Planta (Berl.) 126, 19--23 (1975) 9 by Springer-Verlag 1975
Abscisic Acid and the Response of the Roots of Zea mays L. Seedlings to Gravity* H e n r y W i l k i n s a n d R. L. W a i n Agricultural Research Council Plant Growth Substance and Systemic Fungicide Unit, Wye College (University of London), Wye, Ashford, Kent, TN 25 5AH, U.K. Received 3 April; accepted 7 May 1975 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.
Introduction R e c e n t l y , t h e georeaction of L G 11 m a i z e p r i m a r y r o o t s h a s been shown t o be controlled b y a series of d i s t i n c t physiological processes, which c o m m e n c e w i t h t h e p e r c e p t i o n of white light b y t h e r o o t cap a n d t h e l i g h t - i n d u c e d p r o d u c t i o n of g r o w t h - i n h i b i t i n g factor(s) in t h e cap (H. W i l k i n s et al., 1974a, b ; H. W i l k i n s a n d W a i n , 1974, 1975; Pilet, 1975). T h e geotropie response is t o t a l l y d e p e n d e n t on these two processes since t h e roots of L G 11 m a i z e seedlings, in c o m m o n w i t h those of several o t h e r m a i z e varieties, are n o t p o s i t i v e l y geotropie in d a r k n e s s ( S c o t t a n d M. B. W i l k i n s , 1968; H. W i l k i n s a n d W a i n , 1975; Pilet, 1975). Subsequent steps in t h e i n d u c t i o n of t h e geotropie response are n o t d e p e n d e n t on light a n d can, therefore, t a k e place in darkness. These i n v o l v e t h e l a t e r a l r e d i s t r i b u t i o n of g r o w t h - i n h i b i t i n g factor(s) to t h e lower side of t h e growing zone of t h e r o o t to cause u n i l a t e r a l i n h i b i t i o n of r o o t elongation (Shaw a n d M. B. Wilkins, 1973 ; Pilet, 1973). Since white l i g h t induces t h e p r o d u c t i o n of A B A in t h e r o o t caps of L G 11 m a i z e seedlings (H. W i l k i n s a n d W a i n , 1974), i t was d e c i d e d to establish w h e t h e r a n exogenous a p p l i c a t i o n of this i n h i b i t o r could reduce r o o t e l o n g a t i o n a n d induce geotropie c u r v a t u r e in darkness.
Materials and Methods Fruits of Zea mays L. cv. LG I1 were soaked in flowing tap water for 6 h prior to germination between sheets of Whatman No. 1 filter paper in darkness at 2 5 ~ 1~ C. Seedlings were examined in green light (510-550 nm) and those with primary roots 20-25 mm in length were suspended on nylon net so that the apical 5 mm of root was immersed vertically in known concentrations of cis-trans-DL-ABA solution. After immersion for 2 h in darkness, the roots were washed twice in distilled water and dried on Whatman :No. 1 filter paper to remove all traces of the external ABA solution. The seedlings were then held with the roots orientated vertically in humidified glass petri dishes for 2 h in darkness, to allow further penetration of the inhibitor, prior to turning the roots through 90 ~ for geotropic stimulation in darkness. * Abbreviation: ABA = abscissic acid.
20
H. Wilkins and R. L. Wain
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111 0
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10 -9 CONCENTRATION
I
10 -7 OF
ABA
i
i
o
10 -s SOLUTION
( M }
Fig. 1. The effect of 10-9 to 10-~ l~ solutions of ABA on geotropic curvature (open circles) and elongation (solid circles) of maize roots, after supplying the inhibitor to vertically-orientated root tips for 2 h at 25~ C in darkness. Curvature was assessed after allowing a further 2 h for penetration of inhibitor prior to geotropic stimulation for 1.5 h in darkness. Root length was measured after removing the root tips from ABA solution and again after 3.5 h in darkness. Results for water controls are shown as dotted lines. Data are the mean of at least 27 independent measurements and twice standard errors are represented by vertical lines
Root curvature was assessed, using a shadowgraph technique, after incubation for 1.5 h at 25~ C in the dark. Further experiments were carried out in which the curvature of decapped roots was determined after supplying ABA in the way already described. Elongation of vertically-orientated Zea primary roots was monitored after supplying ABA solutions to the roots in exactly the same way as in the curvature experiments. After washing, root length was measured before and after incubation of the seedlings in darkness at 25~ C for 3.5h.
