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Plant Signal Behav. Mar 2009; 4(3): 183–190.
PMCID: PMC2652524

Arabidopsis root growth movements and their symmetry

Progress and problems arising from recent work

Abstract

Over the last fifteen years, an increasing number of plant scientists have become interested in the Arabidopsis root growth pattern, that is produced on the surface of an agar plate, inclined from the vertical. In this situation, the roots wave intensely and slant preferentially towards one side, showing torsions in the epidermal cell files alternately right-and left handed. In addition, the pattern switches to the formation of large or strict coils when the plate is set horizontally. After this finding, different hypotheses were advanced attempting to explain the forces that shape these patterns. These basically appear to be gravitropism, circumnutation and negative thigmotropism. With regard to the symmetry, the coils and the slanting in the wild-type are essentially right-handed, but mutants were also reported which show a left-handed symmetry, while some do not show a regular growth pattern at all. This review article discusses the earlier as well as the most recent findings on the topic, and investigates the possibility of describing the different mechanisms shaping the root growth patterns via unifying hypothesis.

Key words: root pattern, circumnutation, gravitropism, symmetry, root waving, auxin

Introduction

The main functions of roots are the anchorage of the plant to the soil, and the absorption of water and mineral ions from it. To perform these basic processes, roots developed a complex architecture adapted to fulfil the above functions1,2 in the most optimal way. This architecture is, in part, the result of different kind of root movements.

Whereas, however, the literature, regarding the movements of plant shoots is very large, that regarding the movements of roots is notably scarce.3 In fact, after the researches of the Darwins, reported in The Power of Movement in Plants,4 very little investigation has been done on the subject. However, what is well known is that roots do not elongate in a straight line but by making movements similar to those that also are seen in the emerging plant organs. Root movements can be tropic, i.e., responses to external factors such as gravity, light or humidity, or they can be of nutational character, whereas nastic movements, it appears, have never been reported. In their investigations, the Darwins, studying a set of various plants, ascribed the root movements to gravitropism and to the process named by them circumnutation; a modified form of the term rotating nutation introduced by von Sachs.5 This is a movement that, in contrast to tropisms and nastic responses has been generally considered spontaneous, i.e., not induced by external factors, even though in some cases it can be induced or modified by some agents, such as gravity, and consequently tropistic.6 This topic was extensively studied after the Darwins up to the 1950s, even though rarely in roots, as previously mentioned. Thereafter, it was almost completely abandoned in favor of biochemical researches, until the advent of modern molecular genetics techniques offered new possibilities of investigating the process.

The circumnutation movement is generally helical and elliptic, even though pendulum motions are not rare, and in particular concerning the root, that is the topic of this review, it was shown to occur frequently through movements that have the appearance of flattened helices, when seen on a surface. The process is strictly bound with that of symmetry and torsions seen in plants, which were, until recently, also an almost forgotten topic. A plant describing in its track a helix can generate it to the right or to the left-hand (Fig. 1), but to establish this the concept of symmetry has to be exactly established. In fact, in the past some botanists used to call left-handed what for others was right-handed, and vice versa. This is because the helix described by a climbing plant is different when seen from the outside or from the inside of the helix itself. For instance, with the first definition the Morning Glory (Ipomea sp.) climbs with its shoot to the right-hand, with the second to the left-hand. To simplify the matter it is currently recommended to use the definition applied in physics, in which ambit everything is considered right-handed that turns clockwise, and moves away from the observer. The reverse is left-handed. As well as creating helices during their growth, plants also generate torsions which can also be right or left-handed. For such cases the definition used in Physics should also be applied. As expected, some of the symmetries in plants are genetically determined, like those of the Ipomea purpurea Roth. (right-handed) or those of the Polygonum baldschuanicum Regel. (left-handed), however, some are variable, and thus possibly depend on the environment. As expected, in the plant world the right-handed symmetry is also significantly preferred. Recently, a great help in studying plant movements, torsions and circumnutation, came from the utilization of some model plants. Among these particularly Arabidopsis. The review, will be concerned mainly about the behavior of Arabidopsis roots, with some references to other plant species. Such references will hopefully increase in future years, because some aspects of the problem cannot be easily understood without a larger vision of the plant world.

