), a semi-dominant allele segregating as a single gene mutation, was characterized. ahr1 directed the growth of its primary root towards the source of higher water availability and developed an extensive root system over time. This phenotype was intensified in the presence of abscisic acid and was not observed if ahr1 seedlings were grown in a water stress medium without a water potential gradient. In normal growth conditions, primary root growth and root branching of ahr1 were indistinguishable from those of the wild type (wt). The altered hydrotropic growth of ahr1 roots was confirmed when the water-rich source was placed at an angle of 45° from the gravity vector. In this system, roots of ahr1 seedlings grew downward and did not display hydrotropism; however, in the presence of cytokinins, they exhibited hydrotropism like those of the wt, indicating that cytokinins play a critical role in root hydrotropism. The ahr1 mutant represents a valuable genetic resource for the study of the effects of cytokinins in the differential growth of hydrotropism and control of lateral root formation during the hydrotropic response.     

Hydrotropism in abscisic acid,wavy, and gravitropic mutants of Arabidopsis thaliana

We have developed experimental systems to study hydrotropism in seedling roots of Arabidopsis thaliana (L.) Heynh. Arabidopsis roots showed a strong curvature in response to a moisture gradient, established by applying 1% agar and a saturated solution of KCl or K(2)CO(3) in a closed chamber. In this system, the hydrotropic response overcame the gravitropic response. Hydrotropic curvature commenced within 30 min and reached 80-100 degrees within 24 h of hydrostimulation. When 1% agar and agar containing 1 MPa sorbitol were placed side-by-side in humid air, a water potential gradient formed at the border between the two media. Although the gradient changed with time, it still elicited a hydrotropic response in Arabidopsis roots. The roots curved away from 0.5-1.5 MPa of sorbitol agar. Various Arabidopsis mutants were tested for their hydrotropic response. Roots of aba1-1 and abi2-1 mutants were less sensitive to hydrotropic stimulation. Addition of abscisic acid restored the normal hydrotropic response in aba1-1 roots. In comparison, mutants that exhibit a reduced response to gravity and auxin, axr1-3 and axr2-1, showed a hydrotropic response greater than that of the wild type. Wavy mutants, wav2-1 and wav3-1, showed increased sensitivity to the induction of hydrotropism by the moisture gradient. These results suggest that auxin plays divergent roles in hydrotropism and gravitropism, and that abscisic acid plays a positive role in hydrotropism. Furthermore, hydrotropism and the wavy response may share part of a common molecular pathway controlling the directional growth of roots.     

Complex physiological and molecular processes underlying root gravitropism

Gravitropism allows plant organs to guide their growth in relation to the gravity vector. For most roots, this response to gravity allows downward growth into soil where water and nutrients are available for plant growth and development. The primary site for gravity sensing in roots includes the root cap and appears to involve the sedimentation of amyloplasts within the columella cells. This process triggers a signal transduction pathway that promotes both an acidification of the wall around the columella cells, an alkalinization of the columella cytoplasm, and the development of a lateral polarity across the root cap that allows for the establishment of a lateral auxin gradient. This gradient is then transmitted to the elongation zones where it triggers a differential cellular elongation on opposite flanks of the central elongation zone, responsible for part of the gravitropic curvature. Recent findings also suggest the involvement of a secondary site/mechanism of gravity sensing for gravitropism in roots, and the possibility that the early phases of graviresponse, which involve differential elongation on opposite flanks of the distal elongation zone, might be independent of this auxin gradient. This review discusses our current understanding of the molecular and physiological mechanisms underlying these various phases of the gravitropic response in roots.     

