Resin beads loaded with indol-3yl-acetic acid (J4AA) were used as asymmetrical donors along the elongation zone of intact primary Zea mays L. roots. A strong ...
Plant Physiol. (1987) 84, 1265-1269 0032-0889/87/84/1265/05/$0l .00/0
Maize Root Growth and Localized Indol-3yl-Acetic Acid Treatment A NEW METHODOLOGICAL APPROACH Received for publication October 30, 1986 and in revised form April 6, 1987
PHILIPPE MEUWLY AND PAUL-EMILE PILET* Institute ofPlant Biology and Physiology of the University of Lausanne, 1015 Lausanne, Switzerland ABSTRACr Resin beads loaded with indol-3yl-acetic acid (J4AA) were used as asymmetrical donors along the elongation zone of intact primary Zea mays L. roots. A strong curvature, towards and above the bead, occurred when UIA was applied at a mean distance of 2.20 mm from the tip. No curvature was detected after applications at 3.89 and 5.71 mm from the tip. Correspondence analysis, a new methodological approach in plant hormone studies, permitted the evaluation of the relative influence of several factors on the curvature observed for each root. The parameters considered were the initial growth rate, the exact location of the bead (1.64-2.73 miflitersfom tip) and the quantity of IAA absorbed. Roots which grew rapidly bent earlier than slowly growing ones and the more basal the treatment was, the less curvature occurred. Surprisingly, the amount of IAA taken up (between 1.2 and 2.2 times the endogenous IA content) was found to have no influence on either the time-course or the magnitude of this growth inhibition (curvature). The usefulness of this multivariate analysis is also discussed.
In early studies, the relationships between IAA and root growth were often analyzed by incubating roots in solutions of IAA. The resultant growth effects were normally expressed as a function of the concentration of IAA used (1, 19, 25, 26). Agar-blocks, containing IAA, have also been used in similar experiments (18, 25, 26). Several limitations linked to both techniques have been mentioned: exodiffusion of endogenous IAA (19, 21), real control of the IAA uptake (2, 4), and changes in IAA metabolic pathways induced by application at high concentrations (16, 28, 29). Furthermore, information about the variability of the initial plant material was often obscured by the calculation of mean values (22, 23). It therefore seems more appropriate to use beads as local donors of IAA (5) at physiological amounts (20) on growing roots. Several parameters will characterize each root before, during, and after the application of IAA. Correspondence analysis seemed a useful method to evaluate the importance of various factors which might control the response of the root during IAA treatment. This method was commonly used in other fields (6, 9, 11); but this is, to our knowledge, the first time in plant hormone studies. Results are given concerning the relative influences of the initial growth rate, the location of the IAA application, and the amount of IAA taken up on the growth of Zea mays roots.
MATERlILS AND METHODS Plant Material. Caryopses of Zea mays L. cv LG 11 (Assoc. Suisse des Selectionneurs, Lausanne, Switzerland) were germinated between plastic frames on moist filter paper (24 h imbibition followed by 48 h culture in the dark at 21 ± 1C). Intact
plantlets with straight primary roots (15 ± 2 mm) were selected in dim green light (530 ± 20 nm; 1.2 x 10-2 W m-2; 17) and kept in Plexiglas chambers (darkness, humid air, 21 ± 1C). Bioassays. The technique using resin beads as IAA donors has already been described (20). Amberlite IRA 400 beads (4 mg of 0.45 ± 0.05 mm in diameter; 20-50 mesh, Fluka, Buchs, Switzerland) were loaded in 2 ml of an IAA (Merck, Darmstadt, R.F.A.) at 10-4 M buffered solution (3,3-DGA' 10-2M/NaOH, pH 5.5) containing [5-3HJIAA (1.73 x 105 Bq; CEA, Gif-surYvette, France) as a tracer. Roots were kept vertically for 4 h in the chambers before bead application to permit stabilization of the growth and to enable recording of their growth rates. Beads loaded with IAA (rinsed with 2 ml of 3,3-DGA buffer prior to use) were then placed laterally for 6 h at 2.20 ± 0.03 mm, 3.89 ± 0.05 mm, or 5.71 ± 0.09 mm from the tip. The exact location of each bead was measured on photographs of the roots 15 min after application. At the end ofthe treatment, roots were individually collected for radioactive-uptake measurements (BETAmatic II, Kontron AG., Zurich, Switzerland). Length and curvature of the roots were recorded automatically every 30 min on photographs (Illford FP4, Olympus OM2N). The films were first photocopied (microfilm reproducer) and then analyzed by using a digitizing pad (HiPAD; Houston Instruments, Austin, TX) interfaced with a microcomputer (ABC 80; Luxor AB, Motala, Sweden). Measurements were done along the root axis between the tip and the base of the