The Role of Auxin in the Apical Regulation of Leaf Abscission in Cotton (Gossypium hirsutum L.)

The petiole abscission induced by deblading cotyledonary leavesof cotton (Gossypium hirsutum L. cv. Delta Pine) was acceleratedby the presence of the intact shoot apex or, in decapitatedplants and explants, by application to the stem (proximal application)of indol-3yl-acetic acid (IAA) or 1-aminocyclopropane-l-carboxylicacid (ACC). IAA and ACC accelerated the abscission of debladedpetioles whether applied above or below the cotyledonary node.Transport of IAA to the node was not required for the responseto proximal IAA. [2,3-¹⁴C]ACC was readily transported to thenodal region whether applied to the stem above or below thenode. Application of IAA or ACC to the stem did not induce theabscission of intact leaves or of debladed petioles treateddistally with IAA The acceleration of abscission by proximal IAA, but not thatcaused by ACC, was prevented if explants were treated with a-aminooxyaceticacid (AOA), an inhibitor of ACC-synthase. AOA also preventedthe acceleration of abscission caused by the shoot apex. Theprogress of abscission in debladed explants was greatly delayedby silver thiosulphate (STS—an inhibitor of ethylene action),whether or not the explants were treated with IAA or ACC. Itis suggested that the speeding effects of the shoot apex andof proximal auxin on the abscission of debladed petioles requiresauxin-induced ACC synthesis. The possibility is discussed thatACC may function as a mobile abscission promoter Key words: Abscission, ACC, ACC-synthase, cotton (Gossypium), proximal auxin     

Auxin transport and the regulation of petiole abscission in cotton (Gossypium hirsutum L.): A comparison of indol-3yl-acetic acid and phenylacetic acid

Distal applications of indol-3yl-acetic acid (IAA) to debladed cotyledonary petioles of cotton (Gossypium hirsutum L.) seedlings greatly delayed petiole abscission, but similar applications of phenylacetic acid (PAA) slightly accelerated abscission compared with untreated controls. Both compounds prevented abscission for at least 91 h when applied directly to the abscission zone at the base of the petiole. The contrasting effects of distal IAA and PAA on abscission were correlated with their polar transport behaviour-[1-¹⁴C]IAA underwent typical polar (basipetal) transport through isolated 30 mm petiole segments, but only a weak diffusive movement of [1-¹⁴C]PAA occurred.Removal of the shoot tip substantially delayed abscission of subtending debladed cotyledonary petioles. The promotive effect of the shoot tip on petiole abscission could be replaced in decapitated shoots by applications of either IAA or PAA to the cut surface of the stem. Following the application of [1-¹⁴C]IAA or [1-¹⁴C]PAA to the cut surface of decapitated shoots, only IAA was transported basipetally through the stem. Proximal applications of either compound stimulated the acropetal transport of [¹⁴C]sucrose applied to a subtending intact cotyledonary leaf and caused label to accumulate at the shoot tip. However, PAA was considerably less active than IAA in this response.It is concluded that whilst the inhibition of petiole abscission by distal auxin is mediated by effects of auxin in cells of the abscission zone itself, the promotion of abscission by the shoot tip (or by proximal exogenous auxin) is a remote effect which does not require basipetal auxin transport to the abscission zone. Possible mechanisms to explain this indirect effect of proximal auxin on abscission are discussed.     

STRUCTURAL CHANGES DURING BEAN LEAF ABSCISSION

Anatomical changes in the laminar abscission zone of primary leaves of Phaseolus vulgaris L. ‘Red Kidney’ have been examined in conjunction with the regulation of abscission by growth substances. Quantitative measurements were made of the frequency of vascular obstructions (tyloses, callose plugs). The development of abscission was correlated with an increasing frequency of tyloses and other plugging materials in the xylem of the abscission zone coupled with the dissolution of callose from the abscission zone sieve tubes. These changes were accelerated in petiole explants in which abscission was stimulated by either ethylene or auxin and were suppressed in explants in which abscission was inhibited by auxin.     

