Release Of Lateral Buds Research Articles

Jujube witches' broom phytoplasma effectors SJP1 and SJP2 induce lateral bud outgrowth by repressing the ZjBRC1-controlled auxin efflux channel.

Comprehensively controlling phytoplasma-associated jujube witches' broom (JWB) disease is extremely challenging for the jujube industry. Although the pathogenesis of phytoplasma disease has been highlighted in many plant species, the release of lateral buds from dormancy under JWB phytoplasma infection has not been characterized in woody perennial jujube. Here, two 16SrV-B group phytoplasma effectors, SJP1 and SJP2, were experimentally determined to induce witches' broom with increased lateral branches. In vivo interaction and subcellular localization analyses showed that both SJP1 and SJP2 were translocated from the cytoplasm to the nucleus to target the CYC/TB1-TCP transcription factor ZjBRC1. The N- and C-terminal coiled-coil domains of SJP1 and SJP2 were required for the TCP-binding ability. ZjBRC1 bound directly to the auxin efflux carrier ZjPIN1c/3 promoters and down-regulated their expression to promote the accumulation of endogenous auxin indole-3-acetic acid in jujube calli. Furthermore, JWB phytoplasma infection suppressed ZjBRC1 accumulation and induced ZjPIN1c/3 expression to stimulate lateral bud outgrowth. Therefore, SJP1 and SJP2 stimulate lateral bud outgrowth, at least partly, by repressing the ZjBRC1-controlled auxin efflux channel in jujube, representing a potential strategy for comprehensive phytoplasma-associated disease control and a resource for gene editing breeding to create new cultivars with varying degrees of shoot branching.

Elevated auxin and reduced cytokinin contents in rootstocks improve their performance and grafting success.

SummaryPlant grafting is an important technique for horticultural and silvicultural production. However, many rootstock plants suffer from undesirable lateral bud outgrowth, low grafting success rates or poor rooting. Here, we used a root‐predominant gene promoter (SbUGT) to drive the expression of a tryptophan‐2‐monooxygenase gene (iaaM) from Agrobacterium tumefaciens to increase auxin levels in tobacco. The transgenic plants, when used as a rootstock, displayed inhibited lateral bud outgrowth, enhanced grafting success rate and improved root initiation. However, root elongation and biomass of SbUGT::iaaM transgenic plants were reduced compared to those of wild‐type plants. In contrast, when we used this same promoter to drive CKX (a cytokinin degradation gene) expression, the transgenic tobacco plants displayed enhanced root elongation and biomass. We then made crosses between the SbUGT::CKX and SbUGT::iaaM transgenic plants. We observed that overexpression of the CKX gene neutralized the negative effects of auxin overproduction on root elongation. Also, the simultaneous expression of both the iaaM and CKX genes in rootstock did not disrupt normal growth and developmental patterns in wild‐type scions. Our results demonstrate that expression of both the iaaM and CKX genes predominantly in roots of rootstock inhibits lateral bud release from rootstock, improves grafting success rates and enhances root initiation and biomass.

Rhizobium-induced elevation in xylem cytokinin delivery in pigeonpea induces changes in shoot development and leaf physiology.

Inoculation with Rhizobium strain IC3342 induces in pigeonpea (Cajanus cajan (L) Millsp.) a leaf curl syndrome and elevated cytokinin levels in the xylem sap. High nitrogen (N) nutrition was found to inhibit onset of the syndrome which could then be induced by N-free nutrient after development of seven trifoliate leaves. This provided a new system to study the role of xylem cytokinin in shoot development and yielded plants suitable for determining the rate of delivery of xylem cytokinin to the shoot which for IC3342-inoculated plants was found to be three times that of control plants. Relative to leaves of control plants, the non-curled leaves of these IC3342 plants exhibited higher nitrogen and chlorophyll content and greater photosynthetic rate and stomatal conductance. Induction of the syndrome increased leaf thickness in developing leaves but not in expanded leaves already formed. Diameter of stems and number of laterals were also increased markedly by IC3342 inoculation which in addition induced leaf hyponasty. Exogenous cytokinins when applied directly to control leaves induced leaf curl and increased leaf thickness. The present studies are discussed in relation to the role of xylem cytokinins in plant development and especially the release of lateral buds from apical dominance.