Results T h e geotropic curvature of intact primary roots of L G 11 maize seedlings in darkness at 25~ was assessed after supplying exogenous solutions of ABA to v e r t i c a l l y - o r i e n t a t e d roots. Appreciable root c u r v a t u r e s were detected after supplying A B A solutions r a n g i n g from 10-9 to 10-r M, b u t roots t r e a t e d with w a t e r showed little response to geotropic s t i m u l a t i o n i n darkness (Fig. 1). M a x i m u m root c u r v a t u r e was evoked b y 10-4 M A B A solution a n d this gave a b e n d i n g of 49 ~ i n t h e i n t a c t root. A second series of investigations revealed t h a t 10-4 M A B A produced o n l y a n 11.5 ~ 1.4 ~ c u r v a t u r e i n deeapped roots of these seedlings, comp a r e d with 4=J= 1.6 ~ c u r v a t u r e i n deeapped roots t r e a t e d with w a t e r prior to g r a v i t a t i o n a l s t i m u l a t i o n i n darkness. C u r v a t u r e of t h e A B A - t r e a t e d decapped roots m i g h t well indicate t h a t a small p a r t of the root cap r e m a i n e d after decapping on
some
occasions.
E l o n g a t i o n of vertically-held i n t a c t maize roots was i n h i b i t e d b y 10-dM A B A solution, whilst t h e lower c o n c e n t r a t i o n s of the i n h i b i t o r failed to reduce elongation below the level of t h e w a t e r control (Fig. I). T h e decapped roots were similarly i n h i b i t e d b y 10-4 M ABA, which indicates t h a t t h e presence of t h e c u t apical surface was n o t affecting u p t a k e of t h e compound.
ABA and Response to Gravity
21
Discussion
The root cap controls the response of the primary roots of LG 11 maize seedlings to white light by producing growth-inhibiting factor(s) which are also effective in the induction of geotropic curvature of the root in darkness, i.e. under conditions in which the roots do not normally respond to gravity (H. Wilkins et al., 1973, 1974a, b ; H. Wilkins and Wain, 1975). White light induces the production of ABA in the root caps of these seedlings (H. Wilkins and Wain, 1974), and an "ABA-like" compound is formed on geotropic stimulation of the roots of Golden Bantam maize seedlings (Kundu and Audus, 1974a, b). I t is logical to suggest, therefore, that this inhibitor could well be the one involved in the response of Zea roots to both white light and to gravity. This possibility is supported by the present investigations which show that ABA, applied to the roots, can not only inhibit root elongation but also enables the root to respond to gravitational stimulation in the dark. In other words, ABA acts in a manner similar to that of the light-induced endogenous inhibitor of elongation and, furthermore, can eliminate the necessity for lightexposure as the preliminary step in the geotropie response of these roots. Lateral redistribution of an inhibitor to the lower side of a geotropically stimulated root might increase its effective concentration and so depress the growth rate of the lower half of the organ to a greater extent than the upper half (Hawker, 1932; Went and Thimann, 1937; Audns and Brownbridge, 1957; Bennet-Clark et al., 1959; Shaw and M. B. Wilkins, 1973; Pilet, 