Studies on the abscission of blue lupin leaves

Summary The responses of three types of explants of blue lupin leaves are considered: pulvinar explants, consisting of the pulvinar region alone, petiolar explants, consisting of the pulvinar region plus petiole and laminar explants consisting of the pulvinar region plus leaflets. Abscission is accelerated by removal of the leaflets; removal of the petiole has much less effect. Pulvinar explants fail to abscise in darkness but are the first to abscise in the light. This is in accordance with previous evidence of high light sensitivity of the pulvinar region. Kinetin applied directly to the pulvinar region delays abscission, as does kinetin supplied via the transpiration stream. As shown by experiment, this is probably due to transported kinetin reaching the abscission zones of the pulvinar region. The effects of photoperiodic treatments on explants or whole leaves are described. Abscission in the whole leaf is delayed by short daily photoperiods; the delay reaches a maximum with 8 hours light per day. However, abscission is more rapid in continuous light than in darkness. Removal of the leaflets greatly accelerates abscission even in darkness. The pulvinar explant fails to abscise with photoperiods of 4 hours or less; although it appears to have a long day response, preliminary attempts failed to demonstrate that this is a true photoperiodic response (replacement of a long day by a short day together with a light break). The complex responses of leaves and explants to day length lend further support to the hypothesis that light has effects on abscission other than in photosynthesis.     

Role of IAA-Oxidase in Abscission Control in Cotton

The potential role of indoleactic acid (IAA)-oxidase as an in vivo abscission regulating system in the cotton (Gossypium hirsutum L.) cotyledonary explant was investigated. Phenols (usually monophenols), which are cofactors of cotton IAA-oxidase in vitro, accelerated abscission. Phenols (usually orthodihydroxyphenols), which inhibit cotton IAA-oxidase in vitro, inhibited abscission. Inhibition or stimulation of abscission was accomplished by phenols both with and without IAA. Results were similar when treatments were applied as lanolin pastes to the cut petiole ends or as solutions in which explants were submerged. An abscission accelerating phenol stimulated the decarboxylation of IAA-1-¹⁴C by explants and an abscission inhibiting phenol inhibited the decarboxylation of IAA-1-¹⁴C.     

Methyl jasmonate induces the formation of secondary abscission zone in stem of Bryophyllum calycinum Salisb

Methyl jasmonate (JA-Me) at a concentration of 0.5 % induced the formation of secondary abscission zone and senescence in several types of stem explants (only internode segment, internode segment with nodes and without leaves, internode segment with nodes and debladed petioles) of Bryophyllum calycinum when it was applied in various places of the stem or the debladed petiole as lanolin paste. In the presence of small leaves in stem explants methyl jasmonate also induced the formation of secondary abscission zone and senescence but the presence of larger leaves completely inhibited methyl jasmonate-induced processes. Auxin, (indole-3-acetic acid, IAA), at a concentration of 0.1 % extremely prevented the formation of secondary abscission zones and senescence in the stem tissues induced by methyl jasmonate. Similar relationship between auxin and methyl jasmonate to induce the formation of secondary abscission zone and senescence was found in decapitated shoot of the intact plant. Mechanisms of the formation of secondary abscission zone are also discussed in terms of the interaction of methyl jasmonate with auxin.     

Retardation of abscission of citrus leaf and fruitlet explants by brassinolide

Brassinolide (BR), a novel plant growth-regulating steroidal lactone, markedly retarded the abscission of leaf explants of Calamondin (Citrus madurensis Lour.), when dissolved in water and fed through the petiole. BR was effective at concentrations as low as 0.021 M, and showed a stronger effect than IAA which also retarded abscission. Trifluoperazine (TFP), an inhibitor of the calmodulin-calcium complex, accelerated abscission, and this acceleration could be counteracted by a simultaneous addition of IAA or BR, the effect of IAA being stronger. BR in lanolin applied to the cut surface of the leaf blade of the explant showed a weaker abscission-retarding effect than that applied in water via the petiole. BR and IAA also markedly retarded the abscission of fruitlet explants of Calamondin.     

GIBBERELLIN AND AUXIN RELATIONSHIPS IN ABSCISSION

Gibberellic acid (GA) has no effect on abscission when applied proximally or distally to the abscission zones of debladed petioles of Coleus. Application of GA to the stem apex increases the rate of abscission of debladed petioles. The effect on abscission is accompanied by an increase in the level of endogenous auxin in the stem. Correspondingly proximal applications of indoleacetic acid (IAA) accelerate abscission, whereas the longevity of the debladed petiole approaches that of the intact leaf only in the presence of a continuous distal supply of IAA. No correlation is found between petiole elongation and its longevity. The experimental data support the view that auxin acts at the abscission zone in regulating separation processes and that the effect of GA is through its effect on the level of endogenous auxin.     