Cyclanilide Alterations In Growth Of Five Landscape Plant Species During Container Production Are Location Specific

Abstract Five species of tropical and warm temperate shrubs, bush morning glory (Ipomoea carnea N. von Jacquin subsp. fistulosa (K. von Martinus ex J. Choisy) D. Austin), Indian hawthorn (Raphiolepis indica (L.) J. Lindley), tropical hibiscus (Hibiscus rosa-sinensis L. ‘Brilliantissima’), jatropha (Jatropha integerrima N. von Jacquin), and nandina (Nandina domestica C.P. Thunberg), were grown in 2.8-liter (#1) black plastic containers at two contrasting nursery locations and regimes. In College Station, TX, plants were grown in full sun under high fertility conditions, while in Dallas, TX, plants were grown under 40% light exclusion and a lower fertility level. Plants at each location were sprayed to runoff, either once or twice, with a distilled water only control, a Latron B-1956 spreader sticker surfactant at 0.063 mg/liter (0.063 ppm) in distilled water, or cyclanilide [1-(2,4-dichlorophenyaminocarbonyl)-cyclopropane carboxylic acid] at 56, 112, or 223 mg/liter (56, 112, or 223 ppm, respectively) with the Latron surfactant in a distilled water carrier. Half of the plants in all treatments received a single application, while the other half received a second application two to three weeks later. Although growth responses and release of dormancy varied among species and with cyclanilide concentration, plants of most species had greater release of lateral buds from a dormant or quiescent state under the high irradiance and fertility conditions in College Station, compared to the lower irradiance and fertility conditions in Dallas. However, phytotoxicity symptoms as evidenced by lower quality ratings were also more prevalent in College Station. Phytotoxicity was moderate to severe on nearly all jatropha and tropical hibiscus plants exposed to cyclanilide. Toxicity symptoms and significant growth reductions were present on several species at 112 or 223 mg/liter (112 or 223 ppm, respectively). A second application usually increased the effects on growth responses at all concentrations and often induced phytotoxicity symptoms at 56 mg/liter (56 ppm). The disparate responses between the two locations suggest the importance of local growing conditions on the potential efficacy and predictability of responses to cyclanilide applications.

Spatial and temporal changes in multiple hormone groups during lateral bud release shortly following apex decapitation of chickpea (Cicer arietinum ) seedlings

Although the co-ordination of promotive root-sourced cytokinin (CK) and inhibitory shoot apex-sourced auxin (IAA) is central to all current models on lateral bud dormancy release, control by those hormones alone has appeared inadequate in many studies. Thus it was hypothesized that the IAA : CK model is the central control but that it must be considered within the relevant timeframe leading to lateral bud release and against a backdrop of interactions with other hormone groups. Therefore, IAA and a wide survey of cytokinins (CKs), were examined along with abscisic acid (ABA) and polyamines (PAs) in released buds, tissue surrounding buds and xylem sap at 1 and 4 h after apex removal, when lateral buds of chickpea are known to break dormancy. Three potential lateral bud growth inhibitors, IAA, ABA and cis-zeatin 9-riboside (ZR), declined sharply in the released buds and xylem following decapitation. This is in contrast to potential dormancy breaking CKs like trans-ZR and trans-zeantin 9-riboside 5′phosphate (ZRMP), which represented the strongest correlative changes by increasing 3.5-fold in xylem sap and 22-fold in buds. PAs had not changed significantly in buds or other tissues after 4 h, so they were not directly involved in the breaking of bud dormancy. Results from the xylem and surrounding tissues indicated that bud CK increases resulted from a combination synthesis in the bud and selective loading of CK nucleotides into the xylem from the root.