1973). This possibility is supported by the present evidence that ABA solutions from 10-9 to 10-5 M are not effective in inhibiting root elongation but are effective in enabling the root to respond to gravity in darkness (Fig. 1). Perception of light is a prerequisite for the perception of gravity in some varieties of maize (Scott and M. B. Wilkins, 1968; Pilet, 1975a; H. Wilkins and Wain, 1975) and the root cap is responsible for light-perception (It. Wilkins and Wain, 1974). Consequently, the loss of georeaction on deeapping the roots of such seedlings (Juniper et al., 1966; Gibbons and M. B. Wilkins, 1970; Pilet, 1971) cannot be explained simply as the removal of the site of graviperception, since the lack of response could be due to the absence of the cap in its role as (a) a lightsensor or (b) a light-sensor and gravity-sensor and, subsequently, as a source of light-induced growth inhibitor(s). The present studies support the view that the cap is responsible for graviperccption, since th decapped root is not able to respond to gravity even in the presence of ABA. Thus in intact seedlings the root cap has a dual role, perceiving both white light and gravity. This conclusion does not appear to be supported by data for the movement of ABA in subapical segments of maize roots (Pilet, 1975b). In such segments, lateral translocation of the inhibitor takes place in response to gravity after decapping, indicating that graviperception occurs in the root itself. The most likely explanation of this discrepancy Nes in the fact that the curvature of the segments, used by Pilet, was assessed 7 h after application of ABA. A decapped root regains the ability to perceive light within 5 h of cap removal (H. Wilkins and Wain, 1974) and decapping stimulates the immediate production of amyloplasts, which are thought to be gravity-sensors (Larsen, 1969; Iverscn, 1969), in the decapped apex (Barlow, 1974). I t is possible, therefore, that the decapped root segments used by Pilet partially
22
H. Wilkins and R. L. Wain
r e g a i n e d t h e a b i l i t y to perceive g r a v i t y d u r i n g t h e t i m e allowed for d e v e l o p m e n t of t h e c u r v a t u r e response. Clearly, t h e p r e s e n t i n v e s t i g a t i o n s d e m o n s t r a t e t h a t abscisic a c i d can fulfil all t h e physiological r e q u i r e m e n t s n e c e s s a r y for i t t o be t h e l i g h t - i n d u c e d endogenous g r o w t h - i n h i b i t o r which controls r o o t e l o n g a t i o n a n d t h e c u r v a t u r e of t h e r o o t s in response to g r a v i t y . H o w e v e r , t h e d e t e c t i o n of I A A in m a i z e r o o t apices (Bridges et al., 1973; E l l i o t a n d Greenwood, 1974; R i v i e r a n d Pilet, 1974) a n d demons t r a t i o n s of a d i s t i n c t u p w a r d l a t e r a l t r a n s l o c a t i o n of gibberellie acid in geot r o p i c a l l y s t i m u l a t e d roots ( E I - A n t a b l y a n d Larsen, 1974; W e b s t e r a n d 1V[.B. W i l k i n s , 1974), raises t h e p o s s i b i l i t y t h a t r o o t g e o t r o p i s m is u n d e r t h e control of m o r e t h a n one g r o w t h - r e g u l a t i n g c o m p o u n d .