Hydrotropism and Its Interaction with Gravitropism in Maize Roots

We have partially characterized root hydrotropism and its interaction with gravitropism in maize (Zea mays L.). Roots of Golden Cross Bantam 70, which require light for orthogravitropism, showed positive hydrotropism; bending upward when placed horizontally below a hydrostimulant (moist cheesecloth) in 85% relative humidity (RH) and in total darkness. However, the light-exposed roots of Golden Cross Bantam 70 or roots of a normal maize cultivar, Burpee Snow Cross, showed positive gravitropism under the same conditions; bending downward when placed horizontally below the hydrostimulant in 85% RH. Light-exposed roots of Golden Cross Bantam 70 placed at 70° below the horizontal plane responded positively hydrotropically, but gravitropism overcame the hydrotropism when the roots were placed at 45° below the horizontal. Roots placed vertically with the tip down in 85% RH bent to the side toward the hydrostimulant in both cultivars, and light conditions did not affect the response. Such vertical roots did not respond when the humidity was maintained near saturation. These results suggest that hydrotropic and gravitropic responses interact with one another depending on the intensity of one or both factors. Removal of the approximately 1.5 millimeter root tip blocked both hydrotropic and gravitropic responses in the two cultivars. However, removal of visible root tip mucilage did not affect hydrotropism or gravitropism in either cultivar.     

Molecular mechanisms mediating root hydrotropism: what we have observed since the rediscovery of hydrotropism

Roots display directional growth toward moisture in response to a water potential gradient. Root hydrotropism is thought to facilitate plant adaptation to continuously changing water availability. Hydrotropism has not been as extensively studied as gravitropism. However, comparisons of hydrotropic and gravitropic responses identified mechanisms that are unique to hydrotropism. Regulatory mechanisms underlying the hydrotropic response appear to differ among different species. We recently performed molecular and genetic analyses of root hydrotropism in Arabidopsis thaliana. In this review, we summarize the current knowledge of specific mechanisms mediating root hydrotropism in several plant species.

     

Root hydrotropism of an agravitropic pea mutant, ageotropum

We have partially characterized root hydrotropism of an agravitropic pea mutant, ageotropum (from Pisum sativum L. cv. Weibull's Weitor), without interference of gravitropism. Lowering the atmospheric air humidity inhibited root elongation and caused root curvature toward the moisture-saturated substrate in ageotropum pea. Removal of root tips approximately 1.5 mm in length blocked the hydrotropic response. A computer-assisted image analysis showed that the hydrotropic curvature in the roots of ageotropum pea was chiefly due to a greater inhibition of elongation on the humid side than the dry side of the roots. Similarly, gravitropic curvature of Alaska pea roots resulted from inhibition of elongation on the lower side of the horizontally placed roots, while the upper side of the roots maintained a normal growth rate. Gravitropic bending of Alaska pea roots was apparent 30 min after stimulation, whereas differential growth as well as curvature in positive root hydrotropism of ageotropum pea became visible 4–5 h after the continuous hydrostimulation. Application of 2,3,5-triiodobenzoic acid or ethyleneglycol-bis-( β -aminoethylether)-N,N,N',N'-tetraacetic acid was inhibitory to both root hydrotropism of ageotropum pea and root gravitropism of Alaska pea. Some mutual response mechanism for both hydrotropism and gravitropism may exist in roots, although the stimulusperception mechanisms differ from one another.     

Induction of hydrotropism in clinorotated seedling roots of Alaska pea, Pisum sativum L

Roots of the agravitropic pea (Pisum sativum L.) mutant ageotropum show positive hydrotopism, whereas roots of Alaska peas are hydrotropically almost non-responsive. When the gravitropic response was nullified by rotation on clinostats, however, roots of Alaska peas showed unequivocal positive hydrotropism in response to a water potential gradient. These results suggest that roots of Alaska peas possess normal ability to respond hydrotropicallly and their weak hydrotropic response results from a counteracting effect of gravitropism.     