Figure 1
(A) right-handed and (B) left-handed twining plant. In (A) Ipomea purpurea Roth. (Campanulaceae), in (B) Polygonum baldschuanicum Regel. (Polygonaceae).

Root Movements

When the Arabidopsis primary roots are allowed to grow on an hardagar plate (1.5%), especially if tilted somewhat from the vertical (45–60°), they do not grow straight down, but by making a complicated pattern (Figs. 2A and B; and and3).3). In particular, they wave intensely, slant (or skew) to one side, that is the left if seen from the front, and make a torsion also named CFR (cell file rotation) at the same time, alternatively to the left, when the wave is made to the visual left, and to the right, when the wave is made to the visual right. The waving movement is general in the wild-type, but the slanting is especially strong in the ecotypes Ws and Landsberg, whereas it is weak in Columbia.7 In addition, when the roots are allowed to grow on a horizontally set plate, they form large or strict clockwise coils. Different hypotheses and reviews exist812 on the cause of these movements and on what might be the behavior of the root in a three dimensional situation, outside of the environment of a Petri dish. Unfortunately, this behaviour has been very difficult to reproduce because of the thinness and fragility of the Arabidopsis roots (ca. 0.2 mm). It has, in fact, been studied only once, recording the root movement inside an agar block, and it was shown that roots effectively circumnutate, prevalently to the right-hand.13

Figure 2
(A) Seedlings of Arbidopsis thaliana growing on an inclined hard-agar (1.5%) petri dish. (B) Schematic representation of the slanting produced to the right-hand in the wild type and to the left-hand in some mutants. (C) Darwin's drawing of Phaseolus multiflorus ...
Figure 3
(A and B) Arabidopsis seedlings intensely waving, coiling and slanting to the right-hand from the wild-type ecotype Wassilewskija. (C) right-handed (clockwise) coil from the wild-type showing a strong left-handed torsion. (D) Left-handed (counterclockwise) ...

The first to note a peculiar pattern in Arabidopsis roots was Mirza14 who described the presence of coils, whose shape varied depending on the nutritional medium and conditions of illumination. The waving pattern of Arabidopsis was described firstly by Okada and Shimura,15 who also gave an interpretation of the observations. They stated that the root waving movement is the result of positive gravitropism plus a thigmotropic effect. The latter is the consequence of the fact that the root tip, during the gravitropic reaction encounters an impediment in penetrating the agar surface on a tilted hard-agar plate, and this should induce the switch to the other side. The effect is revealed also by the above mentioned switching torsion. This paper was shortly followed by another,16 in which it was shown that the roots not only wave and present a torsion, but also, in the wild-type, slant towards the visual left. In addition, the roots, when grown on an horizontal petri dish, as already mentioned, always make large or strict coils, in the wild-type, in a clockwise direction. Since, however, the coils are the result of a clockwise movement, and the direction of growth is away from the observer, following the definition given in physics this movement and the resulting coils were considered right-handed in the Simmons et al. paper.16 And right-handed consequently also the slanting towards the visual left side. This because the slanting appears to be simply the consequence of the fact that the half-waves made to the right-hand are deeper than those made to the opposite side.17 Naturally, the visual left-side can became the right if the seedlings are seen from the bottom of the dish as the Masson group (University of Madison, US) do. A method that we would recommend to the investigators not wishing to apply the definition of right-hand and left-hand reported in Simmons et al.16 to avoid equivocal definition of left and right in the description of mutants for instance.

The waving movement in itself, however, is not necessarily coupled with the slanting as Buer et al.18 pointed out, because in some cases the roots wave intensely without any apparent slanting, as is frequently seen in the ecotype Columbia or in mutants. A significant finding was then the observation by Rutherford and Masson7 that the switching from side to side in the waves takes place after half a wave, and not at every wave, as apparently can be thought considering the phenomenon purely visually. The switching apparently brings about an inversion of chirality, as demonstrated by the torsion of the roots, that is alternatively to the left and to the right-hand, as pointed out earlier. The torsion in a waving root, however, sometimes is faint or lacking, as shown by Buer et al.18 but it is always dramatic in the coils. The lack of torsion in the waves frequently observed, generally mixed with waves where is present, seems a consequence of the fact that the space helix, at difference of coils, in the waves is notably loose. The observation of Buer et al. is therefore not surprising.