Transport and metabolism of xylem cytokinins during lateral bud release in decapitated chickpea (Cicer arietinum) seedlings

Although cytokinins (CKs) are widely thought to have a role in promoting shoot branching, there is little data supporting a causative or even a correlative relationship between endogenous CKs and timing of bud outgrowth. We previously showed that lateral bud CK content increased rapidly following shoot decapitation. However, it is not known whether roots are the source of this CK. Here, we have used shoot decapitation to instantaneously induce lateral bud release in chickpea seedlings. This treatment rapidly alters rate and direction of solvent and solute (including CK) trafficking, which may be a passive signalling mechanism central to initiation of lateral bud release. To evaluate changes in xylem transport, intact and decapitated plants were infiltrated with [3H]zeatin riboside ([3H]ZR), a water‐soluble blue dye or [3H]H2O by injection into the hypocotyl. All three tracers were recovered in virtually all parts of the shoot within 1 h of injection. In intact plants, solute accumulation in the lateral bud at node 1 was significantly less than in the adjacent stipule and nodal tissue. In decapitated plants, accumulation of [3H]ZR and of blue dye in the same bud position was increased 3‐ to 10‐fold relative to intact plants, whereas content of [3H]H2O was greatly reduced indicating an increased solvent throughput. The stipule and cut stem, predicted to have high evapotranspiration rates, also showed increased solute content accompanied by enhanced depletion of [3H]H2O. To assess whether metabolism modifies quantities of active CK reaching the buds, we followed the metabolic fate of [3H]ZR injected at physiological concentrations. Within 1 h, 80–95% of [3H]ZR was converted to other active CKs (mainly zeatin riboside‐5′phosphate (ZRMP) and zeatin (Z)), other significant, but unconfirmed metabolites some of which may be active (O‐acetylZR, O‐acetylZRMP and a compound correlated with sites of high CK‐concentrations) and inactive catabolites (adenosine, adenine, 5′AMP and water). Despite rapid metabolic degradation, the total active label, which was indicative of CK concentration in buds, increased rapidly following decapitation. It can be inferred that xylem sap CKs represent one source of active CKs appearing in lateral buds after shoot decapitation.

Stimulatory effect of cytokinins and interaction with IAA on the release of lateral buds of pea plants from apical dominance

Lateral buds of pea plants can be released from apical dominance and even be transformed into dominant shoots when repeatedly treated with synthetic exogenous cytokinins (CKs). The mechanism of the effect of CKs, however, is not clear. The results in this work showed that the stimulatory effects of CKs on the growth of lateral buds and the increase in their fresh weights in pea plants depended on the structure and concentration of the CKs used. The effect of N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) was stronger than that of 6-benzylaminopurine (6-BA). Indoleacetic acid (IAA) concentration in shoot, IAA export out of the treated apex and basipetal transport in stems were markedly increased after the application of CPPU or 6-BA to the apex or the second node of pea plant. This increase was positively correlated with the increased concentration of the applied CKs. These results suggest that the increased IAA synthesis and export induced by CKs application might be responsible for the growth of lateral shoots in intact pea plants.

Indole-3-acetic acid and ball gall development on Solidago altissima.

• Indole-3-acetic acid (IAA) concentrations are reported in ball galls, stem tissue, and in the gall-forming larvae of the tephritid fly Eurosta solidaginis, to gain insight into the mechanism of ball gall formation on the goldenrod, Solidago altissima. • IAA was identified and quantified by gas chromatography-mass spectrometry, with [13 C6 ]-IAA as an internal standard. • Concentrations of IAA were higher in gall tissues than in stem tissues on a weight per stem length basis, but not on a weight per weight basis. First instar larvae of E. solidaginis contained IAA at 9µg g-1 f. wt or 1.21ng per larva, a concentration 33-times higher than in control stem tissues. Gall development was unaffected by the early removal of the apical bud and leaves from galls, and lateral bud release was inhibited by galls in terminal positions in the absence of the apical bud, indicating that ball galls act as sources of auxin. • The presence of an E. solidaginis larva results in higher amounts of IAA in a given length of stem than are found in its absence. E. solidaginis larvae may act as point sources of IAA in developing ball galls.