References Audus, L. J., Brownbridge, M. E. : Studies on the geotropism of roots. 1. Growth rate distribution during response and the effects of applied auxins. J. exp. Bot. 8, 105-124 (1957) Barlow, P. W. : l~eeovery of geotropism after removal of the root cap. J. exp. Bot. 25, 11371146 (1974) Bennet-Clark, T~ A., Younis, A. F., Esnault, R. : Geotropie behaviour of roots. J. exp. Bot. 10, 69-86 (1959) Bridges, L G., Hillman, J. R., Wilkins, M. B.: Identification and localisation of auxin in primary roots of Zea mays by mass spectrometry. Planta (Berl.) 115, 189-192 (1973) E1-Antably, M. M. H., Larsen, P. : Distribution of gibberellins and abscisic acid in geotropieally stimulated Vicia ]aba roots. Physiol. Plant. (Cph.) 32, 322-329 (1974) Elliot, M. C., Greenwood, M. S. : Indol-3yl-acetie acid in roots of Zea mays. Phyteehemistry 13, 239-241 (1974) Gibbons, G. S. B., Wilkins, H. B.: Growth inhibitor production by root caps in relation to geotropie responses. Nature (Lond.) 226, 558-559 (1970) Hawker, L. F. : Experiments on the perception of gravity by roots. New Phytol. 21, 321-328 (1932) Iversen, T.-H. : Elimination of geotropic responsiveness in roots of cress (Lepidium sativum) by removal of statolith starch. Physiol. Plant. (Cph.) 22, 1251-1262 (1969) Juniper, B. E., Groves, S., Landau-Schachar, B., Audns, L. J. : Root cap and the perception of gravity. Nature (Lond.) 209, 93-94 (1966) Kundu, K. K., Audus, L. J. : Root growth inhibitors from root tips of Zea mays L. Planta (Berl.) 117, 183-186 (1974a) Kundu, K. K., Audus, L. J. : Root growth inhibitors from root cap and root meristem of Zea mays L. J. exp. Bot. 25, 479-489 (1974b) Larsen, P. : The optimum angle of geotropic stimulation and its relation to the starch statolith hypothesis. Physiol. Plant. (Cph.) 22, 469-488 (1969) Pilet, P. E. : Root cap and georeaetion. Nature (Lond.) 288, 115-116 (1971) Pilet, P. E.: Growth inhibitor from the root cap of Zea mays. Planta (Berl.) 111, 275-278 (1973) Pilet, P. E.: Effects of light on the georeaction and growth inhibitor content of roots. Physiol. Plant. (Cph.) 33, 94-97 (1975a) Pilet, P. E.: Abscisic acid as a root growth inhibitor: Physiological analyses. Planta (Berl.) 122, 299-302 (1975b) Rivier, L., Pilet, P. E. : Indolyl-3-acetic acid in cap and apex of maize roots: Identification and quantification by mass fragmentography. Planta (Berl.) 120, 107-112 (1974) Scott, T. K., Wilkins, M. B. : Auxin transport in roots. IV. Effect of light on IAA transport and geotropic responsiveness in Zea roots. Planta (Berl.) 87, 249-258 (1969) Shaw, S., Wilkins, H. B. : The source and lateral transport of growth inhibitors in geotropically stimulated roots of Zea mays and Pisum ~ativum. Planta (Berl.) 109, 11-26 (1973)
ABA and Response to Gravity
23
Webster, J. I-I., Wilkins, M. B. : Lateral movement of radioactivity from (14C)GA3 in roots and coleoptiles of Zea mays L. seedlings during geotropic stimulation. Planta (BEE.) 121, 303-308 (1974) Went, F. W., Thimann, K. W. : Phytohormones. New York: Macmillan (1937) Wilkins, H., Larqu6-Saavedra, A., Wain, R.L.: Studies on factors influencing plant roo~ growth. Prec. 8th Int. Conf. Plant Growth Subst. Tokyo (in press) (1973) Wilkins, H., Larqu~-Saavedra, A., Wain, R. L. : Studies on plant growth regulating substances. XXXVIL The effects of light and hormone inhibitors on plant root growth. Ann. appl. Biol. 78, 169-177 (1974a) Wilkins, H., Larqu6-Saavedra, A., Wain, R. L. : Control of Zea root elongation by light and the action of 3,5-diiodo-4-hydroxybenzoic acid. Nature (Lend.) 248, 449450 (1974b) Wilkins, H., Wain, R. L. : The root cap and control of root elongation in Zea mays L. seedlings exposed to white light. Planta (Berl.) 121, 1-8 (1974) Wilkins, H., Wain, R. L.: The role of the root cap in the response of the primary roo~s of Zea mays L. seedlings to white light and to gravity. Planta (Berl.) (in press) (1975)
Abscisic Acid and the Response of the Roots of Zea mays L. Seedlings to Gravity* H e n r y W i l k i n s a n d R. L. W a i n Agricultural Research Council Plant Growth Substance and Systemic Fungicide Unit, Wye College (University of London), Wye, Ashford, Kent, TN 25 5AH, U.K. Received 3 April; accepted 7 May 1975 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.