These are, in general, the characteristics shown by Arabidopsis primary roots growing on a hard-agar dish. They needed at some point to be put together to provide a general picture of the forces that shape the root pattern, and assess what might be the root behavior in a three dimensional situation. This synthesis was furnished in a review by Migliaccio and Piconese8 some years ago. In this synthesis, the waving pattern was seen essentially as the consequence of positive gravitropism and circumnutation acting simultaneously on the growing root. The first factor tending to induce the root to grow down the plate, the second leading the root to accomplish an elliptic movement, similar to that produced on a horizontal plate. The waving however shows a switch from clockwise to anticlockwise at every half turn probably as a consequence of a negative thigmontropic effect, which induces the root to circumnutate to the opposite direction. In addition, in the wild-type, the roots slant to the side that was defined as the right-hand, because the slanting itself is the direct consequence of a dominant right-handedness. In this model the torsions, observed both in the waves and in the coils, are seen as secondary effects, due to the necessity to discharge the energy of a circumnutation helix being flattened on the hard-agar surface. This, however, occurs only in the wild-type, because there is evidence of torsions of different origin in some mutants.19,20 The model does not establish, however, a necessary coupling among waving, slanting and torsions, and refers only to wild-type plants; in mutants the situation can be different. But a coupling exists between coiling and CFR.

The basic idea of the model8 does not stem simply from recent experiments, done exclusively with Arabidopsis, but is significantly connected with the Darwins' work carried out at the end of the 19th century (1880). They studied the root pattern of different plant species with an ingenious apparatus and early photography (Fig. 2C). This consisted of a glass plate, darkened with smoke, on which the roots were made to grow downwards with an inclination of ca. 60 degrees to the horizontal plane, thereby leaving a mark on the smoked surface. These marks were then photographed and interpreted. They were considered as the consequence of a circumnutation process plus positive gravitropism, and also as space helices flattened on the inclined glass surface. The Darwins in fact noted as the roots, during the waving, did not stick to the glass, but raised periodically to complete a circle. By applying this Darwins'model4 to Arabidopsis one is led to formulate the above hypothesis, that differs from what the Darwins reported, only in the fact that the Arabidopsis root switches symmetry every time the root tip hits the hard-agar surface. This can be due to the fact that the Arabidopsis roots are quite thin (0.2 mm) and weak, whereas the plants studied by the Darwins in general develop thick roots. These plants were Vicia faba, Phaseolus multiflorus, Aesculus hippocastanus, Cucurbita ovifera, Lupinus luteus, Quercus robur, Zea mays, Avena sativa. The thickness leads one to think that, in contrast to Arabidopsis, the roots of the above plants, are able to discharge the energy coming from the flattening of the space helix by simply rotating on themselves. Interestingly, the Darwins did not see the slanting, which is visible in some of their picture, but surely the data by them collected on roots where not enough to consider real the process.

In addition, the significant experiments on root circumnutation made in the 1950s by Spurny21 indicate that the roots of pea also make space right-handed helices in their circumnutating movement (Fig. 2D). However, after circling, the roots in this case straightened up, so that the movement was no longer available for study, as happens in Arabidopsis, and a recording camera was necessary.21 Further evidence in favor of the model came then from experiments of growing Arabidopsis seedlings on a three dimensional clinostat (Random Positioning Machine). In these experiments22 (Fig. 4) it was shown that the roots of the ecotypes Ws and Columbia, in simulated microgravitatonal conditions, make only large clockwise coils, while a left handed mutant (1–6C) makes only coils to the left side. Apparently, the lack of gravitropism allows circumnutation to reveal itself in the formation of loops. No switching to the opposite symmetry was apparent, and thus the right handedness of the movement seems demonstrated. Within the same experiments also two agravitropic auxinic mutants were tested, aux1 and eir1/pin2, and the result was that only root random movements were observed, the same behavior that they show in gravitational conditions. The involvement of auxin in the process seems therefore to be also supported.

Figure 4
Arabidopsis plants after a 6-day run on the Random Positioning Machine (three dimensional clinostat). Note the large right-handed (clockwise) coils made on the machine by the roots in the ecotypes Colombia and Wassilewskija. (The pictures are taken from ...