Abscission: the phytogerontological effects of ethylene

The role of ethylene in the aging of bean (Phaseolus vulgaris L. cv. Red Kidney) petiole abscission zone explants was examined. The data indicate that ethylene does accelerate aging in addition to inducing changes in break strength. Application of ethylene during the aging stage (stage 1) promoted abscission when followed by a second ethylene treatment during the cell separating stage (stage 2). The half-maximal effective concentration of ethylene to induce aging was around 0.3 microliter per liter; 10 microliters per liter was a saturating dose. CO₂ reversal of ethylene action during stage 1 was incomplete and gave ambiguous results. CO₂ (10%) reversed the effect of 10 microliters per liter ethylene but not 1 microliter per liter ethylene. The possibility that ethylene not only accelerated aging but was also a requirement for it was tested, and experimental evidence in favor of this idea was obtained. It was concluded that ethylene plays a dual role in the abscission of bean petiole explants: a phytogerontological effect and a cellulase-inducing effect.     

The physiological significance of phenylacetic Acid in abscising cotton cotyledons

The physiological role of phenylacetic acid (PAA) as an endogenous regulator of cotyledon abscission was examined using cotton (Gossypium hirsutum L. cv LG 102) seedlings. Application of 100 micromolar or more PAA to leafless cotyledon abscission-zone explants resulted in the retardation of petiole abscission and a decrease in the rise of ethylene evolution that normally accompanies aging of these explants in vitro. The partial inhibition of ethylene evolution in these explants by PAA was indirect since application of this compound stimulated short-term (<24 hours) ethylene production. PAA treatment partially suppressed the stimulation of petiole abscission elicited by either ethylene or abscisic acid. Both free and an acid-labile, bound form of PAA were identified in extracts prepared from cotyledons. No discernible pattern of changes in free or bound PAA was found during the course of ethylene-induced cotyledon abscission. Unlike indole-3-acetic acid, transport of PAA in isolated petiole segments was limited and exhibited little polarity. On the whole, these results are not consistent with the direct participation of PAA in the endogenous regulation of cotyledon abscission.     

Effect of Treatments with Growth Retardant Chemicals on Petiole Abscission in Coleus

A bscission of debladed petioles of Coleus was observed following spray applications of growth retardant chemicals and particularly of Phosfon D to the foliage. Sprays were applied to some branches, which were left intact (inducing branches), or to adjacent branches the leaves of which were later debladed (induced branches). In all experiments two applications of growth retardant chemicals were made, after which the induced branches were debladed. Treatments on induced branches accelerated the petiole abscission relative to the controls. Treatments on inducing branches, instead, decreased abscission speed of debladed petioles. The evidence suggests that phosfon D affects abscission by interfering with the indoleacetic acid mechanism.     

Mechanism of Action of Abscission Accelerators

Abscission zone explants of Gossypium hirsutum L., Cassia fistula L., and Coleus blumei Benth. were used to investigate correlations between endogenous rates of ethylene evolution and time of abscission. Additions of 0.1 nl/ml ethylene to the explants markedly accelerated abscission; continuous aeration of the explants, to prevent accumulation of small amounts of endogenously produced ethylene, inhibited abscission compared with that of sealed controls. Substances that stimulated abscission simultaneously accelerated ethylene evolution on all three species and at any position of application. The positional effects of auxin are explained as being due to differences in transport in the explant. Thus, distally applied auxin inhibits abscission, regardless of the accelerated rate of ethylene evolution, by being rapidly transported to the abscission zone. Auxin applied proximally stimulates abscission because it is unable to move as rapidly to the abscission zone and the ethylene effect becomes dominant. Ethylene was found to be most effective on aged tissues, and it is concluded that abscission rates are determined by an increase in sensitivity of the tissue to the ethylene that is already being produced.     