Hydrotropism and its interaction with gravitropism in roots

We have studied hydrotropism and its interaction with gravitropism in agravitropic roots of a pea mutant and normal roots of peas (Pisum sativum L.) and maize (Zea mays L.). The interaction between hydrotropism and gravitropism in normal roots of peas or maize were also examined by nullifying the gravitropic response on a clinostat and by changing the stimulus-angle for gravistimulation. Depending on the intensity of both hydrostimulation and gravistimulation, hydrotropism and gravitropism of seedling roots strongly interact with one another. When the gravitropic response was reduced, either genetically or physiologically, the hydrotropic response of roots became more unequivocal. Also, roots more sensitive to gravity appear to require a greater moisture gradient for the induction of hydrotropism. Positive hydrotropism of roots occurred due to a differential growth in the elongation zone; the elongation was much more inhibited on the moistened side than on the dry side of the roots. It was suggested that the site of sensory perception for hydrotropism resides in the root cap, as does the sensory site for gravitropism. Furthermore, an auxin inhibitor, 2,3,5-triiodobenzoic acid (TIBA), and a calcium chelator, ethyleneglycol-bis-(-aminoethylether)-N,N,N,N- tetraacetic acid (EGTA), inhibited both hydrotropism and gravitropism in roots. These results suggest that the two tropisms share a common mechanism in the signal transduction step.     

Novel hydrotropism mutants of Arabidopsis thaliana and their altered waving response and phototropism.

Roots display positive hydrotropism in response to a moisture gradient, which is important for plants to escape from water stress and regulate the directional growth by interacting with other growth movements such as gravitropism, phototropism and waving response. On Earth, hydrotropism is interfered by gravitropism in particular, so that microgravity conditions or agravitropic mutants have been used for the study of hydrotropism. However, we have recently established an experimental system for the study of hydrotropism in Arabidopsis roots that easily develop hydrotropism in response to moisture gradient by overcoming gravitropism. Using the Arabidopsis system, we isolated hydrotropism mutants named root hydrotropism (rhy). In the present study, we examined the hydrotropism, gravitropism, phototropism, waving response and elongation growth of rhy4 and rhy5 roots that were defective in positive hydrotropism. Interestingly, rhy4 roots curved away from the water source and showed a reduced waving response. Both rhy4 and rhy5 showed normal gravitropism and a slight reduction in phototropism. These results suggest that there is a mutual molecular mechanism underlying hydrotropism, waving response and/or phototropism. Thus, we have obtained novel hydrotropic mutants that will be used for revealing molecular mechanism of root hydrotropism and its interaction with waving response and/or phototropism.     

玉米初生根向水性诱导优化试验研究

为了研究湿度梯度对根系向水性反应的影响,采用Takahashi and Scott于1993年创建的方法,设置以下3个试验:1)向水性诱导物不同倾斜角试验;2)根系距向水性诱导物不同距离试验;3)根尖距底部饱和K2CO3溶液不同距离试验。同时,还研究了根长和根系延伸速率对根系向水性弯曲的影响。结果表明,用饱和K2CO3溶液控制湿度时根系的向水性弯曲度明显大于纯水。随着诱导物倾斜角的增大,向水性弯曲增强。与距诱导物3 mm和6 mm相比,根系直接接触诱导物时表现出最大的向水性反应。与根尖距底部盐溶液6 cm相比,相距4 cm时向水性弯曲度增大,这些与根尖周围的湿度梯度增大有关。当根长为1.0、1.5、2.0、2.5、3.0 cm时,短根比长根表现出更大的向水性反应,这可能与其较慢的延伸速率为根系对湿度梯度的反应提供了更充足的时间有关。为了验证这个假说,用相同长度的根系、通过控制不同温度进行试验,结果表明根系的向水性弯曲随温度升高而降低。可见,玉米初生根的向水性反应受环境和根系发育阶段两方面影响。当根系相距诱导物较近、根系周围的湿度梯度较大时,根系向水性反应更强。而且,具有较小延伸速率根系的向水性反应更大。考虑到干旱条件下根系伸长慢、且土壤中湿度梯度大,因而可以认为干旱条件下根系的向水性生长在玉米吸收水分中有重要作用。同时,对根系向水性诱导方法的优化有助于其生理机制的进一步研究。     

Characterization of hydrotropism: the timing of perception and signal movement from the root cap in the agravitropic pea mutant ageotropum