root. For some quickly growing roots, gravicurvature might occur before the end of the treatment thus attenuating the measured angle (20). The maximum curvature was therefore determined and used in the correspondence analysis to describe the response of the root. Correspondence Analysis. The aim of this multivariate approach (3, 9, 1 1) was to evaluate the relative importance of the amount of IAA taken up, the initial growth rate, and the exact location of the IAA-loaded bead on the curvature observed. Each root (object, n = 97) was thus characterized by four parameters (descriptors, p = 4): (a) IAA uptake (after 6 h treatment, values from 238-848 pg (20) per root); (b) initial growth rate (0.5 h before treatment, values from 0.18 to 1.69 mm/h); (c) bead location (values from 1.64 to 2.73 mm from the tip); (d) maximum curvature (from 7.5-69.8°). The matrix, expressing each object according to the four descriptors, was transformed in order to normalize the values obtained for each parameter (10 classes of relative intensity from 0 to 9 and equally distributed between the minimal and the maximal values). A contingency table, which bound roots and parameters, has been established (PRECORRES program; 6) and then submitted to a correspondence analysis (CORRES program; 3, 6, 1 1). This analysis considered simultaneously all the objects in the space (p-1 dimensions) generated by the four descriptors 'Abbreviation: 3,3 DGA, 3,3-dimethyl-glutaric acid.
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and these descriptors in the space (n- 1 dimensions) created by these objects. The best representation of this multidimensional ellipsoid was then obtained by projecting all points (objects and descriptors) along its main axes with the first axis giving the highest variation and the second axis (perpendicular to the first one) describing the following highest variation, and so forth. In this case 100% of the variation was expressed on the three (p- 1) available projections (ri, r2, i3). The influence of a parameter on a root was proportional to the distance between their spatial position which was determined by their factorial coordinates. An iterative clustering method which gathered each root around the nearest parameter was then applied to the root population (6, 7). The factorial coordinates of three independent descriptors (IAA uptake, initial growth rate, bead location) defined the position of the starting centroids (Figs. 2, 3). Mean values (Table II) and rates of curvature (Fig. 3) were then calculated for the groups separated with this method. A multiple regression analysis was also performed using the same root population (REGRESSION program) (13). The validation of the biological significance obtained with these statistical tests was assessed by considering separately the importance of each descriptor. For each parameter, two groups formed from 30% of the highest and 30% of the lowest values were compared for their curvature rates (Fig. 5). All differences between mean values were assessed either by the t test for numerous groups which followed a normal distribution or by the Mann-Whitney test for smaller ones. The analyses were performed on CDC CYBER 7328 and NORSK-DATA ND-540 computers.
projected onto planes (7r,,72, 73). The position of each parameter appeared in three-dimension on Figure 2 and projected together with the roots on planes 7r,, 72, 73 (Fig. 3). Of the total variation, 44.4% was expressed on -rx, 40.7% on 7r2, and 14.9% on 7r3. Each root was clustered around the nearest centroid (IAA uptake, initial growth rate, bead location). Three distinct groups appeared clearly (Fig. 3) and corresponded to the relative influence of each parameter on the magnitude of the curvature. The fourth group (22.7% of the population) was obtained from roots not separated after the first iteration and was not significantly different from the whole population. This group will therefore not be discussed further. The amount of IAA taken up (Table II) has been corrected for a mean of 45.8% of IAA metabolism (20). This corresponded for the whole population to 1.6 times (495 ± 16 equivalent-pg/root) the endogenous content (24). Each group was differentiated from the two other (Table II): group 1 was characterized by a high amount of IAA taken up (685 ± 22 pg/root), group 2 had the highest initial growth rate (0.88 ± 0.06 mm/h), and group 3 the most basal bead location (2.45 ± 0.03 mm from tip); no significant differences were observed between their other remaining mean values. Roots from group 3 had a lower maximal curvature rate than those from groups 1 and 2, characterized by a similar value (Fig. 4). Roots from group 2 bent more and quicker than those from both other groups (Table II; Fig. 4). These results, which has been obtained with the correspondence analysis, were confirmed (a) by usual selection where the importance of each parameter was considered separately: 30% of the highest values and 30% of the lowest values into two