More recently (2008) an experiment of growing Arabidopsis seedlings in true space was performed through ESA by G.F.E. Scherer, and the preliminary results indicate that the wild-types roots make frequently coils (personal communication). Consequently, it was further established that the roots do not need gravity to make coils. However, since the coils seen on the clinostat were always large loops, it is still possible that the thigmotropic effect induced by gravitropism could be involved in the production of the very strict coils in 1 g situation, and this will be in line with the recent experiments of Massa and Gilroy23 on the thatching sensitivity of the root tips and their response.

Since 2001, however, some new experiments, enriching our knowledge of Arabidopsis root movements, and which possibly will change some aspects of the above model, have been reported.

Of these, particularly interesting is the work by Thompson and Hoolbrook,10 who advanced a totally different hypothesis. Following this hypothesis the waving is described essentially as the consequence of the fact that the root tip in its growth along an inclined petri dish encounters an impediment to penetrate the agar as gravitropism induces it, and stops moving. This allows the elongation zone to grow arcuate until the root tip is moved away. It moves then down again until it stops and the elongation zone produces another arc in the opposite direction. This hypothesis is undoubtedly interesting, and it is therefore disappointing that, after a single paper, the research was not extended. However, there are questions that this model does not explain. Mainly, it does not explain the slanting, because if the complete process is passive, why do the roots make larger waves to the right-hand side? Neither does the hypothesis explain the formation of clockwise coils on inclined or horizontally set plates, because if they are the consequence of only physical forces, why do they loop only to the right-hand? In addition, the above cited experiments made with the RPM and in micro-gravity show that the formation of coils does not necessitate gravity. A part of the hypothesis nevertheless can be accepted, admitting that the impediment in the downward growth of the root tip can be bound to a circumnutation movement that is more conspicuous to the right-hand, and will be enlarged by the impediment engaging the root tip.

Another significant contribution to the analysis of the root pattern on agar dishes came from Buer et al.18,24 as a consequence of an analysis on the effect of different environmental conditions, including the presence of some ions in the media, ethylene in the air filling the caps, and the kind of sealing tape. In this paper it is reported that some elements that seem coupled were in fact uncoupled, such as waving and CFR. These data thus seem to prove that the torsion probably cannot be the inductor of the slanting and coiling as it was sometimes supposed,10,20,25 since it is frequently absent, and moreover, a left-handed torsion that induces a clockwise (right-handed) movement in the coils is totally incongruous. An additional interesting hypothesis reported in the paper is the suggestion that the formation of waves could be the consequence of a circadian rhythm that induces the asymmetric growth. This is quite plausible point and supports the circumnutation hypothesis, because this process seems, in fact, due to the diffusion of a wave of growth that goes around helically in the plant organs, possibly controlled by a circadian rhythm. These authors also observed a significant effect of the ethylene present in the air of the dishes in reducing coiling and slanting.

Mutants and Genes

The first wavy mutants (wav1 to 6) were isolated by Okada and Shimura,15 who found differences in the wavy pattern, which in some of them is compressed, in others enlarged, and in some totally lacking. Most of the genes involved were later cloned. The authors did not note the slanting, which was reported somewhat later by Simmons et al.16

The first mutants of slanting were isolated by Rutherford and Masson,7 who named them sku1 and sku2. Both of them were shown to increase the process. The authors report the characteristics of the waves, i.e., wave length, frequency and amplitude, and the variation of slanting. In the same paper, they also showed that the ecotypes Landsberg and Ws produce strong slanting, whereas in Columbia it is notably reduced. The proteins encoded by these genes have as yet not been identified.

Subsequently spr1 and spr2 (Fig. 5A) were isolated by Furutani et al.19 These are mutants, whose roots slant to the left-hand (the visual right) and show a right-handed CFR in the coils, which are left-handed (counterclockwise). spir1/sku6 shows a marked slanting, whereas spir2/tor1 has it reduced, but shows torsion in the leaf petioles. The first left-handed mutant, however, was that isolated by Marinelli et al.26 and named simply 1–6C, a probable allele of the spir genes. SPR1 and SPR2 proteins were identified. SPIR1 bound to the green fluorescent protein (SPIR1::GFP) was found to localize to the polymerising plus-end of microtubules.27,28 SPIR2/TOR with the same technique was found to be localized to the length of the cortical microtubules.28,29

Figure 5
Mutants of the root growth pattern in Arabidopsis. (A) spr1-1, left-handed mutant. (B) clg1, coiling mutant. (C) eir1/pin2, agravitropic and auxin transport mutant, lacking a gene for the auxin efflux. (D) axr2, agravitropic and auxin mutant, lacking ...