In this study, ageotropum pea mutant was used to determine the threshold time for perception of an osmotic stimulation in the root cap and the time requirement for transduction and transmission of the hydrotropic signal from the root cap to the elongation region. The threshold time for the perception of an osmotic stimulation was compared to current estimates of threshold times for graviperception in roots. The time required for transduction and transmission in the hydrotropic response of ageotropum was compared to the time requirement in the gravity response of Alaska pea roots. We determined that threshold time for perception of an osmotic stimulation in the root cap is very rapid, occurring in less than 2 min following the application of sorbitol to the root cap. Furthermore, a single 5 min exposure of sorbitol to the root cap fully induced a hydrotropic response. We also found that transduction and transmission of an osmotic stimulus requires 90-120 min for movement from the root cap to more basal tissues involved in differential growth leading to root curvature. The very rapid threshold time for perception of root hydrotropism is similar to those times reported for root gravitropism. However, the time required for the transduction and transmission of an osmotic stimulation from the root cap is significantly longer than the time required in gravitropism. These results suggest that there must exist some differences between root hydrotropism and gravitropism in either the rate or mechanisms of transduction and transmission of the tropistic signal from the root cap.     

Gravitropism in the starch excess mutant of Arabidopsis thaliana

Amyloplasts are hypothesized to play a key role in the cellular mechanisms of gravity perception in plants. While previous studies have examined the effects of starch deficiency on gravitropic sensitivity, in this paper, we report on gravitropism in plants with a greater amount of starch relative to the normal wild type. Thus, we have studied the sex1 (starch excess) mutant of Arabidopsis thaliana, which accumulates extra starch because it is defective in a protein involved in the regulation of starch mobilization. Compared to the wild type (WT), sex1 seedlings contained excess starch in cotyledons, hypocotyls, the root-hypocotyl transition zone, the body of the root, root hairs, and in peripheral rootcap cells. Sedimented amyloplasts were found in both the WT and in sex1 in the rootcap columella and in the endodermis of stems, hypocotyls, and petioles. In roots, the starch content and amyloplast sedimentation in central columella cells and the gravitropic sensitivity were comparable in sex1 and the WT. However, in hypocotyls, the sex1 mutant was much more sensitive to gravity during light-grown conditions compared to the WT. This difference was correlated to a major difference in size of plastids in gravity-perceiving endodermal cells between the two genotypes (i.e., sex1 amyloplasts were twice as big). These results are consistent with the hypothesis that only very large changes in starch content relative to the WT affect gravitropic sensitivity, thus indicating that wild-type sensing is not saturated.     

Hydrotropism: The current state of our knowledge

The response of roots to a moisture gradient has been reexamined, and positive hydrotropism has been demonstrated in recent years. Agravitropic roots of a pea mutant have contributed to the studies on hydrotropism. The kinetics of hydrotropic curvature, interactions between hydrotropism and gravitropism, moisture gradients required for the induction of hydrotropism, the sensing site for moisture gradients, characteristics of hydrotropic signal and differential growth, and calcium involvement in signal transduction have been subjects of these studies. This review summarizes the current state of our knowledge on hydrotropism in roots.     

Hydrotropism: the current state of our knowledge

The response of roots to a moisture gradient has been reexamined, and positive hydrotropism has been demonstrated in recent years. Agravitropic roots of a pea mutant have contributed to the studies on hydrotropism. The kinetics of hydrotropic curvature, interactions between hydrotropism and gravitropism, moisture gradients required for the induction of hydrotropism, the sensing site for moisture gradients, characteristics of hydrotropic signal and differential growth, and calcium involvement in signal transduction have been subjects of these studies. This review summarizes the current state of our knowledge on hydrotropism in roots.     

Recovery of gravitropic response during regeneration of root caps does not require developed columella cells and sedimentation of amyloplasts

Correlations between regeneration of the root cap and recovery of a gravitropic response were studied using primary roots of Phaseolus vulgaris. After removal of various lengths of the root tip a gravistimulus was continuously given to the root. The statistical analysis of data showed that recovery of the gravitropic response was gradually delayed as the length of the tips removed increased. This suggested that the columella cells of the root cap were involved in gravitropism. When the root cap was completely removed, the roots did not respond to gravistimuli for the first 15 h and began to reorient their growth direction at 20 h. At this time, the columella cells had just begun to regenerate and had immature amyloplasts which did not sufficiently form a sediment. These results suggest that other systems of perception exist in plant cells in addition to the amyloplast-based model of graviperception.