Introduction R e c e n t l y , t h e georeaction of L G 11 m a i z e p r i m a r y r o o t s h a s been shown t o be controlled b y a series of d i s t i n c t physiological processes, which c o m m e n c e w i t h t h e p e r c e p t i o n of white light b y t h e r o o t cap a n d t h e l i g h t - i n d u c e d p r o d u c t i o n of g r o w t h - i n h i b i t i n g factor(s) in t h e cap (H. W i l k i n s et al., 1974a, b ; H. W i l k i n s a n d W a i n , 1974, 1975; Pilet, 1975). T h e geotropie response is t o t a l l y d e p e n d e n t on these two processes since t h e roots of L G 11 m a i z e seedlings, in c o m m o n w i t h those of several o t h e r m a i z e varieties, are n o t p o s i t i v e l y geotropie in d a r k n e s s ( S c o t t a n d M. B. W i l k i n s , 1968; H. W i l k i n s a n d W a i n , 1975; Pilet, 1975). Subsequent steps in t h e i n d u c t i o n of t h e geotropie response are n o t d e p e n d e n t on light a n d can, therefore, t a k e place in darkness. These i n v o l v e t h e l a t e r a l r e d i s t r i b u t i o n of g r o w t h - i n h i b i t i n g factor(s) to t h e lower side of t h e growing zone of t h e r o o t to cause u n i l a t e r a l i n h i b i t i o n of r o o t elongation (Shaw a n d M. B. Wilkins, 1973 ; Pilet, 1973). Since white l i g h t induces t h e p r o d u c t i o n of A B A in t h e r o o t caps of L G 11 m a i z e seedlings (H. W i l k i n s a n d W a i n , 1974), i t was d e c i d e d to establish w h e t h e r a n exogenous a p p l i c a t i o n of this i n h i b i t o r could reduce r o o t e l o n g a t i o n a n d induce geotropie c u r v a t u r e in darkness.
Materials and Methods Fruits of Zea mays L. cv. LG I1 were soaked in flowing tap water for 6 h prior to germination between sheets of Whatman No. 1 filter paper in darkness at 2 5 ~ 1~ C. Seedlings were examined in green light (510-550 nm) and those with primary roots 20-25 mm in length were suspended on nylon net so that the apical 5 mm of root was immersed vertically in known concentrations of cis-trans-DL-ABA solution. After immersion for 2 h in darkness, the roots were washed twice in distilled water and dried on Whatman :No. 1 filter paper to remove all traces of the external ABA solution. The seedlings were then held with the roots orientated vertically in humidified glass petri dishes for 2 h in darkness, to allow further penetration of the inhibitor, prior to turning the roots through 90 ~ for geotropic stimulation in darkness. * Abbreviation: ABA = abscissic acid.
20
H. Wilkins and R. L. Wain
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111 0
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10 -9 CONCENTRATION
I
10 -7 OF
ABA
i
i
o
10 -s SOLUTION
( M }
Fig. 1. The effect of 10-9 to 10-~ l~ solutions of ABA on geotropic curvature (open circles) and elongation (solid circles) of maize roots, after supplying the inhibitor to vertically-orientated root tips for 2 h at 25~ C in darkness. Curvature was assessed after allowing a further 2 h for penetration of inhibitor prior to geotropic stimulation for 1.5 h in darkness. Root length was measured after removing the root tips from ABA solution and again after 3.5 h in darkness. Results for water controls are shown as dotted lines. Data are the mean of at least 27 independent measurements and twice standard errors are represented by vertical lines
Root curvature was assessed, using a shadowgraph technique, after incubation for 1.5 h at 25~ C in the dark. Further experiments were carried out in which the curvature of decapped roots was determined after supplying ABA in the way already described. Elongation of vertically-orientated Zea primary roots was monitored after supplying ABA solutions to the roots in exactly the same way as in the curvature experiments. After washing, root length was measured before and after incubation of the seedlings in darkness at 25~ C for 3.5h.