From these spir genes lefty1 and lefty2 were isolated through mutagenesis.25,27,30 They slant to the right-hand like the wild-type (visual left as the name shows), and encode the genes for alpha-tubulin 6 and 4 respectively. These researches supported a significant role for microtubules in the production of the waving pattern. In particular, it was suggested that the arrays of microtubules in the epidermal root cells are transverse and oriented normally to the elongating root cell files (CFR) axes. This comports that in the wild-type Columbia and Ws (left-hand twisted, as the authors report) the arrays appear twisted to the right-hand, in the mutant spr1 (right-hand twisted) to the left-hand, in the mutant lefty1 (left handed twisted) to the right-hand, and so on. In addition the tubuline arrays appear oriented with a symmetry opposite to the general slanting of the roots.19,20 Furthermore, it was shown that the administration of drugs capable of disrupting microtubules, such as oryzalin and propyzamide can change the twisting of the root cells.31 In support of the above hypothesis, recently, in a new paper,25 thirty two Arabidopsis tubulin mutants were described, showing the disposition of tubuline arrays normal to the twisted epidermis cell files axes, and also of opposite symmetry with respect to the slanting of the roots. However, as is known, in the most of the ecotypes and mutants, the roots slanting make waves, switching the symmetry of torsion at every half-wave, and the slanting is due just to the fact that the half-waves made to one side are larger than those made to the opposite side. Consequently, if the tubulin arrays control CFR and slanting, they have to switch at every half-wave of symmetry, but this oddly is not reported. On the other hand, even though the involvement of tubuline and actine structures in the production of the root pattern seen on the agar plate seems well supported,32 not every researcher found a strong correlation among CFR, slanting and microtubules orientation,18,33 and so doubts persist about this model.

Other significant papers on the involvement of cytoskeleton in the production of torsion have been published. One from Collings et al.34 is particularly interesting. It is about a mutant, i.e., mor1, that in a range of temperature is hypersensitive to cytokalasin D and latrunculin B. These two drugs destroy the actin filaments, influence the tubulin structure and induce swelling in the roots. The mutated gene in sku5 was cloned in 2002 by Sedbrook et al.35 This mutant shows a strong left-handed torsion both in shoots and roots, also when grown inside the agar. This torsion then probably could not be ascribed to the adaptation of the space helix to the flat agar surface, but it seems to be something constitutive. Torsions however, to the right- and to the left-hand, are very common in the plant kingdom and have different origins.36 This finding thus does not exclude the hypothesis that normally in the wild-type the torsion could be the result of the adaptation of the space helix to the plate. Different kinds of torsions can coexist. This topic was a subject of intensive studies up to the 1930s, but after that time very little on the topic has been reported.3

Other significant mutants of slanting, waving and torsions are those isolated by Yuen et al.37 symbols wvd2 and wdl1, which show left-handed slanting and right-handed torsion like spir1 and spir2. wvd1 was produced by constitutive activation of the corresponding gene by the cauliflower 35S promoter. The most conspicuous characteristic of this mutant is that its roots slant without producing a clear waving. It seems a form of growth intermediate between coiling and classic waving. The WVD2 gene was also found to be involved in anisotropic cell expansion and synthesis of microtubules.

A further mutant isolated by the same group38 is rhd3 (root hair defective), which, apart from the defect in root hairs, ascribed to the involvement of the gene in vesicle trafficking between ER and Golgi particles, does not show waving, skewing and CFR. The authors hypothesized that the mutated gene, at least in part, controls these characteristics. They reported, in addition, that some of the already isolated mutants for different phenotypes exhibit similar alterations. Some of these are cob1, cob2, eto1-1, erh2-1, erh3-1. Another isolated mutant, i.e., adk1, recently was found totally lacking of waving and mutated in the adenosine kinase gene.39 This research added this enzyme to the factors affecting the root pattern.