Controlled cytokinin production in transgenic tobacco using a copper-inducible promoter

The cytokinin group of plant hormones regulates aspects of plant growth and development, including the release of lateral buds from apical dominance and the delay of senescence. In this work the native promoter of a cytokinin synthase gene (ipt) was removed and replaced with a Cu-controllable promoter. Tobacco (Nicotiana tabacum L. cv tabacum) transformed with this Cu-inducible ipt gene (Cu-ipt) was morphologically identical to controls under noninductive conditions in almost all lines produced. However, three lines grew in an altered state, which is indicative of cytokinin overproduction and was confirmed by a full cytokinin analysis of one of these lines. The in vitro treatment of morphologically normal Cu-ipt transformants with Cu2+ resulted in delayed leaf senescence and an increase in cytokinin concentration in the one line analyzed. In vivo, inductive conditions resulted in a significant release of lateral buds from apical dominance. The morphological changes seen during these experiments may reflect the spatial aspect of control exerted by this gene expression system, namely expression from the root tissue only. These results confirmed that endogenous cytokinin concentrations in tobacco transformants can be temporally and spatially controlled by the induction of ipt gene expression through the Cu-controllable gene-expression system.

Effect of apex excision and replacement by 1‐naphthylacetic acid on cytokinin concentration and apical dominance in pea plants

As known from literature lateral buds from pea (Pisum sativum) plants are released from apical dominance when repeatedly treated with exogenous cytokinins. Little is known, however, about the endogenous role of cytokinins in this process and whether they interact with basipolar transported IAA, generally regarded as the main signal controlling apical dominance. This paper presents evidence that such an interaction exists.The excision of the apex of pea plants resulted in the release of inhibited lateral buds from apical dominance (AD). This could be entirely prevented by applying 1‐naphthylacetic acid (NAA) to the cut end of the shoot. Removal of the apex also resulted in a rapid and rather large increase in the endogenous concentrations of zeatin riboside (ZR), isopentenyladenosine (iAdo) and an as yet unidentified polar zeatin derivative in the node and internode below the point of decapitation. This accumulation of ZR and iAdo, was strongly reduced by the application of NAA. The observed increase in cytokinin concentration preceded the elongation of the lateral buds, suggesting that endogenous cytokinins play a significant role in the release of lateral buds from AD. However, the effect of NAA on the concentration of cytokinins clearly demonstrated the dominant role of the polar basipetally transported auxin in AD. The results suggest a mutual interaction between the basipolar IAA transport system and cytokinins obviously produced in the roots and transported via the xylem into the stem of the pea plants.

Apical dominance in Phaseolus vulgaris. The triggering effect of shoot decapitation and leaf excision on growth of the lateral buds

It was postulated that the release of lateral buds from apical dominance is triggered by the immediate increase in apoplastic water potential (hydrostatic pressure) that is produced by shoot decapitation and that is rapidly transmitted throughout the plant. In experiments conducted to test this hypothesis the use of a strain gauge transducer capable of measuring bud growth with an accuracy of ± 0.1 μm, showed that growth of the inhibited lateral bud at the primary leaf node of Phaseolus vulgaris (L.) ev. Canadian Wonder was initiated within 1 to 5 s following shoot decapitation or excision of the primary leaves. When only the apical bud was excised the lateral bud showed a brief, transitory growth response of ca 1 min duration, but the axillary buds of the first and second trifoliate leaves were released from inhibition. Decapitation of the shoot just below the first trifoliate leaf induced a lateral bud response characterized by three distinct stages: a) a rapid initial growth response with a mean duration of 4.9 min b) a period of arrested growth, which varied in duration from 2 min to 4 h and c) the subsequent resumption of growth.Excision of both primary leaves induced a rapid but transitory bud response of considerably greater duration than that induced by apical bud excision. Excision of the primary leaves prior to decapitation of the shoot eliminated the phase of arrested growth, which characterized the bud response to decapitation of the intact plant. The rapidity of the bud response to both shoot decapitation and leaf excision and the interaction between the effect of these two treatments are consistent with the hypothesis that competition for water plays a major role in the correlative inhibition of lateral buds.