Inhibition of Abscission of Bean Petiole Explants by Lepidimoide

The effect of lepidimoide on the process leading to abscission was studied in bean (Phaseolus vulgaris L. cv. Masterpiece) petiole explants. The assays, involving observations on the junction of the petiole of primary leaves and the pulvinus, were conducted in the light. Lepidimoide, at concentrations of 1 μm or higher, delayed the abscission process; however, the progression of abscission proceeded at normal rates, and complete abscission resulted. On the other hand indoleacetic acid inhibited the normal senescence resulting in greatly decreased abscission during the observation period. These observations show that lepidimoide only delays abscission, and the kinetics seem to indicate that lepidimoide and indoleacetic acid affect abscission through different mechanisms. Received March 1, 1996; accepted November 4, 1996     

EFFECTS OF INDOLE-3-ACETIC ACID AND PARACHLOROPHENOXY-ISOBUTYRIC ACID ON ABSCISSION IN PETIOLES OF DEBLADED LEAVES OF PHASEOLUS VULGARIS

The effects of indole-3-acetic acid (IAA) and p-chlorophenoxyisobutyric acid (PCIB) on rates of abscission layer formation and abscission were investigated. The primary leaves of Phaseolus vulgaris were used as test material. Treatment at the distal end of one petiole of the pair from debladed primary leaves with 1% IAA inhibited the abscission of that petiole and accelerated the abscission of its opposite untreated partner. PCIB applied simultaneously with IAA counteracted the accelerating effect of IAA on the opposite untreated petiole. This influence increased with increasing concentrations of PCIB. Anatomical studies revealed that PCIB, although it counteracted the effect of IAA on the rate of abscission, had no effect on abscission layer formation. In other words abscission layer formation takes place under the influence of the auxin despite the presence of the antiauxin. The centripetal sequence of abscission layer formation was found in all cases.     

Abscission of citrus leaf explants: no correlation with naphthaleneacetic Acid conjugation in the abscission zone

The role of α-naphthaleneacetic acid (NAA) in the control of abscission in Citrus (Citrus sinensis L. Osbeck) leaf explants and its conjugation were studied in non-aged and 24-hour-aged explants. Dipping non-aged explants in 1.5 micromolar NAA for 15 minutes immediately after excision did not delay abscission whereas 150 micromolar NAA effectively delayed it. As incubation time was prolonged up to 24 hours after excision, the delaying effect of both concentrations gradually increased. In general, both concentrations did not delay abscission when applied to 24-hour-aged explants held for an additional period of up to 24 hours. The uptake and conjugation of ¹⁴C-NAA to glucose and aspartic acid were similar in petiole, abscission zone, and leaf blade of non-aged and aged tissues, for all NAA concentrations. No correlation was established between the kinetics of abscission and the rate of conjugation in the abscission zone.     

Probing Monocarpic Senescence and Pod Development Through Manipulation of Cytokinin and Mineral Supplies in Soybean Explants

Defined solutions containing cytokinin and/or mineral nutrientswere supplied in lieu of the roots through the cut stem baseof soybean explants (a leaf with associated pod and subtendingstem segment) in order to analyze the roles of cytokinin andmineral nutrients from the roots in pod development and foliarmaintenance. In explants cut at early-mid podfill, supplyingonly H₂O accelerated leaf senescence and pod maturation anddecreased seed d. wt relative to comparable parts of intactplants. Zeatin (Z) and/or minerals not only delayed leaf yellowingand the decline in foliar chlorophyll levels and photosyntheticrates but also inhibited leaflet and petiole abscission relativeto H₂O controls. Even large declines in foliar assimilatoryprocesses did not necessarily lead to abscission. Z and/or mineralsalso increased stomatal conductivity throughout podfill. Z showedsome positive synergistic effects with minerals on leaf maintenance.Pod wall, cotyledon and radicle yellowing were delayed by Zand/or minerals but not as much as leaf senescence. Mineralsonly or Z +minerals prolonged seed d. wt accumulation and increasedfinal dry seed wt to a level similar to that for intact plants.Seed growth showed a complex interrelation with pod wall andleaf f. wt and d. wt changes. A decline in cytokinin and mineralflux from the roots appears to be important for pod-inducedleaf senescence; however, pod development, foliar senescenceand their component processes may be affected differently. Thus,even though the explant is a physiological/nutritional moduleof the whole plant, it is influenced by cytokinin and mineralsfrom the roots and therefore only semiautonomous. Glycine max L. Merr. cv. Anoka, soybean, abscission, cytokinin, chlorophyll, mineral nutrients, seed development, semiautonomous physiological modules, senescence, stomatal resistance