RESULTS Importance of a Local IAA Application. Roots treated at the beginning of the elongation zone (at 2.20 ± 0.03 mm counted from the tip) bent towards and above the asymmetrical IAA supply (Fig. 1). In contrast, IAA application at 3.89 ± 0.05 mm or at 5.71 ± 0.09 mm did not induce any bending. Data given in Table I show that the radioactive uptake was identical for both apical bead locations and slightly lower for beads placed at 5.71 ± 0.09 mm. Bead Location, Initial Growth Rate, and IAA Uptake Influences on Root Growth. A correspondence analysis, projecting roots and descriptors in a multidimensional space, was done for the whole population of roots treated at 2.20 ± 0.03 mm. An ellipsoid having main axes (x, y, z) was thus generated and then
Table I. Radioactive Uptake ofPHJIAA by Maize Roots in Relation to the Location of the IAA Application and the Initial Growth Rate IAA-loaded bead, containing ['H]IAA as a tracer, was placed for 6 h at three different locations along the elongation zone of roots of 15 ± 2 mm in length. Mean values calculated for a given number of roots. Bead Initial Number of Uptake of Location Growth Rate Roots [3H]IAA mm+s dpm+ mm ± SE/h dpm SEroot from tip 97 2.20 ± 0.03 0.61 ± 0.03 202 ± 6 202 ± 12 33 3.89 ± 0.05 0.56 ± 0.04 5.71 ±0.09 0.56±0.04 169± 10 36
LLr
12
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I
I
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FIG. 1. Mean curvature rate (in degrees sE/h) of 15 ± 2 mm roots as a function oftime (in h) before and during a local IAA treatment. IAA loaded bead is applied after a 4.25 h growth stabilization period (see arrow) at three different locations (in mm ± SE from tip) on one side of the root.
_
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location
A
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.
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distinct groups (Fig. 5). Except for the mean values given (Fig. 5), each of these selected groups did not significantly differ either from the remainder 40% or from the whole population. (b) By a multiple regression analysis coupled with a Pearson test of correlation. No significant relation was established between the IAA uptake and the maximal curvature rate (R2 = 0.005), which was in contrast dependent of the initial growth rate (R2 = 0.251) and the bead location (R2 = 0.075) parameters. Furthermore, no significant dependence between the IAA uptake and, respectively, the initial growth rate (R2 = 0.012) and the exact bead location (R2 = 0.005) were found by another correlation analysis which plotted one parameter against the other (SCATTERGRAM program; 13). I
DISCUSSION At least three problems have to be briefly discussed. (a) The location of the asymmetrical application of IAA seemed to control the occurrence and the magnitude of the root curvature. (b) The time course of this curvature was dependent to the initial growth rate. (c) The amount of IAA absorbed (in between 1.22.2 times the endogenous content of IAA in whole roots; 24) had no significant influence on the root response. Both acropetal and basipetal IAA movement have been described in the elongation zone of the root (5, 8, 15). IAA metabolism has been reported both in the cortex of root segments treated with IAA (14) and in intact roots after local applications of IAA (20). As the uptake of radioactive IAA applied at 2.20 and 3.89 mm was indentical (Table I) and that similar relative elongation rate have been reported at these two parts of growing roots (23), similar root behavior could be expected for both assays. However, bending towards and above the bead (reflecting a growth inhibition due to IAA) appeared rapidly and only during treatment at 2.20 mm (Fig. 1). Whether curvature was due to a slight basipetal movement of applied IAA (5, 20), to a higher hormonal sensitivity (27) of apical cells, and/or to changes (rates and/or pathways) of IAA metabolism along the root should be
[
i
FIG. 3. Projection of roots and parameters on three planes (r,, 72, T3) defined by the main axes (x, y, z) of the ellipsoid generated by a correspondence analysis. Roots (n = 97) treated at an average distance of 2.20 mm from their tip are described by four parameters: 1, IAA uptake; 2, initial growth rate; 3, bead location; 4, maximal curvature. As the influence of a parameter to a root is proportional to the distance between them, roots are clustered in three groups around the nearest descriptive centroid: group I (5) around the IAA-uptake, group 2 (A) around the initial growth rate, and group 3 (0) around the bead location parameter. The fourth group (+), not separated after the first iteration is not different from the whole population.