Cytokinin activities ofN 6-benzyladenosine derivatives hydroxylated on the side-chain phenyl ring

Cytokinin activities ofN 6-benzyladenosine (bzl6Ado) and its derivatives hydroxylated on the side chain phenyl ring inortho, meta, andpara positions were compared in four bioassays based on stimulation of growth of tobacco callus, retention of chlorophyll in excised wheat leaves, dark induction of betacyanin synthesis inAmaranthus cotyledons, and release of lateral buds of pea from apical dominance. In all these bioassays hydroxylation of the phenyl ring of bzl6Ado inortho andpara positions significantly decreased cytokinin activity. Compared with bzl6Ado, the activity was decreased about 10× in the tobacco callus bioassay and wheat leaf chlorophyll retention test, 100× in theAmaranthus betacyanin bioassay, and 20× and 200×, respectively, in the pea bud test. Hydroxylation of the phenyl ring inmeta position increased activity in the tobacco callus, and wheat leaf chlorophyll retention bioassays, 9× and 1.7×, respectively, decreased activity about 2.4× in the pea bud test and was without effect in theAmaranthus bioassay. Cytokinin activity of themeta hydroxy derivative,N 6-(m-hydroxybenzyl) adenosine, was as high as that oftrans-zeatin in all four bioassays. Possible regulation of biological activities of cytokinins by positionally specific hydroxylation of the side chain phenyl ring is discussed with respect to the reported occurrence of natural purinyl cytokinins with aromatic side chains.

Mechanical perturbation-induced ethylene releases apical dominance in Pharbitis nil by restricting shoot growth

Mechanical perturbation (MP, rubbing) of internodes of Pharbitis nil shoots initiates release of lateral buds (LB) from apical dominance within 48 h. Evidence is presented which suggests that MP promotion of LB outgrowth is mediated by ethylene-induced restriction of main shoot growth. Ethylene production in the internodes is stimulated by MP within 2 h. Effects of MP are mimicked by treatments with 1-aminocyclopropane-1-carboxylic acid (ACC) and are negated by the inhibitors of ethylene production or action, aminoethoxy vinylglycine (AVG) and AgNO 3. The fact that effects of MP, ACC and ethylene inhibitors are observed to occur on main shoot growth at least 24 h before they are observed to occur on LB growth suggests a possible cause and effect relationship. MP also causes an increase in internode diameter. MP stimulation of ethylene production appears to be mediated by ACC synthase. The results of this study and our previous studies suggest that apical dominance may be released by any mechanism which induces ethylene restriction of main shoot growth.

Penicillin inhibition of release of lateral buds of gram (Cicer arietinum) seedlings from apical dominance

The effect of penicillin on apical dominance was investigated in gram (Cicer arietinum L.) seedlings. Application of penicillin to the decapitated stump was found to reestablish apical dominance in preventing the growth of lateral buds. Penicillin was seen to mimic auxin action in this system. Penicillin inhibition of lateral bud growth was relieved by the simultaneous application of antiauxins like 2,3,5-triiodobenzoic acid and maleic hydrazide. An increase in auxin-synthesizing capacity of the treated tissue has been suggested as the possible mode of action of penicillin in the regulation of lateral bud growth.

