Roles of amyloplasts and water deficit in root tropisms

Directed growth of roots in relation to a moisture gradient is called hydrotropism. The no hydrotropic response (nhr1) mutant of Arabidopsis lacks a hydrotropic response, and shows a stronger gravitropic response than that of wild type (wt) in a medium with an osmotic gradient. Local application of abscisic acid (ABA) to seeds or root tips of nhr1 increased root downward growth, indicating the critical role of ABA in tropisms. Wt roots germinated and treated with ABA in this system were strongly gravitropic, even though they had almost no starch amyloplasts in the root-cap columella cells. Hydrotropically stimulated nhr1 roots, with or without ABA, maintained starch in the amyloplasts, as opposed to those of wt. Hence, the near-absence (wt) or abundant presence (nhr1) of starch granules does not influence the extent of downward gravitropism of the roots in an osmotic gradient medium. Starch degradation in the wt might help the root sustain osmotic stress and carry out hydrotropism, instead of reducing gravity responsiveness. nhr1 roots might be hydrotropically inactive because they maintain this starch reserve in the columella cells, sustaining both their turgor and growth, and in effect minimizing the need for hydrotropism and at least partially disabling its mechanism. We conclude that ABA and water stress are critical regulators of root tropic responses.     

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     

The effects of asymmetric calcium application on primary root curvature of ageotropum pea mutant.

Recent studies indicate that roots of ageotropum seedlings can be used to study the hydrotropic response of roots independent of physiological events related to the gravity response of roots. There is evidence that Ca2+ ions are important in both the gravitropic and hydrotropic response of roots. In this study, we have compared three fully graviresponsive pea cultivars and the ageotropum mutant with regard to: 1) general root anatomy, 2) the effects of unilateral Ca application to both the root cap and DEZ region on root curvature, and 4) effects of unilateral application of EGTA to the DEZ region.     

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.     

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.     

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.     

Abscisic acid and the response of the roots of Zea mays L. seedlings to gravity

Summary Exogeneous application of abscisic acid (ABA) to intact roots of LG 11 maize seedlings inhibits root elongation and induces bending of the root in response to gravity in darkness, even though the roots of these seedlings are not normally positively geotropic in the dark. ABA cannot, however, induce geotropic curvature in dark-exposed decapped roots, thus confirming that the root cap is the site of graviperception in the intact root.Abbreviation ABA abscissic acid     

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。     

植物根向水性反应研究进展

根的向水性生长是指植物通过根尖感知土壤中的水分梯度, 向着水势较高区域生长的一种生物学特性, 这一特性对植物从土壤中有效获取水分极为重要。植物向水性研究已成为当前植物学研究的热点领域, 但对于调控这一生理反应的分子机制仍知之甚少。目前的研究表明, MIZ1和GNOM作为植物向水性反应的重要调节因子, 正向调控植物根的向水性生长。此外, 植物激素、光、ROS及钙离子也参与调节植物的向水性反应。该文将从向水性的研究历史、调控基因以及内外因素等方面进行阐述, 便于读者全面了解植物向水性研究进展, 以期为向水性研究提供新思路。     

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.     

Lateral ABA transport in maize roots (Zea mays): visualization by immunolocalization

The intensity of an ABA (abscisic acid) signal as a root-to-shoot signal, as well as its action on root hydraulic conductivity, strongly depends on the distribution of ABA during its radial transport across roots. Therefore ABA was visualized by immunolocalization with monoclonal ABA antibodies under conditions of lateral water flow induced by the application of a pressure gradient to the cut surface of the mesocotyl of maize seedlings. From the labelling of rhizodermis, hypodermis, cortical cells, and endodermis of roots of hydroponically (no exodermis) and aeroponically (with exodermis) grown seedlings it is concluded that the exodermis acts as a barrier to apoplastic transport that controls ABA uptake and efflux, but that the endodermis can easily be overcome via an apoplastic bypass. In longitudinal sections the strongest ABA signals originated from the root cap and the meristematic root tip, which is in agreement with the non-vacuolated cells of these tissues being an effective anion trap for ABA.     

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

Roots of the agravitropic pea (Pisum sativum L.) mutantageotropum show positive hydrotropism, 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 gradlent. These results suggest that roots of Alaska peas possess normal ability to respond hydrotropically and their weak hydrotropic response results from a counteracting effect of gravitropism.     

Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal

Re-orientation of Arabidopsis seedlings induces a rapid, asymmetric release of the growth regulator auxin from gravity-sensing columella cells at the root apex. The resulting lateral auxin gradient is hypothesized to drive differential cell expansion in elongation-zone tissues. We mapped those root tissues that function to transport or respond to auxin during a gravitropic response. Targeted expression of the auxin influx facilitator AUX1 demonstrated that root gravitropism requires auxin to be transported via the lateral root cap to all elongating epidermal cells. A three-dimensional model of the root elongation zone predicted that AUX1 causes the majority of auxin to accumulate in the epidermis. Selectively disrupting the auxin responsiveness of expanding epidermal cells by expressing a mutant form of the AUX/IAA17 protein, axr3-1, abolished root gravitropism. We conclude that gravitropic curvature in Arabidopsis roots is primarily driven by the differential expansion of epidermal cells in response to an influx-carrier-dependent auxin gradient.     