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Table II. Initial Growth Rate, Bead Location, IAA Uptake, and Maximal Curvaturefor Three Groups ofRoots Separated by a Correspondence Analysis and then Clustered Around the Nearest Centroid Three groups of roots are tested according to the relative influence of three parameters (code number 1, 2, 3; see Figs. 3 and 4). Mean values calculated for a given number of roots. Centroid Initial Growth Rate Bead Location IAA-Uptake Maximal Curvature Number of Roots 1. IAA-uptake 2. IGR 3. Bead location
,
14
mm ± SE
from tip
root ±SE
± sE/h degrees dges±S/
0.51 ± 0.03 0.88 ± 0.06 0.45 ± 0.02
2.09 ± 0.04 2.16 ± 0.05 2.45 ± 0.03
685 22 383 15 419 ± 14
24.1 ± 2.1 30.5 ± 1.6 20.1 ± 1.2
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1
1
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highest initial growth rate
1/
21 26 28
1
highest A- uptake
_ FIG. 4. Mean curvature rate (in degrees ± SE/h) of 15 '2mm roots as a function of time (in h) before and during a local IAAtreatment. AA-loaded bead is apmost basal plied after a 4.25 h growth stabilization period (see arrow) on one side of the root at an averge distance of ~~~~~~~~~~~~~~~~bead location 2.20 mm from the tip. A correspondence analysis coupled with a clustering method separates individual roots according the relative influence of three parameters
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4
6
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the curvature. Moreover, as endogenous IAA level has been to maize root growth (22), it seemed relevant to express correlated also these results according to the initial growth rate ofeach root and to the amount of IAA taken up. The multivariate approach ', did not estimate a priori the importance of any descaptors, as did analyses using selections according to only one parameter: growth rate (22), external AA concentration (19, 28). Data indicated (Fig. 4) that the more basal the IAA treatment occurred bB /7 I lower maximal I curvature rate was. t tat Figure 5C shows(group indeed3),a the difference of 39% in the maximal curvature rate between IAA application at 2.52 mm and 1.87 mm from the tip. As a higher growth rate of maize root was related to a _ lower IAA content (22), any relationships between elongation and IAA treatment should be considered in terms of optimal -_ C IAA content for growth: slowly growing roots, characterized by + < a relatively high endogenous LEA level, will react less to any IAA d supply than rapidly growing ones (see model discussd in Ref. _ \ 22). A significant higher curvature for rapidly growing roots than for slower ones was indeed obtained (Table II). Moreover, it is obvious (Figs.< 4 and SB) that for a higher initial growth rate, the response occurred earlier. Although roots from groups 1 and 2 have absorbed, respectively, 2.2 and 1.2 times the endogenous 12L,k,_,_,_,_,_,__ IAA content (Table II), no significant differences were detected
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FiG. 5. Mean curvature rate (in degrees SE/h) of 15 ± 2 mm roots as a function of time (in h) before and during a local IAA treatment. AA-loaded bead is applied after a 4.25 h growth Stabilization period on D one side of the root at an average distance of 2.20 mm from the tip. the whole population (n = 97), 30% of the highest relative values gSL ^RgE I;From q (a, c, e) and 30% ofthe lowest relative values (b, d, f) are separated for each parameter. A, Selection according to the IAA uptake (a = 705 ± 14 57 '§ 2 pg/root, b = 334 ± 7 pg/root); B, selection according to the initial growth rate (c = 0.96 ± 0.05 mm/h, d = 0.36 ± 0.01 mm/h; C, selection 7 8 9 according to the exact bead location (e = 2.52 ± 0.02 mm from tip, f= TIME (h) 1.87 ± 0.02 mm from tip).
LOCALIZED IAA TREATMENT ON GROWING MAIZE ROOTS between their maximal rate of curvature rate (Fig. 4). The model of auxin action (22) can thus still be accepted but only for IAA amounts below 1.2 times the endogenous content (Figs. 4 and 5A). These results lead to some concluding comments. (a) The location of an IAA-loaded bead on the elongating part ofgrowing roots is crucial to induce a curvature. Knowing the exact site of IAA application is essential to report the effects on root growth and therefore the significance of bathing roots in a solution of IAA might be challenged. (b) The initial growth rate, which has been inversely correlated with the endogenous IAA content in maize roots (22), is a good parameter to characterize the physiological status of each root before any treatment. (c) Although IAA must be present asymmetrically at the bottom of the elongation zone to induce curvature, no relation was found according to the amount of IAA taken up (between 1.2 and 2.2 times the endogenous IAA level). Further studies on the role of IAA on root growth should therefore include treatment with lower amounts of IAA together with determination of the rate of IAA metabolism. (c) The correspondence analysis, on individual roots, permits to consider the relative heterogeneity of the plant material (growth rates, hormone amounts, and so forth) and to determine the influence of several parameters on the growth response induced by a local IAA treatment. This multidimensional approach would be especially performing when a higher number of descriptors were used. 1.
2.
3. 4.
5.
6.