Results T h e geotropic curvature of intact primary roots of L G 11 maize seedlings in darkness at 25~ was assessed after supplying exogenous solutions of ABA to v e r t i c a l l y - o r i e n t a t e d roots. Appreciable root c u r v a t u r e s were detected after supplying A B A solutions r a n g i n g from 10-9 to 10-r M, b u t roots t r e a t e d with w a t e r showed little response to geotropic s t i m u l a t i o n i n darkness (Fig. 1). M a x i m u m root c u r v a t u r e was evoked b y 10-4 M A B A solution a n d this gave a b e n d i n g of 49 ~ i n t h e i n t a c t root. A second series of investigations revealed t h a t 10-4 M A B A produced o n l y a n 11.5 ~ 1.4 ~ c u r v a t u r e i n deeapped roots of these seedlings, comp a r e d with 4=J= 1.6 ~ c u r v a t u r e i n deeapped roots t r e a t e d with w a t e r prior to g r a v i t a t i o n a l s t i m u l a t i o n i n darkness. C u r v a t u r e of t h e A B A - t r e a t e d decapped roots m i g h t well indicate t h a t a small p a r t of the root cap r e m a i n e d after decapping on
some
occasions.
E l o n g a t i o n of vertically-held i n t a c t maize roots was i n h i b i t e d b y 10-dM A B A solution, whilst t h e lower c o n c e n t r a t i o n s of the i n h i b i t o r failed to reduce elongation below the level of t h e w a t e r control (Fig. I). T h e decapped roots were similarly i n h i b i t e d b y 10-4 M ABA, which indicates t h a t t h e presence of t h e c u t apical surface was n o t affecting u p t a k e of t h e compound.
ABA and Response to Gravity
21
Discussion
The root cap controls the response of the primary roots of LG 11 maize seedlings to white light by producing growth-inhibiting factor(s) which are also effective in the induction of geotropic curvature of the root in darkness, i.e. under conditions in which the roots do not normally respond to gravity (H. Wilkins et al., 1973, 1974a, b ; H. Wilkins and Wain, 1975). White light induces the production of ABA in the root caps of these seedlings (H. Wilkins and Wain, 1974), and an "ABA-like" compound is formed on geotropic stimulation of the roots of Golden Bantam maize seedlings (Kundu and Audus, 1974a, b). I t is logical to suggest, therefore, that this inhibitor could well be the one involved in the response of Zea roots to both white light and to gravity. This possibility is supported by the present investigations which show that ABA, applied to the roots, can not only inhibit root elongation but also enables the root to respond to gravitational stimulation in the dark. In other words, ABA acts in a manner similar to that of the light-induced endogenous inhibitor of elongation and, furthermore, can eliminate the necessity for lightexposure as the preliminary step in the geotropie response of these roots. Lateral redistribution of an inhibitor to the lower side of a geotropically stimulated root might increase its effective concentration and so depress the growth rate of the lower half of the organ to a greater extent than the upper half (Hawker, 1932; Went and Thimann, 1937; Audns and Brownbridge, 1957; Bennet-Clark et al., 1959; Shaw and M. B. Wilkins, 1973; Pilet, 