Ethylene and apical dominance

Ethylene is involved in at least two discrete mechanisms in the control of apical dominance: the release of lateral buds from inhibition and their subsequent growth and development. Generally, high levels of freely diffusible ethylene in the apical region of the shoot are conducive to lateral‐bud‐outgrowth, while high ethylene levels in the region of the lateral buds themselves tend to be inhibitory. Threshold ethylene levels concerned with the release of buds from inhibition and with the growth that follows may differ between species. Thus, in some species (e.g. Gossypium) lateral‐bud growth proceeds in the continuing presence of ethylene supplied to the whole plant, whereas in others (e.g. Petunia) the growth of the released lateral buds occurs only when the ethylene is remaved.When ethylene production in Pisum nodal sections is enhanced by exomgenous auxin, growth of the attached buds is suppressed. In the intact plant system, unequivocal evidence has not been established for a role of endogenous ethylene acting directly on lateral buds to effect their inhibition. Apical dominance is not affected by the apptication of ethylene antagonists to the lateral buds of intact plants. Results from different studies have been inconsistent regarding the changes in endogenous ethylene levels in the node/lateral‐bud tissue when the plant is decapitated or when auxin is applied to the stump of the decapitated plant to maintain lateral bud inhibition.While exogenous ethylene supplied to the lateral bud generally increases inhibition, the availability of ethylene, regulated endogenously, is essential to the released bud on the decapitated plant in order to sustain its subsequent development into a lateral shoot. There is evidence that, in certain instances, endogenous ethylene may also be essential in the initial stages of bud development, e.g. in thee early growth that is promoted bmy auxin in Phaseolus or by kinetin in Avena.

Release of lateral buds from apical dominance by glyphosate in soybean and pea seedlings

Application of a sublethal dose of glyphosate (N-[phosphonomethyl]glycine) to the seedlings of soybean (Glycine max L. Merr. cv. Evans) and pea (Pisum sativum L. cv. Alaska) promoted growth of the cotyledonary and other lateral buds. The pattern of the glyphosate-induced lateral bud growth was different from that induced by decapitation. Under the experimental condition, glyphosate did not kill the apical buds. Feeding stem sections of the seedlings with radiolabeled indole-3-acetic acid ([214C]IAA) and subsequent analysis of free [2-14C]IAA and metabolite fractions revealed that the glyphosate-treated plants had higher rates of IAA metabolism than the control plants. The treated pea plants metabolized 75% of [2-14C]IAA taken up in the 4-h incubation period compared to 46.5% for the control, an increase of 61%. The increase was small but consistent in soybean seedlings. As a result, the glyphosate-treated plants had less free IAA and ethylene than the control plants. The increase of IAA metabolism induced by glyphosate is likely to change the auxin-cytokinin balance and contribute to the release of lateral buds from apical dominance in these plants.

Does Ethylene Play a Role in the Release of Lateral Buds (Tillers) from Apical Dominance in Oats?

The growth of lateral buds (tillers), which are undergoing release from apical dominance, was measured in upright and gravistimulated intact Avena sativa L. cv. ;Victory' (oat) shoots as well as in isolated Avena stem segments treated with kinetin and sucrose. During release, the tiller bud initially shows a slow rate of elongation accompanied by swelling. It is followed by a more rapid rate of elongation. Ethylene (C(2)H(4)) production in shoot segments containing a tiller bud was found to occur at the onset of tiller swelling during gravistimulation as well as during inflorescence emergence. Exogenous application of indoleacetic acid or C(2)H(4) inhibits kinetin-induced tiller bud swelling and elongation. However, stem segments pulsed for 24 hours in C(2)H(4) or the C(2)H(4) biosynthesis precursor, 1-amino-cyclopropane-1-carboxylic acid (ACC) and then transferred to kinetin and sucrose, showed a significant increase in swelling elongation as compared with segments maintained under the same conditions but without C(2)H(4) or ACC in the pulse. Segments pulsed for 24 hours with kinetin and sucrose plus the ACC biosynthesis inhibitor, aminoethoxyvinylglycine, or the C(2)H(4) action inhibitor, CO(2), then transferred to kinetin and sucrose medium, showed inhibition of tiller swelling during the pulse and of subsequent elongation. These results indicate that C(2)H(4) plays a role in promoting tiller swelling during the onset of tiller release from apical dominance and may act as a modulator hormone in promoting tiller elongation in the presence of cytokinin.