Calcium Requirement for the Induction of Hydrotropism and Enhancement of Calcium-Induced Curvature by Water Stress in Primary Roots of Pea, Pisum sativum L.

Positive hydrotropic curvature in the roots of the agravitropicpea (Pisum sativum L.) mutant, ageotropum, occurred when theroot cap was exposed to a gradient of water potential by anasymmetric application of agar containing sorbitol [Takano etal. (1995) Planta 197: 410]. As previously reported [Takahashiand Suge (1991) Physiol. Plant. 82: 24], in this study the hydrotropicresponse due to unilateral application of sorbitol to the rootcap was totally inhibited by pretreatment with ethyleneglycol-bis-(ß-amino-ethylether)N,N,N',N'-tetraacetic acid (EGTA). However, hydrotropic responseof the EGTA-treated roots was recovered only when EGTA was replacedby a 10 mM calcium (CaCl₂) solution prior to hydrostimulation.A calcium channel blocker, lanthanum (LaCl₃), also inhibitedhydrotropic curvature of ageotropum roots, whereas the hydrotropicresponse was affected by neither nifedipine nor vera-pamil.Application of calcium ionophore, A23187 [GenBank] , resulted in a significantpromotion of hydrotropic curvature. Furthermore, ageotropumroots curved away from a calcium source when an agar block containing10 mM calcium was asymmetrically applied to the root cap. Thiscalcium-induced curvature was found to be accelerated by waterstress and significantly inhibited by LaCl₃. While the calcium-inducedcurvature commenced within 1 h after application, hydrotropiccurvature became visible 3 to 4 h after an exposure to a gradientof water potential. These results indicate that apoplastic calciumand its influx through the plasmamembrane are involved in theinduction of hydrotropism in roots. A gradient of water potentialin the root cap may cause a physiological change that is mediatedby calcium, which ultimately leads to the curvature in the elongationregion associated with the hydrotropic response. (Received October 21, 1996; Accepted January 10, 1997)     

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.

     

Abscisic acid,xanthoxin and violaxanthin in the caps of gravistimulated maize roots

The occurrence and distribution of abscisic acid (ABA), xanthoxin (Xa) and the carotenoid violaxanthin (Va) were investigated in root tips of maize (Zea mays L. cv. Merit). In roots grown in the dark, Va and ABA were present in relatively high amounts in the root cap and in low amounts in the adjacent terminal 1.5 mm of the root. Xanthoxin was present in equal concentrations in both regions. In roots exposed to light, the ABA distribution was reversed, with relatively low levels in the root cap and high levels in the adjacent 1.5-mm segment. Light also caused a decrease in Va in both regions of the root and an increase in Xa, especially in the cap. In the maize cultivar used for this work, light is necessary for gravitropic curving. This response occurs within the same time frame as the light-induced ABA redistribution as well as the changes in the levels of Va and Xa. These data are consistent with a role for ABA in root gravitropism and support the proposal that Xa may arise from the turnover of Va.Abbreviations ABA abscisic acid - GC gas chromatography - HPLC high-performance liquid chromatography - GC-MS gas chromatography-mass spectroscopy - V_a violaxanthin - Xa xanthoxin     

Abscisic acid is a negative regulator of root gravitropism in Arabidopsis thaliana

The plant hormone abscisic acid (ABA) plays a role in root gravitropism and has led to an intense debate over whether ABA acts similar to auxin by translating the gravitational signal into directional root growth. While tremendous advances have been made in the past two decades in establishing the role of auxin in root gravitropism, little progress has been made in characterizing the role of ABA in this response. In fact, roots of plants that have undetectable levels of ABA and that display a normal gravitropic response have raised some serious doubts about whether ABA plays any role in root gravitropism. Here, we show strong evidence that ABA plays a role opposite to that of auxin and that it is a negative regulator of the gravitropic response of Arabidopsis roots.     

ALTERED RESPONSE TO GRAVITY is a peripheral membrane protein that modulates gravity-induced cytoplasmic alkalinization and lateral auxin transport in plant statocytes

ARG1 (ALTERED RESPONSE TO GRAVITY) is required for normal root and hypocotyl gravitropism. Here, we show that targeting ARG1 to the gravity-perceiving cells of roots or hypocotyls is sufficient to rescue the gravitropic defects in the corresponding organs of arg1-2 null mutants. The cytosolic alkalinization of root cap columella cells that normally occurs very rapidly upon gravistimulation is lacking in arg1-2 mutants. Additionally, vertically grown arg1-2 roots appear to accumulate a greater amount of auxin in an expanded domain of the root cap compared with the wild type, and no detectable lateral auxin gradient develops across mutant root caps in response to gravistimulation. We also demonstrate that ARG1 is a peripheral membrane protein that may share some subcellular compartments in the vesicular trafficking pathway with PIN auxin efflux carriers. These data support our hypothesis that ARG1 is involved early in gravitropic signal transduction within the gravity-perceiving cells, where it influences pH changes and auxin distribution. We propose that ARG1 affects the localization and/or activity of PIN or other proteins involved in lateral auxin transport.     

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.