1973). This possibility is supported by the present evidence that ABA solutions from 10-9 to 10-5 M are not effective in inhibiting root elongation but are effective in enabling the root to respond to gravity in darkness (Fig. 1). Perception of light is a prerequisite for the perception of gravity in some varieties of maize (Scott and M. B. Wilkins, 1968; Pilet, 1975a; H. Wilkins and Wain, 1975) and the root cap is responsible for light-perception (It. Wilkins and Wain, 1974). Consequently, the loss of georeaction on deeapping the roots of such seedlings (Juniper et al., 1966; Gibbons and M. B. Wilkins, 1970; Pilet, 1971) cannot be explained simply as the removal of the site of graviperception, since the lack of response could be due to the absence of the cap in its role as (a) a lightsensor or (b) a light-sensor and gravity-sensor and, subsequently, as a source of light-induced growth inhibitor(s). The present studies support the view that the cap is responsible for graviperccption, since th decapped root is not able to respond to gravity even in the presence of ABA. Thus in intact seedlings the root cap has a dual role, perceiving both white light and gravity. This conclusion does not appear to be supported by data for the movement of ABA in subapical segments of maize roots (Pilet, 1975b). In such segments, lateral translocation of the inhibitor takes place in response to gravity after decapping, indicating that graviperception occurs in the root itself. The most likely explanation of this discrepancy Nes in the fact that the curvature of the segments, used by Pilet, was assessed 7 h after application of ABA. A decapped root regains the ability to perceive light within 5 h of cap removal (H. Wilkins and Wain, 1974) and decapping stimulates the immediate production of amyloplasts, which are thought to be gravity-sensors (Larsen, 1969; Iverscn, 1969), in the decapped apex (Barlow, 1974). I t is possible, therefore, that the decapped root segments used by Pilet partially
22
H. Wilkins and R. L. Wain
r e g a i n e d t h e a b i l i t y to perceive g r a v i t y d u r i n g t h e t i m e allowed for d e v e l o p m e n t of t h e c u r v a t u r e response. Clearly, t h e p r e s e n t i n v e s t i g a t i o n s d e m o n s t r a t e t h a t abscisic a c i d can fulfil all t h e physiological r e q u i r e m e n t s n e c e s s a r y for i t t o be t h e l i g h t - i n d u c e d endogenous g r o w t h - i n h i b i t o r which controls r o o t e l o n g a t i o n a n d t h e c u r v a t u r e of t h e r o o t s in response to g r a v i t y . H o w e v e r , t h e d e t e c t i o n of I A A in m a i z e r o o t apices (Bridges et al., 1973; E l l i o t a n d Greenwood, 1974; R i v i e r a n d Pilet, 1974) a n d demons t r a t i o n s of a d i s t i n c t u p w a r d l a t e r a l t r a n s l o c a t i o n of gibberellie acid in geot r o p i c a l l y s t i m u l a t e d roots ( E I - A n t a b l y a n d Larsen, 1974; W e b s t e r a n d 1V[.B. W i l k i n s , 1974), raises t h e p o s s i b i l i t y t h a t r o o t g e o t r o p i s m is u n d e r t h e control of m o r e t h a n one g r o w t h - r e g u l a t i n g c o m p o u n d .