Some of the totally agravitropic auxinic mutants do not show regular waving, slanting or coiling, supporting the hypothesis that auxin transport and action are in some way involved in the production of the wavy and coiling pattern. This leads one to think that auxin controls not only gravitropism and phototropism, but also nutation movements. Of these mutants, only a few are known that are totally agravitropic, whereas most of them are only modestly agravitropic, frequently not over 10–20% of the wild-type. This fact might imply that gravitropism is controlled by numerous genes, and that the absence of one could be compensated by redundancy. Of the totally agravitropic mutants, that show only random movements in the root, aux1 (many alleles), which mutated gene encodes a facilitator for the influx of auxin into cells,40 eir/agr1/pin2/wav6 (many alleles; Fig. 5C) which mutated gene encodes an auxin facilitator for the efflux out of the cells,41 and axr2 (Fig. 5D) which encodes the IAA7 protein, involved in the general auxin action42 should be listed. Also the mutant rcn1, which shows resistance to NPA, and whose mutated gene encodes the protein phosphatase regulatory subunit A, appears connected with auxin action. It has a particular phenotype, because on a horizontally oriented NPA containing agar dish, makes more coils than the wild-type, which is instead inhibited by the drug.43,44

Recently, wag1 and wag2 have been described45 as mutants of some protein kinases related to pinoid. Both show a compressed wavy-pattern, produced also on vertically set dishes. Loss of WAG1 mainly affects the wavelength, whereas loss of WAG2 mainly affects the wave amplitude. Also one of the original WAV genes from Okada and Shimura,46 i.e., WAV2, was cloned resulting in a member of the BEM46 family, within the a/b-hydrolase superfamily. This mutation produces roots with larger waves and increased cell file rotation, but no modification in the microtubules arrays.

Some mutations in the G proteins,47 that induce modification in the root growth pattern both in waving and slanting, have also been described. The reasons for the modifications are unknown, but G proteins notoriously are involved in growth and development processes. Recently, rha1 has also been added to the mutants of the root pattern.48 It shows a reduction of slanting of 40% about with respect to wild-type. This, in spite of the fact that it belongs to the Ws ecotype, that is characterized by a significant slanting. rha1 shows also a modest reduction of gravitropism and resistance to 2,4-D, TIBA, NPA and ethylene in the roots. The gene encodes a heat shock factor, which appears to influence the root pattern. In addition, two mutants very frequently produce coils on inclined plates. These are rgr1/axr4,49 and clg1,50 (Fig. 5B).They also show a reduced number of lateral roots, resistance to auxinic substances and auxin transport inhibitors, together with a moderate reduction in gravitropism. RGR1/AXR4 also results involved in auxin transport through an interaction with AUX1.51

In conclusion, the root growth pattern, as any complex differentiation process, seems to be the result of an interaction among many different environmental and genetics factors. We expect the list of factors modifying the root pattern will keep growing in future.

Looking for a Unifying Hypothesis

Since the discovery of Arabidopsis root coiling by Mirza14 and of the wavy pattern produced on inclined hard-agar dishes by Okada and Shimura,15 various opinions on the different aspects of the process have been advanced, together with a single unifying hypothesis that may explain most of the observed facts.8,12 However, the real question is: how far can we currently advance a unifying hypothesis about the forces that shape the primary Arabidopsis root, which would be credible on the basis of the presently available data? In our opinion the answer is that we could advance it somewhat, even though some doubts would definitely persist.

Such a unifying hypothesis will clearly need to start at the pioneering work of the Darwins,4 which was conducted with equipment very similar to that presently used for Arabidopsis, i.e., a smoked tilted plate and a camera. As discussed above, with this apparatus the Darwins studied the root pattern of several different plant species. In the course of the investigation they found that the roots were growing downwards while circumnutating, i.e., by making elliptical loops that had the appearance of space helices flattened on the glass plate. Post-Darwins more experiments were done on circumnutation, though mainly on shoots and other aerial parts, and only little on roots.3 Exceptions were the experiments conducted by Spurny in the 1950s on peas, who found out that pea roots grew describing right-handed helices in humid air when free.