LITERATURE CITED AUDUS LJ, ME BROWNBRIDGE 1957 Studies on the geotropism of roots. Growth-rate distribution during response and the effects of applied auxins. J Exp Bot 22: 105-124 BANDURSKI RS, HM NONHEBEL 1985 Auxins. In MB Wilkins, ed, Advanced Plant Physiology, Ed 2. Pitman, London, pp 1-20 BENZECRI J-P et al. 1980 L'analyse des correspondances. In L'analyse des donnees, Ed 3, Vol. 2. Dunod Ed. Paris, pp 1-632 BIALEK K, WJ MEUDT, JD COHEN 1983 Indole-3-acetic acid (IAA) and IAA conjugates applied to bean stem sections. Plant Physiol 73: 130-134 DAVIES PJ, JA DORO, AW TARBOX 1976 The movement and physiological effect of indoleacetic acid following point applications to root tips of Zea mays. Physiol Plant 36: 333-337 DELARZE R 1986 Approche biocenotique des pelouses steppiques valaisannes. PhD thesis. University of Lausanne, Lausanne
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7. DIDAY E 1972 Nouvelles methodes et nouveaux concepts en classification automatique et reconnaissance des formes. PhD thesis. University of Paris VI, Paris 8. GOLDSMITH MHM 1977 The polar transport of auxin. Ann Rev Plant Physiol 28: 439-478 9. HILL MO 1974 Correspondence analysis: a neglected multivariate method. Appl Stat 23: 340-354 10. HOFER R-M, P-E PILET 1986 Structural and cytochemical analysis of the cell walls in growing maize roots. J Plant Physiol 122: 395-402 11. LEGENDRE L, P LEGENDRE 1979 La structure des donnees 6cologiques. In Ecologie Numerique, Vol 2. Masson Ed. Paris, pp 1-237 12. MARTIN HV, P-E PILET 1986 Saturable uptake of indol-3yl-acetic acid by maize roots. Plant Physiol 81: 889-895 13. NIE NH, CH HULL, JG JENKINS, K STEINBRENNER, DH BENT 1975 Statistical Package for the Social Sciences (SPSS), Ed 2. Mc Graw-Hill, New-York, pp 1475 14. NONHEBEL HM, JR HILLMAN, A CROZIER, MB WILKINS 1985 Metabolism of ['4Clindole-3-acetic acid by the cortical and stelar tissues of Zea mays L. roots. Planta 164: 105-108 15. PERNET J-J, PILET P-E 1979 Importance of the tip on the [5-3H]-Indole-3y1acetic acid transport in maize root. Z Pflanzenphysiol 94: 273-279 16. PILET P-E 1964 Processus d'induction ou d'adaptation auxine-oxydasiques. CR Acad Sci Paris 259: 1183-1186 17. PILET P-E 1979 Kinetics of the light-induced georeactivity of maize roots. Planta 145: 403-404 18. PILET P-E, ELLIOTT MC 1981 Some aspects of the control of root growth and georeaction: the involvement of indoleacetic acid and abscisic acid. Plant Physiol 67: 1047-1050 19. PILET P-E, MC ELLIOTT, MM MOLONEY 1979 Endogenous and exogenous auxin in the control of root growth. Planta 146: 405-408 20. PILET P-E, P MEUWLY 1986 Local Indole-3-acetic acid application by resin beads to growing maize roots. Planta 169: 16-22 .21. PILET P-E, J-E REBEAUD 1983 Effect of abscisic acid on growth and indolyl-3acetic acid levels in maize roots. Plant Sci Lett 31: 117-122 22. PILET P-E, M SAUGY 1985 Effect of applied and endogenous indol-3yl-acetic acid on maize root growth. Planta 164: 254-258 23. PILET P-E, J-M VERSEL, G MAYOR 1983 Growth distribution and surface pH patterns along maize roots. Planta 158: 398-402 24. SAUGY M, P-E PILET 1984 Endogenous indol-3yl-acetic acid in stele and cortex of gravistimulated maize roots. Plant Sci Lett 37: 93-99 25. Scorr TK 1972 Auxins and roots. Annu Rev Plant Physiol 23: 235-258 26. TORREY JG 1976 Root hormones and plant growth. Annu Rev Plant Physiol 27: 435-459 27. TREWAVAS A 1981 How do plant growth substances work? Plant Cell Environ 4: 203-228 28. VENIS MA 1972 Auxin-induced conjugation systems in peas. Plant Physiol 49: 24-27 29. ZERONI M, MA HALL 1980 Molecular effects of hormone treatment on tissue. In J MacMillan, ed, Encyclopedia of Plant Physiology, Vol 9. SpringerVerlag, Berlin, pp 511-568