Effect of Nitrate Level in Nutrient Solutions on Growth and Nitrate Accumulation in Two Plum Clones

Marianna 2624 (Prunus cerasifera EHRH. × P. munsoniana Wight and Hedr.) and Myrobolan 3-J (P. cerasifera) plum were grown in aerated nutrient solutions to study the relationship between nitrate level and vegetative growth. Shoot-to-root ratios increased in Marianna 2624 and Myrobolan 3-J by 2 and 3.5 times, respectively, as the nitrate concentration of the nutrient solution was raised from 0.25 to 2 mM. The number of lateral shoots on Myrobolan 3-J plants increased significantly at increasing nitrate concentrations, but release of lateral buds from correlative inhibition was not apparent among Marianna 2624 plants. Leaf production and total leaf area also increased significantly as Myrobolan 3-J plants were exposed to 0.75 mM or 2 mM nitrate. The failure of Marianna 2624 plants to respond similarly at higher nitrate levels may be due to a more limited capacity for nitrate assimilation. This interpretation is based on significantly higher concentrations of nitrate in Marianna 2624 roots and foliage than in those organs of Myrobolan 3-J plants.

Transport of benzyl-8-14C-adenine in pea seedlings in relation to stem apical dominance

In intact, decapitated and decapitated indole-3-acetic acid (IAA) treated pea seedlings the translocation of benzyl-8-l4C-adenin (14C-BA) from the roots was studied with regard to the release of lateral buds from apex-induced inhibition. In intact plants (controls) a substantial part of the activity was found in the apical part of the epicotyl. Decapitation resulted in the initiation of growth of lateral buds. As early as 24 h after decapitation and application of14C-BA a significantly higher activity was found in growing lateral buds (cotylars) of decapitated plants than in inhibited ones of intact or IAA-treated decapitated plants. The accumulation of14C-activity in stump tops of decapitated plants treated with IAA was associated with the thickening growth.

Hormonal Regulation of Lateral Bud (Tiller) Release in Oats (Avena sativa L.)

Stem segments containing a single node and quiescent lateral bud (tiller) were excised from the bases of oat shoots (cv. ;Victory') and used to study the effects of plant hormones on release of lateral buds and development of adventitious root primordia. Kinetin (10(-5) and 10(-6) molar) stimulates development of tillers and inhibits development of root primordia, whereas indoleacetic acid (IAA) (10(-5) and 10(-6) molar) causes the reverse effects. Abscisic acid strongly inhibits kinetin-induced tiller bud release and elon-gation and IAA-induced adventitious root development. IAA, in combination with kinetin, also inhibits kinetin-induced bud prophyll (outermost leaf of the axillary bud) elongation. The IAA oxidase cofactor p-coumaric acid stimulates lateral bud release; the auxin transport inhibitor 2,3,5-triiodo-benzoic acid and the antiauxin alpha (p-chlorophenoxy)-isobutyric acid inhibit IAA-induced adventitious root formation. Gibberellic acid is synergistic with kinetin in the elongation of the bud prophyll. In intact oat plants, tiller release is induced by shoot decapitation, geostimulation, or the emergence of the inflorescence. Results shown support the apical dominance theory, namely, that the cytokinin to auxin ratio plays a decisive role in determining whether tillers are released or adventitious roots develop. They also indicate that abscisic acid and possibly gibberellin may act as modulator hormones in this system.