References Audus, L. J., Brownbridge, M. E. : Studies on the geotropism of roots. 1. Growth rate distribution during response and the effects of applied auxins. J. exp. Bot. 8, 105-124 (1957) Barlow, P. W. : l~eeovery of geotropism after removal of the root cap. J. exp. Bot. 25, 11371146 (1974) Bennet-Clark, T~ A., Younis, A. F., Esnault, R. : Geotropie behaviour of roots. J. exp. Bot. 10, 69-86 (1959) Bridges, L G., Hillman, J. R., Wilkins, M. B.: Identification and localisation of auxin in primary roots of Zea mays by mass spectrometry. Planta (Berl.) 115, 189-192 (1973) E1-Antably, M. M. H., Larsen, P. : Distribution of gibberellins and abscisic acid in geotropieally stimulated Vicia ]aba roots. Physiol. Plant. (Cph.) 32, 322-329 (1974) Elliot, M. C., Greenwood, M. S. : Indol-3yl-acetie acid in roots of Zea mays. Phyteehemistry 13, 239-241 (1974) Gibbons, G. S. B., Wilkins, H. B.: Growth inhibitor production by root caps in relation to geotropie responses. Nature (Lond.) 226, 558-559 (1970) Hawker, L. F. : Experiments on the perception of gravity by roots. New Phytol. 21, 321-328 (1932) Iversen, T.-H. : Elimination of geotropic responsiveness in roots of cress (Lepidium sativum) by removal of statolith starch. Physiol. Plant. (Cph.) 22, 1251-1262 (1969) Juniper, B. E., Groves, S., Landau-Schachar, B., Audns, L. J. : Root cap and the perception of gravity. Nature (Lond.) 209, 93-94 (1966) Kundu, K. K., Audus, L. J. : Root growth inhibitors from root tips of Zea mays L. Planta (Berl.) 117, 183-186 (1974a) Kundu, K. K., Audus, L. J. : Root growth inhibitors from root cap and root meristem of Zea mays L. J. exp. Bot. 25, 479-489 (1974b) Larsen, P. : The optimum angle of geotropic stimulation and its relation to the starch statolith hypothesis. Physiol. Plant. (Cph.) 22, 469-488 (1969) Pilet, P. E. : Root cap and georeaetion. Nature (Lond.) 288, 115-116 (1971) Pilet, P. E.: Growth inhibitor from the root cap of Zea mays. Planta (Berl.) 111, 275-278 (1973) Pilet, P. E.: Effects of light on the georeaction and growth inhibitor content of roots. Physiol. Plant. (Cph.) 33, 94-97 (1975a) Pilet, P. E.: Abscisic acid as a root growth inhibitor: Physiological analyses. Planta (Berl.) 122, 299-302 (1975b) Rivier, L., Pilet, P. E. : Indolyl-3-acetic acid in cap and apex of maize roots: Identification and quantification by mass fragmentography. Planta (Berl.) 120, 107-112 (1974) Scott, T. K., Wilkins, M. B. : Auxin transport in roots. IV. Effect of light on IAA transport and geotropic responsiveness in Zea roots. Planta (Berl.) 87, 249-258 (1969) Shaw, S., Wilkins, H. B. : The source and lateral transport of growth inhibitors in geotropically stimulated roots of Zea mays and Pisum ~ativum. Planta (Berl.) 109, 11-26 (1973)
ABA and Response to Gravity
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Webster, J. I-I., Wilkins, M. B. : Lateral movement of radioactivity from (14C)GA3 in roots and coleoptiles of Zea mays L. seedlings during geotropic stimulation. Planta (BEE.) 121, 303-308 (1974) Went, F. W., Thimann, K. W. : Phytohormones. New York: Macmillan (1937) Wilkins, H., Larqu6-Saavedra, A., Wain, R.L.: Studies on factors influencing plant roo~ growth. Prec. 8th Int. Conf. Plant Growth Subst. Tokyo (in press) (1973) Wilkins, H., Larqu~-Saavedra, A., Wain, R. L. : Studies on plant growth regulating substances. XXXVIL The effects of light and hormone inhibitors on plant root growth. Ann. appl. Biol. 78, 169-177 (1974a) Wilkins, H., Larqu6-Saavedra, A., Wain, R. L. : Control of Zea root elongation by light and the action of 3,5-diiodo-4-hydroxybenzoic acid. Nature (Lend.) 248, 449450 (1974b) Wilkins, H., Wain, R. L. : The root cap and control of root elongation in Zea mays L. seedlings exposed to white light. Planta (Berl.) 121, 1-8 (1974) Wilkins, H., Wain, R. L.: The role of the root cap in the response of the primary roo~s of Zea mays L. seedlings to white light and to gravity. Planta (Berl.) (in press) (1975)