The case of Arabidopsis seems a little different, since on hard-agar plates the roots show two patterns. One consists in waving growth, the other in coiling. The two processes can be explained essentially in two ways: either they are prevalently the consequence of circumnuation and positive gravitropism, or they are simply the consequence of an interaction of the growing root with the agar surface, and thus a product of pure physical forces.10 If we examine the coiling first, we see that this behavior is characteristic of plants grown on plates set horizontally or clinostated, but frequently also coils are formed on inclined dishes. Following the theory of physical forces it should be the consequence of the impediment found by the root tip to proceed, that induces the elongation zone to grow arcuate. However, why is the arc always clockwise? Why does it also happen in space where the force of gravity is absent? The explanation could be that the loops correspond to right-handed circumnutation ellipses produced when the force of gravity is absent through clinostating, horizontal plates, or simply the loss of gravisensing in old roots. Thus, they appear as circumnutation helices flattened on an agar plate. The coils however present also a very strong torsion to the left-hand, that is reduced or sometimes lacking in the waves. One possibility here20,25 is that the torsion induces the coiling and the slanting to the right-hand, however the torsion in the waves is frequently lacking,24 so that a different explanation is necessary, this could be that the torsion in the wild-type is simply due to the adaptation of the space right-handed helix to the flat agar surface.

The interpretation of the waving path is more complicated, because in this case the roots switch the symmetry of the movement at every half wave. Here, following the Thompson and Holbrook hypothesis,10 we have the root tip stopped in its movement and the elongation zone making an arch at every half turn in opposite directions. But why do the roots not do this when coils are produced on inclined plates? One explanation we can advance is that the coils on an inclined plate are the consequence of a loss of sense of gravity by the roots, so that the tip is not pressed anymore on the dish. Seen under the pure physical aspect the two processes, i.e., coiling and waving cannot be explained by a single event. On the other hand, the switching of symmetry seems to be due to an impediment found by the root tip that can be called negative thigmotropism, or simply to the inability of the root to move too much up against the gravitropic force to complete a circle, something that becomes easy when the sensing of gravity diminishes, or the plants are set on a clinostat.

Another question is: why the root coils just in one direction and does not switch frequently between clockwise and anticlockwise symmetry as in general the circumnutating shoots do.4,52 However, at difference of shoots, not all roots seem to switch their symmetry frequently, as shown, for example, by Spurny's21 experiments on peas.

A further question is this: why, on vertical plates, waving is barely apparent? One explanation can be that the gravitropic force becames dominant, and little freedom is left for circumnutation, even though circumnutation is still detectable with refined equipment such as that used in Evans laboratory,13 where it was shown that the Arbidopsis roots circumnutate inside the agar in a three dimensional situation. Massa and Gilroy23 however, did not find clear circumnutating movements in studying Arabidopsis roots avoiding obstacles in their movement down the gravity vector. But, as shown already by the Darwins, when gravitropism becomes dominant, circumnutation almost disappears.

Interestingly, Buer et al.18 advanced the hypothesis that the waving pattern could be induced by a circadian rhythm, because the root accomplishes about one complete turn per day. This is a hypothesis which carries credence, and does not contradict the fact that the waves are due to circumnutation. A circadian rhythm can, in fact, control the circumnutating movement, which in this case becomes a tropism. On the other hand, circumnutation in the Darwin's4 view, was a large phenomenon of which tropisms, i.e., gravi-photo-hydrotropisms, apical hook formation, and even nastic movements are simply considered modified forms. This is a point that unfortunately was not followed by latter authors, but is rich in meaning if we want to see the plant movements from the evolutionary point of view. In addition, the Darwinian view of circumnutation also means that not all the circumnuating movements are exactly the same and produced through the same mechanism.

The Arabidopsis root pattern therefore can be probably in part understood today, and explained in a unifying hypothesis, as we attempted to do above. However, to completely understand it, we need more data taken from a larger variety of plant species, and not only from Arabidopsis. So far Arabidopsis has been of great utility for in-depth study of various aspects of plant physiology and genetics, but we ought not to become trapped with just this cruciferous plant. It is essential that we open ourselves to the understanding of the physiology of the whole plant kingdom.

Acknowledgements

We thank Drs. Stefano Mancuso and Frantisek Baluska for their support for this article. The work was supported by ASI (Italian Space Agency, wp1B1212-X1) and by the National Research Council of Italy.

Abbreviations

CFR
cell file rotation
NPA
naphtyl phtalamic acid
IAA
indole 3-acetic acid

Footnotes

Previously published online as a Plant Signaling & Behavior E-publication: http://www.landesbioscience.com/journals/psb/article/7959

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